Piston Selection & DCR

[Its Just Me]

Here's a cold winter day exercise for anyone...

 

I'm trying to "maximize" compression for 87-89 octane fuel.

 

Here is what I know:

-0.030 over Mk IV 454 = 4.280" bore

-4.00 stroke

-6.135" "thumb" rods

-0.042" thick head gasket

-Piston head is 0.012" below deck with my existing 1.645" compression distance pistons (these pistons will be replaced with 1.640" compression distance pistons. I assume I lose 0.005" with this change. Am I thinking correctly?)

-Minimum 124cc combustion chamber (124-127 measured w/124.5 ave) on 781 cast iron heads, stock valve configuration.

 

-The cam I'm using is a GMPP #12366543 hydraulic roller (ZZ502 crate engine). Lift=.527I/.544E, Duration @ .050" = 224I/234E, and 110d lobe seperation, Max RPM = 5800.

 

-I've put a degree wheel on the engine and determined the intake valve closes (0.006") at 66d after BDC, and Intake C/L is 99d after TDC. Do these numbers look OK? I've never done this before, but they seem reasonable (GM doesn't publish the valve timing ie. cam card, so I'm kinda on my own here.)

 

What I'm trying to do is determine how much piston dome I can run, based on Dynamic Compression ratio. I'm looking at either a 22cc dome or a 30cc dome (Speed-Pro Hypereutectic).

 

With this data I compute:

-w/22cc domes: 9.08:1 Static and 7.04:1 Dynamic

-w/30cc domes: 9.68:1 Static and 7.51:1 Dynamic

I've read not to exceed 7.7:1 DCR on 87-89 octane with cast iron heads.

 

Misc supporting info: '71 Monte Carlo SS454, TH400, 2400 stall, 3.31 posi, A/C, Morel street-perf hyd roller lifters, Comp Ultra Pro Magnum roller rockers, Edelbrock RPM air-gap intake, 750 vac secondary holley, MSD HEI street fire distrubutor, 1 3/4" headers, 2 1/2" dual exhaust with h-pipe, flowmaster 50 series mufflers & bumper exit. Eventually looking at an overdrive trans & 3.73/3.55 gears. For street performance and long distance traveling, no track time.

 

I'm leaning toward the 30cc domes but don't want to be missing something and end up with detonation issues. Does anyone see anything wrong with my numbers? Am I missing anything? Any questions back at me?

 

Thanks

Scott

[Sam (Bones)]

I would try to keep your quench distance around .040, by your numbers now and the head gasket you are at approx .059, I personally would use a thinner head gasket or get the block decked

[Mike 57]

What Sam Said!

 

Then go with the smaller domes

[Sam (Bones)]

what Mike said about the smaller domes

 

and with a the 4" stroke of a 454 I would step up to a forged piston too

 

that is pretty close to my current engine and I can tell you it does not like 87-9 octane, it likes and wants 93 octane and I have a lower DCR than your specs, but my quench is around .065 (I didn't build this engine) what I do know is I have...

L2399F .030 forged w/13.8 cc dome

049 heads w/ 110cc chambers

Comp thumper cam 227 & 241 @ .050, 107 lobe seperation

it's a perfect little street engine for what I do with this car now, no more racing, just drive and enjoy it

[Its Just Me]

Thanks for the help guys.

 

I've heard of "quench" before, but never really read up on it. Very interesting and it sound like cheap horsepower while improving efficiency. Win-Win. I've always said an efficient engine will make power, whether you want it to or not.

I like the idea of the smaller domed pistons (22cc) and a thinner head gasket (.027"). This puts me in the .044" quench range and 7.3:1 DCR. I can live with that. It also gives me a bit of a safety factor for the lesser quality gas you find from time to time.

 

Sam, I appreciate your suggestion of forged pistons, and if this was a complete build, they would be a no brainer. But this block was bored for the tighter clearance HE pistons less than 5000 miles ago (by others) and they check out perfectly for HE replacements. The additional cost of the forged pistons and the associated bore/hone to open the clearance would be another step toward divorce court. This project started out as a simple roller cam install/upgrade .

 

Scott

[Sam (Bones)]

darn that snowball

 

and why is divorce so expensive???? (trust me I know)

 

because it's worth it!!!!

[wallaby]

I went through the same process with my engine, and it's tough to find something between 8.5 and 12 to-one compression setup.

The quench is an important aspect of the formula. Keeping the quench area tight helps promote turbulence and eliminates a lot of detonation problems.

Your gut reaction to a detonation problem might be to install thicker head gaskets to reduce the compression, but a thinner gasket will reduce the quench area and may do more for detonation problems than a compression reduction.

 

I had 119cc heads with 19cc (I think) dome pistons. I was just under 9.5 cr static. I never had an issue running pump gas and it was ok with 87 octane for everyday use.

[Its Just Me]

Well, I ordered the 22cc pistons and moly rings today. I also decided to have the block decked to acheive .042" quench. I first considered a thinner head gasket, but the decking seems to be a better long term option. This way, I can run the cheaper, standard thickness head gaskets.

Decking the block will run $135 one time. The thinner head gaskets are $115 every time.

 

Well, I'm off to my shop now to pull the crank out of the block. I'm sending it off for decking and a hone job on Monday. Thank goodness for the long winter...wow, did I just say that?

 

Thanks for the help, I feel good about this route.

Scott

[Sam (Bones)]

wise decision