Revs drop slowly when changing gear ... why?

[NathanUK]

I see what you are referring to now, you are talking about relieving that built up pressurised air.
What I refer to is where the turbo is spinning and therefore will continue to pull air in to itself, whether it is from the air metering unit or recirculated air that it has already pressurised before.

They allowed the decel valve to get rid of the pressurised air.

I'm thinking that the electric idle control valve will eventually vent the boosted air in to the manifold anyway.

So, with a bov the engine should slow down much quicker - more engine braking.

With the recirc valve the turbo should stay spooled longer which sounds good to me.

[911nut]

Originally Posted by NathanUK

I see what you are referring to now, you are talking about relieving that built up pressurised air.
What I refer to is where the turbo is spinning and therefore will continue to pull air in to itself, whether it is from the air metering unit or recirculated air that it has already pressurised before.

They allowed the decel valve to get rid of the pressurised air.

I'm thinking that the electric idle control valve will eventually vent the boosted air in to the manifold anyway.

So, with a bov the engine should slow down much quicker - more engine braking.

With the recirc valve the turbo should stay spooled longer which sounds good to me.

When the high pressure air is exhausted, there is one leak path to the engine; the idle bypass on the throttle body.
When the engine speed drops the vacuum signal goes away at some point and closes the decel valve. The factory had to set-up the Bosch recirc valve and the bypass valve have to open and close at roughly the same time or they would have had a calibration nightmare. So when the decel valve closes the air that's left in between the metering plate and the butterfly goes into the engine via the idle bypass (otherwise the engine would stall).
RPM on a gasoline engine is determined by throttle position. When the recirc/bypass valves dump air to the engine it's the same effect as closing the throttle slowly with your right foot. When the bov dumps that same high pressure air to atmosphere the engine speed drops faster because the throttle is closed and the volume of air entering the intake behind and around the butterfly is chopped off.
So there's no engine braking going on and either way the turbo is spinning in low pressure air. Turbo compressors don't have linear output otherwise none of us would be *****ing about throttle response below 3000 rpm

[NathanUK]

I still think the recirc valve is better as the turbo should keep spinning faster longer. Why waste the energy we've put into the turbo.
As for engine braking, that can be useful but I didn't really drive my 930 long enough to decide if it needed more than it already had. My car has the factory LSD which did seem to make the car push when lifting off compared to my 911 carrera with no LSD. This pushing was easily corrected when the boost came in
I know at low speeds my carrera would benefit from some more engine breaking, especially if you are in 2nd at low revs when you should be in 1st and nearer 4K. Maybe the 4spd 930 tranny makes more engine braking even more desirable?
These cars are already famous for lift off oversteer (loose for USA)!!!

[turbodog]

What if it's something simple. On mine the throttle linkage bushings were worn and the spring weak. This resulted in a slow return to idle, due to a mechanical failure.

[Falk930]

How about limiting the vacuum signal line to the decell valve?
Wouldnt this cause faster drop to idle, but better as total
remove the decell valve?

...something like this:


Falk

[NathanUK]

At a guess, that might actually delay (may also prolong) the action of the decel valve causing the opposite of what you desire?

I think it's best just to cap the vacuum line.

[911nut]

Thinking more about this.....I decided after installing the hybrid bov that I needed a way to vent air when the boost does not achieve maximum (13.1 psi on my car). There are times when it only reaches 10 psi and then there's off boost events too.
Also my desire was to keep the turbo freewheeling during these sub-maximum boost events. This meant that the decel valve could not be discarded, so I disconnect the decel valve output and capped off the hose leading to the intake. Now the decel valve operates normally (in that it opens when receiving it's vacuum signal) and vents to atmosphere as opposed to shoving air into the engine behind the throttle butterfly. This allows the revs drop quickly, which makes for much faster shift, smooth clutch operation and increased clutch life. A side benefit is that there's no need to plug the intercooler openings for the recirc valve and the decel valve.

[NathanUK]

I think you were right to do that as you are not recirculating. However, I still think recirculating would keep the turbo spooled between lifts/shifts longer. It would be interesting to compare and see if I am right.

[p911dad]

I had the same exact problem and it was finally traced to bad ground connections. I recommend checking the several ground points throughout the car(including the one behind the tach). There are at least 4 total points, behind fuel filter, behind tach(accessed through trunk), on the intake manifold and on the transmission. Don't forget the battery and its two points. These cars get "green disease" where corrosion builds up between the mating surfaces of the various connectors.

[911nut]

Originally Posted by gmeteer

I had the same exact problem and it was finally traced to bad ground connections.

gmeteer, the recirc valve (bov) and the decel valve are vacuum operated valves. Electrical faults will have absolutely no effect on them.

Nathan, you seem to be thinking that the turbo speed and the engine speed during the off-throttle period are connected. They aren't. The turbo is spinning at well over 100,000 rpm max. The engine is running at a max of 6200 rpm.
The bov is there to reduce pressure in the intake when the engine is off boost and that keeps the turbo spinning faster than if it's paddling thru high pressure air. The engine revs are dropping to idle during shifting have no effect on turbo speed.
If the turbo is spinning in ambient pressure air it won't care where the high pressure air is being vented to, be it the engine (which is what holds the revs up) or the atmosphere. The effect is the same on the turbo's ability to freewheel.
Over and out.

[NathanUK]

My theory is that the air is recirculated back into the turbo (cold side). Your dumping all of the air out of the turbo so then it has to pull new air through the barn door and the air filter, eitherwise it will have to slow down. Surely it would be much easier for the turbo to have its same air coming back. The actual pressure is not really important, although the IC will add some resistance to the air when recirculating.

Look at this way, what's easier.
A pump pumping water out of a shallow pond through a hose.
OR
A pump on a sealed system just circulating the water round and round?
Maybe the difference is not very much, I'm really not sure.

[911nut]

Originally Posted by NathanUK

My theory is that the air is recirculated back into the turbo (cold side). Your dumping all of the air out of the turbo so then it has to pull new air through the barn door and the air filter, eitherwise it will have to slow down. Surely it would be much easier for the turbo to have its same air coming back. The actual pressure is not really important, although the IC will add some resistance to the air when recirculating.

Take the hoses off the i/c where the bov and recirc vales hook up to. Do you think you will see anything that will recirculate air back to the turbo?
Do this and get back to me.

[NathanUK]

The recirc valve is fitted in the pipe which supplies the turbo with air after the barn door. The hose from the IC after the turbo goes to the recirc valve.

Is a 964t different? I don't know much about the 964t's

[911nut]

Nathan, here's a schematic of how the 930 and 965 induction system functions (source: Porsche 911 Story, 7th edition)

When the throttle closes (7), the engine creates a vacuum which opens the recirc valve (5) and the decel valve (17). High pressure air trapped between the "barn door" (2) and the compressor (4) flows through the recirc valve to the now open decel valve (17) and on to the intake manifold which is at negative pressure (8) because the air will take the path of least resistance.

As you can clearly see the recirc valve is not feeding the compressor but functioning as a pressure relief device.

Porsche actually had a scheme that would keep the turbo spinning faster during shifting on the 965 Le Mans Turbo race car. It involved keeping the butterflies slightly open and continuing combustion with retarded timing. This kept exhaust gas velocity up to the dual turbos and kept them spinning. This isn't practical for a road car due to poor economy.

[NathanUK]

Thanks for the diagram Paul, makes it much easier to understand what happens.

Originally Posted by 911nut

High pressure air trapped between the "barn door" (2) and the compressor (4) flows through the recirc valve to the now open decel valve (17)

Surely that air is under vacuum, not pressure?

Originally Posted by 911nut

As you can clearly see the recirc valve is not feeding the compressor but functioning as a pressure relief device.

I agree that the recirc valve(5) does act as a pressure relief valve as when it opens, the air that is under vacuum after the barn door(2) will equalise with the air under boost. Any excess boost will be used by the engine via the decel valve(17) or the idle control valve(not in the diagram).

Despite any of the above, surely the recirc valve(5) allows the turbo to keep spinning longer as the air being forced by the turbo cold side can re-enter the turbo? If the air was dumped to atmosphere the turbo would cause a vacuum at the barn door(2) causing the plate to open and air being drawn through the air filter assy. This surely should make the turbo slow down much quicker than if it was recirculated? Not too mention even more fuel being wasted...

[911nut]

I think you are missing an important fact: When the butterfly closes there is a spike of high pressure air in the turbo track. On my boost gauge I can clearly see it. The intake manifold has the vacuum at that same increment of time. The air doesn't re-enter the turbo. It is drawn into the engine due to the pressure differential, bypassing the butterfly. If you vented that air to atmosphere (lots of turbo cars are set up that way) there still exists that pressure differential so you still get very effective venting.
By venting the tract the turbo will decelerate more slowly than if the tract wasn't vented at all.
Now here's the issue with recirculating that air; it makes for really slow shifting. You let off the gas, push in the clutch and count "one-thousand-one, one-thousand-two" while the turbo spools down. Now that my engine vents to atmosphere I'm counting the "one" in one-thousand-one shifting and getting back on the gas. At this point the turbo has hardly slowed down at all. I know this because, prior to making the change, I'd get on the gas and at that instant the engine is beginning to accelerate my boost gauge showed 0 psi. Now the turbo is developing 3 psi at that same instant. Instant heat.
I'm never going back.......

[NathanUK]

So you found that with recirculating it took too long for the revs to come down so you could select a higher gear?

[911nut]

Originally Posted by NathanUK

So you found that with recirculating it took too long for the revs to come down so you could select a higher gear?

That's it. I like to shift faster than the recirc scheme would allow. Reduced spool down speed was a bonus.

[NathanUK]

Ah, OK, that makes sense. I don't need to worry about that as I have a G50. I found changing up at the strip fine with my stock engine.
Maybe after mods it may be different.