95mysticta

iTrader: (4)

Hey guys im trying to figure out if my pistons when i put them into the compression calculator is + or - cc? May sound like a noobie question but not sure as the site says its a 5.00cc piston and my engine guy thats putting my recently picked up shortblock back together and wrote down the the pistons to be -5cc on specs of my engine. This makes a huge difference in compression calculating!

Here's the link to the pistons:

http://www.summitracing.com/parts/MLE-SBC250030F05/

fex77k

iTrader: (2)

My 388 I used a -13cc, in my 396 I used a -5cc. A -5cc is pretty common in 350-360 builds, its all about your rods and crank, but I don't think those would be a +5cc.

RamAir95TA

iTrader: (2)

Most compression calculators don't use the same numeric value format for piston dish/volume. Some use +, some use -. You'll have to figure out which format your particular CR calculator uses.

noice

Like said before it depends on the compression calculating program. Those pistons you linked to are flat tops so anything taken out of them makes them +cc. However I've seen flat top pistons with valve reliefs that were call -cc. fex77k above is talking about a Wiesco "flat top" piston with a 13cc relief cut into it, a 13cc relief is a whole crapload of space.

Summit is correct in calling it a +5cc piston, put it into your calculator both ways, the lower compression one is the correct one.

Also what stroke crank are you going to use, what rod length, what is the compression height, how far in the hole is the piston going to be, what thickness head gasket (compressed), and what size combustion chamber.

Edit: So I guess you are using a 6" rod and a 3.48" stroke?

95mysticta

iTrader: (4)

Bore: 4.030
Stroke: 3.48
Deck Height: 0.007
Gasket: 0.039 or .029(Will know when we check the clearances if i run the thinner gasket)
Gasket Bore: 4.125
Heads: 64cc (Contemplating milling 0.020 off to 60cc)

Im using Eagle 6" H-Beams Rods with those pistons and a stock crank.

This is the calculator im using:

http://www.kb-silvolite.com/calc.php?action=comp

Says with a +5cc piston ill be at 10.2 vs -5cc piston will put me at 11.5. I know it sounds like more of a +5cc but just double checking to be sure. More then likely im going to mill the heads to 60cc to bump it up to 10.7. Depending on when my engine guy puts the heads on the motor to check the clearances we might be able to use a thinner gasket on it bump it up to 10.9.

duh

iTrader: (16)

according to your calculator the -5cc would be a dome piston. If you're have valve reliefs and nothing else then you are at +5cc according to them...in actuallity you have a 5cc dish (which would be your valve reliefs).

quik95lt1

iTrader: (7)

wanna make it easy? put in 0 for the piston cc and just add 5cc's to your comb chamber number and you'll get the same results lol............or put in both +5 and -5.......the lower of the two numbers is your compression

1redta

what type of heads are you using? 64cc is a lot for most LT1 heads.

the stock compression is 10.4 most guys with no interest in a boosted car will run into the 11s with no worrys and on some strip car run into the 12 easily with pump gas

I have a stock crank, 6" rod, Mahle power pack bottom end with LE2 heads and the impala .029 gasket, the deck is stock. compression was 11.3-4

quik95lt1

iTrader: (7)

friend of mine just built a 12.3:1 motor and with a conservative tune it will run on pump gas and a bottle of octane booster.........and my old 355 was 11.3:1 and it ran on 93 with no issues at all...........my new motor likes the good stuff though went a little high on this one lol

JAKEJR

I've done the same thing (high-lighted) when the calculator program didn't indicate whether to use a + or -.

Either that or just find a different calculator that instructs you which way to enter the valve relief volume. There are a lot of calculators on-line to choose from.


Jake

95mysticta

iTrader: (4)

Thanks for the help guys looking like i have to take some meat off my fully ported AFRs to get the compression up.

RamAir95TA

iTrader: (2)

****, I run closer to 13.0:1 and run on pump gas just fine. It's all in the tune.

robsquikz28

iTrader: (246)

[QUOTE=95mysticta;14045129]Bore: 4.030
Stroke: 3.48
Deck Height: 0.007
Gasket: .029
Heads: 64cc (Contemplating milling 0.020 off to 60cc)

with the 64cc head I get 10.51 and if your going to mill them to 60cc I got
11.03 compression Ratio

1redta

Octane booster is a JOKE you realize when it says 5 points it means 93 to 93.5 right