Runs great in vacuum, but stutters under boost (Heavily modded)

[HybriDSM]

Runs great in vacuum, but stutters under boost (Heavily modded)

I've been spending the past couple months on weekends trying to get my 1991 GSX back on the road.

Related mods:

Wiseco 8.3 pistons, Manley rods, HKS 272/272, Crower Ti springs and retainers, Aeromotive AFPR, FIC 850cc injectors, 6AN fuel lines, Keydiver chip, MAFT 3.5" translator (blow-thru), 60-1 BB AGP turbo, rewired Walbro 255.


The problem is the car will run perfectly in vacuum, but as soon as it sees any kind of boost it stutters horribly. We did a ton of searches on the topic but none of the answers worked.

For some reason his pocket logger does not like connecting to the diagnostic port, so that is an obvious problem.

There are no boost leaks, the plugs and wires are brand new (Accel 8mm wires and BPR7ES). The coil was working perfectly before the engine work was done, so that really can't be the problem. The fuel system is working properly and the Aeromotive is set at 43psi.

Any help on the topic would be appreciated. We are trying to get this thing to the Shootout and are running out of time.

Thanks.

 

[HybriDSM]

A couple quick points of info:

The car had the aftermarket 850cc injector keydiver installed, and has a brand new knock sensor. The MAFT is "zeroed out" for stock injectors as the ecu is taking care of added fuel trims for the larger injectors. The MAFT can act as if nothing has changed in that regard (according to our friends over at Full Throttle Tech).

As for it being the MAFT, that very well could be the problem, in addition to the timing. I'm thinking we have to start from scratch on the timing since in putting it all back together, I suspect that the car's base timing is off (I.E. not 5* BTDC). I have a timing light but the bastard won't work for some reason (I suspect it isn't picking up positive current from the "terminal" in my engine bay replacing the battery (which is now in the back). Without the working timing light, I can't check base timing which is super important obviously, and without the pocketlogger working consistently, I can't check the timing on top of base either. I'll try and find a way to get the timing light to work in order to set base timing, which is the first step that I should do anyway.

In general, I am a complete noob as far as timing issues go, and even using the timing light is something I have never done before. I'm trying to learn on the fly here a little bit. My roomie, DSM91 has been helping me alot with the more in depth stuff so I am very confident in the put together of the car, but now I'm getting to the part that is unfamiliar territory.

When setting the base timing with the timing light? How do I view the little timing mark relative to the 10 near the crank pulley to ensure I am the proper 5*?

Once I have it set at 5* BTDC, how should I set my timing from there? To break the car in I'll be richening it up per one of your previous posts on breaking in a modded engine and retarding the timing a few degrees is something I will try as well. Just to make sure I have no knock. Any suggestions on exact timing number I should be looking for on the logger?

Thanks again for all the help. At this point I am realizing just how little I know about the electrical and in depth tuning issues about this car, so any insight is much appreciated.

Sincerely,
Matt
'HybriDSM'

 

[flinguist]

The base fuel pressure for a 1G is 38 psig. Fixing that probably won't solve your problem, but it may help in the long run (for tuning).

 

[Eclipse96Ryder]

its ur tuning....make adjustments with ur SAFC..or GMAFT, whichever one you use.....but i had the same problems in my car and i got it tuned a little better and it drove like a dream, no stutters at all...

good luck get that thing tuned