Broke in the 340 a few weeks ago, all went well. It's still in the run stand so I decided to check the cranking pressure yesterday. Using a screw in type compression gauge, the right bank are all 155psi, the left bank are between 137psi and 149psi. The engine was cold when I did the check. I rechecked my ring gap numbers and they're all about the same. I'm thinking maybe a valve issue, both heads were done at the same shop. Does this look like a problem or am I sweating over nothing? What would be an acceptable cranking pressure?
340 cranking pressure
- Thread starter busboy
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goldduster318
Overzealous Car Modifier
Did you do the test with the throttle open? If not, try that again.
IIRC, the minimum acceptable cranking pressure would be 130. On an engine with a static CR around 8.5:1, I would expect numbers around what you're getting, but I bet you're higher than 8.5:1.
Do you have stock or adjustable rockers? If they are just slightly too tight they may be bleeding off just that much cylinder pressure. If you have an air fitting adapter that screws into the spark plug hole, you can rotate each cylinder to TDC and apply air pressure from a compressor to the fitting. If it comes out of the breather, its a ring issue, if it comes out of the carb, its an intake valve, or if it comes out the exhaust, its an exhaust valve. If there is very little to no leakage, you're good to go.
IIRC, the minimum acceptable cranking pressure would be 130. On an engine with a static CR around 8.5:1, I would expect numbers around what you're getting, but I bet you're higher than 8.5:1.
Do you have stock or adjustable rockers? If they are just slightly too tight they may be bleeding off just that much cylinder pressure. If you have an air fitting adapter that screws into the spark plug hole, you can rotate each cylinder to TDC and apply air pressure from a compressor to the fitting. If it comes out of the breather, its a ring issue, if it comes out of the carb, its an intake valve, or if it comes out the exhaust, its an exhaust valve. If there is very little to no leakage, you're good to go.
moper
Well-Known Member
really it should be warm and the throttle held wide open. In terms of the readings themselves, it's still a little early to worry. Let it warm up and see what it pumps.
dgc333
Well-Known Member
The cranking pressure is going to be impacted by the cam too. More duration will lower the readings.
Finally followed up on this. With the motor warm this time, redid the compression check holding the throttle wide open. All cylinder readings are within 5psi at 160-165. The motor is still in the run stand and has less than an hour run time, I just have to get this thing in the car now:-D Thanks to all for the replys.
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