J heads, compression, and today's crappy gas

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Jarlaxle

Jarlaxle

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The title pretty much is the question: how much squeeze can I run without needing race gas or Avgas? I'm considering a car on eBay, but the engine is a 10:1 360 with J heads done by Indy cylinder head. They have 2.02 / 1.60 valves and 5 angle valve job, ported, polished gasket matched and bowl polished. The engine has a comp cams, approx. .484 / .487 lift cam. Will this run on pump 93?
 
I’m betting yes if the block hasn’t been cut and a thinner head gasket installed. Did they calculate compression or go with given ratings ?
 

I did run a 10-1 / 360 on 93 years back. The fuel was E-10 then as it is today. I’m sure the formula has changed on the gasoline since it’s been a good long while. But yea, I did 93 & a 10-1 engine. OH, the cam was a purple 292/.508.

Edit… If I was doing this today, to be honest, I’d opt for a 9.5-1 and no less than a 9.0-1 with the cam you mention.
 
pittsburghracer said:
I’m betting yes if the block hasn’t been cut and a thinner head gasket installed. Did they calculate compression or go with given ratings ?
Click to expand...

Beats me...though I'm inclined to believe numbers from Indy.

rumblefish360 said:
I did run a 10-1 / 360 on 93 years back. The fuel was E-10 then as it is today. I’m sure the formula has changed on the gasoline since it’s been a good long while. But yea, I did 93 & a 10-1 engine. OH, the cam was a purple 292/.508.

Edit… If I was doing this today, to be honest, I’d opt for a 9.5-1 and no less than a 9.0-1 with the cam you mention.
Click to expand...

J heads are small chamber, right?
 
The only way to know the CC of the chamber is to measure. Standard practice back in the day of 30-40k valve jobs was shave the head a little to be sure they were flat. After almost 50 years mine are down to 66cc. I smoothed the sharp edges on the piston and chamber surface and can run 90 octane at sea level with old trw flat top slugs and .039 head gaskets.
 
I’m running 2.02 J heads on my 10.2/1 340 on 91 super clear non ethanol with no problems. I’m using NGK BKR6E spark plugs, with MSD pro billet distro / box, blaster 2 coil, a high flow water pump and Be-cool radiator. I run at 170-180 all day long with no problems what so ever.

RPM air gap, with a Holley 750 DP, and TTI step headers… current cam is a comp XE285HL and I’m changing to a 236/236 on a 107+2 with 540 lift.
 
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toolmanmike said:
No. Read through these and take notes. Small Block Cylinder Heads 'Stock Specs'
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Thanks! Hmm...wonder if 308s might reduce compression...

mderoy340 said:
The only way to know the CC of the chamber is to measure. Standard practice back in the day of 30-40k valve jobs was shave the head a little to be sure they were flat. After almost 50 years mine are down to 66cc. I smoothed the sharp edges on the piston and chamber surface and can run 90 octane at sea level with old trw flat top slugs and .039 head gaskets.
Click to expand...

Do you know the actual compression ratio?
 
Just pay attention to the cranking compression/dynamic.
Much over 170 psi is leaving the ease of tune.. for touchy tune and by varying weather pressure conditions ever changing.
I've run 188 psi with a step head quench piston.
165-170 on open iron with flat top pistons and 170-185 with closed chamber or quench piston when using open keeping the distance within .035. Could say .040 ..but to argue that is wasting my time.
 
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