Hi folks -

I'm rebuilding the TPI 305 in my '92 Z28.I'm going .030 over on the block, and keeping everything else stock. I'm having a hard time finding some hypereurtectic pistons for it. The best ones I've found are some Sterlings, but they drop the CR to 8.95 : 1 and I want to keep the CR the same or bump it up if anything..

anyone have any suggestions?

Thanks!

- nick

 

Nick,

The stock pistons in your 305 should have been cast flat tops with four valve reliefs. Simply by increasing the bore you will be increasing the compression ratio, unless, of course, you are changing the heads.

Stock static compression ratio should have been about 9.2:1. By boring to 3.766", that should increase to almost 9.4:1, all else being equal.

 

I just bought pistons for a 310 I'm having built. I got an engine kit from summit with the Sterling pistons. They say 8.39:1 with 64cc heads. Your 305 should have 58cc heads on it.

I'm getting the block decked for a ZERO deck clearance. With a .040" thick head gasket, 58cc heads and 8cc of valve reliefs, it should be around 9.6:1.

The pistons are H534P...for .030 over bore.

 

Nick:
You have a PM about those pistons ("The pistons are H534P...for .030 over bore.") I looked at them today after we talked. They are flat tops w/ 4vr. All the other brands say 10.0 to 1 for this combo.
Don>>>

 

Thanks guys! I think those are the pistons I need, however I'm still a bit confused with working my CR out with these pistons. If I use the stock 58cc heads, stock GM head gasket at .019 thick, dont deck the block and a .030 bore with these pistons, where would my CR end up?

- Nick

 

That would take you to about 10.1:1. You might want to be careful about using a thinner head gasket on a street engine. Instead of the 0.019" gasket, a 0.031" gasket will still net you 9.8:1 and provide a little more margin of safety. If you want the extra compression pressure, you might want to get the pistons with only two valve reliefs, which wuold bump the static compression back up to 10.3:1 with the 0.031" gasket or 10.1:1 with the 0.039" gasket.

 

Thanks for the info.....

I'm still a bit confused. I just got off the phone with Federal Mogul - and they claim that with the H534P piston with a .030 bore using a .040 thick felpro gasket, and 58cc chambers, compresison would be 8.95 : 1 . If I were to drop to a .019 thick gasket, I'd gain about 1/2 a point so roughly 9.5 : 1

I'm a bit confused as to which set of numbers to go with now. How are you calculating compression out? I'm tyring to figure out how everyone here is coming up with the 10 : 1'ish compression but Federel Mogul is coming up with 9 : 1 'ish compression. I never really trusted FM's numbers, which is what prompted this thread....

- Nick

 

I've been using the DTD compression calculator. Vader also sent me a link to a spreadsheet that does the calcs. Both utilities came up with the same thing.

I played with the numbers a bit. If the total volume above the piston (valve reliefs and volume above the top ring) is 9.5cc, with a deck clearance of .025" the comp ratio is 8.97...real close to what FM told you.

Bore = 3.766
head cc = 58
gasket bore = 3.85
gasket thickness = .040
deck clearance = .025
volume above piston = 9.5cc
=8.97:1



Using 9.5cc for the valve relief volume and a ZERO deck clearance I get 9.46.

Bore = 3.766
head volume = 58cc
gasket bore = 3.85
gasket thickness = .040
deck clearance = .000
volume above piston = 9.5cc
=9.46:1

My original calcs were assuming the volume above the piston was 8cc. It doesn't matter much to me because the machinist is prolly gonna take a little bit of meat off the heads to clean them up. If you could call FM again and ask them the volume of the valve reliefs that would be cool. I'm still kinda guessing on those...

hope this helped some...

 

Nick,

Calculate the area of the bore (3.14159266 · r²) . Multiply that by the stroke to determine the piston displacement of the cylinder.

Determine the total free area of the combustion chamber with the piston at TDC, including valve relief volume, piston dome/dish volume, head combustion chamber volume, head gasket thickness and diameter, top piston ring depth and piston clearance, and deck height/depth.

Compare the total volume of the chamber at the BDC position to the total volume at the TDC position, and you have the static compression ratio.

 

Thanks for the info guys.

Astro - those were the types of numbers I was looking for. It looks like we'll need to deck the block to get the CR up to where we want it..

Thanks guys!

 

yup. That's what I'm having done. The machinst is prolly going to take a tid bit off the heads too...just enuf to true them up.