Compression Ratio

[ChrisCairns]

How could he have made an error in measuring the dish on those? It seems pretty foolproof. I have done it with a burette and a 60 cc syringe with the same result. Maybe he was thinking about someone elses pistons? Your looking good if at 9.34:1 though.

He measured the dish with a caliper. Then did some calculating.

Not the perfect method, even without math errors, but normally it will suffice.

Here's his formula he uses, if you care....

Measure half way up the curved wall of the dish from side to side.....

Then D x D x Depth x .7854 x 16.4.

I said that wasn't the correct formula for volume of a cylinder, memory harkening back a couple of years to school.

He said his formula was for the reciprocal. I left it at that.

But last night for schits and giggles I got out a syringe and without using a burrette it came to 36. Perhaps the valve reliefs are the extra, although it seems excessive. But I'm no longer worried. It's adequate.

 

[ChrisCairns]

Ya got me thinking Bison so I phoned JE and spoke to their technical dept. I gave them the part number and they told me the dish volume of that piston was 37.5. (Should have done that originally....lol.)

Using my machinist's method of Diameter times Diameter times Depth times .7854 times 16.4 it comes out to 3.08 x 3.08 x .285 x .7854 x 16.4 equals 34.82. Close enough.

Now entering that new info into the compression ratio calculator with a .050 compressed gasket thickness we get a CR of 8.54. Using .040 gasket I'll get 8.70.

Dunno if Felpro makes a thinner Lok wire gasket ... the one I WAS going to use has the .050 compressed thickness.

 

[TurboDave]

Well, at least with the 8.5 or so, you'll be able to run some pretty big boost and timing numbers. :wink:

 

[ijames]

Then D x D x Depth x .7854 x 16.4.

I said that wasn't the correct formula for volume of a cylinder, memory harkening back a couple of years to school.

He said his formula was for the reciprocal. I left it at that.

He's right about the formula, not about the reciprocal . Volume of a cylinder is pi * radius * radius * height, and since the diameter is 2 * radius and pi/4=.7854 that's the correct formula. The 16.4 is to go from cubic inches since you took the measurements in inches, to cubic centimeters. Measuring the diameter halfway up the radius is just a rough way to get an average diameter.

 

[ChrisCairns]

Thanks Carl....

My bad memory was telling me the equation was something like pi r squared times the heigth but now you 'splained it I can see it's the same.

It's always a pleasure to read your explanations.

Dave....yessiry. I'd like over 30 since 30 is the most I"ve ever run. But I'm limited by the old FAST which isn't able to use a higher than 3 bar map. At least that was told to me years ago. Perhaps something has changed by now without going to a new DFI (read expensive) box.

 

[bison]

Thanks Carl....

My bad memory was telling me the equation was something like pi r squared times the heigth but now you 'splained it I can see it's the same.

It's always a pleasure to read your explanations.

Dave....yessiry. I'd like over 30 since 30 is the most I"ve ever run. But I'm limited by the old FAST which isn't able to use a higher than 3 bar map. At least that was told to me years ago. Perhaps something has changed by now without going to a new DFI (read expensive) box.

You can go over 30 but you wont have input for boost after 30psi in the fuel map. You should be able to get an XFI box and harness cheap enough and recoup selling the old FAST. Call our bearded clam eating buddy Otto J.

 

[ChrisCairns]

Ahhhh, maybe that's what I was told.

And you also cleared up something else for me. I thought Otto had got crabs, but I see it's clams. Are they more painful? :biggrin: