Boost dropping

[NOWAY55]

OK, I am a little new to this, but looking for some suggestions on what to look at.

If I go WOT, I see a boost spike to about 24lbs and then after about 1-1.5 sec it fades back to 18 lbs and hold pretty good.

If I ease into it and hold say 75% throttle, it will get to about 21-22 and hold that level.

What is causing the spike and drop off when WOT?

Right now I am using a Voodoo MBC and have a Greddy Profec b Spec II on order. I am hoping this will fix my issue. Trying to get to 25lbs steady boost.

 

[canadian]

Re: Boost dropping

You've got all the required supporting mods to run 25psi, correct? Tuned chip and that sort of stuff?

 

[It's just a six]

Re: Boost dropping

Does the computer still control the timeing?
Detonation?

 

[NOWAY55]

Re: Boost dropping

I am running about 20-22 deg timing at this boost level.

No KR (spraying a crap load of meth) A/F is around 10.5

It just wont hold the boost level I want it to.

I would say I have the supporting mods to safely do 25lbs and was told the turbo is good for it.

 

[Quickstop [UK]]

Re: Boost dropping

Get Datamaster before you go any further. it is indispensable for diagnosing problems on these trucks.

Hope you know what you've let yourself in for!

:tup:!

 

[turbodig]

Re: Boost dropping

What's controlling boost now? Bleeder valve?


Might be a little too much alky... not enough heat to generate that kind of boost.

25 might be a bit too much for that turbo. It's out of the efficiency sweet spot at a 2.7
pressure ratio and ~ 57 lb/hr. It should run 20-22 just fine.

http://64.225.76.178/catalog/compmaps/fig9.html


Might also look for exhaust restrictions, or exhaust manifold leaks.

 

[NOWAY55]

Re: Boost dropping

Funny,

I was thinking too much Meth also.

I am still trying to lean it out a little bit.

It was reading 10.4 on my scans but I think the AEM only goes that low. I have never seen anything richer.

I am now shooting for 10.7 to get it off the rich floor where it is. For all I know, the 10.4 may be 9.0 and the gauge just wont go that low.

I got the turbo from Tim's Turbos. I called Tim and he said it should be no problem to do 25lbs.

I am running TunerPro RT to log all my driving BTW that is where I am getting the data from.

 

[turbodig]

Re: Boost dropping

Funny,

I was thinking too much Meth also.

I am still trying to lean it out a little bit.

It was reading 10.4 on my scans but I think the AEM only goes that low. I have never seen anything richer.

I am now shooting for 10.7 to get it off the rich floor where it is. For all I know, the 10.4 may be 9.0 and the gauge just wont go that low.

I got the turbo from Tim's Turbos. I called Tim and he said it should be no problem to do 25lbs.

I am running TunerPro RT to log all my driving BTW that is where I am getting the data from.

You're probably quite a bit too rich... the WB data hits the floor around that level.
Run it at 10.9 or 11.0, and you'll likely get some boost back.

Unless he had a lot of detail specs on your engine build, he couldn't really do anything but guess. Where a motor will sit in a compressor map has to do with how much air it's moving, what your altitude is, how your air intake is set up, lots of things.

Also, don't fall into the trap of thinking that 25 psi will be better/faster than 21. It might not.
(Actually, it's quite likely that it won't)

 

[Tooky]

Re: Boost dropping

I agree with Dig. You have Vortec heads and cam, and my truck does too. I have a "585HP rated" 57mm PT51 turbo and it reaches its limit at 20 PSI. I went to the track and made several runs, the fastest runs were all at 20 PSI and below. The runs at 21-22 PSI were all slower. The turbo is at it's limit. I also found my boost dropping at high RPM in each gear, I believe this is related to me operating the turbo at it's efficiency limit.

This is likely the case for your truck too. However I reached the limit when I was running 11.4 @ 117 MPH. How fast is your truck? With all the mods you have and that boost level, it should be in the 11s or something is wrong (even for a Typhoon). Does it feel like you're making 500-600 HP?

To run 25 PSI on a vortec head/cam engine I would recommend something like a 67mm turbo.

BTW 10:1 AFRs are absurd in my opinion, I spray methanol too and I run 11.6-12.0 AFRs.