Information
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 ]
Calculation
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.]
Comparison
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:
Bore:
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 ]