Milledge Cams
10-25-2003, 11:57 AM
#16
Rennlist Member
Hey Tony, one of the questions that Russ asked: what is the stock cam overlap? I'd also be interested to know that. I also have a very low restrictive exhaust hausing and wondered if that's already a help with the stock cam? With any amount of overlap, i'd assume it would be.
10-25-2003, 04:07 PM
#17
Race Car
The factory 951 camshaft is the same as early 944 camshaft, correct?
Timing specs for early 944 [944-105-155-05]:
Intake Opens: 1 deg after TDC
Intake Closes: 49 deg after BDC
Exhaust Opens: 43 deg before BDC
Exhaust Closes: 3 deg before TDC
There is no intake/exhaust valve overlap.
Timing specs for early 944 [944-105-155-05]:
Intake Opens: 1 deg after TDC
Intake Closes: 49 deg after BDC
Exhaust Opens: 43 deg before BDC
Exhaust Closes: 3 deg before TDC
There is no intake/exhaust valve overlap.
10-25-2003, 06:14 PM
#18
Rennlist Member
PorscheG96; It could mean that the exhaust valve is STARTING to close at 3 degrees BTDC. But, it would take several degrees of crankshaft rotation for the exhaust valve to move up and over the cam lobe. By this time the intake valve has STARTED to open. If this is true, that would mean there is some amount of overlap. Maybe, someone can confirm this. Tony?
10-25-2003, 08:21 PM
#19
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According to the factory repair manual these figures are correct and are measured at 1 mm lift with 0 valve clearance. If You measure valve timing at a lower lift you will have some overlap. However you will probably not have much flow at lower valve lift than 1 mm, and that's probably why Porsche uses that number as a reference.
10-26-2003, 10:57 AM
#20
Rennlist Member
Pardon my ignorance, but what do you mean by "with 0 valve clearance"?
And Tony, with your higher reving engine, wouldn't you still have a potential problem with the hydraulic lifters causing valve float? Maybe, you don't have the stock lifters anymore?
And Tony, with your higher reving engine, wouldn't you still have a potential problem with the hydraulic lifters causing valve float? Maybe, you don't have the stock lifters anymore?
10-26-2003, 12:36 PM
#21
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TurboTommy
>>>Hey Tony, one of the questions that Russ asked: what is the stock cam overlap? <<<
I don't have the spec... but it's not much. The lobe center lines are not very close (which is reflected in the 18" of vacuum on a stock car as well as the silky smooth idle characteristics).
>>>I'd also be interested to know that. I also have a very low restrictive exhaust hausing and wondered if that's already a help with the stock cam? <<<
What is your "very low restrictive exhaust housing"?
Anything you can do to reduce the pressure between the exhaust valve and the turbine will always help with respect several things... and in this context it's the reduction in any reversion that takes place (with the stock cam it's minimal at best).
>>>Hey Tony, one of the questions that Russ asked: what is the stock cam overlap? <<<
I don't have the spec... but it's not much. The lobe center lines are not very close (which is reflected in the 18" of vacuum on a stock car as well as the silky smooth idle characteristics).
>>>I'd also be interested to know that. I also have a very low restrictive exhaust hausing and wondered if that's already a help with the stock cam? <<<
What is your "very low restrictive exhaust housing"?
Anything you can do to reduce the pressure between the exhaust valve and the turbine will always help with respect several things... and in this context it's the reduction in any reversion that takes place (with the stock cam it's minimal at best).
Last edited by TonyG; 10-26-2003 at 05:23 PM.
10-26-2003, 12:40 PM
#22
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TurboTommy
0 valve clearance refers to the fact that it's a hydralic lifter and thus, with the stock cam or any cam with a stock base circle, will not have "lash". Thus it's referred to as 0 (zero) clearance.
On engines that have some means to adjust the lash, then you have to factor in the amount of lash (clearance) into the equation.
The stock lifters are good to 7200 rpms (MAX) with race vavle springs and titainium retainers, with stock vavles.
0 valve clearance refers to the fact that it's a hydralic lifter and thus, with the stock cam or any cam with a stock base circle, will not have "lash". Thus it's referred to as 0 (zero) clearance.
On engines that have some means to adjust the lash, then you have to factor in the amount of lash (clearance) into the equation.
The stock lifters are good to 7200 rpms (MAX) with race vavle springs and titainium retainers, with stock vavles.
10-26-2003, 09:35 PM
#23
Rennlist Member
Thanks Tony for clearing that up about the valve clearance. After posting, I thought about it some more, and figured that's what it was.
If PorscheG's specs are correct there is very little overlap indeed which is a little disappointing. I don't know my exact exhaust housing; the bottom line is the relationship of boost pressure to exhaust back pressure. I tested it and it was 17 psi boost with 6 to 7 psi in the crossover pipe at the higher RPMs, which is very good. The idea is to get good cylinder scavenging providing there is even a small amount of valve overlap. What other benefits would there be with a low restriction exhaust housing when, let's say, there was no overlap whats so ever?
If PorscheG's specs are correct there is very little overlap indeed which is a little disappointing. I don't know my exact exhaust housing; the bottom line is the relationship of boost pressure to exhaust back pressure. I tested it and it was 17 psi boost with 6 to 7 psi in the crossover pipe at the higher RPMs, which is very good. The idea is to get good cylinder scavenging providing there is even a small amount of valve overlap. What other benefits would there be with a low restriction exhaust housing when, let's say, there was no overlap whats so ever?
10-26-2003, 10:33 PM
#25
Rennlist Junkie Forever
TurboTommy
the relationship of boost pressure to exhaust back pressure. I tested it and it was 17 psi boost with 6 to 7 psi in the crossover pipe at the higher RPMs, which is very good.
Uhhh if that were in fact the case, you would have a TIP-IMP <1, which is unhead of on a street car. Your TIM-IMP ratio would be along the lines of a Formula 1 race car if that were the case.
I think you need to double check your gauge/measurements (or rather your mechanic needs to check his).
the relationship of boost pressure to exhaust back pressure. I tested it and it was 17 psi boost with 6 to 7 psi in the crossover pipe at the higher RPMs, which is very good.
Uhhh if that were in fact the case, you would have a TIP-IMP <1, which is unhead of on a street car. Your TIM-IMP ratio would be along the lines of a Formula 1 race car if that were the case.
I think you need to double check your gauge/measurements (or rather your mechanic needs to check his).
Last edited by TonyG; 10-27-2003 at 01:19 AM.
10-26-2003, 11:11 PM
#26
Rennlist Member
Tony, I didn't do the test myself; A very good porsche service/tuner shop did the test. From what I understand that it's really not that unheard of on a street car. I think even a stock 951S (stock boost levels with dropping off at higher RPMs, etc.) acheived a ratio where the intake manifold was slightly higher than turbine inlet pressure.
It's all this obsession with no lag and instant turbo responce that leads to high back pressure figures. You're right, alot of non Porsche OEM cars have higher back pressure than manifold pressure to satisfy the publics' belief that turbo lag is bad. You ever wonder why our cars have that extra little bit of thrust once the turbo comes on compared to other turbo makes with similar displacement, etc?
Why don't you get your back pressure tested; I'm kinda curious in case my test was bulls$%^
It's all this obsession with no lag and instant turbo responce that leads to high back pressure figures. You're right, alot of non Porsche OEM cars have higher back pressure than manifold pressure to satisfy the publics' belief that turbo lag is bad. You ever wonder why our cars have that extra little bit of thrust once the turbo comes on compared to other turbo makes with similar displacement, etc?
Why don't you get your back pressure tested; I'm kinda curious in case my test was bulls$%^
10-27-2003, 01:09 AM
#28
Rennlist Junkie Forever
The skyline that was just tested produced over 37psi of back pressure with a .84A/R garrett running 12 psi boost, with a SFR 6 into 1 custom header.
If it were possible to quite easily have a ratio of less than 1, then cam selection would be a non-issue as reversion would not exist at all...
You should do the test yourself.
If it were possible to quite easily have a ratio of less than 1, then cam selection would be a non-issue as reversion would not exist at all...
You should do the test yourself.
10-27-2003, 04:45 AM
#30
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Last week I tested my back pressure. 4-2-1 exhaust- It shows around 15PS@5000rpm@14.5 PSI boost
This is with a spesial build KKK k26-8-27. What is spesial would the company dont tell the details but they promise low rpm torq.
Ill get it also. 14.5 psi at 2350rpm.
I have a 3.0 l and everything is build for low torq street therefore I have mostly other thing stok. (exept MAF, LindseyII IC, 3" exhaust.)
Belive it or not but changing from 2.5" (witout cat and with 3" end muffer) into Lindseys mild steel thermocoated exhaust, the back pressure dropped down from 22 to 15 psi.
The spool up gives 150 rpm. from 2500 to 2350rpm gives 14.5 psi.
I have done som job arond the valves in the combustion chamber also. But have the stoc cam.
The top numbers do I not have yet and the snow outside today dont looking god for further testing. But the torq is so much on the lower rpm that it dont feel good for the drivetrian or the engine mounts.
sorry for my poor language.
I have a dynochart for an earlyer configuration here (Stock everything exept of chip end muffler and 3.0 engine)
http://no.msnusers.com/GulterKult/sh...oto&PhotoID=15
http://no.msnusers.com/_Secure/0RwDK...39218486075484
This is with a spesial build KKK k26-8-27. What is spesial would the company dont tell the details but they promise low rpm torq.
Ill get it also. 14.5 psi at 2350rpm.
I have a 3.0 l and everything is build for low torq street therefore I have mostly other thing stok. (exept MAF, LindseyII IC, 3" exhaust.)
Belive it or not but changing from 2.5" (witout cat and with 3" end muffer) into Lindseys mild steel thermocoated exhaust, the back pressure dropped down from 22 to 15 psi.
The spool up gives 150 rpm. from 2500 to 2350rpm gives 14.5 psi.
I have done som job arond the valves in the combustion chamber also. But have the stoc cam.
The top numbers do I not have yet and the snow outside today dont looking god for further testing. But the torq is so much on the lower rpm that it dont feel good for the drivetrian or the engine mounts.
sorry for my poor language.
I have a dynochart for an earlyer configuration here (Stock everything exept of chip end muffler and 3.0 engine)
http://no.msnusers.com/GulterKult/sh...oto&PhotoID=15
http://no.msnusers.com/_Secure/0RwDK...39218486075484