Throttle response issue
 

Hey folks. Bought a 930 last fall and basically tore into it without doing much driving because it was leaking oil and had a tired turbo. Now I've got a nice running car that seems pretty fast to me with the following engine ancillary mods:

GHL Headers
RarlyL8/M&K center outlet muffler
934 center outlet WG pipe
Tial 46mm (0.8 bar spring- gauge holds at about 0.9bar @ max boost)
K27 7200 from IA
Kokeln long neck IC
Cheap plastic BOV provided by Kokeln
Leask adjustable WUR w/RPM solenoid

Because I haven't driven a rear-engined car and I plan on attending some DE events when I can fit them into my schedule but until then I'm just learning as I go. I've had some sphincter clenching moments here and there as I figure out things like, "...jeezus...that guard rail sure looks close...guess I shouldn't brake in the corner...stay on the gas.." etc. I imagine I'm a bit touchy with the throttle and tentative- which might be my worst enemy at this point.

One thing that I've noticed which doesn't seem right is when I'm on boost in the 0.6-0.8 bar/5-6000k rpm range and I come off the throttle it cuts out dramatically and feels like I'm running into a brick wall. I've tried to modulate the throttle with slight movements and am having a tough time working with this characteristic. It's a bit worrisome as it abruptly unweights the rear of the car and it feels like I'm not in control...this makes me very hesitant in the corners, around other cars in high speed sweepers, etc. I've already had a few involuntary lane changes due to this issue. Not sure if this is the nature of the car- perhaps newby driver error...or perhaps there's something not quite right in the engine bay. I've noticed a considerable improvement in throttle response since I've eliminated the factory BOV assembly- things happen when I want them to acceleration-wise...but deceleration-wise the cutoff is amazingly abrupt with the slightest of pedal movement. I'd appreciate your thoughts on this one (normal behavior/driver error vs. mechanical issue) as it seems to me that it's only a matter of time before I spin out at an inopportune moment. It's unfortunate as I seem to be "white knuckling" as opposed to attacking the road and having fun. I have an LM1 and am beginning to fine-tune afr's...but I don't think this is an afr issue; seems more related to fuel supply/boost....or sucky driver!

I may be wrong but I believe the decel valve is there to keep this from happening. My understanding is that the decel valve lengthens the time it takes for the engine to return to idle speed when you let off the throttle. Can anyone else verify this?

Yes, the decel valve will dampen that effect.
I have removed my decel valve and modulate the throttle manually. You can train the right foot to keep the revs up between shifts and any other time the clutch is in. The alternative is to add back the decel valve or maybe a different BOV as they have a similar function. The abrupt deceleration you are experiencing is also hard on the turbo and air seals due to the surging that is created when the closed throttle plate reflects the pressure.

This is an inherent characteristic of this engine, and I believe the real culprit in why so many 930s meet their demise tail-first.
The cause (IMHO) is that with this primitive mechanical/pneumatic boost control, when you lift off the gas the BOV relieves the intake pressure (good), but nothing relieves the exhaust pressure. The wastegate is slammed closed by the high intake vacuum so the immediate and abrupt result is heavy engine braking. This braking torque transfers the car's weight off the wheels it is loading, and the triple-whammy of causing abrupt positive camber on an unweighted, torque-loaded wheel. May as well yank up the handbrake!!!
I plan to address this characteristic with a very soft WG spring and electronic boost control along with the EFI in hopes of allowing the WG to relieve some exhaust backpressure when lifting. The more modern turbo cars I've tuned have way less of the driveline-shocking, inertia-reel-testing lift-throttle braking you observe.

....well...I guess the "glass is half full" side to all of this is I think I have good car and health insurance!

I certainly did not intend to come across critically. I happily sold my last "modern" car; fast as hell but heavy, numb and uninvolving. And it was either a 965 or a 2.7RS replica, and this car came up. For all the benefits of a sophisticated contemporary car, please I'll take some idiosyncracies, flaws and character over astringent refinement any day!!!
Hell I'm planning to pull my power steering, and I'd leave out the AC too and put in 1-piece seats if I never had to take my wife anywhere in it.

Fantastic - solving two problems in one hit!

My 86 930 drives like jay71's car, i.e. lift off throttle and my head hits the screen. I had been wondering where to start looking. Sounds like decel valve not working. Like Rarly, I adjust my driving style to compensate but it can still catch you out. :eek:

My 85 930 does just the opposite. When I lift off throttle, it takes several seconds for the revs to drop to idle. Fast gear changes are a real pain do do smoothly because the revs don't drop fast enough. :mad:

I am at work at the moment so I can't stick my head under the lid to see what this valve looks like so I used our host's site to find out more about it. I couldn't find a listing for decel valves on '85 and '86 930s but I assume that my cars have one and its same as listed on 79-83 models.

Can someone please confirm that the decel valve is the likely cause of both problems?

The decel valve makes sense as being the culprit as it was removed along with the factory BOV assembly. If I have to choose between the new setup without the decel vs the old setup with BOV, etc I'll take the new setup and figure out how to "plan" better so the sudden change in momentum and weight shift doesn't cause an accident. I appreciate all the feedback guys- if anyone can think of a bolt-on item- maybe something that would function like the original decel valve without the delay issue that- I'm all ears.

The decel valve is attached to th factory BOV assembly along the midline of the engine bay- it's about 3" in diameter and is a metal saucer-shaped valve with a vacuum hose coming out the middle (I believe). If it's clean I think it has a gold anodized color to it.

The other name for this part is "Vacuum Limiter". This may help you find it on here.

This valve can be refitted to any 930 no matter what mods you have. Just relocate it somewhere.

When I drove my car which was stock I found it over compensated too much for my liking compared to my 911 3.2 carrera. However, compared to my wifes FWD euro box it was pretty good as her car is annoyingly long and makes it very easy to stall the car in traffic.