Stock turbo (CT-26) spool characteristics question

[20pwns]

Hi everyone,
I’ve recently installed a GReddy boost and EGT gauge to monitor my '87 turbo. My mods are: turbo-back, intake and MBC. Otherwise, the vehicle is stock. I've been trying to set the maximum boost to about 0.7bar (10psi) but have been noticing slightly strange boost behaviors. My turbo seems to hit 0.6bar (8.7psi) at around 2700 ~ 2800 RPM but as it approaches 4000 RPM, the boost takes another "leap" reaching 0.85 ~ 0.9bar (12.5psi), which is a lot more than what I want. I was just curious if this is a "normal" behavior for the CT-26.

My past experiences with other vehicles was if the turbo was designed to fully spool at let's say 3000RPM, it meant that it’s also capable of reaching maximum boost at the same time. After that, depending on the tune and/or the turbo size, the boost stays at the desired setting until redline or "tapers" to lower pressures. But in my car's case the boost seems to exhibit a “two-step” behavior where it initially hits at 2700 RPM and gradually works its way to maximum boost at 4000 RPM. And I KNOW the stock CT-26 isn’t a 4 grand-spooling turbo.

If this is, in fact, a normal behavior for the CT-26 is there a mod or an upgrade that I can do to make it hit maximum boost at 2700 RPM and hold it there until redline? But if the above ISN'T a normal behavior, what could likely be causing this problem? Please shine some light into my mystery. Thanks,

 

[Allan_MA70]

you have a decent turbo elbow or what?

 

[oldschool85]

the culprit is that greddy Boost controller.

charlie

 

[PorterzSupra]

The CT 26 is small and spools very quicky under even light throttle, from the time u step on it, it should reach full boost within a second or 2 even in 5th. The turbo is also gear dependant, first gear you may have less boost. I had a BC on mine but noticed a HUGE problem.. it would produce great power from 2 grand to 4 and then after 4 the turbo would go dead and the car would get slower!!! For example in 5th gear at 3 grand my car pulled much harder then 4th gear at 4,500 (even though the boost pressue was the same), the turbo controlled the car, the engine HP was weak (about 150 hp) Now have a T66 turbo and a NA block, it never runs out of breath and before the turbo even spools up I have over 200 HP on the motor alone to get the rpms up real quick, not to mention the turbo body has lower rearend gearing before the turbo even does anything im faster then a NA car. Its a win win situation. I got a little off topic here, basically im trying to tell u the CT26 runs out of breath at 4 grand so dont be suprised when you get alot of power then it dies as the RPMS climb high. I also drove a normal NA car the powerband is MUCH stronger at higher PRM like 4-6 grand then on a turbo. Running the JDM NA block after the knock of death from my turbo motor lol. The reason I dident fix it was because my turbo motor had blown oil seals everywhere, font and rear main, oil pan everything so I bought a NA for like $500. You know how they say to keep what you have its better then a JDM? Well they may be right because my turbo motor had around 130K and the JDM I have has looser bearings and came with a BHG but the way I see it its under warentee so boost it till she blows then the next motor ill have to back it off a tad but yea, 40-60K my ass.. if it ran on factory unchanged oil for 60K maybe

 

[IHI-RHC7]

Uhm, there was something wrong with your car Porter.
The stock CT will hit full boost by 2800 or less on a properly tuned car.
What you are describing, 20pwns is creep. Now the creep is either the greddy boost controller's fault, or it's a restrictive turbo elbow. I've only ever used an AVC-R, so I'm not sure about the greddy's habits.

 

[20pwns]

Allan_MA70

Yes. I have a turbo-back. The downpipe starts directly after the turbo: one of those bell-mouth ones.

oldschool85

I see... So what you're saying is if I remove my GReddy MBC and run a shimmed wastegate, I shouldn't experience the current problem, correct? I will try that. Thanks!

PorterzSupra

Thanks for your input. However what you've talked about doesn't quite relate to my problem. I actually don't have power loss issues after 4 grand. In fact, the turbo pulls "too" strong. What I'm trying to find out is the source that's causing the "boost creep". Thanks.

IHI-RHC7

Thank you. I don't have a AVC-R to try so I'm going to remove my MBC and see if I run into the same problem.


Thanks everyone! Great support. Please submit more replies if you have other ideas!

 

[IHI-RHC7]

Is your wastegate shimmed?

 

[20pwns]

Is your wastegate shimmed?

Yes. Why do you ask?

 

[PorterzSupra]

okay IHI, I think I was giving the stock CT 26 too much credit lol maybe it was 2,800 I know it dont do anything for you by 4 grand