jeff spahn

I drove my car to Vail last weekend and left it there in the house garage (closing weekend at Vail, had to ski) to be picked up by me next month and continue on to Vegas to put my supercharger on Polecat's car. Why, because I am getting a twin screw kit from Hans. So there's the setup.

I parked after driving about 14 hours from home to Vail, Had gobs of power through mountains and AFR was right on. I think the house is around 8300' or so. Vail village is 8022' and house is up the hill a little bit. When I came out to drive around the next afternoon when done skiing the car wouldn't start. Was flooded. Had to put my foot to the floor to get it to start. Same thing next afternoon when I was done skiing again. Foot to floor after it didn't fire right off and it started and also idled about 100-150 rpm too high (the whole time I was driving it there)

What gives? Altitude making initial start too rich (it was tuned with Sharktuner at 760' ASL)? Does the brain not compensate for barometric pressure (I really don't know the answer to this for cars.)? I have a mixture lever in my plane so I do that manually.

All of my other cars start fine at altitude or sea level but they are all 2016 or newer. I did have a problem with the KTM 350 having starting issues at altitude and I had to play with the enrichment button.

So, thoughts? I am returning the car to stock (factory chips and injectors going back in as well) in Vegas and driving it home so I believe it should run fine in the mountains on the way back but I need to understand the mechanics behind this fault.

Thanks

jcorenman

Jeff, I'm thinking that your cranking pulse-width is too long, too much cranking fuel in general but catches up with you at high altitude.

The LH doesn't do any altitude compensation (no pressure sensor), but the MAF is self-compensating for pressure since it directly measures air mass. That doesn't work during cranking however, the LH just throws in some fuel to get things started and then the MAF takes over control.

So hook up the Sharktuner and reduce the cranking fuel pulse-width by a few tenths and see if that helps. Bigger injectors means less pulse-width for the same fuel (the S/C doesn't add any requirement for more starting fuel).

jeff spahn

Thanks Jim. I am taking it out with me when I go back. I'll hook it up and change the fuel pulse-width as you stated. I know its a temporary thing but I want it right.

victor25

Jeff
Jim is correct of course, but don't change it much though, just enough so the car starts ok. Otherwise it will be lean on startups in Vegas where Polecat lives.

victor25

Actually the more I think about it, the more I think you should just leave it alone. The car has started great with no issues for the last few years, and it warms up just fine, correct? So what if you have to open the throttle up a little bit on cold start in the mountains. How often does that happen? With having injectors in the car twice as big comma that adjustment is also going to be twice as sensitive.

jeff spahn

Ok. I'll leave it and if it won't start ever while up there then I'll change it.