Turbo "backpressure" ?

[TTM]

Hello & Happy New Year to everyone

A question I never dared asking before, one especially for lightly modified 944 turbo owners (preferably the cars, not the owners) : how "bad" to the engine can be the "hiccup" that happens when lifting off the throttle under full load (say 1.2 bar in fifth gear) ? Feels like a brake parachute. Is this related to "turbo backpressure" ?
Did some runs on the Autobahn recently and noticed how I need to lightly modulate the throttle in order to avoid that.

PS : gratuitous pic of terrible winter weather on the continent for the last two weeks.

 

[pauly]

Don't you have a dump/blow off valve fitted ?.

 

[Diver944]

Not sure I've ever come across this occurance with a 944T engine [&:] The lower compression turbo engine of 8:1 reduces the engine braking compared to a normally aspirated car and lifting off always feels very gentle in comparsion (no matter how hard I'm pushing)

 

[333pg333]

Thom, perhaps is a fueling or ignition issue rather than just a backoff issue? I have a similar thing going on and to me it seems like a tuning symptom rather than just a backoff one. Remind me how are you tuning your engine at the moment?

 

[TTM]

Paul, I agree, I have to say the few standard turbos I got to drive always felt like they was very little engine braking, well at least compared to my S2.
Have you noticed if the intensity of this engine braking decreases with peak boost pressure ? If I run it at 1.0 bar or less it certainly feel less intense.

Patrick, I'm not tuning my engine at all. I only use a knock detector for now, but will add a Zeitronix when the new engine is getting in.

 

[333pg333]

Ok so how long has this been going on and what has changed?

 

[sawood12]

Are you talking between gearchanges the moment you depress the clutch and lift off the accelarator? I've often thought that when lifting off abruptly the 'jolt' is quite severe, though i've never suspected that anything is wrong - it sort of makes sense in that you're under acceleration with quite alot of torque being transmitted through the transmission and the wheels then as soon as you dump the clutch or take your foot off the accelerator all that torque dissapears immediately and the kick in the back you were getting immediately drops off. If you modulate the accellarator when lifting off then obviously you are getting a gradual reduction in the torque delivery from 100% to 0% over a couple of seconds period of time rather than instantaneously.

I can only assume that the cars and transmission is designed for this. There is a bit of give in the clutch which will absorb some of this (assuming you have a stock clutch) but it does feel quite severe. I've just got used to honing my clutch/accellarator coordination to be a bit more mechanically sympathetic to the transmission, but occasionally if I get the timing wrong I do notice it.

 

[TTM]

Even if the engine and the transmission can stand it it doesn't feel quite right.
Anyway, pleased to see it's fairly normal. Will have to try braking with the left foot while modulating the throttle down.

 

[sawood12]

I've also noticed that it can be just as severe if not moreso with turbo diesels. I think the key is to be smooth when lifting off the accelerator and depressing the clutch.

 

[333pg333]

I thought you were talking on backoff while still in gear? Partial throttle. No? Are you saying when you go to change gears?

 

[TTM]

Not an issue when changing gears (in that case the driver would need to relearn how to change gears smoothly[]), only when having to lift off the throttle at high speeds in top gear because the Schmidt in front in his diesel Merc has just jumped on his brakes at 150 mph.