Increased intake duration vs exhaust?
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TECH Senior Member
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Increased intake duration vs exhaust? We all know our intakes are terribly restrictive. I am wondering if any of you guys have considered upping the intake duration and have a split duration cam favoring the intake side?
If you have a GREAT exhaust setup. 1 3/4 or 1 7/8 long tubes, dual 3 inch with Xpipe and bullets for example, your exhaust side is not really that restricted provided head flow numbers on the exhaust look good. If you went with a bias towards intake duration I wonder how it would work out?
Say a 228/224 XE-R .588/.585 based cam on a 112 Lobe seperation?
Or a 228/226 based cam?
Anyone attempt this or know of anyone that has tested with it?
We all know with a restrictive exhaust system more exhaust duration helps rid the exhaust charge and pull in a fresh intake charge. But with a very free flowing exhaust system your duration requirement on the exhaust will be lower than that of a restrictive setup.
Why isnt the opposite case true considering our restrictive intakes?
Thanks fellas,
Chris
<small>[ March 28, 2002, 10:14 AM: Message edited by: Chris ARE 360 ]</small>
If you have a GREAT exhaust setup. 1 3/4 or 1 7/8 long tubes, dual 3 inch with Xpipe and bullets for example, your exhaust side is not really that restricted provided head flow numbers on the exhaust look good. If you went with a bias towards intake duration I wonder how it would work out?
Say a 228/224 XE-R .588/.585 based cam on a 112 Lobe seperation?
Or a 228/226 based cam?
Anyone attempt this or know of anyone that has tested with it?
We all know with a restrictive exhaust system more exhaust duration helps rid the exhaust charge and pull in a fresh intake charge. But with a very free flowing exhaust system your duration requirement on the exhaust will be lower than that of a restrictive setup.
Why isnt the opposite case true considering our restrictive intakes?
Thanks fellas,
Chris
<small>[ March 28, 2002, 10:14 AM: Message edited by: Chris ARE 360 ]</small>
Launching!
Joined: Dec 2001
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From: Houston
Re: Increased intake duration vs exhaust? Chris as cams become bigger and more race type. There usraly take that direction... it not normal too see it on street stuff. a very normal setup is where you have say a 260/264 ---> .670/.670 notice the lift is the same but the Ex still has more duration giving the EX a less radical ramp. Don't forget that on a single pattern cam the intake already have a huge advnatage. having bigger valves.
Launching!
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From: North Tonawanda - New York
Re: Increased intake duration vs exhaust? Chris, i may be mistaken but I think TPIS has cams ground like that. They may also increase lift on the exhaust side to help makeup for the lower duration. They list the specs of the LS1 grinds on their website. It's an interesting thought !
- Mitch R.
- Mitch R.
Launching!
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From: Houston, Texas F-Body Heaven!!!
Re: Increased intake duration vs exhaust? I think there would be a few advantages of going with this type of grind. First, there would obviously be more power because of the increasing intake duration. You could increase the intake duration to new limits because with a lower exhaust duration the total duration would remain relatively the same. That leads me to my second point. Less overlap...
Current thinking tells us that as the durations increase the engine has to spin faster and faster to make power, and the idle rpms have to be raised more and more. This is because of the overlap associated with big duration cams. If you lower the exhaust duration overlap is reduced, and more intake duration can be added. Then the rev limiter and idle can remain within decent limits. Which leads me to my third point...
With the decreased overlap, less compression would bleed off in the lower rpms thus increasing low end torque (or at least keep from losing low end torque) while still increasing upper rpm hp because of the greater intake durations.
Just my .02
'Los <img border="0" alt="[Chevrolet]" title="" src="graemlins/camaro.gif" />
<small>[ March 29, 2002, 10:46 AM: Message edited by: Carlitos ]</small>
Current thinking tells us that as the durations increase the engine has to spin faster and faster to make power, and the idle rpms have to be raised more and more. This is because of the overlap associated with big duration cams. If you lower the exhaust duration overlap is reduced, and more intake duration can be added. Then the rev limiter and idle can remain within decent limits. Which leads me to my third point...
With the decreased overlap, less compression would bleed off in the lower rpms thus increasing low end torque (or at least keep from losing low end torque) while still increasing upper rpm hp because of the greater intake durations.
Just my .02
'Los <img border="0" alt="[Chevrolet]" title="" src="graemlins/camaro.gif" />
<small>[ March 29, 2002, 10:46 AM: Message edited by: Carlitos ]</small>
Re: Increased intake duration vs exhaust? Its funny you mentioned this Chris. I was on the phone one time with Crane and they were telling me that a lot of racers order ls1 cams with the intake higher than the exhaust and they were producing awesome #'s. The guy was suppose to call me back and give me the name of the racers, but he never did.. So I figured he was just blowing smoke up my *** and looking for a test mule..
TECH Addict
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From: College Station, Tx
Re: Increased intake duration vs exhaust? </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by Carlitos:
<strong>With the decreased overlap, less compression would bleed off in the lower rpms thus increasing low end torque (or at least keep from losing low end torque) while still increasing upper rpm hp because of the greater intake durations.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Overlap and compression "bleed-off" are 2 different events and are not related. Overlap is determined by the exhaust closing and intake opening event. Dynamic compression is determined primarily by the intake valve closing event.
Chris
<strong>With the decreased overlap, less compression would bleed off in the lower rpms thus increasing low end torque (or at least keep from losing low end torque) while still increasing upper rpm hp because of the greater intake durations.</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Overlap and compression "bleed-off" are 2 different events and are not related. Overlap is determined by the exhaust closing and intake opening event. Dynamic compression is determined primarily by the intake valve closing event.
Chris
Launching!
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From: Houston, Texas F-Body Heaven!!!
Re: Increased intake duration vs exhaust? </font><blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr /><font size="2" face="Verdana, Helvetica, sans-serif">Originally posted by ChrisB:
<strong>[QUOTE]Overlap and compression "bleed-off" are 2 different events and are not related. Overlap is determined by the exhaust closing and intake opening event. Dynamic compression is determined primarily by the intake valve closing event.
Chris</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Oops! Yeah, you're right. Just got off a 24 hr shift when I added that. Hadn't had my nap, yet. <img border="0" alt="[boring]" title="" src="graemlins/gr_zzz.gif" /> I was confused a bit. <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" /> I was probably thinking of bleeding off intake charge in FI applications with tight LSAs. Muh bad. I'm not even sure if that sounds right, right now. I just woke up. <img border="0" title="" alt="[Razz]" src="gr_tounge.gif" />
<img border="0" alt="[Chevrolet]" title="" src="graemlins/camaro.gif" />
'Los
<strong>[QUOTE]Overlap and compression "bleed-off" are 2 different events and are not related. Overlap is determined by the exhaust closing and intake opening event. Dynamic compression is determined primarily by the intake valve closing event.
Chris</strong></font><hr /></blockquote><font size="2" face="Verdana, Helvetica, sans-serif">Oops! Yeah, you're right. Just got off a 24 hr shift when I added that. Hadn't had my nap, yet. <img border="0" alt="[boring]" title="" src="graemlins/gr_zzz.gif" /> I was confused a bit. <img border="0" title="" alt="[Confused]" src="images/icons/confused.gif" /> I was probably thinking of bleeding off intake charge in FI applications with tight LSAs. Muh bad. I'm not even sure if that sounds right, right now. I just woke up. <img border="0" title="" alt="[Razz]" src="gr_tounge.gif" />
<img border="0" alt="[Chevrolet]" title="" src="graemlins/camaro.gif" />
'Los
