J head max compression

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daniel9843

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Hi there i was just wondering if i could have some help, i have a La 360 disassembled. With forged pistons TRW 40 thou over zero decked block with J heads that are at 62cc which on my calculator says comp ratio will be around 10:8-1 . Is this to much comp for iron heads what will be the out come if i fit them up to the block will it get to much detonation.
 
depends on what fuel your going to run and how much timing. for pump gas i would say it was to high because you would be right there on the edge. i was told 10-10.5:1 for cast iron and 11:1 for aluminum as a reasonable safe max but i would keep it around a full point lower than those numbers just to be extra safe.
 
With 98 octane and enough cam duration i'm sure it's doable. You could also thicken up the head gaskets for a little insurance.
 
That's a little harder to tell. The rest of the combo will determine what will be tolarable for what your using the car for......trans, gearing, weight, intake, etc. I'm going to say as a general number somewhere around 240*@ .050. intake duration. Would probably be best to speak to a cam grinder and get a recommendation that fits just what you need.
 
Daniel, i should also ask if your piston deck height will be near zero as you stated?

Reason i ask is, you need to be about .020 down with the .050 gasket to be at 10.8 to 1, in my rough calculations with a small valve notch. Differences there can drasticly change the calculations.
 
yes the piston will be at zero deck height. the rest of the running gear is 3000 stall convertor, 3.89 gears, the heads have 2.02 inlets 1.60 exhaust, 27 inch tall tyres, body weight 3400 pound
 
I used a better calc with a 5cc notch and came up where you did, so i think your 10.8 is probably close.

I think you'll be ok with the above mentioned duration, but some members here use a dynamic comp. calculator that takes into account cam timing also. I hope one will chime in and give there thoughts on your build. Sounds like it's gonna be a nice one :D.
 
Were the heads milled? J heads are typically between 71 to 73 CC.
 
98 octane in Europe and AU is the equivolent of 93 in the states. IMO - you'll need to be very careful with the cam choice - and it may still have problems. Plus - cuttting a J head to 62ccs is leaving them thin at the gasket surface...
 
moper said:
98 octane in Europe and AU is the equivolent of 93 in the states. IMO - you'll need to be very careful with the cam choice - and it may still have problems. Plus - cuttting a J head to 62ccs is leaving them thin at the gasket surface...
Click to expand...

moper said:
That's a lot of milling...
Click to expand...

Thanks for posting Dave. I didn't even consider the fuel differences between countries.
 
Chassis plays a role in detonation vs compression. Heavy car with freeway gearing and a stock 1700 stall converter = detonation. Light car deep gears 4000 stall far less chance of detonation. Carb and ignition tuning play a role as well.
 
Skrews nailed it,head on.A bigger cam,looser converter,deep gears really help take the"load" off the engine,reducing the need for octane needed.
 
Absolutely doable with 98 and the rite cam. Pick a cam that will put your dynamic compression down around 8.5 and you can run 93 without any problems. I had a 11:1 (static) X headed 340 with a mopar .508 cam and it ran perfect on 91. Cam has a lot of duration so it bleed off some of the cylinder pressure. My compression was 175 and it ran strong on 91. No spark knock
 
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