Vince5

Hi,

I have heard different theories about the pros and cons of removing the air pump (the system that pumps fresh air into the exshaust).

I am in the process of lightening the car (1991 model) so taking it away is very tempting (less weight on the rear wheels)

For those who have already done it what was the effect appart for the polution ? More lag ? More power ?

Thanks for sharing your experience

DrJupeman

I have a similar question, so I'm hopeful someone will share their opinion!

cobalt

My car came with the air pump disconnected since I have the B&B headers. I don't know how the car was supposed to perform prior to it being disconnected but it appears to be fine. I believe the air pump is only used upon initial warm up to basically cheat emissions requirements imposed at the time. My car supposedly passed both CA and NJ emissions testing with the pump disconnected.

Vince5

Cobalt - Since your car is modofied it's going to be hard to compare but out of curiosity at what rpms do you get full boost let's say in 3rd when you floor it ?

cobalt

I wish I could give you an honest answer. I was running the car with a bad BPV and the boost was dropping very quickly between shifts. I replaced the BPV with a billet one and had about fifty miles to try her out before we were hit with a series of snow storms that left the roads in bad disrepair. That was back in December. Its not worth venturing out until the roads are fixed and the salt is gone. Even my Pickup hates it right now.

I don't recall what the numbers were since I only had fifty miles to try out the new valve but I recall it being in the 22- 2400 rpm range for full boost. I wish I could take her for a spin to let you know for sure but it isn't worth damaging a wheel. I will let you know the first chance I get. I sure hope its soon.

Vince5

Thanks Anthony !
Full boost at 2200/2400 rpm you are a lucky guy ! I get full boost much later (3500/4000 may be) ! But you do have a lot of mods

cobalt

Vince5

I wish I had a chance to take her out this Saturday, the weather was nice. Now we are back to 15 degrees and potential snow. I will have to wait again. I would not trust those numbers just yet. Like I said I only had fifty miles to run her with a working BPV and I remember full boost coming on much earlier than before. But my gage was giving me funny readings and these darn things don't read above .7bar. I would also say I was loosing some top end with the power coming on much earlier.

While I am at it does anyone know of someone who can rework these gages to read higher (accurately). I know the tachs on the 993 turbos read higher? Are the 993 tachs compatable with the 964's?

69COU

Vince

I am with Anthony on this as i recently changed my CBV and would say 0.7 is on gauge at about 2500 rpm

USED THE FORGE MOTORSPORT VERSION. best £100 upgrade i have made, since changing the wastegate spring!

Cheers
Pierre

Staffan

Vince
There are some posts on the subject on the 930 board, saying that cars without air pump seems to be doing fine at smog inspection.

Based on that information I will disable the air pump (but use the cat), if I pass the smog test with some margin I wil remove the whole thing.

L8Apex

Guys,

I have been told by Porsche techs that the air pump on the c2 turbos is more than just an emissions part. The cylinder head has a threaded air injection port that is supplied by the air pump which lowers the cylinder head temp. They have seen c2 turbos come in with heavy oil consumption with broken air pump belts. It seems that if the cylinder head gets overheated the valves cannot disipate the heat properly endng up with valve guide failure or premature worn valve guides (in low milage cars) which eventually allows oil to pour into the cylinder and at worst the valve will break.

This leads to another question of the negative effects of worn valve guides. Will the increase in cylinder temp mixed with oil also damage or wear out the rings on the pistons? Maybe Adrian or Stephen will have some answers.

Vince5

The forge unit is in and a 0,85b spring as well but I haven't had the chance to test all this, I am looking forward to getting my car back

Vince5

I am not overly anxious about polution, last time I passed with a very wide margin and there is always the solution of runing a slightly poor mixture when you go there (now who did I learn that from )

It's mostly lag I am concerned about

Vince5

Interesting
I will ask my mechanic next time we talk

Staffan

I think these techs are wrong, pumping air into a fire doesn't cool it down.

Anyway, my understanding of the air pump (think I read about it in Adrians book) is that it blows air into the cylinders (exhaust side) when the engine is cold to improve "combustion" (not sure I am using the right word here).

When the engine reach a certain temperature a change over valve directs the air to the cat instead once again to improve "combustion" (same thing here, not sure about the wording, more effeciently burn fuel).

Anyway, my understanding is that it only pumps air into the cylinders for a short time before directing the air to the cat.

Vince5

Thanks Staffan. Then there should not be any difference in lag. Basically appart from polution there should not be any drawbacks