340 cylinder pressure

[polywideblock]

when doing a compression check, what should a 71 340 cylinder pressure be ?

got results from mechanic , all except one cylinder 157 psi other 158 . :eusa_think:

know nothing about motor(other than it runs great when it does ) its as it came from the US

670 street avenger on rpm air gap, stock exhaust manifolds stock ignition

 

[moper]

The pressures you record are the sum of a few parts. Generally if the readings are above 125psi, and are within 10% of each other, you're fine. It sounds like you are right there. It's in good shape. Better than good to be honest.

 

[polywideblock]

so 157 psi would be considered normal , any idea how to work out if I've got an 8.5 or a 10 :1 motor from my cylinder compression

 

[moper]

There is no way to work out static compression from pressure readings. A small cam in a low compression engine may have higher pressure readings than a high compression engine with a big cam. Take a head off and measure. That's the only way to know what you have.

 

[Cranky]

According to my trusty Motor's Auto Repair book, 155 + or - 20 psi for a 70 340.....and 110 for a 71 but it also says that static compression was lower for 1970 than 71. I'm thinking misprint there. From what I recall back then, compression was 10.5 in 70 and 10.2 in 71. Chrysler dropped the compression on everything in 71 except the Hemi and the baby Hemi (340). They were reduced but not very much. Are you having any pinging issues with it? Had a 71 Cuda and it would run fine on 93 pump so long as the engine temp didn't go over 180 degrees which was hard to keep it from going over that in Houston's summer time heat. Just adding one gallon of av gas to top off a fill up was enough to cure that problem.

 

[polywideblock]

thanks for the info guys , seems to be right on the money cylinder pressure wise .
never had a pinging problem(that I've noticed) , but have been adding octane booster / lead replacement since the first tankfull of gas over here ( was told it was a high compression motor and needed an additive :eusa_think: )

 

[Cranky]

If your fuel is good enough you may not need any additives unless the heads are stock and do not have hardened exhaust seats. Never had a problem with them not having hardened seats though. Can't remember exactly what pressure mine had but think it was just a bit above yours and like I mentioned, there never was a problem unless my engine temps went over 180. This car was driven back and forth to work regularly and on the hot days, I just kept my foot out of WOT. Anything short of WOT wasn't a problem so it was still able to be brisk without pinging. Drove that car for 15 years!

 

[polywideblock]

stock heads , we have 91 , 95, 98 RON fuel at the pump. I use 98 + octane booster and a lead substitute + stable , put in when I refill the tank . smells pretty funky coming out the exhaust :faint: