How to find the compression ratio on my "Mystery Motor"?

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SlimPikins

SlimPikins

Pikins, Slim Pikins.
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A small amount of history:
I have a 440 with a date stamp of 76. When I bought the car the only thing I knew for sure about it was, it had a set of 915 heads (in unknown condition) and it was suppose to have been rebuilt (unknown date).

This week I pulled the motor out and started tearing it down. I have found that is has .030 pistons and an unknown aftermarket cam. The pistons are flat top with no valve reliefs in them. The pistons sit a ways down in the cylinder, but I'm not sure exactly how much. The 915 heads have had hardened valve seats put in them, and the valves are aftermarket but very weak.

Is there a way that I can measure the bore, stroke, depth the pistons are in the cylinders, combine that with the factory rating of 78cc the closed chamber heads are suppose to be to find out what compression ratio my motor has?

Any help would be greatly appreciated.

Slim.
 
SlimPikins said:
A small amount of history:
I have a 440 with a date stamp of 76. When I bought the car the only thing I knew for sure about it was, it had a set of 915 heads (in unknown condition) and it was suppose to have been rebuilt (unknown date).

This week I pulled the motor out and started tearing it down. I have found that is has .030 pistons and an unknown aftermarket cam. The pistons are flat top with no valve reliefs in them. The pistons sit a ways down in the cylinder, but I'm not sure exactly how much. The 915 heads have had hardened valve seats put in them, and the valves are aftermarket but very weak.

Is there a way that I can measure the bore, stroke, depth the pistons are in the cylinders, combine that with the factory rating of 78cc the closed chamber heads are suppose to be to find out what compression ratio my motor has?

Any help would be greatly appreciated.

Slim.
Click to expand...

Google... Compression ratio calculator It will ask you for all of that info, Including head gasket thickness and other things of that sort and it will do the math for you :)

Joe
 
Thanks Turbojoe - Been wanting something like that for a while. Sometimes I forget to use that Goggle search.

Ma Snart
 

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you need to know how far down the pistons are and the bore size, look for a number on top , head gasket thickness , and the head chamber CC . that will get you close enough.
 
First, find a piece of ¼” or thicker clear Plexiglas that is about 8” square.
Drill a ¼” hole approx 2” from one edge of the Plexiglas sheet in the middle.
Level your head on a bench with the combustion chamber facing up.
Clean the combustion chamber with a wire wheel of there is a lot of carbon buildup.
Put in a spark plug.
Stick the Plexiglas plate to the head over the combustion chamber using Vaseline.
Fill a large syringe (from a vet supply or pet store) with rubbing alcohol and note how much is in the syringe.
Fill the combustion chamber until there is no air left under the ‘glass.
Write down that CC amount.
Now we have to calculate the volume created by the head gasket. If you are using the typical .020” steel shim gasket, measure the approximate diameter of the gaskets bore hole (say, 4.5”).
So we multiply Pi time the radius squared times the vertical measurement to get the volume of the head gasket.
3.1416 (Pi)x5.0625 (the radius squared) =15.90435x .020” (head gasket installed thickness)=.32cubic inches, which translates to 5.244cc’s
Add that value to the combustion chamber volume.
Since (in your case) your pistons are below deck height at TDC, just slime up the “glass” plate again and put in over a cylinder that is at TDC. If the block is still in the car, just make sure the offset fill hole is at the top edge of the cylinder.
Now fill that volume with rubbing alcohol and note the amount.
Now measure your bore size.
I will use the stock numbers for example:
4.32” x 3.75”, so 3.1416 (Pi) X 4.6656 (the radius squared) = 14.657 x 3.75” (the swept volume or stroke)= 54.9654 cubic inches. This is the total volume for each cylinder. If you multiply that number times 8 cylinders, you get 439.72 cubic inches, or 440!!
54.9654 cubic inches = 900.721cc’s
The compression ratio is the difference between the total volume above the piston and the swept volume (stroke).
The typical 906 head has a combustion chamber volume of 78.5 cc, so I will use that.
78.5 cc’s plus the head gasket volume of 5.244cc’s and the volume between the piston top and deck (say 60cc’s, that’s a total of 143.744cc’s
Now we add 900.721cc to 143.744cc to get our total cylinder and head volume (1,044.465cc)
Finally you divide 143.744cc by the total cc’s (1,044.465) to get your true compression ratio: 13.76:1!
Clear as mud?
 
mhuppertz said:
First, find a piece of ¼” or thicker clear Plexiglas that is about 8” square.
Drill a ¼” hole approx 2” from one edge of the Plexiglas sheet in the middle.
Level your head on a bench with the combustion chamber facing up.
Clean the combustion chamber with a wire wheel of there is a lot of carbon buildup.
Put in a spark plug.
Stick the Plexiglas plate to the head over the combustion chamber using Vaseline.
Fill a large syringe (from a vet supply or pet store) with rubbing alcohol and note how much is in the syringe.
Fill the combustion chamber until there is no air left under the ‘glass.
Write down that CC amount.
Now we have to calculate the volume created by the head gasket. If you are using the typical .020” steel shim gasket, measure the approximate diameter of the gaskets bore hole (say, 4.5”).
So we multiply Pi time the radius squared times the vertical measurement to get the volume of the head gasket.
3.1416 (Pi)x5.0625 (the radius squared) =15.90435x .020” (head gasket installed thickness)=.32cubic inches, which translates to 5.244cc’s
Add that value to the combustion chamber volume.
Since (in your case) your pistons are below deck height at TDC, just slime up the “glass” plate again and put in over a cylinder that is at TDC. If the block is still in the car, just make sure the offset fill hole is at the top edge of the cylinder.
Now fill that volume with rubbing alcohol and note the amount.
Now measure your bore size.
I will use the stock numbers for example:
4.32” x 3.75”, so 3.1416 (Pi) X 4.6656 (the radius squared) = 14.657 x 3.75” (the swept volume or stroke)= 54.9654 cubic inches. This is the total volume for each cylinder. If you multiply that number times 8 cylinders, you get 439.72 cubic inches, or 440!!
54.9654 cubic inches = 900.721cc’s
The compression ratio is the difference between the total volume above the piston and the swept volume (stroke).
The typical 906 head has a combustion chamber volume of 78.5 cc, so I will use that.
78.5 cc’s plus the head gasket volume of 5.244cc’s and the volume between the piston top and deck (say 60cc’s, that’s a total of 143.744cc’s
Now we add 900.721cc to 143.744cc to get our total cylinder and head volume (1,044.465cc)
Finally you divide 143.744cc by the total cc’s (1,044.465) to get your true compression ratio: 13.76:1!
Clear as mud?
Click to expand...

My brain hurts but I think I get it. ;)

Thanks.
 
Just to clarify, A 906 head doesn't have a CC volume of 78, its 90-92cc, now a 915 head has a CC volume around 78cc, unless thats what you meant to say.
 
JoeDust451 said:
Just to clarify, A 906 head doesn't have a CC volume of 78, its 90-92cc, now a 915 head has a CC volume around 78cc, unless thats what you meant to say.
Click to expand...
\
Only a 915 that has been cut will be around 78 , the set I have are almost 82
 
JoeDust451 said:
Just to clarify, A 906 head doesn't have a CC volume of 78, its 90-92cc, now a 915 head has a CC volume around 78cc, unless thats what you meant to say.
Click to expand...

Yes, thanks. I used the advertized blueprint volume of the closed chambered 915 mistakenly.
:tard:
 
mhuppertz said:
Yes, thanks. I used the advertized blueprint volume of the closed chambered 915 mistakenly.
:tard:
Click to expand...

Sounds like you used NHRA blueprint specs which are no where near actual .

NHRA for a 906 is 79.5 cc , as said above 906's are 90-92 cc
 
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