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Old 05-18-2002, 02:50 PM   #1
Eric SS
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Exclamation I have a camshaft question. Intake vs. exhaust lift

OK, I have always worked on american cars (mostly chevy V8's) and as I was looking around the net, I got some specs off of Cobb's website:

Intake Lift: .375"
Exhaust Lift: .310"

That surprised me. On all the engines I have worked on, The intake and exhaust lift is either the same, or the exhaust lift is larger than the intake. Does anyone know why Subaru decided to use more intake lift than exhaust but still go with more duration (232 vs 236 seat to seat) on the exhaust lobs. IS the info backwards on cobb's site (which I doubt)?
Thanks

Eric
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Old 05-18-2002, 11:38 PM   #2
Koopa
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to the best of my knowledge the longer exaust duration helps mostly for aditional bost or if your are running nitrous. you tend to loose low end but gaind top end power. and since these are 4 cyl they need to be reved more any ways. you will never got the low end out of a 4 that you can with a 6 or 8 cyl. at least not with out a lott of $$$$ and time....
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Old 05-18-2002, 11:52 PM   #3
Eric SS
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Quote:
Originally posted by Koopa
to the best of my knowledge the longer exaust duration helps mostly for aditional bost or if your are running nitrous. you tend to loose low end but gaind top end power. and since these are 4 cyl they need to be reved more any ways. you will never got the low end out of a 4 that you can with a 6 or 8 cyl. at least not with out a lott of $$$$ and time....
True that the longer your duration on the exhaust side compared to the intake makes for better nitrous power due to the exhaust scavenging but I seriously doubt that subaru had that in mind when they designed the cams

Eric
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Old 05-19-2002, 02:17 AM   #4
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Quote:
Originally posted by Imprezinator


True that the longer your duration on the exhaust side compared to the intake makes for better nitrous power due to the exhaust scavenging but I seriously doubt that subaru had that in mind when they designed the cams

Eric
yes but it ALSo helps tremendously on NA cars(same as nitrous more or less) but on a turbo car you want theoretically a cam with zero valve overlap to prevent reversion...eric i'll e-mail ya some specs when i get a change(i have to do a lil measuring).

jeremy
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Old 05-19-2002, 02:25 AM   #5
Eric SS
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Originally posted by HndaTch627
yes but it ALSo helps tremendously on NA cars(same as nitrous more or less) but on a turbo car you want theoretically a cam with zero valve overlap to prevent reversion...eric i'll e-mail ya some specs when i get a change(i have to do a lil measuring).

jeremy
Thanks Jeremy. I know you know a lot more about high revving japanese 4-cylinder engines than I do. call me naive but I've just never heard of an engine with more intake lift than exhaust lift but I guess 4 bangers do that.

Eric

PS: Haven;t seen you around in a couple days

Last edited by Eric SS; 05-19-2002 at 02:30 AM.
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Old 05-19-2002, 02:47 AM   #6
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Just posting to join thread...I am kinda intrested in hearing the answers. I would think it is because our cars can't breath worth crap but I guess that is the wrong side :/

btw hndatch627 lost his internet...and that would be the only reason you don't see him post

-neal
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Old 05-19-2002, 02:49 AM   #7
Eric SS
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btw hndatch627 lost his internet...and that would be the only reason you don't see him post

-neal
hehe.. what happened???

Eric
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Old 05-20-2002, 11:27 AM   #8
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On a small block chevy the exhaust ports are small and you need a lot of lift and duration to expelled the gasses. My smallblock nova's cam is a lunati w/ .500- .515 lift 256-270 duration @.050. Valve overlap also helps with a scaveging effect and acts like a vacumm to pull more into the cylinder on the intake side. Or something like that. I know the principals but have a hard time explaining it. I guess the subaru 2.5 is more efficient at getting the exhaust out of the cylinders. Aren't the spicey cams more like 410 lift and 224 duration?
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Old 05-20-2002, 10:10 PM   #9
Eric SS
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Originally posted by STEALTHBMBR
On a small block chevy the exhaust ports are small and you need a lot of lift and duration to expelled the gasses. My smallblock nova's cam is a lunati w/ .500- .515 lift 256-270 duration @.050. Valve overlap also helps with a scaveging effect and acts like a vacumm to pull more into the cylinder on the intake side. Or something like that. I know the principals but have a hard time explaining it. I guess the subaru 2.5 is more efficient at getting the exhaust out of the cylinders. Aren't the spicey cams more like 410 lift and 224 duration?
Dude, you are running a crap load of duration for such small lift.

Eric
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Old 05-21-2002, 03:04 AM   #10
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Quote:
Originally posted by Imprezinator


Thanks Jeremy. I know you know a lot more about high revving japanese 4-cylinder engines than I do. call me naive but I've just never heard of an engine with more intake lift than exhaust lift but I guess 4 bangers do that.

Eric

PS: Haven;t seen you around in a couple days
it's not that...just compare the size of a stock impreza exhaust port to that of a stock SBC and you'll see a difference, also take into account that 4 valves will flow more then 2.

jeremy

p.s.: AT&T sucks my arse
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Old 05-21-2002, 03:14 AM   #11
Eric SS
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Quote:
Originally posted by HndaTch627
it's not that...just compare the size of a stock impreza exhaust port to that of a stock SBC and you'll see a difference, also take into account that 4 valves will flow more then 2.

jeremy

p.s.: AT&T sucks my arse
True. Does this mean that Subaru's intake ports are weak then?

Eric
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Old 05-21-2002, 07:43 AM   #12
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Quote:
Originally posted by Imprezinator


Dude, you are running a crap load of duration for such small lift.

Eric
I know, but the motor likes it. It has small intake runners, only 175cc, so a lot of lift insn't needed. Open the valve to .500 and leave it open a long time. My nova runs 10.50's with it.
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Old 05-21-2002, 11:41 AM   #13
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Quote:
Originally posted by Imprezinator


True. Does this mean that Subaru's intake ports are weak then?

Eric
not necesarily becuase they move the volume needed for the motor, the weakness falls in the intake plenum design...that's why when you put a CAI on you are sacrficing torque by removing that main air box...the damn plenum is so tiny it doesn't hold enough air to feed the motor instantly....why else would the car have such a massive throttle body. *Shrug*

Jeremy
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Old 05-21-2002, 07:56 PM   #14
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the really interesting thing Eric is that the phase II SOHC motor has 1.75:1 intake rockers and 1.5:1 exhaust.

I'm sure it's in the ports and valve sizes. although the way the second (rear) exhaust port takes a wicked bend, you wouldn't think that exhaust would outflow the intake.
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Old 05-22-2002, 02:14 AM   #15
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Originally posted by ColinL
the really interesting thing Eric is that the phase II SOHC motor has 1.75:1 intake rockers and 1.5:1 exhaust.

I'm sure it's in the ports and valve sizes. although the way the second (rear) exhaust port takes a wicked bend, you wouldn't think that exhaust would outflow the intake.
i agree 100% just one of those mysteries i guess *shrug*

jeremy
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Old 05-22-2002, 05:14 AM   #16
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I believe the bore also plays a part in this comparation - with the cam shaft profile. When the mixture of air is burnt the pressure inside the cylinder is much higher than the pressure of the atmosphere. There for the force pushing the gasses outside is stronger than the air pushing through the intake valve.

The air drag coefficent plays a part here. The air drag is highly crowing function of velocity. However, when the force pushing is larger the velocity of the material (gas) is always larger than with the smaller force. I hope you understand what I mean.

The pressure is proportional to the volume which is proportional to the bore. Two engines with different bore, but otherwise similar: The bigger bored engine has smaller pressure inside it when the same amount of air/fuel mixture is burnt -> the force is smaller. This isn't quite good example but anyhow.

Ports' design, cam shafts, valves and other factors plays of course a role here. But the above mentioned explains somehow why the exhaust ports don't have to flow as good as the intake ports - the measurements are done with a stabilised pressure.

I'm no expert, but some thoughts...



tony

edit:some typos eliminated
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