LT1-7

We’re trying to figure out why my brother’s boost keeps dropping after 6500rpms. He’s running an XS TO4S. Last week on the dyno it boost to 20 and then drops to 16 after 6K. He was running a 10psi spring which we think was the problem since boost controllers can only add about 70% of the spring (according to Apexi). So now he installed a 14psi spring. The drop in boost is less but still drops. It’ll boost up to 20 then drop to 18 after 6500. At first I though we were running out of turbo but then realize the wg is still opening all the way to redline. We’ve tried both the Greddy Profec B controller as well as a Blitz DSBC Spec R. Both the same.

Wargasm

Is it possible that there is excessive backpressure in the exhaust manifold pushing the wastegate open?

Brian

Wargasm

btw, got any dynos of 20 psi (I know not complete to redline)

SPOautos

You should put your hand in front of the bov and make sure there are no leaks. Also, check to make sure all the IC pipe clamps are nice and tight

What are the specs on the turbo?

STEPHEN

pp13bnos

iTrader: (1)

Did Ralph say anything about my 25lb+ dyno run? CJ

LT1-7

wargasm, the exhaust manifold is from XS, it's a cast iron. I guess it could be the problem but highly doubt it. Lots of people are using this manifold with great luck.

Dyno #'s are pretty good but we're loosing lots of power on top beause of the boost drop. HP peaks at 6000 at 420hp I believe, somewhere close to that. When boost dropped, so did the hp curve. I think we would have hit 500rwhp if that 20psi would have stayed

SPO, that was tried at one of the very first runs. We put a buntch of washers in the Turbo SX BOV, nothing was bleeding off.

CJ, yeah Ralph told us about that awhile ago. Sounds like it scared the crap out of all of you huh

XSTransAm

dont you want it to taper a little to redline... 2psi taper is good IMO.

Garrett

I was losing power on the top end do to a restrictive filter. Also, a boost leak might be happening under load over 6500 rpms that might not be affecting anything until that load.

LT1-7

TransAm, I don't see any reason anyone would want boost to taper off unless they're at the threshold of fuel or what the engine can handle. Looking at the graph you can really tell he needed that couple pounds. It just goes up then levels off. Looks like he would have gotten at least 475 if the boost kept up

Garrett, all the IC pipes are on with T-bolt clamps, no leaks there. I believe the intake manifold gaskets were changed not too long ago also. The filter is an HKS 4" inlet

I think I just might have found out what the problem is, tell me if this is possible.

The boost controller's stepper motor inlet is also tied in with the BOV. I'm thinking the BOV somehow bleeds some of that pressure off after a certain amount of time, therefor lowering the pressure the stepper motor's out pressure

XSTransAm

the boost controller inlet should come off the compressor of the turbo. the bov should come off the manifold. This will give you the best boost control. having the two on the same line is never good.

LT1-7

boost controller inlet from the compressor of the turbo? That's the first I've heard of this. Is that how most people are routing it? I though manifold pressure is most accurate when controlling boost?

XSTransAm

you will get the quickest boost response from the turbo when using the compressor nipple. thats how the factory system does it. and how most cars do it (wrx evo, ect..)

maybe your turbo dosent have a nipple on the compressor housing?

XSTransAm

im sorry let me restate myself, running the wastegate off the compressor of the turbo will allow for better boost control. However it will not help with spool up.

just to be crystal clear