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Old 10-27-2012, 06:01 PM
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Piston Question

I pulled the heads on my '74 Nova 350 today and found these pistons. The motor originally belonged to a '78 truck. I am guessing it was a rebuild that that one of the p/o's put in the car. I am trying to calculate compression ratio (ballpark) in order to get a cam that will work with this motor. I believe that these are dished pistons and I assume the "30" represents a .030 over bore. When using these online compression calculators how much - negative cc's would you guesstimate on these pistons? Also, when measuring piston to deck clearance am I correct that the raised outer ring area of the piston is the area that I need to be measuring from?


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Old 10-27-2012, 07:09 PM
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From what I see, its a 8.0:1 compression dish top piston for a 350 bored .030". Memory serves the 350 around 78 had 8.0:1 compression and made 260 hp.
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Old 10-27-2012, 07:21 PM
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Quote:
Originally Posted by bubba-21 View Post
From what I see, its a 8.0:1 compression dish top piston for a 350 bored .030". Memory serves the 350 around 78 had 8.0:1 compression and made 260 hp.
Except these are rebuilders pistons that are .020" short. So compression ratio will be less.

Yes you measure from the outer ring. You will probably find they are over .045" in the hole..
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Old 10-27-2012, 07:41 PM
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True, I didn't take that into account. If you are rebuilding it, I'd throw in a set of flat tops. They will give you a considerable gain in power.
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Old 10-27-2012, 08:04 PM
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I came up with .031 in the hole using a magnetic dial indicator. I will confirm with a straight edge and feeler gauges to be sure.

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Old 10-27-2012, 11:59 PM
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Quote:
Originally Posted by JonnyRed View Post
I came up with .031 in the hole using a magnetic dial indicator. I will confirm with a straight edge and feeler gauges to be sure.

Thanks
Never mind the feelers. Use the dial indicator to set the piston to TDC, then zero the indicator to the deck. Then measure the pistons at the flat ring like you said. Take your measurements over the wrist pin (3 and 9 o'clock as viewed from along side the engine) to remove the piston rocking in the bore from skewing the measurement.

Those pistons have a dish and they're beveled around the outer edge. This makes taking a depth measurement of the dish and using the diameter of the dish to figure the volume. You can use grease to seal the top ring land and measure the space above the piston to the deck, the same as you would measure a combustion chamber, using a piece of plexiglass or a CD to cover the hole. More on doing this on a budget: CCing cylinder heads.

Once you have that volume, add the head gasket bore and thickness along w/the cylinder head volume (measure it- published figures are almost always wrong). Then you will know what the static compression ratio REALLY is.

I use this static compression ratio calculator. Whatever calculator you use, be sure to have the right sign for a dish. Some use a negative number, others a positive number to indicate a dish.

A recent post w/more info: Wanting more power.......but not quite sure what I have to start with
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Old 10-28-2012, 01:35 AM
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Originally Posted by bubba-21 View Post
True, I didn't take that into account. If you are rebuilding it, I'd throw in a set of flat tops. They will give you a considerable gain in power.
"Throwing in a set of flat tops" isn't the fix for more power, figure out your c.r. with different piston options. Flat tops might give you what you need, but you need to consider chamber volume and other variables to make it right. Read the links Cobalt provided.
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Old 10-28-2012, 10:21 AM
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Keep in mind if you replace the pistons that rebuilder pistons also come with flat tops and you have to be careful as some sites, such as ebay, the seller will falsify the information so make sure you check the information at more than one web site. Factory 350 Pistons comp height is 1.560. Rebuilder pistons have a comp height of 1.540
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Old 10-28-2012, 10:51 AM
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Quote:
Originally Posted by cobalt327 View Post
Those pistons have a dish and they're beveled around the outer edge. This makes taking a depth measurement of the dish and using the diameter of the dish to figure the volume.
The above should read:

Those pistons have a dish and they're beveled around the outer edge. This makes taking a depth measurement of the dish and using the diameter of the dish to figure the volume difficult.
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Old 10-28-2012, 04:25 PM
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For doin' the math, figure 'em at 13 cc's.

Cylinder volume 727 cc's
Chamber volume 76 cc's
Piston crown volume 13 cc's
Piston deck volume 6.5 cc's
Steel shim head gasket 3.5 cc's
Total of all five values 826 cc's
Total of four values (less the cylinder) 99 cc's
Divide 826 by 99 and find 8.34:1 SCR

A fellow might use a cam something along these lines with such a motor.....
http://www.race-mart.com/Crane_Cams-416204.html

Last edited by techinspector1; 10-28-2012 at 04:52 PM.
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