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Reason for Exhaust Bias of GM Cams?

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Old 02-03-2005, 07:37 PM
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Originally Posted by Unaffiliated Racing
compared to other heads the intake to exhaust ratio is very good...like I said it was mainly because of emissions.
Old 02-04-2005, 10:35 AM
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So if the ASA and GrandAm cam have been optimized by GM for max performance with headers, why do most aftermarket cams have designs with less exhaust bias? I'm sure GM did extensive testing.
Old 02-04-2005, 10:51 AM
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I wouldn't say most aftermarket cam's have less exhaust duration. There are a lot of traditional splits, and are getting to be more all the time. Traditional splits are about all Comp Cam's sells. Most of the reverse splits are custom grinds. The intake is more sensitive to lift, while the exhaust likes duration.
Old 02-04-2005, 11:47 AM
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For example, here is Futral's cam specs from their website.

Why is the relationship between the intake and exhaust durations so different from the GM max performance designs?

FM-F2 224"/224" .588"/.588" 112 or 114
FM-F4 226"/226" .575"/.575" 112 or 114
FM-F5 224"/228" .588"/.576" 112 or 114
FM-F6 226"/230" .575"/.595" 112 or 114
FM-F7 224"/230" .588"/.595" 114 *
FM-F9 232"/236" .585"/.600" 114 *
FM1-F10 228"/228" .576"/.576" 112 or 114
FM1-F11 228"/230" .576"/.595" 112 or 114
FM1-F12 230"/230" .595"/.595" 112 or 114
FM1-F13 230"/232" .595"/.585" 112 or 114
FM1-F14 232"/234" .598"/.598" 112 or 114
FM1-RJ 236"/239" .600"/.611" 111

* recommended for blower/nitrous applications
Old 02-04-2005, 11:56 AM
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No Cam company's keep many reverse splits on the shelf. Why? Because there are very few cyclinders heads in the world where the exhaust side out flows the intake side. Most reverse splits that cam companys have are some of the turbo grinds and that is because of how a turbo works. A reverse split really wouldn't do you any good on a typical head.
Old 02-04-2005, 12:37 PM
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Check out what Ed "The Camfather" Iskederian has to say about longer exhaust duration:
http://www.iskycams.com/techtips.php#2003
Old 02-04-2005, 03:21 PM
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Originally Posted by Unaffiliated Racing
No Cam company's keep many reverse splits on the shelf. Why? Because there are very few cyclinders heads in the world where the exhaust side out flows the intake side. Most reverse splits that cam companys have are some of the turbo grinds and that is because of how a turbo works. A reverse split really wouldn't do you any good on a typical head.
the FM 13 is standard split at .050 (230int. 232exh.) but at .200 lift it's a slight reverse split(151int. 150exh.) so is it a standard split cam or reverse split?remember that dur.at .050 lift is only one part of the cam spec,you have to take the dur. at the other lift points to get a true picture of a cam profile.if i were to guess,i would think that a lot of futrals cams follow this example.

Last edited by 66deuce; 02-04-2005 at 07:20 PM.
Old 02-04-2005, 03:28 PM
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Ed is Cool.. .tells it like it is.

Chris
Old 02-04-2005, 03:43 PM
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Originally Posted by nuzee
Check out what Ed "The Camfather" Iskederian has to say about longer exhaust duration:
http://www.iskycams.com/techtips.php#2003
Interesting theories & ideas.
Old 02-04-2005, 03:55 PM
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i would also recommend the "Cam Disscussion" threads from awhile back.their long,but well worth the time for anyone that wants to learn about cams.

Last edited by 66deuce; 02-04-2005 at 04:00 PM.
Old 02-04-2005, 04:20 PM
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For an LS1 head a reverse split makes more sense too me, but then again specs @.050 mean very little to nothing.
Old 02-04-2005, 04:39 PM
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I find it funny...

Anti-reverse guys claim, they are only good for top end power. But yet, I've seen others claim they kink off power up top.

However, Ed Iskenderian writes:

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.
Old 02-04-2005, 04:40 PM
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Can I be the guy who says its neither?
Old 02-04-2005, 04:43 PM
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Originally Posted by MUSTANGEATER
Can I be the guy who says its neither?
Definetly. And to fill in your next words....match it to your setup, don't follow camps.

Close?
Old 02-04-2005, 04:46 PM
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Originally Posted by SportSide 5.3
Definetly. And to fill in your next words....match it to your setup, don't follow camps.

Close?
Are you working for Ms. Cleo now-a-days?
Old 02-04-2005, 04:46 PM
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****, i took her job...kicked her on the street.

Old 02-04-2005, 05:26 PM
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Well I've probably spec'd out more LS-1 cams than most people around....99% of the time I will use a traditional split unless it's a turbo grind. Yes, granted the LS-1 head has a very good intake to exhaust flow ratio..it still lacks a little on the exhaust side.
Old 02-04-2005, 06:24 PM
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I think there is no question that a traditional split is better for a normally aspirated LS1, but the best split ratio may not be as big as those GM off-road cams have.
Old 02-04-2005, 06:45 PM
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Originally Posted by Cal
I think there is no question that a traditional split is better for a normally aspirated LS1, but the best split ratio may not be as big as those GM off-road cams have.
Probably the most impressive, forum favorite ots grind. The F13, is a 230* 232* .595 .585 @ 112

An intake crutch is in place once the intake valve is cracked open enough to where the head flows decent.

Any way it's put, it comes down to the setup and type of usage. There are to many varieties of normally aspirated setups to say, all will make more power with more exhaust lobe.
Old 02-04-2005, 07:18 PM
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Originally Posted by SportSide 5.3
Probably the most impressive, forum favorite ots grind. The F13, is a 230* 232* .595 .585 @ 112

An intake crutch is in place once the intake valve is cracked open enough to where the head flows decent.

Any way it's put, it comes down to the setup and type of usage. There are to many varieties of normally aspirated setups to say, all will make more power with more exhaust lobe.
i agree.it all depends on your combination.and i would be willing to bet that a good number of "standard split" ots cams are in fact reverse split or at least single pattern once you get above .050 lift.


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