A question about octane and compression

**BOWSER** · 06-29-2019, 06:20 AM

Hello everyone. I recently purchased a 2.5 225 promax that has all the goodies like aluminum flywheel, weisco pistons, Brucato ACU, bobs tuner, shaved heads etc. I haven’t taken it to have a leakdown performed yet but it’s pulling 140lbs of compression across all 6. Previous owner says run premium with Aces so got a jug of Aces that will be here on Wednesday. I’m gonna hang the motor on the boat tomorrow morning and would like to go for a test run. Boat has a full tank of premium, but I don’t want to risk hurting the motor by running too low of an octane for it. Can anyone recommend a fuel additive that I can purchase around town that will get me by in a pinch? If the tank wasn’t full, I would just try an AV gas mix but I need to displace about 10 gallons before I can consider that option. Any ideas?

**rock** · 06-29-2019, 06:44 AM

At 140 you are safe with premium. No need for av gas or race fuel.

Rock

**BOWSER** · 06-29-2019, 06:50 AM

The motor was put together at the beginning of last year and has been run for a while. I’m not sure what the original compression numbers were but I am assuming that the heads were cut to 34cc since that seems to be a popular number among these motors. Still good to go with premium?

**rock** · 06-29-2019, 07:03 AM

Most guys shoot for 140 as it is safe with premium and with Aces can usually tolerate the occasional low octane you get at on water gas facilities.

Rock

**XstreamVking** · 06-29-2019, 07:14 AM

One figure left out of the mix. What is the timing set at? Compression is one factor, timing is another. 93 should be good. But check the advance setting.

**BOWSER** · 06-29-2019, 07:31 AM

That’s a good point. Previous owner says it runs a little fat which is fine by me and I guess a common thing for the ACU. I’ll put the gun on it tomorrow once I get everything plugged in and set up, and get back to you. Thanks

**FMP** · 06-29-2019, 08:01 AM

Good pistons in it , tighten it up a little bit more

**Merc 2.5** · 06-29-2019, 08:29 AM

I agree with everyone bout fuel, as long as u have a decent gas station u can halfway trust to keep fresh fuel , 93 is fine , I run 23deg timin with 145-150lbs and haven't had any issues , also weight of the boat plays a factor to, just dont lug motor down with high compression and high timin,,let it spin up

**jarrettolson** · 06-29-2019, 08:43 AM

Kind of a sideline question but at what cc and compression is it best to run AV gas?

**Merc 2.5** · 06-29-2019, 08:48 AM

155+ ,will use a mixture, .most these guys know the rite concoction for diff compression #s

**rock** · 06-29-2019, 09:00 AM

This is some info I as given forever ago by a Mercury tech at Mercury racing. 130 or less 89 octane, 145... 91-93, 165-175... 101, 175+... racing fuel only but nothing over 108. Do not know how accurate it is but this is what I was told. That being said several years back I bought a boat with a stock (other than the cut heads) 260 that had the comp at nearly 180 and they said it never got anything but 89 octane. I would never try that but the motor still lives.

Rock

**Motv18** · 06-30-2019, 10:41 AM

What’s the max rpm ? Aces I would splash or vp if say turning 7500 any length of time just for how hot the bores would be getting.

Theirs a minimum so so it doesn’t knock. then theirs the ratings that produce the most power, along with burn rate, and then theirs to much, and slow burns. Just like a/f ratios with its bell curve and oxidized fuel adjustments.

It should run fine on a fast durn alcohol free 91-93. But they might be guessing or did a test that showed aces bosted them a bit as it will speed up the burn so that might of only had slow pump gas.