alright i want to build a 396 with 3.875 crank and 5.75 rods. my question is how do you know what hiegth pistion to get. i mean how do you know what pistons to get so you dont have the pistons hitting the head and sticking out past the deck.

and what is the stock deck hiegth on the lt1.

 

Can't answer your first question but to your question stock deck hieght on a lt1:
http://www.97ta.com/lt4/lt459.jpg

at least on mines anyways.

 

another question with a 396 using 3.875 crank and 5.75 rods, what cc pistons do i need to hit about 11.5 to 1 or 12.1 to 1 CR.

 

Once you find out the deck height and compression height of the piston. Use the bore and compression height/deck height to figure out the volume in the actual cylinder when piston at TDC, then use same volume formula for the gasket (bore and compresed thickness). Then add in the volume of the combustion chamber with plug in. should be ~58cc stock.

Then figure out the cylinder volume at BDC based on stroke and compression height of the piston. Divide the smaller into the bigger (?? think so) and you have a ??:1 ratio.

There is a much much easier way to figure it out, I believe the one I used was on either an orange county fbod assoc, or georgia, probably georgia, but do a search on yahoo for compression ratio calculators.

 

IE....

3.875" crank
-8cc dish pistons (valve reliefs)
58cc combustion chamber
.024 gasket compressed thickness (imp SS i believe)
.02 shaved off deck


comes out to 395.422 CI
and 11.63:1 static compression ratio

 

You weren't paying attention in that other thread!

Stock block deck height is 9.025". The compression height you need is calculated as follows.

block deck height - (stroke/2+rod length+gasket height+desired piston to head clearance) = compression height

If you wanted 0.045" piston to head clearance your combo would work out to the following (using your figure for gasket height).

9.025 - (3.875/2 + 5.7 + 0.024 + 0.045) = 1.319". This would be a custom piston BTW, so I hope you aren't wedded to these numbers. I think it's good you want to learn more, but the guy building the engine needs to be involved in these claculations. You probably want to avoid the expense of custom pistons and should therefore choose a proven combo, unless you know some speed secrets

Rich Krause

 

thanks alot you guys that helps alot. and rick thanks for being so patient.

i look at all these kits and there all so different that helps alot.

 

Rich, the equation, that you presented, is incorrect. I would state it the following way.

9.025 ( if indeed that is what you actually have) (+) .024 - (3.876/2 + 5.7 + .045) = comp. hgt. of piston.

I'm confident in your case, it's merely a typo.

If indeed that .022" indicator reading was performed on an oem rotating assembly, you could easily be .010-.015" under std. (9.025) at that point. I've seen oem part stackup total 8.990".


Note (hopefully) of interest:

9.025" is the blueprint spec for the distance from the deck to the crank centerline. There is no guarantee what you have, is the blueprint spec noted, with or without prior machining. A good shop will remove stock from the block deck, referencing from the crank centerline. The amount removed is secondary to the actual distance, or deck height obtained. When you give them your block, they won't ask how much you want removed, but what deck height you want to achieve. When finished they will inform you what distance they ended up with, with amount of stock removed noted for reference only. Thot you might like to know.