930 Boost/heat question

jt1 · 08-29-2006, 04:15 PM

I took my recently acquired 930 to a track day this weekend. Overall I was very pleased with the cars performance, but have a question about the boost. Normally during street driving, or the first session at the track, when you got to full throttle, the boost will jump to 0.3 bar, then around 3500rpm it will go to 0.8 bar. As the session progressed and the engine heated up, the rpm when it went to 0.8 bar progressively went up. By the end of a session, it would get to 5K rpm before it went to 0.8. The boost would jump suddenly, not build gradually, and it was worse in the afternoon sessions when it was over 90deg ambient. The car did not seem to run hot, it never got over the second mark on the oil temp guage.

Car is a 83 Euro 930, stock except for Group B cams. Recent rebuild with excellent leakdown. Stock Euro exhaust, no muffler.

Is this the IC heating up, a wastegate problem, turbo going south, or something else? This is my first Porsche and my first turbo car, so I'm on new ground.

Thanks for the help.

John

WydRyd · 08-29-2006, 06:04 PM

I'd think it's the heat soak from the IC heating up. When my car has sat for a while and the IC has had time to heat soak, there's a definately increase in "lag" until the IC has had time to cool down after a few minutes of driving.

If you're "on it" for an extended period of time, then your IC will begin to heat soak and affect performance.

Are you running stock IC, or an aftermarket full-bay unit?

RarlyL8 · RarlyL8's Garage

Most likely it is your intercooler becomming heat soaked.
What type intercooler do you have? The thick street coolers are good for short bursts but suffer heat soak under long pulls. The thinner full bay track units are less prone to holding heat.
Another issue is the air going through the intercooler. If air is getting around it you will heat soak more quickly.

WydRyd · 08-29-2006, 06:11 PM

Agree. You should have some sort of skirting around the intercooler to trap and direct the air THROUGH the core, instead of around it.

jt1 · 08-30-2006, 03:30 AM

The car has the stock IC, with the stock skirting around it. It also has a aftermarket GT style wing, and while the skirting and the wing match up well, there's a lot of vented area in the wing that is outside the skirting, so air could bypass the IC.

Can somebody explain the physics of this? I understand that the IC heating up reduces HP, because the intake air is hotter and less dense. I'm not grasping how the hot IC reduces the rpm at which the turbo builds full boost.

Thanks-

John

Noah930 · Noah930's Garage

Hotter air = less dense = harder for those available air molecules to spin up the turbo?