I've never seen this problem before. Car is a '93 with auto to manual conversion. it has a full 3in exhaust (bar down pipe), and K&N panel filter. Everything works as it should, prim turbo makes a steady 12psi as does the secondary turbo for a short time, but then as rpm increases boost drops back to 7psi by about 6000rpm. It seems like a ported engine with too small a compressor, only everything on this car is stock.

I tried a the non-seq mod temporarily and aside from the expected lag boost still fell away at higher rpm.

The only difference is the turbos are from a '96 (no pills).

I'm guessing it's something to do with the factory boost control? Has anyone come across this before? It can't be a boost leak or the prim turbo wouldn't hold 12psi.

I've searched an found just about every boost issue under the sun except this specifically.

 

Stock pre cat could be your problem. Might be clogged.

 

boost falls off from the factory

 

Boost does fall off somewhat, but should not fall below 8psi at redline. BTW, if you have the stock ECU, 12psi is too high...

 

Check the turbo troubleshooting link in the FAQ thread for other ideas also.

 

Thanks for the ideas guys. I have looked through the faq but am still none the wiser. The car has no cat converter at all. In my experience 11-12psi is fairly normal with a large exhaust and intake done. Yes ECU is stock (auto model). I will swap it out for a manual ECU but I'm not holding my breath.

 

Hi.

I have an auto-manual conversion as well, and was running the stock Auto ECU. I was also getting a drop to 8psi at 5500-6000rpm. I don't have a pre-cat (car is JDM). Seems to be the way the stock Auto ECU is setup.
Have a look at the boost and dyno graph here:

https://www.rx7club.com/showpost.php...&postcount=404



Despite my initial suspicions a hose had popped off on the dyno (it must have popped off on the way home), the same thing occured after double checking everything again, and continued to do so for a number of months. Other people have reported similar things on the ausrotary forum.

A few months ago I plugged a powerFC in (without touching ANYTHING else), and ta-da, 10-8-10 exactly.

Therefore I am led to believe that the stock Auto ECU runs 8psi boost at the high end. Until decent dyno and boost graphs of a dead stock car on the manual ECU show otherwise, I also have a hunch the manual ECU does the same thing.

 

Interesting.
After paying a bit more attention to the boost gauge it's dropping to more like 8psi, rather than 7psi as I first thought. I swapped in a manual ECU and there was no change, so I think I'll leave it at that, since it sounds like it's normal. I can always ditch the factory boost control solenoid and fit a manual boost tap later on down the track.

 

I just went through a similar diagnosis. My car boosts to 11-12 on the primary, and at the higher rpms in 3rd gear I'm seeing 8 psi. I was told that this is normal.

 

just about every factory turbo car has boost that tapers off at high rpms. From the factory the Evo X will peak at about 23psi at 4000rpm and fall off to 14psi by 7500.

 

I don't think a stock FD holds 10 psi until redline, unless you use an EBC.

My FD is stock, except CB-CAI and DP.
I get 11-8-10..dropping to 8 psi at 5500 rpm.

 

Interesting to hear more people reporting the 8psi drop.

Seems as though the whole 10-8-10 stock thing is a bit misleading.

I would really like to see a dead stock dyno and boost graph, preferably on a chassis dyno. Would be also interesting to see the base PowerFC map on a stock car dyno as well (I may be getting this done as a baseline run sometime in the future).