New to turbos and need some help.

Hi guys. I have recently purchased a 1991 GT4 Group A Celica, you guys call it the All Trac. I am new to turbo cars and have very limited knowledge about working on them. The car is pretty much stock and I thought I would install a 3" exhaust from the dump pipe as they tell me you can get an increase in power and the turbos breath easier with a bigger exhaust system. But since installing it, even though it feels there has been a power increase, the car seems to cough and splatter as it hits the higher rev range. What I mean by that is that the turbo doesn't seem to wind up smoothly, the power seems to jerk on and off. I have hit the search button in trying to find answers to my questions but I can't seem to locate anything specific.

I read of boost creep, reflashing of the ECU, waste gate not coping with higher boost, things I don't really understand as yet. What am I missing here after installing the exhaust system? Is there anything else I should have done or should be doing to make the car run smoother?

Thanks for any information you can give me.

through my research, for a pretty much stock GT4, I have read that a 2.5 system would do much better until you reach into the 300++ HP range.

the same reason the celica has a 8 port intake manifold, skinier longer path at low rpm to hold the backpressure in a way, then when you get into the high revs, it switches the intake manifold to a shorter fatter path.

sort of.. I think.... my opinion with my limited understanding of all this.

hmm.. I know this doesn't help you right now, just somethin that came to mind..

imagine putting to your mouth and blowing through a straw, or blowing through a fat pipe, blowing through a fat pipe at the begining.. it's like... BLAAAHHHH... it switches over to the fat pipe when revs/air is pumping lots of pressure.. low rpm with fat pipes, you lose somethin....

all this ^ ...I think.. not 100%... that's my first thought from what you said. (basically, I dunno., just some thoughts..)

(imo, I would grab a manual boost controller and up the boost with a 3" system, and there is a mod to have the intake sitchover to the other intake runners at lower rpm than original.)

also, I would check all the spark plugs, distributor cap/rotor/wires.. make sure they're up to the task... distributor timing also..

check all intake hoses going to the turbo for minute cracks/leaks, and coming out from the turbo, and from intercooler to the throttle..
make sure air filter is not totally disgusting clogged... check all vacum lines around the engine/around the throttle body/intake manifold..
dont put 80's octane gas. 94+

that's all I can think of ... i dunno...

Well thanks for your help there. The first part of what you said didn't make much sense to me but the latter I will try to do in regards to checking all spark plugs, hoses, etc.

As for fuel, I never use anything under 98 octane.

And what exactly do I want that mod to the intake switchover to do at lower RPM?

After reading what you wrote about your issues, then reading again what I wrote, It didn't make much sense to me either man..lol don't mind me.. :crazy:

hope some pros around here come and help you out man, and probably give me one upside the head for not knowing wtf im saying and spewing nonsense.. :notworthy:

I said 94, cuz that's the max we have around here in Toronto, Canada.. sucks i know... 98 is good shit.. I wish!

and with 3" pipes, I thought about upping the boost to use those 3" pipes, and thought to suggest about setting the car to switover the intake runners(or whatever that part is called exactly that does the switover) those cables/gears/spring mechanism on the side of the intake manifold to switch the intake over to the higher power range at a lower rpm, was just a thought.. but i dunno how that helps your emediate issue.. sry man.. what just talkin some shit thinking out loud about that 3" system you put on the car. I guess thinking about what I'm about to do to my RC ya know.. lol


but reading your post again on a fresh day, I'm a nobody when it comes to diagnosing turbo engines, but jerking feeling power on and off when punching it.. sounds to me like if the wires, spark, ignition, dist/cap/rotor is all good, i would look to the fuel system, the fuel filter maybe clogged, or fuel pressure problem at the injectors.. something along those lines... some vacum leakage etc. like i meantioned before.. i dunno man... could be some sensor, maybe engine light is flashing, or maybe flashing but you don't know cuz bulb is dead.. check codes in the engine bay in that diagnostic plug.. lots of info on here how to that if need be, but if you see the engine light flash on when you start the car.. it's not that.. just writing things in my head as they're coming to me what i would think about if it was my car... sry man, dont really know.. I think on the weekends it's freakin dead around here..lol.. I'll let someone who really knows wtf they're talkin about chime in now.. that's all I got.. good luck man.

A exhaust system is not going to cause the symptoms you listed. I would take a good look at the ignition and fuel system and make certain everything was reconnected correctly when the car was worked on. A high RPM miss is a pretty good indication of an ignition problem-- loose plug wires, old plugs, wrong plug gap, coil problem, etc.

A 3" exhaust will produce more top end power, all other things being equal, due to a reduction in back pressure and is often the case an associated increase in boost. It's virtually impossible to run anything other than a 3" size as hardly anyone makes one in 2.5", unless you have something custom fabricated.

The size of the exhaust is not especially critical, beyond removing back pressure post-turbo. What will affect things quite a lot is the size of the down pipe. A 2.5" down pipe will produce considerably more torque, and about the same power on a CT housing turbo than a 3". It also fits better in most cases.

The best overall setup for a CT26 is a 2.5" down pipe mated to a 3" exhaust.

Simba,

You think the lack of back pressure from the new exhaust here is just causing boost creep and he is hitting boost / fuel cut?

unless he has a downpipe he probably isnt getting any boost creep. i dont remember ever getting much if any until i put my berk tech dp in and that is with a full 3" all the way. now if he does have a downpipe i could see him hitting fuel cut very easily for my 3" turbo back i hit 1.1bar in all gears other then 1st.

Go over all the maintenance bits, I'd say do the fuel filter first, double check your air filter, spark plugs, wires, cap, rotor, and check for boost leaks.

98RON is the same as 94AKI available in the US...

Hes not using any fancier gas than anyone else...

Anyways, I know you say the car is mostly stock except for the DP, but some of the cheaper ebay style (SSAC / XSPOWER) downpipes have a problem where the wastegate flapper is impeded and the turbo will spool above the stock "safe" threshold (10-13 psi). There are ways to eliminate or increase the fuel cut threshold. Search for fuel cut defender.

The first thing you need to do right away is get a boost gauge so you can monitor your boost pressure. If you notice boost creep (pressure creeping above 7-8psi) and then you hit fuel cut at between 9 and 13 psi then chances are that your downpipe is preventing your wastegate from opening correctly. You can remedy this problem by grinding the DP and matching the outlet to the gasket on the inside of the DP. The other better option would be to replace the DP with something quality from either ko-racing (they make a killer 2.5 in DP) berk or aussie.

Just to help clarify, the wastegate is a bypass for exhaust gasses to "go around" the turbine wheel of the turbo. The farther that this gate is open, the less your turbo will spool. Once the desired amount of boost is achieved the wastegate opens proportionally to maintain the desired boost pressure. When you pull off your DP you will notice on the turbo side that there is a large hole that has a fan in it (exhaust turbine) and this wierd oval shaped piece with two circular protrusions on it. This is the wastegate flapper. As boost pressure increases, that little guy opens up. If you want to see inside it there is a little circlip that you can remove on the actuator that will let you freely swing it open to see whats inside.

Please PM me if you have any other questions regarding turbos or the 3sgte and their operation.

This sounds like boost creep and hitting fuel / boost cut.

I concur with Simba and others in that a downpipe back exhaust system swap, with all other factors being equal, wouldn't be enough to cause any real problems.

It doesn't sound like boost/fuel cut to me. That type of problem is pretty obvious... boosting full maxing everything out and all of a sudden BAM! Check engine light. Zero power. Usually stuck in limp mode. But at the very least, you would slam from power to nothing and it would be pretty clear what's going on I think. Zoolander, describe your situation and symptoms with more than one sentence and better suggestions might be made.

Sounds like an independent problem that is being attributed to last worked on thing of the car, aka typical rationalizing.


If I had to guess, it sounds like a leak to me that is intermittent based on pressure.

-Towels

Just realized this thread is pretty old.

Did you ever get this resolved BlueSteel?

-Towels

sounds similar to an issue i had a while back. i replaced the cap/rotor, wires and plugs and it cleared it all up.