quench & squish


Staff member
quench area:
Quench and Squish area explained.jpg

http://www.chevyhiperformance.com/tech/ ... index.html

http://www.chevyhiperformance.com/techa ... index.html



[color=#008000[b]]read thru these threads also[/b][/color]







detonation and rapid heat increases can ruin piston ring seal to the bore wall



Mahle pistons are designed for specific applications with the alloy that is best suited for that particular application.
but keep in mind the better versions of forged pistons tend to have the advantage in both strength and heat tolerances

......................Piston Alloy Comparison
...................................................... 2618
High silicon............................................No silicon
Low expansion........................................expansion
Tighter piston-to-wall clearance................More Piston-to-wall clearance needed
Quiet Operation......................................Noise when cold
Less ductile............................................More ductile
More stable & consistent.........................Higher resistance to detonation
Longer life cycles....................................Shorter life cycles
A zone in the combustion chamber where the flat area of the piston at top dead center is very close to the matching flat area on the combustion chamber in the cylinder head. Because the piston and cylinder head is significantly cooler than the as yet unburned part of the fuel-air mixture (i.e., end gas), they pull the heat from the fuel/air mix trapped between them at tdc , . Because the end gas is now cooler, detonation is quenched or reduced. this mass of fuel/air mix is thrown into the center of the combustion process much like a raw egg would be rapidly exiting from the area between a table top and a rubber mallet if you rapidly smacked down on it, but unlike the egg the combustion chamber contains and directs the splash in a known direction due to its design and clearances However, the process does form unburned hydrocarbons. the quench area is between the piston at top dead center and the flat surface of the cylinder head it consists of the distance between the two, limited to the compressed head gasket thickness and the distance below or above the block surface the piston extends,the OBJECT of including an EFFECTIVE quench area is to force a jet of rapidly moving fuel/air mix to shoot out into the combustion chamber, resulting in a far faster and more efficient burn in the combustion chamber, clearance of less than about .036 results in the piston hitting the heads in some applications, distances over about .044 results in a far less effective jet of fuel/air mix and potential for detonation to occur in the quench area, remember it only works if the head and piston get so close that theres a boundary layer of semi cooled air that's so tight it can,t ignite.
quench acts a bit like throwing a cup of gasoline, hard and fast into a camp fire.......it results in a huge increase in the burn rate compared to pouring the gas slowly into a fire

keep in mind ONLY pressure resulting from the fuel/air mix occurring AFTER TDC adds power or torque, any pressure or fuel burnt during the compression stroke tends to reduce power
if the piston was .020 down the bore at TDC and the compressed head gasket was .024 the quench distance would be .044, but in reality at high rpms its a bit tighter because pistons and rods stretch at high rpm levels just a bit.
try to keep the QUENCH in the .038-.042 for the most effective results, naturally milling the heads has no effect on quench, but milling the block does, simply because while milling the heads raises compression, by making the combustion chamber smaller in most engines it doesn,t change the quench distance, milling or decking the block changes the distance between the piston and head surface so that does alter quench.

An area in the combustion chamber of some engines where the piston squishes or squeezes part of the fuel-air mixture at the end of the compression stroke. As the piston approaches top dead center, the mixture is pushed out of the squish area and this promotes turbulence, further mixing of the fuel-air mixture and more efficient combustion

run less than about .037 thousands and at high rpm levels the pistons might hit the cylinder heads, run more than about .044 thousands the QUENCH effect of forcing the fuel air mix to the center of the cylinder from the cylinders edge area looses both speed and effectiveness, remember the quench area must be so tight that virtually all the fuel/air mix is forced (squished) into the center area and none is allowed to burn until its squirted into the burn area increasing turbulence and burn efficiency
in theory the much better quench, combined with the shorter more compact area the flame front needs to cover and the far higher turbulence combine to allow more of the pressure to build AFTER the crank passes TDC on the end of compression and beginning of the power stroke

its mostly an advantage in that you get a more even and FASTER burn in the cylinder and less chance of detonation, simply because both the lower time and faster pressure curves favor the ignition flame front vs detonation
look, it takes approximately 40 thousands of a second for the flame from the ignition to cross a 4.25" bore,at low rpms and still takes about 15 milliseconds at high RPM due to the much faster movement of the compressed fuel air mix in the cylinders, lets look at what that means
if the Chevy plug is located 4/5ths of the way to one side that's a time of about 32 thousands for the pressure to build as the flame travels 3.4" in the Chevy but in a compact combustion chamber it could only take the cylinder flame front less than 10-20 thousands of a second to travel across the combustion chamber for a complete burn at low rpms, this of course speeds up as the swirl and turbulence increase with increased engine RPMs but the ratios stay similar. this results in more usable energy WORKING on the piston AFTER IT PASSES TOP DEAD CENTER ON THE POWER STROKE. BUT MODERN WEDGE combustion chambers use increased QUENCH to speed the flame front and lower the burn time combined with a smaller combustion chambers.
the difference may be easier to grasp if you think of the quench area as a significant part of the total combustion chamber volume,that's forcing its potential fuel/air mix into the central combustion chamber as a jet of highly compressed F/A mix, like the difference between lighting a cup of gasoline by simply placing it next to a camp fire vs throwing it violently into a camp fire
naturally you need to verify the piston to deck height




the quench (squish) at .035 would be unlikely to be a problem from the octane being to low, angle, but rather a huge potential for the piston too head contact as the rpms build and the rods stretch on the exhaust strokes where there's little compression to slow the rod stretch as they play (crack the whip) at high rpms and if you don,t think rods and pistons expand under high rpm and high heat then you've probably never seen the marks the heads quench areas leaves on heads/pistons at quench distances lower than about .037 which Ive found its best to exceed and stay in the .038-.042 range.
yes its entirely possible to run a .035 quench, but you better be running top quality components and measure very carefully.
get out a .035 feeler gauge and look at it on edge then rock the piston in the bore slightly and you think about it a bit.





http://em-ntserver.unl.edu/Mechanics-Pages/Luke-schreier/unzip/Tension and Compression in Connecting Rods VI.htm

the quench/squish area is generally located opposite the spark plug location side of the bore, and there's generally a matching flat area on the combustion chamber

here the dome and spark plug side is on the right and the flat quench/squish is on the left



here is a cylinder head

the spark plug and dome goes to the exhaust side,(lower edge) the quench squish area is the matching flat area towards the intake on these sbc heads (upper edge in this picture)

the idea, is that as the piston almost impacts the head the two flat areas force the cylinder volume trapped between the flat areas violently toward the spark plug , this speeds the burn rate and tends to limit detonation

think of clapping your hands together violently with a raw egg in your palm, that raw egg will be forced away from the impending impact and squeezed out to spray to the sides, the cylinders volume get sprayed the same way but limited by the bore and chamber in only towards the spark plug direction, thus a high velocity mist of gas and air gets thrown into the area of ignition just as the burn starts, speeding the burn rate and lowering the distance the flame must cross

look at this picture, in a real engine the distance is very tight unlike the diagram and a good percentage of the cylinder volume is compressed and forced into the dome/spark plug area
generally you measure the piston to block/deck height and then add the compressed head gasket thickness to the deck height distance to find the heads distance from the piston, but cross check with
a dial indicator on a deck bridge


and a dial indicator


and re- check with plasti-Gage during the pre- assembly


http://www.kb-silvolite.com/article.php ... ad&A_id=56

http://www.kb-silvolite.com/article.php ... ad&A_id=36


http://www.kb-silvolite.com/test/articl ... ad&A_id=35

and before someone points out that plasti-Gage won,t measure .038-.042, yeah! your correct by itself it wont but Ive cut dime size end tabs from a cheap feeler gauge

next time your looking
for some reason most auto parts stores don,t seem to have feeler gauges but do have (TAPPET GAUGES)
http://www.jcwhitney.com/jcwhitney/prod ... map=27994G

and I usually use a .035 tab with a cross of plasti-gauge on top set on the quench area, its easily placed and removed and measured after spinning the engine over and there's no chance of damage to the piston or head,
lots of guys just use soft solder and a machinists mic., and it works fine in most cases, the potential problem with that is that solder forces the piston to compress the solder and it takes a good deal MORE force to compress solder than a thin thread of plastic, when the rods in compression you'll get a similar clearance, but when the rods whipping the piston around on the exhaust stroke under TENSION that quench distance tends to be closer to the head as the rod,piston and clearances tend to stretch out due to heat and inertial loads due to centrifugal forces, you need to be sure the piston wont hit the head on both the exhaust and compression strokes and holding the piston down, away from the head crushing solder tends to give a slightly wider clearance because it tends to rock the piston in its bore, firmly away from the quench clearance on its piston pin.
yeah! Ive done it both ways and the difference is minor but there is still a difference of a couple thousands, probably not significant in most cases but get that clearance real tight and it might be critical info to know

ID suggest trying for a MINIMUM of .038-.044 on a 496 BBC for quench as the long stroke bbc, or any BBC run over about 6000rpm with a 4" or longer stroke) tends to allow the quench clearance distance to tighten up a bit at high rpms, as parts stretch under stress/inertial loads
Last edited by a moderator:


Staff member
if your getting PINGING/DETONATION
ask yourself these questions, because without info your working blind

remember to get the quench clearance between the head and piston in the .038-.044 range, and check you ignition advance curve, and timing






and at WHAT rpm does the ignition advance reach full advance?
whats the normal engine temp?
what octane fuel are you using?
whats the engines compression ratio?
have you changed cams?
whats the fuel/air ratio?
have you verified TDC?
which spark plugs and gap are you using?
what carb jets and power valve are you using?
whats your fuel pressure?
have you tried retarding the ignition timing?
is the distributor set with a known advance curve?

http://www.projectpontiac.com/ppsite/co ... iew/16/30/



Id call the piston manufacturer and get their tech guys feed back, you'll generally want about a .005 piston to bore clearance with forged pistons but every piston design and manufacturer will have the required specs on file for every piston they make ,that should be carefully followed
don,t forget to ask about ring end gap,ring thickness,ring back clearance and ring groove size, quench distance, compression height, valve notch clearances and cylinder bore hone grits to be used,what piston pin and retainer should be used and what the pistons should weight, always ask "what else should I know?" and "what would be YOUR suggestions here?"

theres dozens of factors that will effect your engines tendency to get into detonation.








Staff member
Magnificent Quench

What is the most, exact precisely defined occurrence in all piston engines? It isn’t ignition timing, combustion, crank indexing, or valve events. It is Top Dead Center. You can’t build an engine with an error at Top Dead Center because TDC is what everything else is measured from. Spark scatter, crank flex and cam timing can move, but TDC is when the piston is closest to the cylinder head in any one cylinder. The combustion process gets serious at Top Dead Center and about 12 degrees after TDC, most engines want to have maximum cylinder pressure. If maximum cylinder pressure occurs 10 degrees earlier or later, power goes away. Normal ignition timing is adjusted to achieve max cylinder pressure at 12 degrees after TDC. If your timing was set at 36 degrees before TDC that is a 48 degree head start on our 12 degree ATDC target. A lot of things can happen in 48 degrees and since different cylinders burn at different rates and don’t even burn at the same rate cycle to cycle, each cylinder would likely benefit from custom timing for each cylinder and each cycle. Special tailored timing is possible but there is an easier way—“Magnificent Quench”. Take a coffee can ½ full of gasoline burning with slow flicking flame. Strike the can with a baseball bat and you have what I would call a “fast burn”, much like what we want in the combustion chamber. The fast burn idea helps our performance engine by shortening the overall burn time and the amount of spark lead (negative torque) dialed in with the distributor. If you go from 36 degrees total to 32 degrees total and power increases, you either shortened the burn time or just had too much timing dialed-in in the first place. If you have really shortened the burn time, you won’t need so much burning going on before Top Dead Center. Now you can retard timing and increase HP. Did you ever have an engine that didn’t seem to care what timing it had? This is not the usual case with a fast burn combustion but an old style engine with big differences in optimum timing cylinder to cylinder will need 40 degrees of timing on some and others only need 26 degrees. If you set the distributor at 34 degrees, it is likely that 4 cylinders will want more timing and 4 cylinders will want less ( V-8). Moving the timing just changes, which cylinders are doing most of the work. Go too far and some cylinders may take a vacation. Now what does quench really do? First, it kicks the burning flame front across and around the cylinder at exactly TDC in all cylinders. Even with spark scatter, the big fire happens as the tight quench blasts the 32 degree old flame around the chamber. Just as with the coffee can, big flame or small flame, hit it with a baseball bat and they are all big instantly. The need for custom cylinder-to-cylinder timing gets minimized with a good quench. The more air activity in a cylinder you have the less ignition timing you are likely to need. When you add extra head gaskets to lower compression you usually lose enough quench that it is like striking the burning coffee can with a pencil. No fire ball here and that .070-.090 quench distance acts like a shock absorber for flame travel by slowing down any naturally occurring chamber activity. A slow burn means you need more timing and you will have more burn variation cycle-to-cycle and cylinder-to-cylinder, result more ping. Our step and step dish pistons are designed not only to maximize quench but to allow the flame to travel to the opposite side of the cylinder at TDC. The further the flame is driven, the faster the burn rate and the less timing is required. The step design also reduces the piston surface area and helps the piston top stay below 600 degree f (necessary to keep out of detonation). All of our forged pistons that are lower compression than a flat-top are step or step dish design. A nice thing about the step design is that it allows us to make a lighter piston. Our hypereutectic AMC, Buick, Chrysler, Ford, Oldsmobile and Pontiac all offer step designs. We cannot design a 302 Chevy step dish piston at 12:1 compression ratio but a lot of engines can use it to generate good pump gas compression ratio. Supercharging with a quench has always been difficult. A step dish is generally friendly to supercharging because you can have increased dish volume while maintaining a quench and cool top land temperatures. You may want to read our new design article for more information. ".

By John Erb
Chief Engineer
KB Performance Pistons


Staff member
Plastigage, a registered trade mark, is used to determine the clearance between a bearing insert and its journal. For the occasional engine builder, using this product is a much cheaper alternative to purchasing sets of OD and ID micrometers, which will easily run into a thousand dollars or more.

The proper way a machinist would determine bearing clearance would be to measure the OD of the journal diameter of a shaft and subtract from that the ID of the installed bearing insert, the difference being the clearance.

The clearance provides space for several events, first shafts and blocks are seldom if ever exactly straight so the clearance provides mechanical space where the shaft can find the center between it and the block. The clearance is then taken up by the oil pumped into the clearance such that the shaft is supported on a wedge of oil all the way around the journal. The clearance also provides a leakage rate for the oil thru the bearing to remove heat and debris, and keeps lubrication on the bearing surfaces which would be worked out by the mechanical forces if these were lubed with grease.

If the clearance is too tight, the shaft and block cannot find a center that provides an oil wedge all around the bearing, then there will be metal to metal contact. This problem is made worse by expansion of parts when they heat up and expand into the clearance. If the clearance is too wide an oil wedge cannot form because the leakage rate out of the bearing is too high, again metal to metal contact will occur.

The factory and bearing manufacturers specify a range of acceptable clearances for your engine. Running on the lower side is good for commuter street use as it minimizes oil loss thru the bearing which reduces the work the oil rings have to do in-order to keep excessive oil thrown from the crank off the cylinder walls. It also allows the factory to run a smaller hence cheaper oil pump, this also reduces parasitic power losses from the crankshaft which improves fuel mileage on their government mandated CAFE tests. Performance use dictates that clearances be set toward the high side to allow for greater thermal expansion that comes with the heat of competition. These wider clearances are usually accompanied with a high volume oil pump to insure enough oil is pumped thru the engine to provide wedge development inside the bearings. This always results in tremendous amounts of oil being slung off the crankshaft which if left uncontrolled will overcome the oil rings ability to scrape it off the cylinder walls. To control this oil, competition engines will include scrapers and windage trays to peel this extra oil off the crank and return it to the pan.

Clearance between the crank's rod journal and bearing, while lacking the long alignment issue of the main journals to block, are subject to many of the same problems in developing an oil wedge between the journal and the bearing insert. There are additional problems with the rods in that there are alternating loads between power stroke and the other strokes that the oil wedge must protect the journal and bearing from. This is made more difficult by the fact the bearings are rather narrow making it easy for the forces on the power stroke to blow the wedge out. Plus these bearings carry the greatest loads which always hit in exactly the same place, so impact and heat are big problems for these bearings requiring a lot of oil flow thru them to cool the working surfaces. Again the manufacturer specifies clearances and the same rules apply here as with the main bearings regarding street to competition use.

The choice of bearing material also affects what clearances should be used. Typically copper/lead, copper/tin, copper/babbitt bearings should run toward the low side to the middle of clearances, aluminum bearings should be set from the middle to upper band of the clearances.

If you're building a full out race motor you can run wider clearances than recommended, but this is the ozone of pros who usually can afford to replace the motor if their clearance choices are wrong. Don't go there unless you've got big bucks and intend to run with the big boys.

Plastigage comes in several size ranges, so you need an idea of where your clearances should be falling in-order to get the right stuff. If the clearances are wrong the only choice is machining. Too tight and the shaft may have to have one or more journals turned down. If the clearance is too wide then either the shaft has to be replaced or it can be turned to a standard undersized with new specific bearings for that undersize. If the shaft is a little too large or small, most bearing companies and the factory include a selection of bearings that range from +/- .0001 to .00015 or .0002 inch around the nominal dimension so you can dial the clearance in pretty well.

Bogie posted this info above


Staff member
kpforce1 posted this

"Though it has been explained many times to countless gear heads (myself included), it still appears that many people do not understand what quench is and why it is so important. I thought I would take a stab at providing the fullest explanation I could with some illustrations.

To begin, we always say quench when referring to a distance from piston to head. For me to fully explain quench I will use two phrases 1.) Quench and 2.) Quench effect (could also be called squish). Quench effect is the actual process of squishing the air and fuel mixture into the combustion area while quench is nothing more than the piston "in the hole" depth (or deck clearance) plus the compressed head gasket thickness. You could also say that quench is the distance from the flat portion of the piston to the bottom flat surface of the head. Seems pretty simple right? Now I’ll start with the confusion.

Quench is NOT affected by the volume of a dome/dish/valve relief or the head combustion chamber size. Only the distance from the top of the piston (flat portion) to the bottom of the head affects quench. The quench effect will vary depending on how much “quench area” there is on your setup, but that will be discussed later. Here is a picture to illustrate what quench is (piston deck clearance + compressed gasket thickness is your quench).

The reason valve dishes, relief’s, and domes do not count for measuring quench is because it’s the distance measured between the flat area on the piston surface and head surface that comes within the desired quench (for my explanation I will use a quench of .05”). This quench area forces all of the A/F mixture in the cylinder into the actual combustion chamber when the piston is at TDC (top dead center). For example, if you are looking at the surface of a dished piston you will always notice a fairly large flat area opposite of where the sparkplug would protrude into the chamber. This area extends out and surrounds the dish itself... this is the potential quench area for the piston. The “actual quench area” is where the distance between the flat surfaces of the piston and head are at or EXTREMELY near our .05” quench. Look at the picture of the head and piston below to see the flat areas I’m talking about.

If you are looking at a domed piston the same type of quench area can be observed around the dome. The flat top piston may seem different because the whole surface is flat except for valve relief’s but it just has a lot more potential quench area than a domed or dished piston. You could say that the head will determine how much “actual quench area” you will have because pistons are designed around the head chamber. The only reason for domes and dishes are to change the volume of the combustion area. Here is a picture of an O.E. flat top, dished, and dome piston from left to right.

Now lets look at what happens in this "quench area". The arrows represent the motion of the A/F mixture.


Notice in the picture above that the A/F mixture in the quench area is being forced into the combustion chamber. The tighter the quench, the more velocity the A/F mixture gains while being forced into the combustion chamber. The higher the velocity of the mixture the more "mixed" the A/F mixture will become leading to a higher percentage of burned A/F. More turbulence is derived from this higher velocity as well. While turbulence is a bad thing when talking about induction, you want as much turbulence as possible in the combustion process. The more turbulence experienced while the piston is ascending to TDC makes for an extremely homogenous A/F mixture which = more efficient burn = more power with less unburned A/F.

Head chambers are designed around creating the most efficient burn cycle possible. The greater your quench, the more area you are essentially creating for "pockets" of the air and fuel (A/F) mixture to get into. Because these pockets of A/F mixture are not in the primary combustion area, they may not ignite with the rest of the mixture. These unburned pockets are essentially a ticking time bomb because they have a tendency to [detonate] before the piston is back at TDC. Usually when this happens the piston is moving upwards while the detonating pocket of A/F mixture tries to force the piston back down. This is why detonation is so damaging to your mechanically personified creation (your engine… lol sorry, I get carried away writing stuff like this). Detonation places added (unwanted) stress on the piston and works against the rest of the engine, robbing you of power and causing headaches.

Here are some excerpts from Chevy High Performance that I though would help you to determine what quench is right for your application.

1.) "Tightening the quench area often results in the reduction of ignition timing requirements. This can then lead to a reduction in negative work (the cylinder pressure rising while the piston is still approaching TDC). This often is evidenced by a gain in low- and mid-range torque."

"There is plenty of discussion about the net effect of squish and quench. While it’s doubtful that this will ever amount to more than a few horsepower in any street application, it does offer some distinct advantages when it comes to increased engine efficiency, better fuel mileage, and driveability. If you’ve ever wondered why certain engines run better than others, this could be one reason why."

2.) "All of the respected engine builders who we’ve talked to are firm believers in minimizing the quench clearance. According to Ken Duttweiler, the tightest quench he recommends is around 0.050-inch. He has built engines with far tighter clearances than this, but much of this depends on the piston-to-wall clearance. All pistons tend to rock slightly as they transition through TDC and this rocking motion reduces the piston-to-head clearance. Smaller-diameter pistons with tight piston-to-wall clearances don’t rock nearly as much in the cylinder bore compared to larger-bore pistons with wider piston-to-wall clearances.

Since piston clearance plays such a big part in piston-to-head clearance, it is possible to run a piston-to-head clearance tighter than 0.040-inch if you feel brave. Noted horsepower hero John Lingenfelter says that clearances of 0.037 to 0.040 inch are possible, but you must know what you’re doing. The late Smokey Yunick also recommended a quench clearance of 0.040 inch as a safe but critical clearance."

I want to intervene here because one variable they do not mention here and should be is the pistons material. Hypereutectic pistons have a lower thermal expansion rate compared to a forged piston so they can have tighter initial piston-to-wall clearances and will not "rock" in the bore as much - especially when cold. Forged piston engines are "looser" when cold and can have substantial piston rock that needs to be considered when choosing quench (I’m sure you have heard people talk about piston noise when first starting a forged piston engine). Because forged pistons have a higher thermal expansion rate than hypereutectic pistons, they have a higher initial (cold) piston-to-wall distance but will yield a tighter piston-to-wall clearance (within the hypereutectic piston range) once normal engine operating temperature is attained. Of course this is all based on thermodynamics and some other fun jazz.

What is minimum piston to head clearance?

Rod material, the mass of the piston, and piston-speed are the factors that determine this. Steel rods in a big block usually require .045. Steel rods in small blocks require at least .036. Most imports can get by with as little as .030. Aluminum Rods generally require .010 more clearance than steel rods. *Remember, the compressed gasket thickness can vary from .025 in steel shim applications to .040 for composite and up to .100 for some copper gaskets.

Continuing on with the Chevy High Performance article -

3.) "All the engine builders we spoke to mentioned that tightening the quench (reducing the piston-to-head clearance) to get it under 0.050 inch will increase the static-compression ratio, but this tighter clearance also creates a more powerful squish effect. This additional turbulence creates a more homogenous “soup” in the chamber, reducing the harmful effects of lean air/fuel ratio pockets. With all other variables being equal, this contributes to creating an engine that is less prone to detonation."

In conclusion, to chose the right quench for your application you should consult with your engine builder or look at other experienced home builders proven combinations. Be realistic when choosing a quench distance as well. Based on what I have discussed with you all in this write-up you should understand that for a stock street driven motor up to a higher performance street motor there is no need for the risk of running .042” or tighter quench. For even hotter street/strip cars you could push it up to the edge with .037”. Flatout race cars… do what you please because you will put as much time on your engine in a season as you would in a week of running around in the daily driver. I guess this paragraph could be taken lightly for I am not building your engine, but I would highly recommend to consider what I have stated.

While the ideas are the same for pretty much any engine (loosely speaking), my analysis is primarily for 1st generation chevy's (Lxx/LTx series). 3rd generation engines (LSx/LQx series) are somewhat different (piston is actually out of the hole as opposed to being at zero deck height and below) but it is still the same principal.

All information was found through various online searches, articles, and my interpretation. Images were used from the Chevy High Performance article on the same subject HERE

And that ladies and gentlemen, is the down and dirty of quench and pretty much anything related . Hope this helps with some peoples quest on the ultimate street machine"



Staff member
the best solution from a performance perspective is to do the required calculations to select the longest length connecting rod and the lowest weight piston,
of a decent design that will reduce the reciprocating mass significantly more.
the tall deck has a 10.2" deck height, a good dual plane aluminum high rise intake manifold will tend to provide the best compromise if you use a low compression and mild cam duration,
while it might seem like a waste of time, now, reading the links and sub-links will provide a good base to work from, later and save you a great deal of wasted time and money

you have a choice, you can slap the components you own together, now and live with what you have built regardless of the results , or you can put some real thought into making the result perform and carefully select parts and significantly boost power... yes that routes more expensive up front, but in the long term it tends to get better results and cost LESS.
common BB CHEVY piston compression heights are

remember the blocks deck height, minus the piston pin height minus 1/2 the crank stroke will equal the required connecting rod length
the blocks deck height, minus the connecting rod length, minus 1/2 the crank stroke. will equal the required piston pin height

if you wonder why I suggest using SCAT (H) beam style cap screw connecting rods vs stock or most (I) beam designs this picture should show the increased cam to connecting rod clearance

After market performance ,big block connecting rods come in several common lengths








notice the pin height in the pistons pictured above allow a longer or shorter connecting rod length


heres a selection of commonly available big block chevy connecting rod lengths






your going to want the longer length and 150%-200% stronger aftermarket connecting rods with the much stronger 7/16" ARP rod bolts if your building a tall deck BBC engine,
so if your trying to build the best combo, you should select the longest and strongest connecting rods that allows you to select an off the shelf compression height piston to save money,
keep in mind head gaskets come in head gaskets come in .010 steps from about .020-to-about .80 and blocks generally measure 10.223 if that O.E.M. block has not previously machined, try too get the quench in the .040-.044 range,
you can get the piston thats compression height is .010-.015, .020 , .025, below or above the deck height, and with a matched head gasket get the quench correct after measuring the deck height, and compression height.




I recently helped one of the local guys assemble a 496 BBC 4.25" stroke
engine using 6.385: SCAT rods with 12.7:1 pistons thats being built
I watched him start to install the first piston with the dome facing the lifter gallery, or upper side of the cylinder... I waited until he had started to install the connecting rod cap on that first rod and asked him to rotate the crank to TDC I handed him a bridge and a dial indicator




and asked him to verify the deck height.....honestly I had a real hard time not laughing.....I think most of us realize that we all made similar mistakes.... no harm done (YET).
after a few seconds, I suggested he check the spark plug clearance with the head just laid on the block with an old head gasket...yeah, he caught the mistake then!
















The high performance race engine by definition indicates that limits are going to be pushed. As far as pistons are concerned that limit is peak operating cylinder pressure. Maximizing cylinder pressure benefits Hp and fuel economy. Considering the potential benefit, owners of non-race engines from motor homes to street rods also look to increasing cylinder pressure. Increasing the compression ratio is one sure way of increasing cylinder pressure, but camshaft selection, carburetion, and supercharging can also alter cylinder pressures dramatically.

Excessive cylinder pressure will encourage engine destroying detonation, and no piston is immune to its effects. An important first step is to set the assembled quench ("squish") distance to .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance (the distance your piston is down in the bore). If your piston compression height (not dome height) is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of this piston that would contact a similar flat area on the cylinder head if you have zero assembled quench height. In a running engine the .040" quench usually decreases with RPM to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity across the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, and speeds flame travel after TDC to increase power. On the exhaust cycle, some cooling of this piston occurs due to the closeness of the hopefully cooler cylinder head. The power increase occurs because the shock wave occurs at exactly TDC on all cylinders, every time. It tends to make all cylinders alike and receive more identical flame travel speed. Spark scatter tends to be averaged with the TDC kick received from a tight quench.

Some non-quench engines, such as '68 and later Chrysler V-8's, can be converted to quench type with pistons such as the KB278, KB280, KB372, and KB373. Most Mopar cylinder heads recess the quench area into the head, so a raised area on the piston is necessary to get the close collision. If you are building an engine with steel rods, tight bearings and pistons, modest RPM, and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Unfortunately, the operating quench height varies in an engine as RPM and temperatures change. If aluminum rods, loose pistons (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit most of the benefits of the quench head design and can push the engine into severe detonation.

The suggested .040" static quench height is recommended as a good average dimension for stock rod engines up to 6500 RPM. Above 6500 RPM, rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or thick head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will likely create more ping at 9.5:l than you had at 10:1. One way to cheat the system is to make sure the piston of choice is light on quench side and to make sure the piston of choice is light on quench side and heavy on spark plug side. As RPM increases the piston tries to cock away from quench surface, allowing a tighter quench at most all RPM. The suitable way to lower the compression is to use a KB Dish Piston. KB Dish Pistons (reverse combustion chamber) are desinged for maximum quench area. Having part of the combustion chamber in the piston can improve the shape of the chamber and flame travel. The Step Dish is sort of an upscale version of our reqular configuration. It allows some piston weight reduction and allows the quench action to travel further across the chamber. It is especially favored when large dish cc's are required.

When detonation occurs, stock type pistons usually break the 2nd and 3rd ring lands. The massive strength of the KB 2nd land prevents this from happening. Detonation destroys by heat, and aluminum does melt. By providing extra-strength ring lands, we postpone piston failure due to detonation. We say postpone rather than eliminate because continued detonation creates hot parts that act as glow plugs. The out-of-control combustion creates welding temperature. Melt down is only seconds away!

For a successful performance engine use a compression ratio and cam combination to keep your cylinder pressure in line with the fuel you are going to use. Drop compression for continuous load operation, such as motor homes and heavy trucks, to around 8.3:1. Keep a cool engine with lots of radiator capacity. Reduce total ignition advance 2? to 4?. A setting that gives a good Hp reading on a 5-second dyno run is usually too advanced for continuous load applications. Normally aspirated drag race engines have been built with high RPM spark retard. The retard is used to counter the effect of increased flame travel speed with increased combustion chamber heat. "Seat of the pants" spark adjustment at low RPM will almost always cause detonation in mid-to-high compression engines once the combustion chamber and pistons are at full operating temperature.

Accumulator Groove is the groove between the 1st and 2nd compression ring. It does make the piston lighter, but the real purpose is more abstract. Pressure spikes that get trapped between the 1st and 2nd compression rings tend to unseat to top ring. This action encourages ring flutter and loss of piston ring seal. The void created by this groove between the rings tends to average the spike pressure of combustion, keeping the pressure low enough to prevent lifting the top ring while maintaining some pre-load on the 2nd (oil scraping) ring.

Top Ring End Gap is often a major player when it comes to piston problems. Most top land damage on race pistons appears to lift the land into the combustion chamber. The reason is that the top ring ends butt and lock the piston at TDC. Crank rotation pulls the piston down the cylinder while leaving at least part of the ring and top land at TDC. Actual running end gap will vary depending on the engine heat load. Piston alloy, fuel mixture, spark advance, compression, cooling system capacity, duty cycle, and Hp per c.i. all combine to determine an engine's heat load.

Most new generation pistons incorporate the top compression ring high on the piston. The high ring location cools the piston top more effectively, reduces detonation, smog, and increases Hp. If detonation or other excess heat situations develop, a top ring end gap set toward the tight side will quickly butt, with piston and cylinder damage to follow immediately. High location rings require extra end gap because they stop at a higher temperature portion of the cylinder at TDC and they have less shielding from the heat of combustion. At TDC the ring is usually above the cylinder water jacket. The current design KB Pistons do a better job of keeping the rings cool.

If a ring end gap is measured on the high side, you improve detonation tolerance in two ways. One, the engine will run longer under detonation before ring butt. Two, some leak down appears to benefit oil control by clearing the rings from oil loading. A small amount of chamber oil will cause detonation and significant Hp loss. The correct top ring end gap with KB Pistons can be 50% to 100% more than manufacturer's specs. Design changes have been made that reduce top land problems dramatically. Read more detail on this in the "New Piston Improvement" article.

Ring Options of 1/16" or stock 5/64" are offered in many KB applications. The 1/16" option reduces friction slightly and seals better above 6500 RPM while being considerably more expensive. Stock (usually 5/64" compression rings) work well and help with the budget. Metric ring options are becoming more common.

Piston to Bore Clearance of .0015", .0020", .0035", and .0045" were wide-open throttle dyno tested. After 8 hours of maximum torque and 7? hours at maximum Hp, the pistons were examined and all looked new, except the tops had normal deposit color. Even with 320?f oil temperature, the inside of the piston remained shiny and completely clean. Excess clearance has been shown to be safe with KB Pistons (no reported skirt cracks in 13 years). The added skirt stiffness of all KB Pistons reduces piston rock, even if it is set up loose. Less rock allows you to run a tighter quench. KB Hyper-eutectic Pistons with over .002" clearance may make noise. As they get up to temperature they may still make noise because they have a very low expansion rate. Our hyper-eutectic alloy not only expands 15% less, it insulates the skirts from combustion chamber heat -- when the skirts stay cool they don't grow. Running additional piston clearance because friction is reduced can sometimes have a short-term Hp improvement.

Pin Oiling should be done at pin installation. Either pressed or full-floating, pre-lube the piston pinhole with oil or liquid pre-lube -- never use grease. (If you are using a pressed pin rod, be sure to discard the spiral pin retainers.) All KB Performance Piston sets supplied from the factory include a tube of Torco / MPZ engine assembly lube. A smooth honed pin bore surface with a reliable oil supply is necessary to control piston expansion. A dry pin bore will add heat to the piston rather than remove heat. All pistons are designed to run with a hot top surface, cool skirts and pin bores. High temperature at the pin bore will quickly cause a piston to grow to the point of seizure in the cylinder.

Marine Applications generally require an extra .001"-.003" clearance because of the possible combination of high load operation and cold water to the block. A cold block with hot pistons is what dictates the need for extra marine clearance.

John Erb
Chief Engineer
KB Pistons
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