does race gas substancially decrease idc?

[vintagemuscle]

right now i am running 100 octane on the street and i am seeing 92% idc running 17psi on my 14b. fuel pressure is set at 45psi at idle. i plan on going to the track this weekend but i would like to raise the boost to 20+ psi. would running 120 octane lower my idc's enough to run 20+ psi? i will be getting dsmlink shortly and some 850cc injectors but i want to see what i can do with what i have first. i believe i have a fairly good tune with using just the maft but is 92% idc a little high for just 17psi on the 14b? i also have 272's which i think is why i'm seeing such high idc at redline. also, could i raise my base fuel pressure anymore to help?

 

[andymoraitis]

Raising fuel pressure will add more volume to the equation and lower IDC's but at the expense of a solid air/fuel ratio. While our motors are less sensitive to A/F than they are to timing, it's still a good idea to stay in the high 10's or around 11:1 and around 12:1 on race gas. Since your fuel pressure is already 8 psi over base, you're already running rich and I certainly wouldn't ever run that high over base pressure. As it stands, you could be pushing enough volume through the rail to wash fuel past your rings and dilute your oil.

Race gas doesn't lower IDC's. Since it has a slower burning flame front, it allows for greater timing and boost without running into knock. Basically it provides you with a cushion to turn up the wick, but if the airflow demands increase, or remain constant, IDC's will follow suit. While your IDC's ideally shouldn't exceed 85%, you're not in what I would refer to as a danger zone, but you're approaching it and are stressing them which could lead to early failure. You're also correct that your cams are adding more air to the mix and therefore demanding more fuel, which will in turn drive IDC upwards.

If I was tuning your car, I'd add a bunch of race gas, turn the fuel pressure back down to 40 and see how much boost I could get away with. Leaning it out with lower pressure will provide better performance on slightly less boost (with more timing) than drowning the motor in fuel and adding more boost.

Let me know if that makes sense.

Andy

 

[bmwisthekey]

usually being able to lean the car out safely in addition to turning the boost up while using race fuel will lower your IDC's

 

[vintagemuscle]

thanks for the help guys and for correcting me.
andy, what you said makes perfect sense. tomorrow i will lower my fuel pressure back down to 40 with my same tune and see what happens.

 

[andymoraitis]

Give it a try. You'll very likely run better and you can still lean it out a bit and log for knock. I think for track only duty, you'll be OK with your IDC's that high. You just don't want to be there all the time.

Good luck and let us know how it goes.