why a bit of extra cam lobe exhaust duration?

grumpyvette

Administrator
Staff member
Chevy 4 Life said:
Grumpy, great info, I had a few mins, so I read the majority of what you have detailed and it all is starting to make a lot of sense. Well it looks like I am on the fence, if I wanted more HP on the top end, I would get better heads and possibly a good single plane, or what I think makes sense for the RPM range I want Idle-5000 at most for street, is switch out the Comp 280 cam and lifters, or either the XE262 or the Lunati Voodoo cam. Do you think with my 1 5/8" headers and dual exhaust, 700r4 and 3.70s that those 2 smaller cams would work better? What do you think of the Split duration cam like the XE262, as I have only ran single duration cams. Thank you so so much for all the helpful info, I could only hope to learn half of what your brain holds.

IF the cam you select has the correct LSA and duration too effectively match the flow characteristics, of your heads,headers,and cam timing, and deal with the exhaust restriction, displacement and compression ETC. adding a bit of extra exhaust duration helps compensate for and tends too help an engine with a slightly restrictive exhaust by allowing the exhaust a bit more time to blow down and in theory scavenge the cylinders, yes the 1 5/8" headers tend to be a bit more restrictive especially when a stock exhaust systems bolted on behind it so yes a longer exhaust duration may help compensate.
each change you make in component selection tends to effect the efficiency of the other components, ideally the selection is made so all the components match the intended displacement, compression and intended operational rpm band so as to maximize the power potential
valve seat and back face angles ,valve diameter and valve lift and duration effect the flow thru the curtain area

keep in mind that valve may be forced off its seat, too full lift and re-seating 50 plus TIMES A SECOND at near 5500 rpm, so theres very little TIME for gases to move through the very restrictive space between the valve seat and valve edge
vgd4.jpg

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

porting+valve_area.jpg

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[ quotee=isky cams posted this]

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.
clay5.jpg

almost every mechanics tool box needs a few basic measuring tools and supplies, dealing in proven facts as to correct clearance sure beats guessing
12cal.jpg

clay6.jpg
https://www.amazon.com/Claytoon-Set...d=1466872286&sr=8-17&keywords=plastilina+clay

http://www.utrechtart.com/Plastalin...currency=USD&gclid=CN3G75zOw80CFQgaaQodKbgFjA
- See more at: http://www.iskycams.com/tech-tips-2000.html#2003[/quote]
A CORRECTLY TUNED SET OF HEADERS , MATCHED TO A CORRECTLY DESIGNED CAM TIMING HAS A SIGNIFICANT EFFECT ON INTAKE FLOW AND CYLINDER SCAVENGING EFFICIENCY, EXHAUST SCAVENGING CAN BE 5 TIMES STRONGER THAN THE PISTON, MOVEMENT INDUCED NEGATIVE PRESSURE (VACUUM) IN THE INTAKE RUNNERS
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Id also point out that cam timing matched to the exhaust scavenging has a huge effect on potential intake flow rates
exhaustpressure.jpg

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THESE LINKS SHOULD HELP

http://garage.grumpysperformance.com/index.php?threads/is-backpressure-hurting-your-combo.495/


http://garage.grumpysperformance.com/index.php?threads/port-speeds-and-area.333/

viewtopic.php?f=52&t=506&p=626#p626

viewtopic.php?f=52&t=1070

viewtopic.php?f=56&t=495

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viewtopic.php?f=56&t=10419

viewtopic.php?f=56&t=1166

viewtopic.php?f=69&t=9930&p=38054#p38054

viewtopic.php?f=52&t=10705&p=46769&hilit=+using+charts#p46769
 
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