[Skippy597]

Just ordered pistons and now of course I'm wondering if I should have gone for more compression.
Here's the specs

400 block zero decked 4.15 bore
Scat forged 4.25 crank
Scat H beams 6.8 rods
Ross/butler Pistons 22cc dish
Speedmaster 72cc heads (CC'd right at 72cc)
Cam is going to be a comp/butler 236/242 @ .050 roller cam

M20 trans / 3.36 gear

Should put me right at 10:1 if I calculated right. But now I'm wondering if I should have gone with 20cc and brought it up a hair. This is a hot street car that will see lots of miles so I was thinking 10:1 is safe.

The pistons were the last piece of the puzzle so I got them ordered last night. However I'm sure I can still change the order if needed.

I realize I'm splitting hairs over a few tenths but it was a thousand dollars for the pistons and want to make sure I'm right the first time.

[b-man]

Call them back and change it to 20ccs.

The set I ordered for my 421 (Ross from Butler) were supposed to be 26ccs but when they were checked by my builder they were 27.2ccs.

Either way now now’s the time, don’t regret it later.

[Skippy597]

Originally Posted by b-man

Call them back and change it to 20ccs.

The set I ordered for my 421 (Ross from Butler) were supposed to be 26ccs but when they were checked by my builder they were 27.2ccs.

Either way now now’s the time, don’t regret it later.

Alright good call, also didn't think about removing the hard edges off the pistons and chambers. I just got off the phone and they are switching them to 20cc. Very helpful people there at butler!

[AG]

Decrease the dish by 5 cc's and shoot for 10.5:1 static.

[steve25]

Agreed with the above, kind of!

It takes at least a 4 CC change to do anything positive in terms of power improvements.

A 2 CC change is is just polishing your shillelagh!

For your motor to end up with a compression of 10.5 to 1 you need a total CC volume above the piston at TDC of 99.15 CCs.

I can't see why you need a 22 CC dish.

This is what I come up with for total CC numbers which assumes zero deck clearance.

Chamber volume 72.CCs
Head gasket volume 9.4 CCs
Zero deck. 000
Valve notch volume/ flat top with 2 valve notches. 4 CCs
Ring land volume 1.5 CCs.
Total 86.9 CCs

So you only need a dish of 12.25 CCs to end up with a 10.5 to 1 ratio

If you wanted a ratio of just 10.1 your total volume needed would be 104.66 CCs, and still that's a total dish volume of only 17.76 CCs.

[mrennie]

You didn't mention what octane you plan to use. Many cars with aluminum heads and 230+ duration cams are running 10.5:1 on 91 octane.

[Speargun]

I've run several aluminum head combos around 10 - 10.5:1 on 90-93 octane without issues, but I also pick my cams to keep the DCR at, or just below, 8.3:1 to help avoid detonation.

UEM Pistons has a good calculator to figure SCR & DCR.
UEM Pistons Compression Ratio Calculator

[Skippy597]

Planning on 91 octane as that's what's most available here and we have it with no ethanol. All the calculators I used online (all vary to some degree) come up with about 10.15-10.25:1 with a 20cc dish. Figures the actual numbers are a bit lower should leave me at damn close to 10:1... I think. Dont think it's worth the slight power increase to go to 10.5:1 and worry about detonation. We do get some very hot days in the summer. I figure 10:1 is a safe but nice compromise

[steve25]

Your compression ratio is only going to be about 9.5 with a 20 cc dish.

Try this go to a compression calculation site and input 106.9 CC for the chamber size.
I get this number by your 72 CC chamber.
Your 20 cc dish.
4 CCs for valve notch .
9.4 CCs for head gasket.
1.5 CCs ring land volume.
Zero deck .

Leave blank in the formula what I listed here.

The result is 9.5 .

Now do the same with a total chamber size of 96.9 which would be for a 12 cc dish.

[Skippy597]

It is my understanding that the 20cc dish includes the valve reliefs also. The entire piston cc is 20cc or there abouts.

[Skippy597]

Using your numbers Steve I get 9.8:1 but using 102.9 assuming the total piston volume is 20cc puts me around 10.1:1. I've been using Wallace racings calculator and with putting in the head gasket info and adding additional 1.5cc for piston to ring land volume I get 10.05:1

[Skippy597]

Maybe I should drop to 18cc? I wonder if it's too late to change... Again

[Skippy597]

Just got off the phone with butler. The 20cc is the total piston volume including valve reliefs, he recommended staying with the 20cc cut and that puts me just above 10:1 with a nice bumper zone for bad gas. Very helpful guys there and I'll guess we'll see but I am glad I dropped it to 20cc instead of the original 22cc.

[Stan Weiss]

What I get.

Stan

[Dragncar]

These Pontiac builders recommend 9.9- 10.25 CR not just because of bad gas. Its for people who do not or can not stay on top of their tune.
I would get 10.5- 11-1 and just buy name brand gas from a very busy gas station and be done with it.
Myself, I would buy new style pistons and spend extra to get the new coated , metric, ported rings. Total Seal.

[Skippy597]

It's not an all out performance build so the safety margin is fine with me for the very slight power loss. Verdoro is very much enjoying his stroker build with the same cam and iron heads running at only 9.5:1. Yes, I may be sacrificing some, but not a lot.

[i82much]

do those heads have a heart-shaped chamber?

[Skippy597]

Originally Posted by i82much

do those heads have a heart-shaped chamber?

Yes, they're a modern chamber design.

[mrennie]

It is your build and you need to be comfortable with the end compression. Don't stress

My GTO motor is around 10.3:1 with KRE heads. Pulls like crazy and I have zero regrets about the compression.

When I play with dyno software, the increase in HP and TQ for 1/2 a point of compression is not significant on its own.

[Skippy597]

Honestly I probably should of insisted this morning about going to an 18cc dish but I think it's too late now. But in reality either way it's not much to be gained or lost.