00CaliforniaZ28

iTrader: (11)

Probably going to be ordering a TSP 402 on Monday, and Im trying to do my final build in my head and tweaking the CR. I just wanted to make sure Im doing the math right...

Stroke 4.00"
Bore: 4.010"
Pistons: -10cc
Deck Clearance: .007 out (I cant find specs on TSPs exact number, but I figure that would be on the safe side as an estimate)
Head gasket thickness: .051
CC Volume: 62cc

= ~11.2:1, yeah?

Im stuck with 91 octane here in California, so Im even thinking of going to a -15cc piston to keep the CR a bit safer since its going to be my DD as well and I need reliability, and its not a max effort build. I could then go with a .040 gasket, and be just a tad under 10.9:1, which would seem a bit safer.

And that route also leaves me in a good position to go FI down the road when its not my driver. I can swap out the 62cc heads for some 72cc's and be in the mid to high 9s to push some good boost through.

Anyway, what are your guys thoughts on what CR I should be shooting for on 91 octane and on the safe side?

JimMueller

iTrader: (9)

Assuming a gasket bore of 4.010 and not including top ring land clearance, I'm getting 11.2:1, and quench being .044. If you switch to a 15cc dish and a .040 gasket, I show 10.88 and .033 quench - the latter of which is dangerously low. Of the combinations you mentioned, 15cc with a .051 gasket would be 10.6/0.44 quench, that would be my personal preference with 91 octane.

With the 15cc/.040 setup and assuming a 6.125 rod, keep the advertised IVC at 65+ if you want your DCR below 8.6. For the 10cc/.051 setup and a 6.125 rod, keep the advertised IVC at 71+ to keep your DCR below 8.6. With 10.6/0.44, keep the adv IVC at 64+ if you want to keep DCR <8.6.

I'll let the boost guys advise on preferred boost SCR... I thought you get by with a few pounds of boost in the low 10's, but more than that you wanted mid 9's.

00CaliforniaZ28

iTrader: (11)

I guess I can split the difference and go with a .045 gasket and -15cc pistons. I get a SCR of 10.75, and a quench of .038. Seems like thats a fair compromise? And thank you for the reply!

Nimitz87

iTrader: (17)

stock the pistons are coming out of the hole so your deck height should be negative.

I'm assuming your using the GM MLS gaskets which have a gasket bore of 4.060 iirc.

that puts you at 11.02:1

72cc heads (317's) would put you right at 9.93:1

I'm using the DCR calculator by pat kelly found in the stickies.

Jim why is .033 quench "dangerously low" ?

Chad

JimMueller

iTrader: (9)

It depends upon your parts combination and how high you're going to spin the motor. Metals expand when heated, and also grow a tiny bit at high RPMs (say, 7000+?) and you don't want your valve cheating on you with your piston Most recommend keeping the quench/squish between .035-.040.

Here's a good thread.

DVS99TRANS

iTrader: (7)

I'm pushing past 11-1 and doing fine on 90 octane with my 408 (-2cc Wiseco), .045 MLS gaskets and undecked 5.3 heads Suprised it isn't knocking on me, but if your thinking about boost later on. Definitely get the lower compression now before you get the urge to go FI like me. It may hurt your NA # a little, but not when you add a charger to it though

Buddy of mine had an identical setup to my car and only made less than 35rwhp difference with 9.6-1 cr on his 402.

00CaliforniaZ28

iTrader: (11)

Thanks guys, I ended up ordering the -15cc pistons. This way I can just mill the heads to 60cc to get my SCR at 11:1, and then if so desired down the road, I can swap out for some 72cc heads and be at 9.5:1 for some decent boost.

Now the hardest part.... waiting! Hopeully itll only be about 8 weeks or so!