12:1 compression ratio
 

just finished rebuilding the first of two 502 engines for the cafe racer. Since there are no dynos in panama and all i have to go by is the compression ratio, was wondering what kind of hp would the engine put out considering 12:1 compression? no blowers, just naturally aspired.

Need a lot more info than that; Heads, cam, carbs? also @ 12:1 can you say "The Pin Has Been pulled." That is A LOT of compression in a boat engine. You running straight race fuel?

Hope there is lots of GOOD gas in panama !!!!!!!!!!!!!

!!!!BOOM!!!!! :lolhit: I say way too high!

problems
 

big compression like that you are asking for trouble.looks like you will be burning the realll expensive stuff.i can only imange how much race gas cost in panama.

gas in panama is 91 and 95 octane at the marina.

then it doesn't matter how much it costs because you are only going to use about 3 gallons before the pistons melt.

if you are locked into 91 /95 then you better get it down to 9.5 or so or start buying pistons in bulk.

Yup................. Im at 9.8 and on the teeter of oh $hit.
-Mike

started the engine for the first time , runs nice, but on JET FUEL... guys were right. as usual. will jet fuel cut the cheese?

octane
 

i would think you would need aleast 110-115 octane to support that kind of compression,maybe more.

12:1 on pump gas or nav gas = :poopoo:

no way, in a car you can run 11:1 with aluminm heads on 94. 12:1 in a boat I agree is high but 104 octane should do it. Turbo Blue baby:coolcowboy:

NO,BUT YOUR ENGINE WILL,try some flex fuel.

Fixx
 

JET FUEL????? jet fuel is JP-5 which is kinda like kerocene and has alot of additives in it to keep it from freezing in high altitudes..

i hope you were refering av gas than that will work for a little whyle until all the chemicles does a number on all the internals..if your thinking E85 which will also work but you need to have a special carberator set up for e85 (alchol) which also eats regular & rubber fuel lines..

yup, aviation gasoline, my bad. im just new to all this 12:1 compression issue, kind of bummed really, and will probably use 95 with some kind of octane boost?

Fixx
 

are you sure its 12:1? what pistons are you using.head cc's.shave one point of compression with the aluminum heads, also don't go above 32degree advanced on the timing or try 30* and see how it runs, they do it with blower motors and all blowers do is raise the compression by stuffing the engine full of air...octane booster is a banded if you have high compression..you may want to install a knock sensor gauge just to monitor the engine for detonation also..

post a good pic of your pistons,,would be helpful..

timing is 32, specs of the pistones are the following:

Bore (in) 4.530 in.
Bore (mm) 115.062mm
Piston Style Dome, with single valve relief
Piston Material Forged aluminum
Compression Distance (in) 1.395 in.
Piston Head Volume (cc) -21.00cc
Wrist Pin Style Press-fit or floating
Pin Diameter (in) 0.990 in.

would lowering the timming help?

are you kidding ?? av gas has the wrong additives for auto or marine use, can use blended but bottom line is comp WAY to high....you can retard the timing to the point it wont run worth a crap, you are wasting time and $$$

100 octane LL av gas works well in 10:1 motors as long as you jet for it. at the comp numbers he is talking about about it needs a good 103 or 105 "race fuel" and while retarding the timing some may help a bit, the more you retard the timing the higher the chamber temps get so ultimately you end up in the same place anyway. you could do that if you got stuck w/ bad gas and wanter to get home at low revs but there is no way that that is a viable long term solution.

the solution is to fix the motor mechanically or by fuel by the barrel... or pistons by the case.

Cafe,

This calc will bring you +/- .1 pt for a given varience in deck height, chamber volume, and gasket volume. You should be able to radius the crowns to bring you closer to 10:1 if necessary. Try to avoid the thcker gasket approach to further reduce the CR.

Bob

big diffrence in a boat vs a car.the boat motor runs under 100% load all the time,unlike a car.i say use the highest octane you can find.better to be safe than sorry.after all,gas is the cheapest thing about boating.

Fixx
 

keep the timing where its at and listen for detonation,,install a knock sensor and gauge


like bob said you could radius the piston crown to relieve some compression, also get rid of the sharp points on the piston's, this will help with detonation. Take Bob's advise,he wont steer you wrong..if you use a thicker mls head gasket, i wouldn't go no more the .051 thick make sure you use head studs.. i run 10.2-1 in my little boat,355sbc on 93 fuel for 10 yrs,never no issues.. keep it cool and it should be fine..

What heads are you using? Are you sure you're 12:1? There are many ways to crutch your engine and make it run but none of them are right. Especially given the fact that you're building a second motor. The right thing to do and cheapest in the long run is to pull your motor apart and have the domes cut down. They are mos likely a solid dome and any competent machine shop will be able to mill the domes easily.
Also mrfixx the increased cylinder pressure from a SC is different then a high static comp ratio and running a thicker gasket to compensate on a NA motor is probably worse then running proper quench.. My .02 :drink:

Fixx
 

Also mrfixx the increased cylinder pressure from a SC is different then a high static comp ratio and running a thicker gasket to compensate on a NA motor is probably worse then running proper quench.. My 02..


that would depend on his deck height,, cheers :)

You are correct,the calculated compression ratio is that posted in my attachment. His actual mechanical compression ration is :

thanks for the input guys. will set the timing to 30, as well as use blended gas.

Cafe,

I would recommend against retarding the timing to prevent detonation as it will elevate your EGT figure significantly. Your actual compression ration is not 12:1. It ranges anywhere from 10.350:1 w/ 121 cc combustion chamber to 10.783:1 w/ 116 cc combustion chamber. These are easily managed numbers with 95 (RON+MON)/2 fuel.

Bob

Here are the 116 cc chamber calcs on your engine:

Get in touch with Bob and he can set you up with cams to get your dynamic compression where it needs to be to make your engines happy for a long time. After he's done mine though.. :signs069: :angry-smiley-044:

Fixx
 

+1

thanks for the input.

Years ago I bought a Bahner jet boat with a 30 over 454 corvette block and I was told it had 13-1 pistons. I was also told to run atleast 100 octane fuel. The guy I bought it from said to use 1 qt. toluene to 10 gals premium fuel, which would make 100 octane. I purchased the toluene from the Shell petroleum plant at $30.00 for five gals. I could also run AV gas 50/50. The motor was said to have approx. 700 HP. On the compression guage it would pump up to 250 PSI.

I ran that motor for 3 years and ran it frequently at 6300 RPM with no problems. I finally had to give it a valve job because one of the valve springs broke (it had double springs) . The block had metal O-rings machined around the bore.

I always set the timing by ear and retarded it if I heard any pinging.
It ran approx. 90 MPH and was a wild ride. I never had any other issues with that motor.

The motor was built by a shop called Wilder Racing in Pomona CA. This was in the early 80's.

I guess it also depends on who built the motor and how you take care of it.

so you took a 700 hp big block at 13 to 1, hand mixed some fuel for it out of whatever you could find on the day, set the timing " by ear and ran it at 6300 revs...

well... assuming that any that is even remotely accurate all you have demonstrated are all of exactly the wrong things you can possibly do with a good piece of hardware..

you are correct in one thing.. it all depends on who is taking care of it...

and i wouldn't let you take care of my lawn mower.

I won't argue with you because you feel you know it all!!!!! I am just telling you what I bought, what I was told, and my experiences running it!
This was the early 80's and that was how they ran then! The guy that built the motor was well known for winning races!
I didn't say I had the exact specs on the build, but I did say what the compression guage read 250 PSI!!!

Yeah my lawn mower runs great too and it's 17 years old!!!!

:party-smiley-004:

That's some funny sh!t there !!!!!!!:bong::bong:

I dont know how guys used to get away with "timing by ear". Especially in a boat with 13 to 1 comp at 6300 RPM????? To me, It just goes to show, sometimes when you just dont give a Sh!t, you get away with murder! The second you start worrying is when thing start going wrong!

Everything in a boat with more then 10:1 compression needs aluminum heads, max 34* timing and very good fuel ( 98 + OCTANE ) if u want to keep it alive.

12:1 min. 110 Octane !!!!!

But what do i know.

( On your pistons i would mill them down so the dome is gone )

The only way to know what compression these engines have is to do all of the math. My 548's have 10cc dome pistons and my compression worked out to 10.0 to one. I doubt if 22cc dome in a 502 works out to 12 to 1. Probably more like 11 to 1. Still high but workable with a mix of racing gas and 93. It probably needs about 100 octane to live. I'd mix half and half 110 racing gas with 93. Or just change the pistons.

Ha, Rmbuilder beat me to it. I should've read the whole thread before I opened my mouth. Thanks for the info.