I'm not quite sure this is a boost creep, or a boost spike problem i'm having. I described this in my project log. I'm running gt2560, 5o 750cc, no intercooler (i have 2 to choose to mount sitting in my garage now chillax), wastegate pressure (hooked to compressor housing), megasquirt, FM 2.5 all the way back with a test pipe.
I get a solid 10psi up until about 5500rpm give or take then i start to climb in pressure. I've been seeing it top out at 15psi around 7000rpm. But the other night we seen it hit 20psi.
I've taken off the actuator arm and the flapper moves a full 90* nice and easy. The arm is super hard to pull, and i can only pull it maybe 1/4 - 1/2 an inch. But i assume that's the way it should be??? How, or can i even run a MBC on this external wastegate? I mean, if it's a 10psi WG as is. I have no idea.
Also, when we were tuning, we may have set base timing on the wrong mark, could that be it? It will be another week before i can check this.
Or maybe my 2.5" exhaust with a test pipe isn't restrictive enough?
I know you all want me to say F it and run ALL of the boost. But i kinda don't feel like building this spare motor just yet.
Build thread = slowest turbo miata ever
(and it's not living up to it's name overboosting like it is)
I wound up porting the turbo. I only had a couple drives with it together before I found out the 2 middle cylinders are F---ed. I will admit I didn't go rape it to truly test the boost stability, however what romping on it I did do it held the pressure I had set it at. Well actually I was messing with a new MBC so don't "know" what I set it at, but it would hit 8psi on wastegate can only and stay there. Couple clicks on the MBC and it would hold 9psi, then clicks and 10psi. So as far as I can tell porting the hot side fixed my issue.
Don't think it's your exhaust setup, the separated gas design is the one that can tend to boost creep. FM's the gasses combine pretty early which tends to help wastegate/boost control.
How much preload on your wastegate flapper? Double check to make sure everything is tight and no possible air leaks on the hose from compressor housing to wastegate.
I suspect the turbine housing needs a little porting, rest of your setup seems pretty good. But, I'd double check everything else first of course, before you start creating more work for yourself. Check the base timing ASAP if you're unsure, but I don't think it's related to this.
Awww hell. What a bunch of work this is going to be. I should bought a Borg Warner.
I'm going to check everything over, base timing, and this little exhaust leak i have and all everything else. But i think you're right it might need to be ported. If i have to take this turbo off and send it in and all that i might just man the F up and run 15psi like a boss... Till she blows then rebuild.
If you are not able to get a replacement for that actuator and do not wish to go externally gated I would recommend an upgraded actuator that you can actually change springs in like the Turbosmart IWG units which run about $200
There is also the option of the Kinugwara IWG units which run about $90 and also have good reviews