Low end power question.

[80083r]

So I'm new to being a "car guy", pretty good on basic theory or the science behind things, but not so knowledgeable about specific automotive things. Terminology tends to catch me out - I don't know what things are called, or where they are located. I'm sure there are other gaps in my knowledge that I don't know about yet, but on to the question.

My 89 turbo is stock as far as I know, with the exception of the air filter, which is a K&M, I think. Runs well, pulls hard, I love the car - it's the most powerful vehicle I've ever owned, love it.

But...

There is a lack of low end power, and it seems like it takes about 1/2 a second to start with any kind of power. This may point to a problem, but I'm not sure, it's also my first turbo.

More details: I'm comparing starting from a light (not racing, just normal driving) between my supra and the little 4 bangers I'm used to. Normally, I push the gas, and the car goes. With the supra, it's like I push the gas and... wait, now it goes. Is this normal? just an older car? I'm not talking about waiting for the turbo to spool up, that pause is pretty good, and the power starts pretty smooth from about 3000rpm and up. I'm talking about from 1000rpm, easy start normal driving - the pause is microscopic, but I "feel" it. It seems like I have to slip the clutch quite a bit to keep from stalling, or wind up to 1500rpm and "chirp" the tires.

To forestall some questions - it's not because I'm bad at driving a standard. I'm not the greatest, but I've had 20 years of experience or so and never had any problems.

It's also possible I'm just paranoid and the issue is totally in my head.

Theoretical question - what are the problems / implications with changing to flat topped or NA pistons and using a smaller turbo? I'm not interested too much in huge power at the top end, just looking for a little more low end torque and a faster spooling turbo. I may have asked this question before, I can't remember, I'm spinning in circles with the list of things I want to learn about and do with this car.

 

[Dan_Gyoba]

Well... though there's certainly a difference between higher RPM power, and what you get off the line, it's hard to quantify what that difference is. I know that while I was NA, the low end had more grunt, but not very much. It's certainly not hard to get the turbo's tires spinning off the line, which means that there's as much low end as I need for the tires I've got. It should be nice and smooth off the line, and you should get decent power delivery right off the line. No need to chirp the tires anyway.

Some FI problems will manifest in ways that are consistent with what you describe. Intake leaks will do this, since the metered air isn't right. It's worse at low load/throttle, since the unmetered air is a greater portion of the total. I had a similar experience when I forgot to tighten a clamp on the intake pre-turbo, but that was a bigger issue where the RPM would initially drop when I opened the throttle before coming back up. Maybe a smaller version of that? Probably worth spending some time diagnosing the intake plumbing anyway.

For your theoretical question: I don't think that the stock turbo is a problem with NA pistons. The forged pistons that I installed are certainly not flat topped, but are rated for 9:1 static compression, so they're not far off the 9.2:1 compression of the NA pistons. Personally though if I were going to tear down the block and replace the pistons, I'd just get forged ones and be done with it... Oh wait. That's what I did do... Anyway, for your purposes, the CT26 is a "smaller" turbo, still is even upgraded with a 50 trim wheel.

 

[80083r]

Would FI or intake leaks also cause stumbling idle when starting hot? I haven't had that issue for a while, but I did occasionally when I first got the car 2 years ago.