True boost?

[jasoncitrano]

I was told that any turbo engine that you use a boost gauge on whatever reading you are seeing is not whats actually going into the engine. They said that becuse there is some valve overlap the exhaust pressure leaks up into the intake manifold causing the numbers to show higher then they really are? is this true or does the 4g63 have very minimal valve overlap?

 

[turboinside]

False: there is a minimal amount of over lap but it is there to create a scavaging effect in the exhaust. This helps dispel burnt fuel from the cylinders. The read out you get from a gauge is how much presure is in the intake manifold. I think whom ever you heard this from was mistaking P.S.I. for C.F.M. Just because there is 12 P.S.I. of boost in the intake, doesnt mean your motor is inhaling it all

 

[tstkl]

does it matter?

 

[94Jettameowpsst]

i think you'll only have a lot of overlap if you have an auto motor w/ auto cams. I have no boost leaks what so ever and when i got my car dyno'd it rose to 17psi for about 4 seconds then fell off to 14 by 6k rpm. No explanation besides stock auto small POS cams

 

[turboinside]

I think most cars lose a little boost when the rpm climbs. I have a '99 GST and it climbs to about 15 p.s.i. the falls to 12 p.s.i. at about 5100 r.p.m.

 

[herostar]

I'm going to have to disagree with you about losing boost as engine rpm's climb. Ever hear of boost creep? It's the complete opposite. As far as your engine not seing "true boost", well yes, there is valve overlap. But it's only for a short time. When the valves do overlap the exhaust valve is just about closed when the intake valve is just opening. The intake valve is open for quite a long time after the exhaust valve is closed which allows your "true boost" to fill the combustion chamber. So yes, when your gauge reads X psi, then X psi is going into your engine. If the overlap was big enough to cause you to lose boost pressure, it would be open during the compression stage of combustion and there would be no pressure in your cylinders. No compression = not a running engine.

 

[TurboChrged93]

I think most cars lose a little boost when the rpm climbs. I have a '99 GST and it climbs to about 15 p.s.i. the falls to 12 p.s.i. at about 5100 r.p.m.

Negative. It all depends on your setup. I didn't take the time to look @ your mods however I'm guessing by your opinion given above... you have a stock setup. Number 1, the t25 runs out of breath around high RPM. Second, if you have a restrictive exhaust you will lose boost due to the exhaust flow not being able to keep up with the turbo's flow.

 

[tstkl]

Negative. It all depends on your setup. I didn't take the time to look @ your mods however I'm guessing by your opinion given above... you have a stock setup. Number 1, the t25 runs out of breath around high RPM. Second, if you have a restrictive exhaust you will lose boost due to the exhaust flow not being able to keep up with the turbo's flow.

couldnt have said it better, thats why when you get a new exaust sometimes the psi increases...

 

[turboinside]

Negative. It all depends on your setup. I didn't take the time to look @ your mods however I'm guessing by your opinion given above... you have a stock setup. Number 1, the t25 runs out of breath around high RPM. Second, if you have a restrictive exhaust you will lose boost due to the exhaust flow not being able to keep up with the turbo's flow.

Yes I do have the stock turbo, But I do have a full custom exhaust 2.5" with mandrel bends, high flow cat and DTM style muffler. When I stated that most turbo cars will loose boost at higher R.P.M., I meant cars with small or stock turbo's. I guess I should have been a little more clear on the subject. If you have modified or replaced the turbo with a bigger one then you would most likely build boost to redline. As for me, I don't think that I will replace the turbo. It is a daily driven street car with no intetions of going to the track. I have other cars for the track. Besides, I have seen 300+ Wheel horse power on the stock turbo's!

 

[Defiant]

I was told that any turbo engine that you use a boost gauge on whatever reading you are seeing is not whats actually going into the engine. They said that becuse there is some valve overlap the exhaust pressure leaks up into the intake manifold causing the numbers to show higher then they really are?

Your source's ears are held apart by air pressure.

 

[jasoncitrano]

Not to revive an old thread but i figured it out. He was acutally right. You need an exhaust manifold pressure gauge and a intake manifold pressure gauge you subtract the exhaust from the intake and theres your true boost.