nuclearfishin

Went to the strip last night and did some runs (no data logging however) and can confirm the car seems to be running good. I was trapping at 127 mph, however my ET's were 11.7!!!! My 60 foot times tell the whole story, they're around 2.1 which is horrible compared to what I used to run (1.5-1.6). For some reason the car is just not building much boost during launch mode. It used to build much stronger, now it gets a couple psi and just stays there. The first 100 ft the car is barely rolling out, then the power kicks in and it goes like a bat out of hell. The car is holding 21+ psi of boost during the run, so I don't think it's a boost leak problem. Any other ideas that would relate to launch only but not affect the car otherwise?

MKIVdan

Is this an auto car? Maybe a torque converter issue?

nuclearfishin

Yes, tip car. Would a torque converter issue explain why the car can't build boost during launch? I can't get normal boost before I even lift off the brake. As far as I can tell all other symptoms seem normal (quick, smooth shifts, no noise, no shuddering, etc.).

TT Surgeon

Have you changed the tranny fluid to a hi temp version?
When I used to race s/g w a powerglide and a trans brake it as easy to cook the fluid, and it just wouldn't hold stall.
The tq converter will affect stall speed if the veins are bent or failing, extremely unlikely. I'd bet your ecu is pulling timing under load, usually a self preservation move to save the engine. I'd data log it and check those afr and ign angles.

Kevin

If you have less timing and less LOAD request== slower time. Boost doesn't equal more power IF the ECU is clamping down on the output.

Igooz

Nuclear, most of the newer cars with electronic throttle pedals (w/throttle position sensor) have a safety feature which works as follows: If your foot is on the brake (brake switch active) and your foot is on the throttle (tps > x), after a few seconds the ECU removes power until brake switch goes inactive. This is an FMEA feature in case of stuck throttle pedal and to avoid unintended acceleration, etc...just try it at home driving around town tonight and you will see...

What are you doing different now than before? (I sold my 997 TT Tip 3+ months ago so I can't test if for you).

TT, you should post a picture of your '55 with 2 spd PG! but you can't trans brake a 997TT...you can only foot brake it or line-lock it.

TT Surgeon

"Trans brake is a solenoid operated valve in the trans valve body. It essentially locks the tranny in foward and reverse at the same time until the actuating button is released. Really puts a shock on the internals but is instant engagement of foward gears. They are also used mostly with other electronic devices, for quicker reaction times and consistency in et's. Line lock is also a solenoid but is installed in the hydraulic brake line, to lock up the front brakes for burnouts and some even use them on the rear for launching(never seen them work for that to well). They are button or switch operated too, but only releases the brakes and has nothing to do with the trans"...TCI racing.
Here is a typical trans brake used in drag racing. I'm sure it could mechanically be put into a tip, but as igooz pointed out the ecu/tcu would probably freak out.

MKIVdan

Im not too familiar with the Tip auto trans but if you are brake boosting off the line and the car isn't moving out like it used to or building any boost then I would def look into the trans/converter first.

As TT said, if there is an issue with the converter it wont hold a stall.

Do you notice anything different with the stall speed while brake boosting at the line from when you were running 11.0's with 1.5 60s to when you running now (11.7 with 2.1 60's)?

Is it still holding the same rpm from before and now while brake boosting?

Next time you are at the track you should try to use the ebrake while brake boosting to see if that helps. I used this ebrake method when I had an auto MK4 Supra to help build more boost on a stock stall that would only go up to 2100rpms and 2300-2400rpms with the ebrake, the result was 1.9 60's down to 1.7 60's

nuclearfishin

All good ideas and suggestions, thanks for the continued support as I troubleshoot the issue. To answer your question, I don't recall the exact stall speed rpm when the car was running fast, I normally paid attention to the boost which was always higher. It's definitely possible the lower boost is due to a low stall rpm as opposed to a boost problem like I originally thought. I'll try and test with the ebrake and see how that works. Is there any durametric data that could help identify a bad torque converter?

nuclearfishin

I just tested the stall rpm and here is what I found. The rpm and boost are the same whether i use the brake pedal or ebrake. Rpm's max out at about 2100 and boost max is about 10 psi. What is the typical stall speed for a 997.1 tt? Does this sound more like it points to the torque converter?

MKIVdan

Not sure what the stall speed is for a stock Tip tranny but 2100 sounds pretty average for most auto cars.

so currently you are at 10psi in its current set up.

How much boost were you building before when the car was running faster times?

nuclearfishin

Good question, I didn't have my durametric back then so I'm not sure of the exact boost. I do know the gauge in the car used to go higher if that's a reliable measure. Let's assume 2100 rpm stall is typical for a stock tip, If the torque converter were going bad, is it possible to have fairly normal stall speed but still bad acceleration when the brake is released? It's the 1st hundred feet or so in my runs where the car is not accelerating, and then it goes great as evidenced by the 127 mph trap.

MKIVdan

The more and more I think about it, I think I am ruling out your torque converter. If you are holding 2100rpms brake boosting and its not going higher than that then it sounds like your torque converter is working fine.

A bad converter wouldn't let you trap 127mph.

earl3

What has changed since you were running 1.5-1.6 60's and now?

Your issue still screams moderate boost leak to me. The turbos will just overspin themselves to make boost once they're within boost threshold -its the beginning of the threshold moving way right in the RPM range you'll most easily spot when you have a leak. Have a shop that knows these cars pressure check from the turbos (or DIY).

I think your car has a 10 sec / 130+ mph pass in it with race gas.

BTW, will get you my CMS IC Logs ASAP -they're on a different computer I use for datalogging so gotta go grab them.

nuclearfishin

Boy I wish I knew what changed. The car was just pressure tested to 22 psi by a shop I trust. My durametric says I'm making 23 psi boost at peak, although like you said it could be the turbos overcompensating. I tend to agree with the boost leak comment because the turbos aren't building near the pressure they used to for brake boosting and launch (according to the dash gauge).

Question about pressure test--on a 997, isn't the test normally completed on the turbo exit hose that goes to the intercooler? Is it possible there's a leak prior to that, and if so, how would you test for that (I have one of the boost leak testers from 911tuning). I know on a 996 it's easy to test the inlet side, but on a 997 it looks like a pita.