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AAV removal, why? Bigger CBV.

I hear some are taking the AAV's off there turbos. Why is that?

I thought the AAV was there to bypass air around the Throttle Plate between shifts.

It works along with the Compressor Bypass Valve to help keep the compressor wheel spinning so boost stays up and it also makes things easyer on the turbo.

I even added a larger flow adjustable CBV that I set up in a reverse orientation and was able to get no boost drop between shifts on the factory boost gauge.

Compressor pressure would hold it open from one side and vac wold pull it open from the other side at cruse and I would set is so when boost built in the manifold it would use the mechanical advantage of a biger piston in the valve to push it closed when accelerating. I use the adjustable spring tension to fine tune it.

It also made it so I had faster boost onset with accel from cruse. I would get instant .2-.3 bar boost from cruse at freeway speeds if I recall.

My reference is the 91 -94 C2 Turbos and it has been a couple of years.
Old 06-08-2008, 10:26 PM
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Crotchety Old Bastard
 
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There is the throttle bipass valve, AAValve and AARegulator.
Some folks (myself included) have removed the AAR. It bipasses air only during warm up and a miniscule amount when closed. That miniscule amount is compensated by adjusting the throttle air bleed valve.
I use my right foot to keep the turbo in boost between shifts.
Can you elaborate more on the custom Recirculation Valve (BOV), sounds interesting.
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Old 06-09-2008, 04:00 AM
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So the AAV is not that about 3" round can w about two 3/4" tubes and a small vac line?

If not, I know what you are talking about. Why remove that? They do fail and I had to replace it. It dose not effect performance.

It has been a while so my terminology is rusty.

If I remember about the CBV, it was an adjustable aftermarket unit that had a piston that slides to close it off. The top was about 2" round and had a larger position that acted on the valve. With the stock orientation the valve piston closed perpendicular to the air flow.

I flipped mine so valve piston had to close against the air flow.

I think the valve is meant to operate only on down shifts to bypass air the keep the compressor from stalling and to take load off the shaft.

The way I set it up it was open all the time except under boost. This lets air bypass at idle and at cruse.

Remember, the pressurized air is pushing the valve open now and the manifold vac is pulling it open.

Not until the throttle is opened for accel and boost starts to build in the intake manifold is there enough signal to open the valve.

This only works when the piston is much bigger than the piston on the valve side as it needs the leverage to close the valve.

I measured the vac level at idle and at curse. Then I used a hand vac pump to set the spring tension of the valve on a bench. So it would stay open at idle and at cruse vac.

I wanted to try a bigger CBV but did not want to cut up my S2 inter-cooler on my C2T.

I did notice a diff in how boost came on and keeping boost up between shifts.

Thx.
Old 06-09-2008, 07:03 AM
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Sorry should not have said :
" Not until the throttle is opened for accel and boost starts to build in the intake manifold is there enough signal to open the valve."

It should have said - enough signal to close the valve.
Old 06-09-2008, 07:07 AM
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I disabled mine because the engine slows down between shifts faster.
Other than that I see no difference in boost lag or popping on decel, but some people do get more popping in the exhaust with it removed or disabled.

It's an annoying vacuum limiter being called a deceleration valve when installed in an '87 930.
It's purpose is to lower hydrocarbon emissions on deceleration, and it has no effect on boost lag between shifts on my car.
Old 06-09-2008, 07:19 PM
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Yes, I was going to remove mine for the same reason. I had a 914 and removing it helped a bunch.

However, reducing rpms between shifts is not the best performance goal on a turbo. But, removing it dose make daily driving a bit more palatable. Especially if you have one that is not working very well.

Yes it may help emissions some but on a turbo it also reduces the shock the turbos can get when the throttle plate is slammed shut. Letting some air pass around the throttle plate helps to keep boost up between shifts.

The Compressor Bypass Valve also helps reduce lag and stress on the turbo.

One passes air around the throttle plate, the other around the compressor wheel. Together there is more benefit than if there is just one. Look at the size of the tubes plumbed into both.

One well respected tuner actually used to relocate the deaccel valve to make it work more better. I wish I knew what he was doing and how.

I do not know how well they do or do not work with the old style 930 set up. They have a different over-boost set up. I am used to the C2T set up which is the same as 930's with aftermarket IC's.

Regards.
Old 06-09-2008, 08:38 PM
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Just so'z we're all clear, like Brian said you can remove your Auxiliary Air Valve, it only helps with cold warmup, but a Compressor Bypass Valve (aka "teh blow off") should NOT be removed or disabled. It's job is to dump intake pressure when the throttle snaps shut (like during an upshift). Without this pressure relief the intake pressure will spike and your turbo will experience a reversion that will not only stall it (causing increased lag after shift) but could kill it.
If anything, when I yank out that other plumbing crap I'm going to plumb in a second redundant CBV off my intercooler. Get that boost dumped fast so the compressor can "freewheel" against as little air as possible.
As for the vacuum limiter, I understand its intent, if not its execution. If it's a question of engine-braking force limitation, there's much more of that from the manifold-pressure controlled wastegate than from off-throttle vacuum.
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Old 06-09-2008, 09:49 PM
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Quote:
Originally Posted by 911st View Post
Yes, I was going to remove mine for the same reason. I had a 914 and removing it helped a bunch.

However, reducing rpms between shifts is not the best performance goal on a turbo. But, removing it dose make daily driving a bit more palatable. Especially if you have one that is not working very well.

Yes it may help emissions some but on a turbo it also reduces the shock the turbos can get when the throttle plate is slammed shut. Letting some air pass around the throttle plate helps to keep boost up between shifts.

The Compressor Bypass Valve also helps reduce lag and stress on the turbo.

One passes air around the throttle plate, the other around the compressor wheel. Together there is more benefit than if there is just one. Look at the size of the tubes plumbed into both.

One well respected tuner actually used to relocate the deaccel valve to make it work more better. I wish I knew what he was doing and how.

I do not know how well they do or do not work with the old style 930 set up. They have a different over-boost set up. I am used to the C2T set up which is the same as 930's with aftermarket IC's.

Regards.
I understand how all of it works and when people talk about it here they are coming from different year cars which have some different parts, etc, and some folks don't actually have a 930, instead they have a 930 motor in a different 911 chassis so some of these parts are a little different again.

My car is not stock and that changes the way mine reacts again...
Previous owner was tracking the car and he put in powerhaus group B cams which make lots of power from 4500-7200 rpm and none below 4000 rpm. feels like 50 horsepower until 4000rpm and then it's 400+hp all at once.
These cams suck for street driving but thats whats in there right now. I have a set of SC330 cams and 12 rebuilt rockers but havn't installed them yet
It also has B&B headers and muffler, and I put in a garretson long neck intercooler with C2type recirculation blow off valve in the big rubber elbow. It is a Turbo XS valve to replace the bosch unit and it has a brass piston and adjustable spring tension.
The car also has a 4 month old K27HFS turbo with adjustable boost valve set at 1.1bar, and an IA modified fuel head, and Brain Leasks adjustable control pressure regulator or WUR and an rpm switch operated solenoid in the boost signal hose going to the WUR.
I have the deceleration valve in there right now hooked up between the intercooler and the intake manifold, but at this time I have the small vacuum line that operates it disconnected and plugged so it never opens and is the same as plugging the 2 bigger hoses that bypass the throttle body.

The turbo XS cast aluminum C2 type blow off valve with brass piston and adjustable spring tension is there, hooked up the normal way and working just fine.
I can put the vacuum line back on to the decleration valve in about 10 seconds, it's easy when you have the long neck intercooler (because there is no intermediate manifold under there taking up all the room) and have it fully operational again but it doesn't have any noticable effect on lag that I can tell.
Now that you have said there is a difference i'll put the vacuum line back on it and try it and see...

What I do notice and don't like is when driving around town at normal legal speeds the engine decelerates between shifts slower than
I like and I hate that... I want it to decelerate as fast as possible with that heavy stock flywheel in there so I can shift to the next gear quicker and have the revs perfectly matched.

I'm coming from light flywheel multi weber carbed cars in the past and I don't want slow engine deceleration, and there is no turbo effect on my car when driving around town so turbo lag is not an issue. I never get into any boost at all because there is none until 4500rpm with these cams.
I have to be on open road or entrance ramp to get into boost and then it's pretty intense.

I'm still experimenting with system pressures with the shims in the IA head and then trying different threshold, steady cruise and boost enrichment settings on Brian's adjustable WUR, and rpm switch points on the summit racing digital rpm switch. It takes time though and lots of test runs...

I enjoy reading your posts on 930 CIS.
Hopefully I'll get mine working the way I want it someday.
I have the LC-1 and G3 gauge installed so I can monitor AFR's.

Thanks for your cool information.
Old 06-09-2008, 11:19 PM
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