04 GT Dyno results :(

[CrazyHorst]

Originally Posted by BNaylor

For exhaust back pressure testing one of the easiest and inexpensive tests to run is an exhaust back pressure test with a regular automotive vacuum gauge 0-30 in-hg connected at an intake manifold vacuum source. After connecting the gauge start the engine at idle, slowly increase engine speed to 3,000 RPM, engine vacuum should be equal to or higher than idle vacuum at 3,000 RPM. If vacuum decreases at higher engine RPM's, excessive exhaust back pressure is present meaning some sort of corrective action should be taken.

I'm not sure what this test will prove other than that the vacuum gauge is working correctly.

The intake manifold vacuum is based on the restriction past the throttle blade....and has got to decrease as the throttle opens wider! Granted 3000 rpms in neutral (no load) doesn't take much throttle opening so there isn't bound to be a big change...at WOT it should be nearly zero (recalling WOT in neutral won't do a lot other than making some paniful cyclic noises).

There is always some "spitback" into the intake tract as the intake valve opens...consider that the combustion chamber has exhausted very close to slightly above atmospheric and that the intake is below atmospheric. This is the prime reason a DOHC cylinder head "falls on its face" at low rpms. However the vacuum gauge cannot detect the spitback unless there are gross problems like an unintended banana up the tailpipe.

[BNaylor]

Originally Posted by CrazyHorst

I'm not sure what this test will prove other than that the vacuum gauge is working correctly.

Actually it does more than that and you answered your own question. Plus it doesn't cost much for us poor DIY types that are more practical than theoretical in addressing tech issues. A very cost effective test regardless.

It will detect any gross exhaust back pressure issue or restriction whether it is a clogged CAT converter or even a banana stuck in the exhaust. ......

Here is some anecdotal proof:

Click here<---Be sure to read it in its entirety

Post #23

Originally Posted by cool02grandprix

ok I went and got a vacuum tester like you have.....

at idle I get 13-14 in hg
rev to 2000 to 3000 and it drops to basically to about 0 (zero) and then at idle back to 13-14 in hg

Obviously not a reliable scientific method to fine tune your exhaust system or if you are looking for horsepower. Here is a good link from Thrasher Engineering on some more practical DIY testing:

Click here

Exhaust Backpressure Study

Replacing the stock production exhaust system with a low-restriction, free-flow one is usually one of the first modifications made to any vehicle in the name of performance. We all know they're louder, but how much performance do they really add? We've all seen supposed dyno tests, usually run by the exhaust manufacturer's themselves on their own dyno, indicating vast power gains, and psychologically, we always equate a healthy exhaust rumble with increased power in the seat of the pants, but how much power are we really gaining? To find out, we're running a simple backpressure study, and our results will be posted here as they come. Admittedly this study is not totally scientific as there are many uncontrolled variables, but it should be sufficient to provide a rough estimate.

It is generally accepted by automotive engineers that for every inch of Hg of backpressure (that's Mercury - inches of Hg is a unit for measuring pressure) approximately 1-2 HP is lost depending on the displacement and efficiency of the engine, the combustion chamber design, etc. Our sources indicated that in the case of the L67 3800SC, 1HP per inch of Hg is reasonable.

1 inch Hg backpressure = 1 HP lost

For reference, we have the following conversions factors:

1 ATM = 14.7 PSI = 76 cm of Hg = 29.921 inches of Hg = 1.013 bar

[rickisrad]

I think you are right B- I will go on how it rides on the street and track. I notice a huge difference with the mods I have so that is all that really matters. I am going to put the HV3 insert in there within the next 2 months and soon after I will get the ER rockers. I want to put a cam in it but really do not want to pull the engine. The shop I work at is getting sick of me having my car in there anyways