fleg1

I have gm 502 crate engines that were set up for marine use they do have crane 741 or 731 cams I don't remember which one. They have 8.75 to 1 compression Gm says to use 92 octane. The question I have, is the compression low enough to warrant it necessary to use 93 octane? I just don't see why I need to use premium gas when HP 500's run on 87..

Any help would be appreciated..

MILD THUNDER

iTrader: (3)

I have some insight on this, using some calculations on intake valve closing point with those cams, cranking psi, and some other thoughts. However, I remember you pretty much calling me a dumb a$$hole on here a few months back, along with some other insults, you telling me I don't know $hit about engines, , etc. SO, I'll keep my thoughts to myself!

Eliminated572

Who built the motors and what do they have to say about octane? MT is correct about dynamic compression vs. static...that is what will determine octane requirements. What is your cranking psi using a compression gauge? Where is total timing set at?

fleg1

Not sure what the cranking psi is, the timing is at 36*

Engines came right from GM we changed the oil pans to 8qt, put teague oil adapters, took out oil bypass valves and plugged the outer hole, changed the flywheels and water pumps and put the accessories on them.. They run excellent just was curious if I actually needed to use 93 octane..

Here is a link with some specs!!
http://www.jegs.com/i/Chevrolet+Perf...68778/10002/-1

fleg1

..

fleg1

LOL!! appreciate your input..

mike tkach

iTrader: (3)

i also have insight on your question but you made it perfectly clear that you know more about engines than i do so il keep my insight to myself,feel free to correct my spelling though.

Eliminated572

HP500's were engineered, tested, and proven for their octane needs. Your crate motors on the other and were not designed to meet this low octane, much less designed for contant load marine use. Stick with what they intended octane wise. Why question their design when the price difference between the fuels is peanuts in comparison to the bigger picture? My 2 cents

fleg1

Thanks I appreciate your thoughts!

mcollinstn

Chances are, you can probably run 87 octane.
But you'll have to test it yourself.
If you have a good ear, you can listen for pinging and if you hear any then it's clearly not enough octane for your setup (although changing your ignition curve may give you enough leeway to run it).
If you don't HEAR pinging, then make a few runs, make a wide open run and then check the plugs - you'll be looking for any signs of little metallic dots, which are a sign of detonation.

Hot weather will lower the detonation threshold, so test during HOT weather.

If your tests do not reveal any detonation evidence on the plugs, and you can't hear any detonation, then run the cheap stuff.
But occasionally pull a plug and check if you do.

MC