dholling13

After extensive research and a few changes, my 91 c2 Turbo is sitll seeing crazy spikes in boost pressure on the Andial Gauge I have mounted in the car. This episode started out when I removed the Cat from my car and went with a by-pass pipe and stock muffler combination. The car has b&b headers with a 1 3/4" pipe going to the wastegate... At first, I had a 1 bar andial spring in the stock gate and it was over boosting. So I went with some good advice and hung the stock gate on the wall and installed a tial gate with a .9 bar spring. The car was still over boosting, so I replaced the over boost switch and the gate spring with a .7 bar spring. At 5k rpm's the gauge is registering between 15-16 psi's but I can hear the gate opening up.

In 2 months I am going to a DE at VIR, and I want to be able to go near redline on the track. This can't be something new since we have been doing this mod for years. Any thougts without me shipping my car to Stephen? Something I would like to do but not until major surgery is needed. I have included some pictures of what projects I did: Steering Wheel, bye bye bumperettes, Filter Kit, Extinguisher etc, and the vacuum line that splits to the boost gauge. I was also curious, does anyone know what this round gold vacuum unit does?

Thanks agin,
Drew

cobalt

Again beautiful turbo you have there.

Something does not add up. The only thing I can think of is if you are using your stock tail pipe and WG cat that you may have a restriction in the WG cat and it is not dumping the exhaust even though the WG is opening when it should. You might try testing the car on the dyno and verifying that the Andial gage is accurate (they usually are) and see how the chart looks. Is it a slow creep or a drastic spike?

Good Luck

dholling13

The cycle begins at 2k rpm's where it builds up to 12 psi's, then at 4k it hits 14, then 5k it's creeping up near the 15-16 range and finally i let off the gas it surges forward and drops back down to zero... I gutted the wastegate pipe check out the picture.

underpsi

i believe this is caused by the design/position of the inlet wastegate pipe from the B&B headers; which is not the ideal design.

dholling13

That's what TPC said could be a problem; but it usually has to do with diameter of the pipe itself. Most pipes are 1 5/8's that cause this problem, mine is 1 3/4... As far as a design flaw, do you mean the pipe is branching off at the wrong point? I would like to hear what the Doc has to say about this...

38D

The gauge shows 12 psi at 2,000 RPMs? There's no way that can be. The gauge must be wrong.

underpsi

yes i believe the angle of where it branches off is not idea which causes a late response from the wastegate. it is not necessarily the size of the tubes but the angle in relation to the exhaust flow. I also have b&b headers and it always over-boost 1-2 psi initially then it corrects itself.

dholling13

Colin,

I miss stated the initial it's closer to 2.5k rpms when it spools up to 10-12 psi's... Do you know what that gold colored round vacuum disc does?

cobalt

From what I have heard the B&B system is one of the better designs.

I have the B&B and have 0 issues. I am at full boost by 3200 rpms using a cat. I hit 14 PSI and it creeps ever so slightly up to 14.8 PSI at redline.

If you have no cat and you are getting this much creep over the set pressure it sounds like you are still having a problem with the WG.

dholling13

What I have heard is because the cat has been removed there is almost no back pressure now. The turbo spools up and flows much faster than before. But there has to be another reason why the gate isn't dumping the excess boost?

SonnyV

That is a deceleration valve. It helps to keep the rpm from dropping too fast when you let off the gas. From what I remember, some people removed them out of track cars.

Sonny

Y65MPH

Drew,

Does the throttle plate close when you "overboost"? Or are you just seeing issues on the guage. I had the same problem.

Craig

underpsi

yes this issue is more pronounced without the cat due to no back pressure; which will allow the turbo to spool quicker and overboost before the wastegate is able to react in time at your desired boost setting.

38D

you must have an issue on your control line for the wastegate, or the wastegate itself has an issue. There is no way you'd have that much overboost that early, cat bypass or not.

nathanUK '81 930 G50

I agree. It is also known as a vacuum limiter.