Hot Rod Forum banner

What pistons are these?

15K views 16 replies 4 participants last post by  Slammedsi 
#1 ·
Pulled the head off a 355 I had built several years back. Maybe y'all can tell me what CC pistons these might be, So I can more accurately find the compression ratio. There is a number stamped on them, Its H345NP and they are .030 over. I'm unsure of the brand.



 
See less See more
3
#4 · (Edited)
Yeah, the eyebrows are 5cc's, but the part number bothers me. I'm thinkin' they're 1.540" pistons instead of 1.560", even though the link by SixShooter45 says 1.560". There's something strange about that part number and there is conflicting information available.

Bring any cylinder to TDC. Stand a steel rule on its edge across the bore, about a half inch from the edge of the bore. Insert feeler gauge blades until you determine the exact piston deck height (measurement from the crown of the piston to the block deck). Play with the crank a little one way and then the other to make certain that you have found TDC. Insert the feeler gauge blades at 3PM or 9PM as you are standing at the side of the motor to prevent rocking the piston on its pin at Noon or 6PM.

If it's a virgin block (never decked) and the pistons are 1.540", I would expect to find a piston deck height of around 0.045". With a conventional thickness head gasket of 0.040", the squish (or lack of squish) with this arrangement would be 0.085". If the piston deck height plays out at around 0.025", then the block is virgin and the piston is 1.560".
 
#9 · (Edited)
OK, lets run the SCR to make sure we're on the same page.....
.7854 x 4.03 x 4.03 x 3.484 x 16.387 = 728cc's in the cylinder
Edelbrock 60895 64cc's in the chamber
.7854 x 4.03 x 4.03 x 0.028 x 16.387 = 6cc's in the piston deck height
Piston crown = 5cc's
GM 10105117 composition gasket (can be used with aluminum heads according to GM) 6cc's
If this gasket does Brinell the heads a little, it will only take a very light cut on the mill to bring them back to dead flat for using them on your next build.
728 + 64 + 6 + 6 +5 = 809cc's total.
Deduct cylinder cc's from total cc's, 809 less 728 = 81cc's
Divide 809 by 81 and find 9.99:1 static compression ratio with a 0.056" squish. (Call it 10 to 1.) Ideal squish is 0.035" to 0.045", but we cannot always have "ideal".
10:1 should be fine for aluminum heads and pump gas on the street. Use a camshaft with 0.050" duration of somewhere between 212 and 230, depending on operating range you want. The sweet spot for 10:1 SCR would be a cam with 220 degrees duration on the intake valve.
An Edelbrock Performer RPM high-rise dual-plane will make more power from idle to 6000 than any other intake manifold. Max power from a 350 calls for a 750 carb (vacuum secondaries if using a tight converter, double pumper with a loose converter or stick shift). If a fellow is not after max power, a 500, 600 or 650 will work fine. Those fellows looking for max fuel mileage will use a RPM Spreadbore intake manifold and mount a Quadrajet on it.

If you'd like, I can DynoSim the package with your cam choice.....or my cam choice.....
 
#10 ·
From a thread at Speedtalk:
Post by BigEd36 » Wed Dec 12, 2012 6:42 pm
According to the Speed Pro electronic catalog hypereutectic piston H345ACP (545 grams for +.030") has a 1.548" compression height, hypereutectic piston H345NCP (546 grams for +.030") and H345NP have a 1.560" compression height. Cast piston 345NP has a 1.54" compression height.
If this is correct, your pistons are hypereutectic and have a 1.56" CH.

If you measured how far down the piston is at TDC and got 0.028", that would tell me that the block has not been decked (stock is nominally 9.025", which gives 0.025" "piston down the hole" w/a 1.56 CH piston). The "extra" 0.003" is well within tolerances.

OR

That could also mean the block has been decked ~0.020", and the pistons are 1.54" CH. If you look at the block deck surface, the factory finish is different than the finish that almost all machine shops will leave after milling the decks.

So in the end, w/o measuring the piston CH and/or the block deck height, you're going to be guessing. But either way, figuring the compression ratio is unaffected by how the piston deck height got to be what it is (unmilled deck and 1.56" CH or milled deck and 1.54" CH).

Static compression ratio calculator
A couple calculators for dynamic compression ratio:
Kelly DCR calculator
Wallace Racing DCR calculator
KB/Silvolite DCR calculator
 
#11 ·
Couldn't help myself.....
Screwed a hydraulic flat tappet cam into the block, Crane 274-HO6, retarded 4 degrees, 750 carb, small tube headers with mufflers.......
Cam installed at 102 degree intake centerline angle, 110 degree exhaust centerline angle on these 0.050" numbers......
Intake opens (3) degrees BTDC
Intake closes (35) degrees ABDC
Exhaust opens (35) BBDC
Exhaust closes (3) ATDC

RPM....HP....TQ
2000...148...387
2500...188...395
3000...240...420
3500...295...442
4000...343...450
4500...383...448
5000...414...435
5500...420...401
6000...388...340

Max volumetric efficiency 89.9% @5000
Max BMEP 193.8 lbs @4000
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top