RV cam
Logistics Pro
Joined: Jan 2011
Posts: 4,762
Likes: 54
From: Antelope Valley ,CA
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Cargo Master
Joined: Jul 2010
Posts: 2,099
Likes: 8
From: York, Pa
here's a calculator if you know all your specs
Engine Compression Ratio (CR) Calculator
Engine Compression Ratio (CR) Calculator
Post Fiend
Joined: Dec 2007
Posts: 8,521
Likes: 16
From: Lost
Those 390/410 pistons will sit down in the hole about .100 lower than the old ones. In a 410, they gave ~10.5:1 CR, but in a truck 390 it was much closer to 8:1 CR.
Swapping those 390/410 pistons for a set of 360 pistons should put it back into the low-mid 9's, and I'll bet they will even look like the old ones. Ask the guys at napa to compare your old pistons to 360 pistons, and you'll likely see a close resemblance.
I'm running a mid 9's CR 390 (360 pistons), with a factory GT/CJ cam, on regular. It does like the mid grade a bit better, but will run on the lower end stuff. Timing is the key here. This engine is backed by a C6, which does help to minimize the lugging that can be accomplished with a manual.
Swapping those 390/410 pistons for a set of 360 pistons should put it back into the low-mid 9's, and I'll bet they will even look like the old ones. Ask the guys at napa to compare your old pistons to 360 pistons, and you'll likely see a close resemblance.
I'm running a mid 9's CR 390 (360 pistons), with a factory GT/CJ cam, on regular. It does like the mid grade a bit better, but will run on the lower end stuff. Timing is the key here. This engine is backed by a C6, which does help to minimize the lugging that can be accomplished with a manual.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
I made a mistake in my figurin. I missed the rod length by a full tenth. Rogue's 8 to 1 figure is correct. In any case, if that cam's specs are the same as those on the same grind in a 335 eeries motor, it'll still work fine as that 400 I ran one in had around an 8 to 1 comp ratio.
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Cargo Master
Joined: Jul 2010
Posts: 2,099
Likes: 8
From: York, Pa
Well no need to worry about those low compression pistons. Napa did take them back and gave me a set of the HF395P which had the same compression height as the ones I took out. Tommorow I think I might be close enough to start putting some of it together.
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Cargo Master
Joined: Jul 2010
Posts: 2,099
Likes: 8
From: York, Pa
Well after about 3 weeks of waiting for machine works and parts since I started this project it's soon done. I know it seems like its been a lot longer but really has moved along nicely for the amount of work that's been done.
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