whodathought38SS

I need a little help, what is the max compresion you can run a BBC 540(4.530 bore X 4.25 stroke with 6.385 rod) on 87 octane fuel? I was guessing 9.0:1 with aluminum heads, crane cam PN 139021 and 34-36 deg total timing and a max RPM of 5500. I am sick of hunting down 92 octane and then praying its good. Thanks for the help.

1CE

double ck. but 4.530 x 4.25 should be 548ci/run no t-stat/keep timing @ 34* or less/be sure you read your plugs brown to light brown/u-should be good to go/why not chuck in some octane boost here and there to be sure ? ps or ask Mr.horsepower1 here on OSO he would know best.

whodathought38SS

yeah I know you are right about the cubic inch size of the motor, I was useing 540 as a generic term for a 4.25 stroked 502. I should have been more specific, sorry for the confusion. I really don't want to have to add anything to the fuel, I would rather go down to a lower compresion ratio like 8.6:1 if that is what it takes.

I have been running supercharged 468s (4.311 bore X 4.0 stroke) with 8:1 and 7 psi of boost qith the same cam for about 5 years. I loved those motors but I lost one to bad fuel this past July and have been thinking about going bigger cubic inch and N.A. to get the same power (650-700).

ECeptor

I've always found this article to be a great reference on compression.

http://www.popularhotrodding.com/tec...ech/index.html

I would think a 9.0:1 NA engine w/ aluminum heads would be no problem on 87 octane.

mrhorsepower1

I would stay around 9.5:1 max on the compression and about 34 degrees timing. You will be fine.

Chris Sunkin

I was looking at the GMPP sheet on their HO454. They claim 92 is required at 8.75CR. That seems awfully conservative but I've never tried running regular unleaded in a boat so I can't say for sure.

I think before I tried it, I'd want to add in one of those knock sensor/controller's like th J&S Safeguard.

whodathought38SS

I think the Merc. 454 and 502 mag run about 8.6:1 on 87 right??

MRHORSEPOWER1 do you meen 9.5:1 for 92 or 87?

Just trying to make sure I understand we are talking about static CR right? I gave the cam and motor specs because I know there is a difference and I am unclear on how to figure Dynamic CR. JE makes a nice easy to get piston set that will make about 9.5:1 but I thought that was for 92 octane.

Again thank you all for the help, I really need all I can get LOL.

mrhorsepower1

If your N/A , yes I have run 9.5:1 compression on 87 octane with aluminum heads. I would not run a thermostat for the water, keep the timing at 34 total, and it would be wise to run a thermostatic oil filter assembly to build proper oil temps and keep the engine free of moisture .

Chris Sunkin

Dean,

How much difference do you consider the aluminum heads make in offsetting compression? Are they equivalent to a 1 point drop?

whodathought38SS

Thanks MRHORSEPOWER1!

I have a stainless marine crossover/Tstat kit with larg oil cooler and a high flow 140deg thermostat with four 1/8" holes in it from the supercharged motors. But I did not have an oil thermostat.

I used to get a lot of moisture in those motors if I did not run them very long and very hard.

So than plan would be to get rid of the thermostat and put in a 1/2" restrictive washer and a oil Tstat?