I recently did a 40-70 3rd gear boost check on my car and noticed a strange boost pattern:

8 psi starting at 3100 rpm
Boost starts climbing at 4300 rpm
Peaks at 11 psi at 4400 rpm and then drops
Back to 8 psi again at 5100 rpm
At 5400 rpm (70 mph) the boost starts to drop and ends up at 5 psi at around 6600 rpm

I'm not sure where to start looking so any directions would be greatly appreciated. The pertinent mods on my car are:

Mild streetport
Cheap bastard intake mod
DP
Pettit Unlimited ECU
RB Dual Cat-back

The boost readings were taken on the Autometer gauge that came with my Pettit Column mounted pod and the gauge line is plugged into the UIM nipple.

 

The 10-8-10 boost pattern refers to max psi on primary, the psi during the transition, and the max on both respectively. That means your pattern is actually 11-8-5 using that nomenclature. If it's actually 11 psi on primary this may mean a mild boost creep due to your intake and cat back mods. More importantly, the 5 psi on both turbos speaks of different issues. There are a number of problems that can cause low pressure once the secondary kicks in, so just search for those bad boys.

 

Thanks for the correction. I'll do a search for troubleshooting ideas based on my (corrected) boost pattern.

To add to the confusion, a short time after I did the boost test (15-20 min), I did a standing start launch at 4000 rpm and couldn't get more than 2-3 psi through 1st and 2nd gear.

 

I'm having the same issue. Low to no boost in lower gears but only sometimes, it's very frustrating.

I've decided to pull the turbos off and perform a proper non sequential conversion which will simplify the boost control (will replace with Greddy EBC) and hopefully give me reliable boost.

I would like to hear if you fix it what it was though, just for interest's sake.

Good luck.......remember to report back once you do fix it to help others. There's too many problem threads where the results and cause of the problems aren't reported back at the end which doesn't actually help anyone fix a similar problem.

 

How low does the boost drop after 4400 RPM? Had you done any mods/repairs just before it started acting this way?

I think one possibility is that your turbo pre-control may be sticking.

This site is a good reference for debugging the sequential turbos:
http://www.autosportracetech.com/RX-...leshooting.htm

 

Thanks, I've been looking at that site.

The boost basically drops linearly from 11 psi at 4400 rpm to 8 psi at 5100 rpm, then stays at 8 psi from 5100 rpm to 5400 rpm, after which it starts dropping linearly from 8 psi to 5 psi at 6500 rpm. I've got a video but I dont' know how to post it.

So instead of replacing stock parts, could this be something that a boost controller would fix? Doesn't a boost controller by-pass the turbo pre-control?

 

On sequentials, no the boost controller only replaces the wastegate. The whole system is essentially the same thing but with a better wastegate controller.

I think that a boost controller would be a good first step, but it's not likely you'll get a 10-8-10 as a result. At least with that in place you don't have risk of a sudden boost spike killing your engine, and you'll be able to adjust the boost to exactly 10psi.

Dave

 

Oops, I mis-read your original post. Since your boost is effectively dropping as revs increase, have you ruled out a clogged cat?

 

Ditto on the possibility of a clogged cat...

Dave

 

I have to admit I haven't thought about the cat, it was replaced about 4 years ago. Would a clogged cat allow boost creep to 11 psi on the primary?

 

I think it's possible: if 11PSI @ 4400RPM maxes out the cat, as the revs increase further the backpressure on the turbines would build due to the higher flow rate.