Compression Ratio Information / Calculation / Comparison

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The change in horsepower due to the change in compression ratio is relative but not directly proportional. That is to say that a change from 8:1 to 9:1 will give you a larger increase than would the change from 13:1 to 14:1. I seem to recall for every one point change around 7:1, that is the change from 7:1 to 8:1, would be slightly more than a 3% power increase. Once you get up to around 13:1, that same one point change is only good for about a 1.5% power increase.

The rule of thumb for the compression ratios run in most street engines is: for every point change in the compression ratio your power output will change by 2%. Using this rule of thumb on an engine that produces 400 hp, every 1 point change in compression ratio will result in approx. a 8hp change in output.

One thing that you have to remember is that this is a static model. The only variable changing is the compression ratio. Most of the time when a compression ratio change is made, significant other factors are changed which can significantly affect power output.

On avaliable pump gasoline it probably could be argued that your power might actually increase. This would be true if your compression ratio were high enough to force the use of a retarded timing curve (due to pre-ignition).

The TRW L2323F forged piston which is rated at 10.25:1 comes out much less if you were to actually compute the mechanical compression ratio. Two factors that reduce the mechanical ratio are the as follows:

1. The steel shim head gaskets Olds used had a compressed thickness of only .017". The common Felpro head gasket is .043".

2. When you have a valve job performed, the valves have their margin reduced, reducing their slight protrusion into the combustion chamber. Valve seats are slightly recessed into the head by the grinder. Both of these increase the head's combustion chamber. If any or all seats are replaced, this could go either way.

These two factors can increase your combustion chamber volumn by 5 to 7 cc's. This is enough to significantly change the compression ratio.

Consider the use of the car when determining the compression ratio. Your camshaft profile probably has the most significant impact on what mechanical compression ratio you should run. A longer duration camshaft will allow you to use higher mechanical compression ratio pistons because it lowers the effective compression ratio by keeping one or both valves open slightly into the compression stroke.

Be careful about the effects of production tolerances on compression ratio. Simply selecting a set of pistons labeled as 9.0:1 is not enough - you have to take into account the real combustion chamber volume, head gasket thickness, piston dish volume, and piston deck height. In reality, these dimensions are usually on the large side, resulting in less than the advertised compression ratio. While this is certainly safe from a detonation standpoint, it is not particularly healthy for performance.

The bottom line is that during any quality engine rebuild, it pays to take the time to check all of the factory dimensions. You may decide that it's not worth the time and expense to correct discrepancies, however at least you'll make that decision consciously.

[ Thanks to GABowles, Joe Padavano for this informatiton ]


Put the head on something so you can move it around, V- type stands work well. Install a spark plug and tilt head so you have high side. Make a cc'ing plate out of a piece of clear plastic by drilling a ¼" or so hole in the plastic. Put a thin coat of grease on the head or piston to be cc'd. Smush the plate onto the head or piston, with the hole at one edge of the chamber. Measure how much fluid will fill the chamber. Water with some dish soap helps to break the surface tenson of the water. The air will self purge as you fill the chamber. Sometimes you have to tap the head a little to get rid of the bubbles.

A Burette is probably the nicest setup. A 60cc, or a 20cc syringe for pistons with a small dish, can be used with good results. An accurate turkey-baster would work just fine, graduated in cc's, or you can convert to cc's.

Elevate the hole, so it's at the top of the chamber, and the water [or oil or whatever] will drain away. Water can be used on the bench-done parts, and engine oil when cc'ing the total clearance volume on the assembled engine.

Typical Olds 425-HC-T Engine CRatio specifications:
4.125" Bore
3.975" Stroke
0.015" Deck clearance [piston top is this far below block's deck surface]
4.250" Gasket hole diameter
0.045" Gasket thickness when installed
2.750" Piston dish volume
0.045" Piston dish depth
10.5:1 Stock CRatio.

[ Thanks to Chris Witt, Walter for this information ]

Volume Formulas

Changed cc's to cubic inches by dividing by 2.54^3. Then, using CR=BDCV/TDCV, where
BDCV = Volume at BDC, = TDCV + piston displacement
TDCV = Volume at TDC, = amount of oil required to fill the combustion chamber at TDC, as measured per above.
piston displacement = pi/4 * bore^2 * stroke

pi = 3.14159
Bore = 4.125"
Stroke = 4.250"
1 ml = 1 cc

From the above, we can easily calculate:

cc       in^3     What
6984.0   425.00   Total engine displacement
 870.5    53.10   Cylinder displacement [piston swept volume]
   4.4      .27   Piston dish volume
   3.3      .20   Deck clearance volume
  10.5      .64   Gasket hole volume [stock is more like 4-5cc]
  18.1     1.11   Total Clearance Volume, except head
  98.1     5.99   Total Clearance Volume, including std. 80cc head

Which yields a CRatio of 9.87 with an 80cc head, or 10.78 with a head shaved 0.045" [head loses 1.01cc per .005" cut]

Note: each cc of volume in the Clearance Volume affects the CRatio about 0.1 point at these numbers [bore, stroke, etc.]


An overbore of .030" or .060" makes a difference of up to about 3% in the cylinder displacement, which is practically negligible in the grand scheme of things.

Amount to cut the head in order to achieve the stock advertised compression ratio:

Std    .030   .060   C/R     Engine
.032   .026   .019   10.5    425HC Toro/SF pistons, 80cc heads
.033   .026   .020   10.25   425HC Std. 455 HC pistons, 80cc heads
.040   .033   .026   10.25   455HC Std. HC pistons, 80cc heads
.090   .082   .074    9.00   455LC Std. LC pistons, 80cc heads
.152   .147   .142   10.25   350HC Std. HC pistons, #8 [79cc] heads
.078   .073   .067   10.25   350HC Same pistons, #5-6-7 [64cc] heads
.134   Why bother?    9.00   350LC std. LC pistons, #8 [80cc] heads
.060   .053   .047    9.00   350LC same pistons, #5-6-7 [64cc] heads
.047   .042   .036   10.25   330HC HC pistons, 64cc heads
.061   .055   .049    9.00   330LC LC pistons, 64cc heads
.037   .030   .024   10.5    E400HC dishless flattop pistons, 80cc heads
                             Early 400 engine, 4.000 bore x 3.975 stroke
.049   .042   .036   10.25   L400 HC pistons, 80cc heads
.051   .043   .035    9.00   L400 LC pistons, 80cc heads

Well, after some careful measuring it was discovered that the 425 Toro/Starfire engine had higher compression pistons than the std. 425 HC. At first glance they appear identical, but the T/S pistons have a slightly shallower dish, like .040" rather than .060", which cc'd out to 4.5cc for the T/S pistons and 7cc for the lesser-car pistons. Each cc is about 0.1 on the compression ratio, so the Toronado/ Starfire pistons offer about 0.25 more compression ratio, all else equal.

Rule of Thumb:
Basically, to get the 'advertised' CRatio with today's .045" gasket, you have to cut the heads about .050". Since the gasket is about .025 or .030" taller, you are really only moving the heads .020 or .025" closer to the block- hardly worth milling the intake/head face to match.

Also, there's no way to get even a 9:1 CRatio with #8 heads on a 350. In fact, with low-comp pistons, the CR calculates out to about 7.5:1 !!! For a 9:1, you'd have to mill a ridiculous amount, like 0.125", off the heads. Yuck.

[ Thanks to Chris Witt for this information ]

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