Odd boost creep issue

[Ligouri Rd]

For reference I am running the original Boost Designs manifold and down pipe with a T3/T04E 57 trim 63 A/R turbo. I have a real Tial 38mm waste gate with a 5.6# spring. The waste gate signal is coming directly off of the compressor housing. The exhaust is three inch diameter from the down pipe necking down at the hollowed out secondary cat and then to the N1 cat back.

I have been having intermittent issues with the boost rising well above the spring rate. I think I have it nailed down but I don't exactly understand it. With my Apexi N1 silencer installed I get pretty consistent boost around 8 psi rising to but not over 10 at 5500rpm and higher. This is not a problem since the A/F never goes above 12 at this boost level. Now with the silencer removed the boost keeps rising up past 15psi. This was a little scary last weekend as I watched the A/F ratio rise above the safe 11-12 range; needless to say I immediately got off the throttle.

I know that the boost designs manifold is prone to creeping but what makes it work ok only when the silencer is in? Without the silencer is their so much flow that what is trying to come out of the waste gate is being forced back by what is coming from the turbine?

I have a track day coming up at the end of September and would like to run without the silencer to get better pull but I am afraid I will over boost while I am trying to concentrate on the track. I guess I could also bite the bullet and put in some 470cc injectors, but I have no idea if the boost will stop at 15psi.I could also flip the wastegate around and run open dump, but that ain't exactly great for daily driving. Lemme know whatchall think, Thanks.

[GEO]

Your problem is msot likely the exhaust gases being dumped back into the downpipe from the wastegate.

[klattr1]

obviously when u use the silencer, its slowing down the velocity of the exhaust. enough to make it less efficient. if you air fuel ratios are jumping along with the spike with no silencer, then put that shizzle back in. no reason to risk blowing a motor over not having a silencer. you arent the only one thats had that issue. the solution: hasnt been found yet other than running a different manifold/exhaust setup.

[SHIFT_Lock]

Well I just went through this with a customer and decided that the 90degree elbow for the wast gate was causing a "path of least resistence" effect.

What is basicly happening is with no back pressure from a cat or silencer the exhaust gases at peak boost is causing "vaccum" of sorts. only because of the 90 degree bend. So in a sense it is easier for waste gate to stay closed or near it rather than follow the exhaust gas into the down pipe and not out the waste gate. If that makes since.

I don't really think that it would matter if the WG was to dump to the atmosphere or reroute back in. If you are not running a cat most likely putting one on will take care of the problem. Or get rid of the 90 degree and put a flange rite on the collector.

Remember this is all just theory also. But I think it makes since. Anyone else have any theories?

Hope that helps......Scott
Modified by SHIFT_Lock at 4:03 AM 9/7/2005

[rn240sx]

Quote »I know that the boost designs manifold is prone to creeping but what makes it work ok only when the silencer is in? Without the silencer is their so much flow that what is trying to come out of the waste gate is being forced back by what is coming from the turbine? [/quote]Prone to boost creep........ And they know this..?? Thats sad.

[SHIFT_Lock]

With my Apexi N1 silencer installed I get pretty consistent boost around 8 psi rising to but not over 10 at 5500rpm and higher. Now with the silencer removed the boost keeps rising up past 15psi.

With the design of this wastegate set up it can and will.

[klattr1]

my boost creeped (on BD manifold) with open downpipe and a dumped to the ground wastegate setup. guess it was too efficient (no backpressure)

i had a 1 bar WG spring and i could hit 22-23 psi at 7000 rpms. luckily i used race gas the whole time.

[Ligouri Rd]

Huh! I didn't event think about the elbow leading up to the wastegate. It makes sense that any vacumme caused by a siphon effect of the elbow would subtract from the pressure being used to open the wastegate.

A high flow cat is a costly solution but ultimately seems to be the best course of action. Based on what I am seeing hopefully a cat should add a small enough amount of restriction closer to the manifold for the wastegate elbow to function properly. My hollowed out secondary cat is pretty hack anyway. Thanx

[Ligouri Rd]

Well I installed a high flow cat (3" flowmaster from PDM) this weekend in hopes of adding enough restriction to allow the wastegate to open properly. So far it hasn't made a difference, the boost still spikes past 10 psi. There is a minor exhaust leak I have to fix tonite where the downpipe meets the new cat. Apparently the downpipe flange has warped at bit over time. I highly doubt fixing this leak will have much effect. I guess I will be racing with my silencer in, no sense in wrecking all the hard work I put into this motor. Unless anyone else has some ideas...

...I guess since Ivan has reappeared practically a block away from where I work I could ask his advice (Although AMS was formerly a big no-no of a place to go if you didn't happen to own a DSM).

[sunnys14]

how come everyone has boost creep problems with their BD manifold while i dont? weird stuff here...

[Ligouri Rd]

Would the wastegate signal line make any difference? Klattr where are you picking up your waste gate signal?Currently I am picking up the signal directly from the compressor housing with an 18" length of reinforced vacumme line. My boost gauge is teed off of the fuel pressure regulator. My diverter (Forge, with lightest spring installed) valve is mounted fairly near the compressor as well. I know this sounds stupid, but could the diverter valve be opening a little, relieving pressure at the compressor and not sending the proper pressure to the wastegate? I know its usually discouraged to do this but, would relocating the wastegate signal to the intake manifold make any difference?

[klattr1]

try bypassing your diverter and run the side port wastegate line directly to the compressor housing with no interruptions and then check back with me.

but the thing is, if that was the problem, the car would overboost directly all at once, instead of boost creeping towards higher rpms or redline. which makes me think its more of a scavenging issue.

[scarboroughdub]

i think your best bet to see if its the wastegates plumbing due to boost designs manifold is to dump the waste gate to the atmosphere and see if you still experience boost creep if not problem solved and you may need to route the waste further down the dp and at a 45 deg angle.