I'm getting my build a little more solidified, and I just wanted to bounce my planned parts list off of my fellow TGOers.

Here's where I'm at:

L98 Block .040 over
Scat 9000 Pro Lightweight 3.75" stroke
Eagle SIR rods 6"
Mahle Powerpak flat top pistons (5cc)
LT4 HOTcam
AFR 195cc heads (67cc)
Fel-Pro 1010 (.039" compressed, 4.166 bore)
Deck height .010"

I'm still quite the novice at engine building, only having one pretty textbook 355 build under my belt. So I don't know if deck height changes relative to the piston. Is it safe to assume that the Mahles will sit the same .010" in the hole as my Speed Pro hypers did?

If deck height remains the same, then that leaves me with .049" quench, and 10.5:1 SCR. Which seems pretty high, although I am using an aluminum head. What I'd really like is for someone to comment on my DCR and also double check that I'm using the right values. As based on my reading, my DCR seems manageable on pump gas.

Here's what I'm inputting into the calculator:

8 cylinders
4.040" bore
3.75" stroke
67cc Combustion chamber
.039" gasket thickness
4.166" bore
.010" deck height
5cc dish

Intake Closing Angle of 68.5 (LT4 HOTcam)

For a calculated SCR of 10.51:1
and a DCR of 8.14

If I zero deck the block, I come up with a 10.75 SCR and 8.34 DCR. The 10.7 seems quite high for pump gas, but the DCR comes out pretty reasonable. Any one successfully run such high CR with a similar combo?

Thanks for any tips or advice!

 

No. Not about whatever it was you thought you were asking about; nor ANYTHING ELSE.

IMO that's too much compression for that teeny little stock cam. Especially with NO quench like that, at .049".

But, that all depends on what your deck height ACTUALLY is, and what the chamber cc's ACTUALLY are. Those can vary quite a bit from "spec". If you're going to play the skate-on-the-edge-of-disaster gamer with your compression, you need to have some good, solid, MEASURED, actual numbers; not "assumptions" or catalog specs.

 

Well they are MEASURED numbers. With my Speed Pros I had an actual .010" deck height, and my AFR 195s measured out at an actual 67ccs. I just wanted to know whether the deck height would change with different pistons. So what you're saying is that deck height can vary between different brands "flat-top" pistons?

 

If the piston pin height is different, then obviously the deck height would change. The mahle pistons are probably .020" higher in the hole then speed-pros. Make sure to measure it.
10.5 is pretty high, but with AL heads it's very close to acceptable.
8.14DCR? You sure you have that right?

I have 10:1 with a DCR of 8.3:1, and i'm using iron heads. With 92 octane and full timing i'm on the safe edge, at 4000' elevation here.

I'd say measure the deck height with your new pistons before doing any math. Then try to stay in the low 8's for DCR and you should be ok with premium fuel.

 

But of course. There's no "law" that says they all have to be the same.

Or for that matter, even from one "flat-top" piston to another, within the same brand; depending on the purpose. In the simplest case, Chevy 302, 327, 350, & 383 pistons are all 4" bore (or overbores of it), identical in every way, only with different pin heights. With "custom" pistons, you can have them drill the pin hole ANYWHERE you want.

You gotta watch out for the "specs"; some of the piston mfrs (*cough* TRW* cough*) will give you a "spec" that ISN'T for the actual piston they put in the box, but rather is the "stock" spec for the OEM motor. So you buy a piston that says it's 1.56", but you get it and discover it's REALLY 1.54" .... .020" farther down in the hole. The rationale being, if this is a "replacement" situation and the block has been decked or align-bored, any error that occurs will be less catastrophic if it results in lower compression, than if pistons start crashing into the heads.

 

I can't believe the guys at the machine shop didn't mention the deck height would change with the compression height. I guess I should have known, but I've never built a stroked motor, and this is only my 2nd SBC build so I've never really encountered it. I don't understand why they didn't mention that the deck height would change the whole time I was telling them I wanted to get in the .040" quench range and wanted zero deck height.

Not exactly sure, but I can't see anything wrong with the figures, and all the numbers were crunched through a couple of different calculators. I've seen other HOTcam builds on CZ28.com with similar SCR & DCR relationships, so the best I can figure the HOTcam is just a lot wilder of a cam where it counts in DCR than most people give it credit for.

It's a moot point now I suppose. I talked with the machine shop about decking the block as I'd decided to go 3.480/5.7" with 1.550" compression height Mahles which would have put me .020" in the hole vs my old Speed Pros. After some discussion, I changed that to a 3.5" stroke using a Eagle forged 1-pc crank. That should put me at zero deck height, and just leave a 1010 gasket for quench. Compression works out to 10:1 SCR and a 7.8:1 DCR. Passed a gas station today, and saw 93 is up to $2.75. Wonder if it'll run on 87...

 

My 388 with 62cc fast burn alum heads/1.25" Compression Height -16cc dished JE forged alum pistons .002 in the hole/ Zero Deck/.043" quench/SCR=10.2:1/DCR=8.36:1 runs on 87 octane with no problems.