PSI Compression on 75 350 factory engine
September 29th, 2019, 05:05 PM
#1
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PSI Compression on 75 350 factory engine
What would the cylinder compression psi be on on 1975 350 4 bbl from the factory when new? I know they were around 8.5:1
I'm getting 185-190 psi on my 75 350 after the rebuild with #5 heads,bored .030" over, new cam, new rings & pistons.
Just wondering how much it would increase?
Thanks
I'm getting 185-190 psi on my 75 350 after the rebuild with #5 heads,bored .030" over, new cam, new rings & pistons.
Just wondering how much it would increase?
Thanks
September 29th, 2019, 06:18 PM
#2
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Actually factory compression should be around 7.9 to 1 when actually measured, 8.5 to 1 is very optimistic. I got a 140 to 142 psi on a 76 350, same motor with a Performer cam installed, still had shim head gaskets. I would assume flat top pistons with that cranking compression? What cam? I have 150 to 155 psi with a 214/214 cam with #6 heads that are 62 to 64cc measured with the stock 76 short block and .028" head gaskets for 9 to 1 compression.
September 30th, 2019, 10:54 AM
#3
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Be aware that the camshaft intake closing event will have a large affect on the pressure readings - earlier intake closing will "trap" more air and give a higher pressure reading. So two engines with the same static compression ratio but different camshafts will have different PSI readings.
September 30th, 2019, 06:19 PM
#4
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The cam I'm running is a Lunati,10420701... intake duration duration 213, exhaust 219.
Running Sealed Power Pistons L2321F30. These pistons are not flat tops.
Not sure how much the heads were machined to clean them up?
New valves with 3 angle grind. The compression is somewhere between 182 -185 psi... It was a year ago I did the test.. Can't find my notes now,The more I think about it the the PSI was on the lower to mid range of 180's.
Running Sealed Power Pistons L2321F30. These pistons are not flat tops.
Not sure how much the heads were machined to clean them up?
New valves with 3 angle grind. The compression is somewhere between 182 -185 psi... It was a year ago I did the test.. Can't find my notes now,The more I think about it the the PSI was on the lower to mid range of 180's.
September 30th, 2019, 08:41 PM
#5
Out of Line, Everytime😉
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You probably still have 9.6 to 1 compression with those pistons. That cam isn't that big, especially for that compression ratio. Basically, you gained 40 psi over a stock mid 70's 350. How does it run in comparison?
October 1st, 2019, 04:33 AM
#6
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The engine runs extremely well. Huge difference with power. I'm very happy with it. I used the thornton exhaust manifolds with new duals.
Spent a lot of time playing with the lean quadrajet carb. Jets, rods, etc. & timing.
Spent a lot of time playing with the lean quadrajet carb. Jets, rods, etc. & timing.
October 2nd, 2019, 04:10 PM
#10
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Apples and Oranges but I had a quick cylinder compression test done on my engine a few years ago.
At about 110,000 miles it showed 190 PSI.
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