Stumble on boost
 

My setup:

Stock greddy manifold, turbo and DP.
Starion I/C with JimB pipes.
VTA Bosch BOV with check valve
JimB MBC (connected between outlet on turbo and WG)
Bipes
BEGI AFPR
MSD 2225 aux fuel pump
1.8l injectors
FM O2 Clamp (not installed)
323 GTX PCV (plumbed into intake)

I'm running 7psi. I've tuned the AFPR to have ~37psi at idle, ~50psi with vacuum line disconnected. I have added the restrictor to give me more fine control over rate of rise. At full boost I'm seeimg just almost exactly 100psi. So I think I'm pretty close on fuel, while being a little on the rich side.

The car runs great, pulls nice and hard. Boost holds very steady at 7psi. The BOV seems to be doing it's job, when I lift under moderate boost I get a nice whoosh, under heavy boost it lets out a higher pitched whoosh (chirp, whistle).

The issue. Mostly have noticed this in second gear, but is present in third just not as pronounced. Under boost if I go from WOT to partial throttle I get a stumble and the car jerks a bit until I either completely lift or go back to WOT.

I have two thoughts on this. Change the BOV from VTA to recirculating. Maybe I should do this anyway? Or maybe the O2 clamp will smooth things out a bit.

Any suggestions? I really want to get on the dyno and make sure my AFR is on target and up the boost to 9-10psi. Before I do that I want to get the car running as good as possible (Dyno time is 180-200 per hour).

Once again, excellent resource we have here!

Jay

i think its the BOV if you dont have a aftermarket computer or piggyback it will stumble. on my 90 it would turn the check engine light on so i just recirculated. just make it recirculating. less wooooosh but it wont give problems. also it would create a large rich condition. and for 7 psi boost its around 92 psi of fuel.


anyone correct me if im wrong

Nope you got it spot on. The ECU has already metered the air that is "wooshing" out. Therefor it adds the correct amount of fuel to burn with the air that isn't there... basically everytime this condition happens the ECU tries to flood the engine. Turning your fuel pressure down a little will help, but I think the best route is re-circulation until you have a stand alone, or possibly an EMU (since they can run off MAP).

It should also have to do with the o2 clamp. Your dumping in a lot of fuel and then the ecu wants to pull fuel to go lean. Probably going from open loop back to closed is causing the stumble, the o2 clamp will smooth that out.

If the bov doesn't leak at idle then you shouldn't really worry about it, you'll just see rich conditions when you shift.

Issue was running the BOV VTA. Did the required mods to recirculate and it is perfect. Can feather the throttle underboost and the car is as smooth as silk.

I would assume as I lifted the BOV was activating and was causing the car to go rich and then stumbled before it stabalized. And now it is nice and quiet unless I get on the throttle.

Jay

FM sells a VTA BOV that'll work with the stock ecu, but it is pricy. I still don't understand why some people prefer VTA. I'll take the quiet, stealth option anyday. :)

It might not leak at idle but you are still going to get a rich condition when shifting or lifting.

I can't speak for the newer FM valves, cause I've got the older one. It can be adjusted so that you VTA above 20 inches Hg. It gives a nice "Pop" between shifts and closes at idle. The only time it seems to remain open is when your foot is off the gas on a long downhill, when you aren't fueling much at all..

My Bosch valve must have been opening very early for the problems it was causing. While in second gear going from WOT to 1/2 throttle HIGH in the revs it would buck like crazy. It works really well now that I've plumbed it back into the intake. And I also suspect the check valve was leaking at idle since I was having the occasional problem with the idle hunting up and down, now it is just like it was pre-turbo.

I'm also having an issue with running rich. Which may have amplified the problem with the BOV VTA. Sorting that out now, but being cautious. Should get on the dyno this weekend. Just after that the emanage should be here and I get to start all over again! :)

Jay