MSefi.com read only forum (Jan.15,2018)

Hey all.

So for a while now we have had our 280z running with an MS2 v3 board. Last week we were doing some street tuning and pushing 10psi the car was running great up until about 5200rpm where I noticed the injectors were starting to max out. I saw the duty cycle hitting 92% or so and decided that we aught to bump up the fuel pressure to keep from running the injectors static.

Bumped the base fuel pressure from 40psi to 45psi.

Problem is, since last week the ambient air temps have dropped nearly 20 degrees F. During that run last week, at 10psi and 5000rpm or less, A/F ratios were in perfectly safe levels around high 11.8:1 with intake air temps around 98 degrees. Today IATs were around 78 degrees.

Today we took the car out after bumping up the fuel pressure and as we boosted the car to the same level, A/F ratios were getting dangerously lean, up in the high 13's. I realize that some IAT corrections are in order and intake charge temps have quite a bit to do with overall mass of air entering the engine, but can a 20 degree drop in IAT account for at least 2 points leaner A/F ratio?

Hiya, Random.

A temperature drop from 98F to 78F will result in a 3.7% denser charge. A fuel pressure increase from 40 to 45psig will increase fuel flow through the injector orifices by 4.4%, so this is a bit of a wash. I think part of your problem is that you are into the non-linear range of your injectors, and you have unfortunately no real way of determining what flow you'll get with any changing conditions.

I would suggest your best bet is to swap injectors to at least 20% bigger, and try again.