Pump Gas

[springs]

Need some input. What's the highest compression ratio you can run on pump gas without adjusting timing?

[Fish boy]

Dunno know about i/o's but if you are asking about OB's, 145lbs, or 34cc heads is about as high as you can go and still run 93 octane.

[springs]

BBF aluminum heads

[Sam]

I run your typical marine grade gas (89 octane) with 9to1 compression. So far I have had no issues. As for timing that will very from boat to boat.

Sam

[Mr X]

8.5:1 if it is carburated and no knock sensor.

[smoothie]

I have a BBC with 9.8:1 and alum heads I always use 93 oct,from what I have been told you can go alittle higher with the alum heads because they run cooler.

[Pismo]

Yes, the aluminum heads most definitely buy you a point, so your up to 10:1 on 89 octane typical marine gas.

[Sam]

Pismo, can you exponud on your statement, I'm not 100% clear on what your saying.

THX
Sam :bonk:

[Rootsy]

since aluminum dissipates heat more efficiently than iron your combustion chamber temperatures will tend to be a bit lower... This allows you to run a higher static compression ratio. As a gas is compressed it generates heat. Heat is what causes detonation by either elevated temps imparted to the incoming fuel air which raise them to their self ignition temperature upon compression or by causing hot spots in the combustion chamber. if you can get rid of the heat of combustion in a quicker fashion it is not transferred to the fresh air/fuel mixture which keeps you away from having it self ignite....

personally... for a marine motor that is under constant strain, especially at high rpm .. running ignition timing right wehre the motor will make greatest ooomph.... i'd be stepping up to premium for anything over about 9.5 - 9.6 with iron heads... 10:1 with aluminum would be on the edge of my "warm fuzzy"... Keep in mind combustion chamber configuration, valve material, piston crown configuration, quench distance and A/F ratio are ALL going to play into this general rule of thumb. You also have to consider that we run our engines (non closed cooling) much cooler than an automobile engine which adds to that margin of safety but which also robs us of power....

i run 9.4:1 (measured) static 8.6:1 (calculated) dynamic compression ratio with an "ideal" if not skoash tight quench distance... more than static you really should look at dynamic compression ratio... this translates into your compression pressure in psi... it is dependent not only upon static compression ratio but also upon valve timing events... the higher your pumping pressure... the better the fuel you better be running... for any arbitrary setup...

i run premium because it is a heckuva lot cheaper than putting a hole in a piston should something go sour... and i sure as heck can't hear what is going on under my hatch at WOT...

Sorry walt... didn't mean to tell ya how to make a watch...

JR

[Sam]

James, I knew this would draw you out ;) . Good to hear fom ya, you need to come out and play more often.

Sam :D

[springs]

Using a standard gasket thickness all the online compression calculators give me anywhere from 10.1 to 10.46.

[Marlin275]

You also have to consider that we run our engines (non closed cooling) much cooler than an automobile engine which adds to that margin of safety but which also robs us of power....
JR


How much power/faster would closed cooling give back?
What is that in mph? :rolleyes:

[onesubdrvr]

How much power/faster would closed cooling give back?
What is that in mph? :rolleyes:

I'm sure you know this, but keep in mind, even with F/W cooling, boats still are not closed cooling,....... But, good question, one I'm curious to have answered. Maybe there's a project for me!!, get a radiator, find a place to mount it, with a fan,....hmmmmmmm,....

Wayne

[Jamesbon]

I'm @ 10.01:1 as measured by piston domes, head gasket thickness and head c.c. volume...in a 502 with nasty cam. Hate to see my upcoming fuel bills. I'm under the assumption that 10:1 is the max for pump gas in iron headed engines. There's a few here who already run this combination without incident. :)