Compression Calculator

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10.5ish

 

All other things being equal on a 302 with 4:030 bore (pistons, deck height, gasket thickness), 11.5:1 CR with 64cc head ends up at 10.3:1 with 72cc head.

 

Basic math is correct....but it could be an even bigger drop if the current combo is 64cc heads, undecked stock block, and a steel shim head gasket but then a thick composition head gasket is used during the head swap.

That would drop compression a little more than another half a point. down to 9.6:1.
Gonna be pretty gutless below 4500 rpm I would think, on the street.
Will need a LOT of rear gear to be any fun.

Another thing to realize here is that with those old TRW's compression ratio rating was only achieved with a blueprinted, minimum volumes and clearances build(Virtually every TRW piston is spec'd like this)..... a typical garage type rebuild will only get 10.8 to 11.0:1 at best with those pistons.....so your final ratio of this 72cc head swap could be even lower than these initial calculations.

 

just changing to 66-68 cc aluminum heads will add power and likely not ping because of "Al."
maybe even 64cc aluminum heads. Which camshaft are you running?

 

Thanks for answers (!), for me 10.3 would be a good number, with alum heads (I have in-hand which an acquaintance passed along) and some cam duration it should be good on the street/highway w/ pump gas. I'd like to drive this thing but keep the bottom end.

I don't know the cam specs, just a "what-was-in-there" solid lift, I'll have to find a p/n on it or measure.

Will post a pic or two when the project is running.

 

If the engine is a part, now is the time to blue print it. There is 450 hp available in that engine,,,easy.
11:1 cr is ok with short stroke and aluminum heads like 190 cc profilers

 

Here you get into the problem with aluminum head’s and their preferred head gaskets. Aluminum is softer that cast iron and more thermally dimensionally active so is subject to abrasion around the fire ring. Typically aluminum uses a multi-layer gasket which is frequently thicker than that used by the factory. The thicker gasket increases chamber volume by more than the difference between 64 to 72 cc chambers.

Several other things happen as well because the thicker head gasket increases the which increases detonation tendency regardless of lower compression ratio. This can be something of a technical minefield where solving one problem creates another.

Most generally it is recommended that switching to aluminum head’s also includes decking the block so you can play with dialing in the .035 to .040 squish/quench clearance with a thicker gasket. Many head suppliers recommend a .043 inch thick composite gasket. Given an unmolested SBC starts out with .025 inch between the piston crown and the chamber step without going to FelPro’s .015 thick rubber coated shim gasket it’s hard to get the S/Q clearance in the optimal range. The next step up is their .019 shim gasket. Shim gaskets require that both block and head have really good plainer surfaces to prevent compression, oil and coolant leakage.

GM production aluminum head’s used a .053 thick gasket which with a .025 crown to deck clearance of .078 inch. They sacrificed some amount of chamber turbulence and end burn heat sink for the purposes of meeting production goals and minimizing warranty exposure. This is viable with aluminum that iron wouldn’t tolerate because aluminum moves heat much faster so runaway end burn temps that the quench cycle tamps down are not such a big problem. A note for posterity is Ford ran about a .080 clearance on the M400 engine with an iron Cleveland type head, they ran into big time ping trouble with this engine on modern gasoline. This much maligned engine can be made into a screamer if you work this issue.

So it would be helpful to know if the engine you have has a decked block and what thickness is the current head gaskets. If the block has been decked than the factory stamped codes on the right side block pad just ahead of the cylinder head would not be there as the milling operation would cut them off. gating at the thickness of the head gasket is trickier if you can find an overlap of head to block deck sufficient to get a feeler gauge in there this can be measured without pulling a head. Obviously a thirty over motor has been touched so what the factory used for gaskets may not be the same as this build.

Not knowing the year of the engine id this is a short pump older version there are aluminum head’s being sold that look like the old original cast iron head’s that lack accessory mounts on their ends. Chevy SBC Assembled Aluminum Camel Double Hump 461 Cylinder Head

Bogie

 

Thanks for the more-info!

I am pretty interested in those aluminum camel-hump cylinder heads.

 

They're not terribly impressive if you look at the airflow numbers.

Trick Flow has a aluminum "Double Hump Clone" DHC-175 head that is much better, you can get it in both early and late hump versions, no bolt holes in the ends for accessories or with bolt hole styles.

Trick Flow® DHC™ 175 Cylinder Heads for Small Block Chevrolet TFS-30210006 - TrickFlow.com

The Speedway hump heads are imports, if that matters to you. Same head is also the Flo-Tek 180 (just no humps on the outside.)

 

Also interesting. The TF site does show 60cc chambers, and not available 'til mid-May, both possible issues here.

 

I know very little about head gasket technology, but I used the Mahle 5746 .026 gaskets with Blueprint aluminum 64cc heads on my SBC crate, and it seems to be working fine. Block was GMPP original (no decking) and the heads were new.

I was trying achieve the opposite effect and get the compression up on my low compression crate engine, so maybe it’s not what you want.

 

Question: W/ 302 Chevy 4" bore 3" stroke (Z-28 motor) .030"-over, old TRW pistons rated at 11.5:1, orig. 64cc heads:

What would be the likely resulting CR of switching to aftermarket 72cc aluminum heads and no other changes?

I can work it all out and have a 50/50 of being correct (past experience, lol) or someone else could probably nail it off the top of their head. Am just trying to make something more streetable without getting into the short block.

-Loren

What is the piston to deck height.??

 

9.7 maybe