Boost question after rebuilt and head work

YELLOW 94 · Aug 19, 2008

Before I rebuilt my engine my boost gauge was reading 12psi. For the rebuild I went 20 over on the cylinders and ported the heads. This is the only change that was done to the engine.

I went to the dyno on friday and was watching the boost gauge and I only saw 8lbs of boost. Why do you think the boost is down. On the first pull I only saw 5 pounds of boost so we tightened the belt. 8lbs was on the 2nd and 3rd pull.

This weekend I gonna go a compression test to see if maybe I have valve hung open, or just go ahead and readjust all the valve.

Any thoughts would help. By the way the car put down 410 at the wheels uncorrected with 8 degrees of initial timing.

Thanks
Orlie

Yellow302 · Aug 19, 2008

higher flow = less back pressure = less boost

so your moving more air but there is less restriction.

ProKiller · Aug 19, 2008

YELLOW 94 · Aug 19, 2008

I have a hard time believing that I could lose 4 pounds of boost on my gauge. I understand what your saying about seeing less boost on the gauge. One thing I did add was a one inche spacer between the intakes. Im thinking that this spacer will add volume to my intake will that contribute to my gauge seeing less boost.

later
Orlie

Black95GTS · Aug 19, 2008

YELLOW 94 said:
I have a hard time believing that I could lose 4 pounds of boost on my gauge. I understand what your saying about seeing less boost on the gauge. One thing I did add was a one inche spacer between the intakes. Im thinking that this spacer will add volume to my intake will that contribute to my gauge seeing less boost.

later
Orlie

Did you check for leaks? I highly doubt you would lose 4 pounds of boost based on the modifications you did.

Adam

YELLOW 94 · Aug 19, 2008

Im with u Black95GTS I do not believe I could of lost 4 pounds of boost on the gauge. I will look for any leaks in the system.

later
Orlie

Yellow302 · Aug 19, 2008

ive got some ****ty digital gauge and it changes every time i start the car.

toyman · Aug 19, 2008

YELLOW 94 said:
Im with u Black95GTS I do not believe I could of lost 4 pounds of boost on the gauge. I will look for any leaks in the system.

later
Orlie

I lost boost with my change (heads & cam) but it was between 2 & 2.5 psi. I am running a speed density setup and there is a MAP psi reading shown on the dyno report. Are these readings only shown because of my setup?

YELLOW 94 · Aug 19, 2008

I just thought of this, I also changed my cam, I went from an e cam to a trickflow stage 2 cam. Ill check it out this weekend and report back. Thanks for the help.

later
Orlie

final5-0 · Aug 19, 2008

I gotta agree with the folks who say better flow will mean less boost

It should be less but like others have said ... maybe something else
is allowing some of the boost to escape

Grady

Zero Signal · Aug 19, 2008

Yeah you just need a smaller pulley or larger blower. Aint physics a bitch.

Black95GTS · Aug 20, 2008

YELLOW 94 said:
I just thought of this, I also changed my cam, I went from an e cam to a trickflow stage 2 cam. Ill check it out this weekend and report back. Thanks for the help.

later
Orlie

Well that certainly changes things. I'd still check for a leak, but that is a "bigger" cam, so that may be the culprit. Boosted guys pop gaskets on dynos all the time, as I probably don't need to tell you.

Adam

Vipersix · Aug 20, 2008

Boost pressures depend primarily on the displacement of the engine, the displacement of the blower, and the drive ratio of the blower. Changing any of these items will alter the resulting pressures experienced. The boost formula: (25.58 x blower displacement x drive ratio) / (engine displacement) - 14.7

For example: 302 cu. in. with a 161 blower and 1.95:1 drive. (25.58 x 161 x 1.95) / 302 = 26.6 - 14.7 = 11.9 psi

Boring .020 over results in 305 cu. in. (25.58 x 161 x 1.95) / 305 = 26.3 - 14.7 = 11.6 psi

Other factors such as camshaft timing, valve size, and port size will also change the boost but I don't think it would be enough to justify a drop of 4 psi.

My vote goes for checking for leaks.

4Jenna · Aug 21, 2008

How much flow did the cylinder heads pickup after the porting?

Is your boost refrence still pulling from the same spot as before?

YELLOW 94 · Aug 21, 2008

To be honest with you, I have no idea what the flow numbers are.

Yes the gauge is located in the same position.

later
Orlie

4Jenna · Aug 25, 2008

Depending on how much the heads picked up the boost could drop a good bit.

Check the vaccum line that goes to your boost gauge for kinks, leaks, etc.

94GTLaserRC · Aug 25, 2008

Another possibility that wasnt mentioned is the belt. Did you check the belt for slippage? Is it tight enough on the pulley?

Let us know if you find the problem.

RC

YELLOW 94 · Aug 25, 2008

94GTLaserRC said:
Another possibility that wasnt mentioned is the belt. Did you check the belt for slippage? Is it tight enough on the pulley?

Let us know if you find the problem.

RC

The belt is tight, I didnt get a chance to look at the car this weekend. So it will have to wait till next week.

later
Orlie

Boost question after rebuilt and head work

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