how your cam LSA effects your compression ,torque , DCR


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and very much appreciate the sites financial assistance a great deal.


grumpyvette said:
THANK YOU! I TRULY DO APPRECIATE the INSIGHT and informational support you provide
and very much appreciate the sites financial assistance a great deal.

Thank You Grumpy.


Staff member
I had a couple guys ask individual e-mail questions on this thread, so I realized I need to drop back to some basic concepts that seem to be over looked
duration on a cam lobe is the factor that determines how many degrees the valve is off its seat and potentially flows gasses in or out of a combustion chamber over the piston.
lift on the cam lobe obviously dictates how far the valve opens ,
but its duration that determines how long its open, and once a valve lift exceeds about 1/4 its diameter its flow rates close to maximized, so the longer its kept over that lift ratio the more effectively it will flow . but remember the longer the duration used on the intake valve the further the piston moves in its rotation.
when I was younger, it did not mater if you liked
HOWARD CAMS ,http://www.howard
crane (386)310-4875

crower 619.661.6477

erson 800-641-7920


ISKY 323.770.0930

clay smith 714-523-0530

every one had about the same basic catalog
you turned to the brand or manufacturer ....CHEVY,FORD,DODGE,AMC


then you looked for the type of cam you wanted...SOLID OR HYDRAULIC, FLAT TAPPET OR ROLLER CAM

there was at least a couple pages of options starting at the milder end and progressing to the wilder and longer duration versions, some had detailed suggestions as to required gearing,compression car weight, trans stall speeds etc.

and if you needed help with a selection
the tech guys link phone number was on each page!

today they have a tech number to call, and some generally clueless nerd clicking off choices using a computer flow chart to select your cam, and after talking with at least a dozen, I'm convinced 2/3rds have never changed their own oil or spark plugs let alone done a cam swap!

yes there ARE cam selection soft ware programs, but the days of talking to a really experienced engine builder mechanic or racer are long past in my experience, in fact talking to some of these guys on tech desks its rather obvious they are checking off a computer check list and don,t have a #$%%&^ clue what half the terms they use mean!


but keep in mind the port flow rate is restricted by both the intake cross sectional area and the duration , or time the valves off its seat WHILE theres a significant pressure differential between the intake port and the cylinder, the intake runner can be designed to induce inertial pressure in a set rpm band, but its the exhaust flow that's far more influential at helping to drag in the next intake charge into the cylinder as its fast moving gasses exit the exhaust, thus the overlap period is vitally important to maximizing cylinder fill efficiency, once you reach the rpm range where there's a significant inertial energy level in both the intake and exhaust to draw in and cause the exiting gases to be drawn out of the cylinder.
if you do some research you quickly find that the engines power curve very closely relates to its cylinder fill efficiency,

you also find that the intake port cross sectional area and valve size plus the cam duration ,lift and overlap have a huge effect on the engines ability to operate efficiently art any rpm range, because of either flow restriction if the port cross sectional area or cam timing are restricting flow or low port gas velocity levels, if the compression ratio,exhaust scavenging ,engine displacement or cam timing are not maximizing the flow rates in the ports



tighter LSA tend to increase the OVERLAP but thats not always the case



DURATION AND LSA are NOT necessarily related




related info you might want to read thru carefully ... torque.htm ... enging.pdf
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Staff member
your going to find that reading thru the links will be useful ... ewall.html ... ction.html
and in general , Id suggest matching the cams duration and lift to the application, going as high on the displacement and compression ratio as you can, and when selecting a cam for a street/strip car, going a bit on the low side of a given suggested duration range and with a tighter LSA tends to give the most useful power band on a street/strip engine ,and installing cams STRAIT UP, vs 4 degrees advanced, in many applications, but be aware that most cams tend to be made on a wider LSA with more duration, because a LONGER listed duration, and still maintaining a fairly decent idle quality on a cam seems to help sales more than actual power in a useful power band.
and yes obviously you must use well matched ,carefully thought thru,components , in the engine's exhaust,transmission, drive train gearing etc. that match the applications intended rpm and power band to be able to see the results the difference in cam timing can potentially make,


example, heres a solid roller cam from crane that Ive seen used several times on 12:1-13:1 compression 496 big block engines, that has always produced good power.
but youll never see that power if you select restrictive heads,intake or headers or gear the car so it won,t operate efficiently in the intended power band, a cam like these two below needs to be matched to cylinder heads and an intake designed to take full advantage of that .714 lift and ports that flow more than 350cfm, matched to headers that will efficiently scavenge a 496-540 displacement big block chevy above 5500rpm. slap a good performance cam like these linked too below, in an engine with a restrictive intake , small carburetor, on stock peanut port heads and matched to a restrictive stock exhaust and either cam will potentially result in a real "dog" that can barely function. keep in mind one of the strong points of that tighter LSA cam timing is only useful if the exhaust scavenging of the cylinders is efficient in the intended power band, and effective at drawing in intake flow during valve over lap.


Heres a very similar cam, but having seen several friends try both cams I can tell you the first one was preferred by the three guys I know who tried both cams and yes swapping ROLLER cams between guys cars to test them out is rather common among several of my friends


grumpyvette said:
These LS-X Engines behave Similar to A Pontiac. 400- 455 Engine Grumpy.
108 Lobe Separation was the Winner.

Pontiacs Perform Strong with 112-102 Lobe Separation. Cams.
Bigger the Cubes, 455 Preferred , The Tighter the Lobe Separation you can use.
Its really the Long crank stroke and Very high piston speeds in Feet pet second traveled doing the work.
Altering Rod center to center lengths 98 degree Lobe Separation cams can and have been used in 455's and a Race Winner.


All the Power in the world does no good if the tires can not hook up Grumpy.
And Driver cant handle either .

Dialed in Combo and Car and Driver.
Summed up fast.
Lots involved as you know too.


Staff member
most of the more popular current cams are designed much more for easy compatibility with EFI and the related SENSORS, and rather comparatively smooth low rpm idle characteristics than simply too maximize the engines power curve,thus the old muscle car rolling burble or rumble in the exhaust sound is also reduced.
thus the use of the now rather common, wider 112-116 LCA vs the older design cams with the tighter 104-108 LCA angles.
that are far less sensor friendly,due to low rpm reversion pulses that tend to drive sensors in EFI rather crazy, but the tighter lobe center angles are usually better when you use a carburetor as a source for atomized fuel, and the tighter LCA also tend to maximize the exhaust scavenging thus they get a bit more efficient cylinder fill and burn maximizing the engine efficiency.
the issue is also complicated by the current need by the manufacturers to reduce exhaust emission levels and the tighter LCA is a huge problem in that re-guard for the manufacturers in that below about 2000rpm theres a marked increase in emissions if you use a tight LCA in a cam design along with enough duration to maximize peak higher rpm power.




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I am aware if down faults of tight LSA Cams with EFI Grumpy.

There is a way to Tune EFI I have read Tight LCA Used.
Alpha Numeric EFI Programming.
Just the Coolant Temp sensor and TPS Used.
All out Racing only.


Staff member
557B2100 said:
Building-up an engine from a ZZ4 shortblock....Will be putting AFR 180cc heads on it...I've read in several sources that a single-pattern camshaft is recommended with these heads...I've also been told that these magazine articles just bunk....Looking at this cam:

Need opinions and recommendations.

Chevy V8 bore & stroke chart

262 = 3.671" x 3.10" (Gen. I, 5.7" rod)
265 = 3.750" x 3.00" ('55-'57 Gen.I, 5.7" rod)
265 = 3.750" x 3.00" ('94-'96 Gen.II, 4.3 liter V-8 "L99", 5.94" rod)
267 = 3.500" x 3.48" (Gen.I, 5.7" rod)
283 = 3.875" x 3.00" (Gen.I, 5.7" rod)
293 = 3.779" x 3.27" ('99-later, Gen.III, "LR4" 4.8 Liter Vortec, 6.278" rod)
302 = 4.000" x 3.00" (Gen.I, 5.7" rod)
305 = 3.736" x 3.48" (Gen.I, 5.7" rod)
307 = 3.875" x 3.25" (Gen.I, 5.7" rod)
325 = 3.779" x 3.622" ('99-later, Gen.III, "LM7", "LS4 front wheel drive V-8" 5.3 Liter Vortec, 6.098" rod)
327 = 4.000" x 3.25" (Gen.I, 5.7" rod)
345 = 3.893" x 3.622" ('97-later, Gen.III, "LS1", 6.098" rod)
350 = 4.000" x 3.48" (Gen.I, 5.7" rod)
350 = 4.000" x 3.48" ('96-'01, Gen. I, Vortec, 5.7" rod)
350 = 3.900" x 3.66" ('89-'95, "LT5", in "ZR1" Corvette 32-valve DOHC, 5.74" rod)
364 = 4.000" x 3.622" ('99-later, Gen.III, "LS2", "LQ4" 6.0 Liter Vortec, 6.098" rod)
376 = 4.065" x 3.622" (2007-later, Gen. IV, "L92", Cadillac Escalade, GMC Yukon)
383 = 4.000" x 3.80" ('00, "HT 383", Gen.I truck crate motor, 5.7" rod)
400 = 4.125" x 3.75" (Gen.I, 5.565" rod)
427 = 4.125" x 4.00" (2006 Gen.IV, LS7 SBC, titanium rods)

Two common, non-factory smallblock combinations:

377 = 4.155" x 3.48" (5.7" or 6.00" rod)
400 block and a 350 crank with "spacer" main bearings
383 = 4.030" x 3.75" (5.565" or 5.7" or 6.0" rod)
350 block and a 400 crank, main bearing crank journals
cut to 350 size

ALL production big blocks used a 6.135" length rod.

366T = 3.935" x 3.76"
396 = 4.096" x 3.76"
402 = 4.125" x 3.76"
427 = 4.250" x 3.76"
427T = 4.250" x 3.76"
454 = 4.250" x 4.00"
477= 4.5" bore x 3.76" stroke
496 = 4.250" x 4.37" (2001 Vortec 8100, 8.1 liter)
502 = 4.466" x 4.00"
557T= 4.5 bore 4.375" stroke
572T = 4.560" x 4.375" (2003 "ZZ572" crate motors)

T = Tall Deck

ALL production big blocks used a 6.135" length rod.
Yes, I've read Comp Cams five stage build-up....However, they used RHS heads, not AFR's....One of the articles where a single-pattern cam is recommended is this one: ole... and this heads test

"This head is the smallest of AFR’s line. It is intended for engines up to 350ci. The flow numbers are especially impressive on the exhaust side of this head, leading us to believe that a single pattern cam may be the best choice here. The E/I relationship is outstanding with and 84%, almost higher than any other head in this category."

I love discussions like this!
because it forces you to THINK THINGS THROUGH!
OK , first question?,
what will result from "OVER SCAVENGING THE EXHAUST"

second question"
if the exhaust durations "LONGER" does that infer its TOO LONG? (too long in reguards to what?)

if its too long compared to the INTAKE duration, maybe the INTAKE duration NEEDS TO BE INCREASED TO MATCH??
how does the compression and valve diam. come into play?

the answers going to change with each application, the longer exhaust duration allows a RESTRICTIVE exhaust system more TIME to allow escaping exhaust gases to flow,
thus in theory reducing effective back- pressure
if your running the correct headers with a low restriction exhaust it tends to allow some of the intake runner fuel/air to flow through the cylinder,
and follow the previous mass of exhaust out the exhaust valve during the over-lap period, thus reducing fuel efficiency a bit, as thats wasted fuel,
but its not generally hurting power, just fuel mileage
theres several factors at work,that CAN,T be ignored as they work in concert, your port flow, valve curtain area,
cam duration, lift and LSA all control FLOW rates, but proper exhaust header design has a huge effect on the cylinder scavenging effiecincy
lets do a bit of math, and keep in mind that a correctly designed header and exhaust system, if matched to the correct cam timing can significantly increase the engines potential power/rpm band
port cross sectional area can be measured and the stall speed , accurately calculated, as can the required matching header configuration, and cam timing, yeah! it takes some reading but the infos readily available


just a bit of info on intake gaskets sizes to match port cross sectional areas


Calculating the valve curtain area
The following equation mathematically defines the available flow area for any given valve diameter and lift value:
Area = valve diameter x 0.98 x 3.14 x valve lift
Where 3.14 = pi (π)
For a typical 2.02-inch intake valve at .500-inch lift, it calculates as follows:
Area = 2.02 x 0.98 x 3.14 x 0.500 = 3.107 square inches
SO lets do a bit of math
a cylinder head with a 2.02' intake valve and a cam with a .450 lift at the valve with a 1.5:1 rocker will in theory produce a valve curtain area of 2.79 sq inches, swapping to a 1.6:1 ratio increases the lift to .480 lift 2.98 sq inches, increasing the available port flow potential at least in theory by about 6%, but keep in mind the port can only flow at full valve lift for the limited time the valve remains at full lift and if the narrowest section of the port cross sectional areas less that the valve curtain area that not the valve restricts flow


if you were to look at a performance big block chevy cylinder head your largest standard intake valve size is either a 2.19" or in a few cases the larger 2.3" valves
a bit of math shows that you won,t reach the max potential flow until valve lift reaches or slightly exceeds about .575-.600 inches of lift with a big block chevy
and a bit more math suggests a minimum of 4.2 square inches of port cross sectional area would be about ideal to match that potential flow,
if you built a 496 BBC stroker that 4.2 SQ inch port would max out at about 6000 rpm and would be best matched with a single plane intake and a cam with a tight 105-106 LSA






[isky cams said]
Longer Exhaust Duration: Is this really necessary?

Most stock camshafts from American production V8, V6 and 4 cylinder engines manufactured today are ground with the longer exhaust lobe duration. Or, another way of looking at this is that they are ground with shorter intake durations! The former embraces the viewpoint that either the Exhaust Ports or Exhaust Pipe system is somewhat restrictive, and is in need of an assist. The latter suggests that the intake system is rather efficient and cam timing can be trimmed back a bit with out much sacrifice in power, in order to maximize throttle response and cruising efficiency.

Take your pick here. There is no absolutely correct viewpoint - because both are probably true! In a stock engine running at conservative RPM levels, for the sake of overall efficiency, fuel economy and a quiet smooth running engine, this staggering of intake and exhaust duration is quite common and appropriate.

However, High Performance is another thing entirely. Change one factor, let's say in this case, the exhaust system (installing headers and larger pipes) and you have just negated in most cases, the need for that longer exhaust lobe. Now couple this change with a different intake system and camshaft and you have really scrambled the equation. But, wait just a moment. Why is it that so many people (racers & cam grinders alike) insist on running a cam with longer exhaust duration regardless of what equipment is employed? The answer is "habit". Most of them have been somewhat successful in doing it their way and will probably never change unless virtually forced by circumstances to do so.

Before we go any further however let's review what it actually is we are trying to do with an engine when we attempt to make more power. Our best result comes when we are cognizant of the fact that an engine is basically an air pump. We pump it in and out (although in a different form) and we have problems when one side or the other is restricted. Balance or the equilibrium or flow should be our objective, unless of course we are not trying to make more horsepower!

Example #1 (Oval track racing) Here, I have often observed that the most experienced drivers are those who are most likely to run a single pattern (equal on intake and exhaust duration) cam. Why? Because such cams always, I repeat always make more torque! These veterans have a more educated foot and greater experience in feathering the throttle in the corners. They can therefore, utilize the benefit of added torque, in the lower to mid RPM range, to their advantage.

Their counterparts, the younger drivers on the circuit, generally are not as experienced and may at times actually get "crossed up" in the corners especially with a lighter car or when they are learning the ropes. In their case, a longer exhaust duration is often the more appropriate choice. It will often help them to drive better, more "flat footed" if you will, without consequence. But please for the sake of accuracy, let us be truthful. The benefit comes from an actual bleeding off of low to mid range torque, which is always what happens when Exh. Duration is lengthened, not from any improvement. The improvement, (if any) would come because of an improvement in scavenging at the extreme upper end of the power curve and would usually be marginal at best. Yet the so-called "extra power" potential of a longer Exh. Duration cam is most often why they are touted - power most people are backing away from at the end of the strait away!

Example #2 (Drag Racing) At the drag strip it's a little different and I feel more honest. Here, racers have long enjoyed longer exhaust and longer durations across the board (If I may add specifically for the purpose of "killing" low-end torque) to keep the tires from too easily breaking lose. This has been successful and sometimes actually results in a slight increase in top end power - something you can actually use in drag racing since it is a full throttle endeavor through the lights. Keep in mind here though, it's quite possible that a longer duration cam overall would have done just as well or better. In other words if you needed that longer exhaust for top end, perhaps the intake could have benefited from such a lengthening as well.

One of my favorite expressions is how "The Drag Racing mentality has infiltrated the ranks of Oval Track". Many have crossed over and made the switch in the past 10-15 years and some have brought their preconceived notions about how to cam an engine with them. A few may actually read these concepts and if they do so will at least come away with a better understanding of what they are doing. On the other hand they also could find that this information might actually help their cars to run just a bit faster!

Note: Readers may find Camfather Ed Iskenderian's Top Tuners Tip #33 "Can an Exhaust System Over-Scavenge the Combustion Chambers" to be a relevant precursor.

Tech Tip - 2004
Intake Restriction and Over Scavenging: "Waste not...Want not!"

It is certainly an over simplification to make the statement "that which is not wasted, should be inducted". However, in the case of restricted intake systems and in particular 2-BBL carb rules, it is not far off the mark. Engines with such restrictions are "choked off" to the point where they will not run much past 6500 RPM (if even that high) without dropping off sharply in power. You might have trouble running very fast yourself if someone had your windpipe choked down to say 50 or 60% of it's normal capacity. Under such conditions, would you volunteer to give blood at the Red Cross? Of course not, but without knowing so, racers often do the equivalent with their engines by running a camshaft better suited for a 4-BBL class! How So?

If you'll recall in last months tech tip: "Longer Exhaust Duration: Is This Really Necessary?" I discussed how, through habit, many racers and cam grinders alike are predisposed to running camshafts with longer exhaust durations, whether they need to or not! Well, in the case of restricted intake applications, if there was ever a situation in which you'd want to avoid the longer exhaust "trap" it's here! Especially the 8, 10, 12 or even longer degree spreads, I often discover people employing.

Use such a cam at you own risk - and don't be surprised to find that your exhaust temperatures are unusually high. Your headers in fact may even glow cherry red. There is a very good reason for this. Raw (unburned) fuel is burning "late" or in the pipe (header/manifold). You may have a good equilibrium of flow going here but there is just one problem. Much of what should be inducted into the cylinder is being scavenged out the exhaust! You see, although back pressure in an exhaust system can be restrictive, the only thing that could be even worse is a reduction of it to the point where you are now, in effect pulling a vacuum. In the case of an intake restriction, very slight back pressure is preferable to avoid "over scavenging".

Yes, Yes I know. You are probably thinking "what's wrong with a little scavenging?". Well, nothing if you can afford it. But with intake restrictions (either small 2-BBL carbs and/or restrictor plates) you must be very careful. You already have reduced intake potential and therefore simply cannot be cavalier about valve overlap and scavenging or you'll be way down on power and have those nice bright cherry red pipes to show for it! Case in Point: One racer who called me was in this exact situation and was running, not surprisingly, a 14 Degree longer exhaust duration. It was Friday afternoon and he needed a cam the next day for the last "points race" of the season and UPS had already picked up at Isky. "Too Bad" I said, "You don't have a set of those low ratio break-in rocker arms because they could really help in this case". " I do have some" he said "but they are only 1.2:1 ratio - is that okay?" I told him to use them (on his exhaust valves only of course) and he finished the race 2nd having come from the back of the pack. Later we made him the right cam so he could avoid this make shift approach.

Unfortunately, the symptoms are not always as obvious as in this case to allow for a speedy diagnosis. Also, it's not only longer exhaust duration that causes the problem. Although it is usually the primary offender, it is often coupled with too close a lobe separation angle of say 104 Degrees. A widening to 106 Degrees or preferably 108 Degrees (some go even wider) is usually prudent.

I am not absolutely dead set against a slightly longer exhaust duration in these cases as a 2-4 Degree longer exhaust lobe is permissible under some circumstances (if your running a completely stock exhaust system including mufflers for example). Each case is different, depending upon the equipment employed. I might even recommend shorter exhaust duration to some; if I feel they have "overdone" their exhaust ports and or exhaust system a bit. What matters is the end result and if you're out of balance on one side simply employ what I call the "Great Law of Compensation" to bring you back to that equilibrium of flow.

So, how can you tell if you may need to make some of these changes in your camshaft? Well, short of trying a lower exhaust rocker arm ratio, you can increase exhaust valve lash .004" - .008" temporarily to see if there is any improvement. You can also try and increase restriction (smaller headers or pipes, or in the case of open headers a longer collector) and simply observe the results. Remember, "One test is worth a thousand expert opinions". Keep this old axiom in your "tool box" and you'll be ahead of the game. How do you think Smokey's shop got to be "The best Damn Garage in Town" anyway? Yes, he had those country smarts, but his experiences in racing and his willingness to test are legendary!

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