Isky cam's interesting comments about split duration cams - Chevelle Tech
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  #1  
Old Sep 7th, 07, 5:44 AM
Tokyo Torquer Tokyo Torquer is offline
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Default Isky cam's interesting comments about split duration cams

Isky says longer exhaust duration cams simply kill low-mid range torque and no more...and that single pattern cams alway make more power...

"...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. ..."
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  #2  
Old Sep 7th, 07, 5:55 AM
dreis454 dreis454 is offline
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Default Re: Isky cam's interesting comments about split duration cams

we'll see what UDHarold or dacaman has to say about this!

to me, this sounds very old school....maybe they need to get into the 21st century

I'm stickin with my Lunati voodoo 268
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  #3  
Old Sep 7th, 07, 10:30 AM
CDN SS CDN SS is offline
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Default Re: Isky cam's interesting comments about split duration cams

My understanding one of the reasons that split duration cams were developed to make up for cylinder head flow differences between intake/exhaust ports .....so not sure a blanket statement can be made on which type makes most power cylinder head design needs to considered in the combination ???
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  #4  
Old Sep 7th, 07, 10:31 AM
fabio fabio is offline
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Default Re: Isky cam's interesting comments about split duration cams

the bigger exhaust duration allows the engine to hang on at the upper revs.
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  #5  
Old Sep 7th, 07, 1:05 PM
66 BRONZE 66 BRONZE is offline
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Default Re: Isky cam's interesting comments about split duration cams

No cam expert here, but my voodoo 262 in my 327 has alot more torque and top end power with the 1.6's over the 1.5's. I guess it's a matter of the combination of parts.
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  #6  
Old Sep 7th, 07, 4:08 PM
GRN69CHV GRN69CHV is offline
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Default Re: Isky cam's interesting comments about split duration cams

To a point, I tend to agree with both views. Although, I have become a fan of similiar duration cams [my current cam is 233/236 @ .050 hyd roller on 112LSA]. It does have a lot of midrange power, yet still revs to 6300+.
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  #7  
Old Sep 7th, 07, 4:49 PM
cstraub cstraub is online now
 
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Default Re: Isky cam's interesting comments about split duration cams

Duration split is dependent on I/E ratio. If the duration split is calculated to take advantage of what this percentage is then a a very broad powerband can be achieved.
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  #8  
Old Sep 7th, 07, 4:52 PM
kirkwoodken kirkwoodken is offline
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Default Re: Isky cam's interesting comments about split duration cams

Like GRN69CHV, I feel strongly both ways. You have to get it out before you can get it in. Street cars with typically restrictive exhaust systems should benefit from a little more time to get the exhaust out. That said, if the head flow has the proper bias between intake and exhaust, there should be no reason for unnecessary blowdown.

In my opinion, the most important part of any cam design, with any timing, is to clean the exhaust gas out of the combustion chamber. Left over exhaust gas is the greatest killer of power at any RPM. When an engine "comes on the pipe", it runs better because residual exhaust gases have been removed from the chamber.

To settle this argument, one only needs to perform standard lash loops on the dyno to determine what a particular head/piston/manifold/carb/compession setup wants. I'm confident that most competent tuners are this sharp, and I would think it would be mentioned in the Isky article. Maybe the Isky article should have been about lash loops and teaching how to determine what cam YOU NEED for YOUR engine and the way YOU drive.
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  #9  
Old Sep 7th, 07, 4:54 PM
cstraub cstraub is online now
 
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Default Re: Isky cam's interesting comments about split duration cams

Quote:
Originally Posted by Tokyo Torquer View Post
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!
I'm glad Isky says this so I ask. . .How can you calculate, figure, select the cam for a given engine without taking the flow numbers in account? We are talking about the item that controls the air potential in and air potential out....
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  #10  
Old Sep 7th, 07, 5:46 PM
Rmchevelle Rmchevelle is offline
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Question Re: Isky cam's interesting comments about split duration cams

Quote:
Originally Posted by SNAP View Post
No cam expert here, but my voodoo 262 in my 327 has alot more torque and top end power with the 1.6's over the 1.5's. I guess it's a matter of the combination of parts.
Any E.T.'s on that combo yet? Sounds interesting.
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  #11  
Old Sep 7th, 07, 9:03 PM
UDHarold UDHarold is offline
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Default Re: Isky cam's interesting comments about split duration cams

Of course single pattern cams make more torque. That's why Pro-Stocks use 20* bigger exhaust cams at .050, to kill off ALL the torque in the engine.........
There are all kind of reasons to run single-pattern cams.
There are just as many, or even more, reasons to run dual-pattern cams.
There are even reasons to run reverse-pattern cams.
It all boils down to two things, What the cylinder head flow needs, and what the customer wants the engine to do.
I have made lots of hydraulic cams that are single-pattern. I have made hydraulic cams that are smaller, and bigger, that are dual-pattern cams. Both types worked fine for what they were supposed to do. I even tried to talk Holley into 1 or 2 single-pattern VooDoo cams, but then, Comp didn't make any single-pattern Xtreme Energy cams.
At UltraDyne I had a single-pattern flat tappet finish 2nd in the Winston Amatuer Championship.
There are reasons for everything. The trick is knowing the right reason for any application.
I sold 2 reverse-pattern roller cams to a customer today. About 90% of what he buys, and he buys them 2 to 6 at a time, are reverse-pattern. He also helped me develop the 283/289F6 flat tappet, and he's bought a lot of those.
My most popular flat tappets are 6 to 8 degrees larger on the exhaust than on the intake.
The most popular rollers are mostly 8 degrees larger, some only 4, some 16 to 20.
A cam should always be made for what the customer wants the power band to be like.
All of the above are the correct answers.

UDHarold
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  #12  
Old Sep 7th, 07, 9:12 PM
zdld17 zdld17 is offline
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Default Re: Isky cam's interesting comments about split duration cams

Quote:
Originally Posted by SNAP View Post
No cam expert here, but my voodoo 262 in my 327 has alot more torque and top end power with the 1.6's over the 1.5's. I guess it's a matter of the combination of parts.
I did not hear anyone mention that even running increased rocker arm ratios , is also another way to increase durations. Am I on or off with this theory?
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  #13  
Old Sep 7th, 07, 10:45 PM
M.Maner M.Maner is offline
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Default Re: Isky cam's interesting comments about split duration cams

IMHO most honest street engines don't turn enough rpm to outpace the exhaust.
Mike
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  #14  
Old Sep 7th, 07, 10:53 PM
Troy70SS Troy70SS is offline
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Default Re: Isky cam's interesting comments about split duration cams

How is Rocker Arm Ratio going to increase duration? Lift sure but duration is ground in right? You lost me on that one.

Troy
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  #15  
Old Sep 8th, 07, 6:14 AM
dreis454 dreis454 is offline
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Default Re: Isky cam's interesting comments about split duration cams

Quote:
Originally Posted by zdld17 View Post
I did not hear anyone mention that even running increased rocker arm ratios , is also another way to increase durations. Am I on or off with this theory?

rocker ratios do nothing for duration...only lift
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matching # LS5 454+.060" 9.2 :1, 2.19/1.88 valves,Comp HR cam 289/295 234/240 .613/.613 110* (dyno'd @ 557hp @ 5400 & 583tq @ 4100)
matching # M22 w/CF dual friction clutch.
Holley 750 HPs dp(82751) on Weiand Stealth & MSD Pro billet HEI
Hedman Elite headers/Flowmaster exh. (w/super 40s)
TC#3095
Garage find parked from 1978-2004

http://macswebs.com/ls5registry/bal/bal.htm
http://www.chevelles.com/showroom/sh...0&ppuser=25834
http://ssregistry.macswebs.com/1972/1d37w2b_reis.htm
http://www.youtube.com/watch?v=YqZkVw6xMkM
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