I'm building a fresh 493" EFI'd pump gas RB motor that had 10.2 to one compression. I had been running the mechanical DC carbs, but am switching to a Holley HP unit with coil-on-plug and a Wilson ported Victor single plane with their throttle body.

Does anyone know how much I can bump up the compression using pump gas while injecting straight methanol controlled by the Holley computer? I've heard I can run at least 11 to 1, but have never used methanol injection before.

It depends on how much methanol you spray, the more you spray, the HP ECU will pull fuel out to get the AFR where you set it. if it rattles add more Meth

Any reason why you wouldn't run E85?

:iagree
I run local pump E85 in my S/P car with 15.0t o 1 compression and love it
Try it if you can get it their Good power and very good repeatability at the tracks

Repeatability? ... so you’ve found consistency in the E85 mix in oReGoN ??

Have a nice day

He said EFI so no need to worry about repeatability.

Use a fuel composition sensor and set it up to automatically compensate for varying levels of alcohol.

Yes straight methanol will make more power than E85 but hard to justify if you don't need all of what it has to offer. (and the complications that go along with it)

E 85 does NOT contaminate the OIL( ) like straight methanol alcohol does
Ethanol alcohol has less BTU per gallon than Methanol alcohol does, I think Methanol has 85,000 BTU per gallon and Ethanol has 83 or 84,000 BTU.
I'm not sure how many a gallon of pump E85 has with the additives and gasoline to keep them mixed

Normally aspirated motors on methanol only; need from 5.5 afr to 4.9 aft depending on compression. E85 is about 9.6 AFR and pumpgas about 12? Afr for e10.

A charactoristics of methanol is its' ability to cool the mixture. I understand it takes five times as much heat energy out of the mixture as g gasoline. Blown alky motors run an AFR very fat; like 3.8 AFR. That allows a lot of boost along with 12-1 compression. But be aware methanol is a poison and soaks through the skin. So if you chose to use it keep in mind no skin contact and don't inhale fumes. When processed by the human body it changes to a poison that can eventually blind or kill.

I hadn't really considered it, but now that you ask and Cab recommended it, I've been looking into it. It would be nice to have 13 to 1 compression with no fear of detonation. The Holley HP set-up has an input for the E85 sensor and adjusts it accordingly. The only issue becomes where is e-85 available on the open road. There are lots of options near me, but fewer on the open road.

That's what I'm using (HP efi with composition sensor). Tons of E85 availability as far as I will probably drive this car but can see it could be a problem if you planned a long trip.

I have mine set up to put 6deg additional timing in at 65% alcohol and above (as I've found that range to take as much timing on most everything I've calibrated as 85%+) and pull it all out as it approaches 10%.

So if I couldn't find it on the road shouldn't be a problem to run straight 93 octane for a few tanks just to get where I'm going (and not have to adjust anything).