Deck height to Compression ratio

[cstreit]

I cannot seem to find my Bruce Anderson Book to look up the formula for calculating compression ratio...

Can someone either give me that formula or let me know how much dropping the deck height by, say, .005" would affect the compression ratio?

Thanks,

[J P Stein]

As I recall, he says 1mm (.04 in. rounded) off the deck height is worth a .5 increase. It should be a linear progression.

[ChrisBennet]

Compression ratio is not too hard to figure out with pistons that have a flat ridge around the edge like JE's.
The basic idea is to figure out the volume of the cylinder with the piston at the bottom of it's stroke (Vbdc) and divide it by the volume of the cylinder at the top (Vtdc) of it's stroke.
CR = Vbdc / Vtdc
Vtdc = Vhead - Vpiston_dome + (A * deckheight)
Vbdc = Vhead - Vpiston_dome + (A * deckheight) + (A * stroke)
A = bore area = PI * r^2 = PI * (.5 * bore dia) * (.5 * bore dia)

Let me know if you want more detail (like how to measure piston dome volume for example).
-Chris

[jgparker]

I agree that 1mm should give somewhere around .5 in compression increase, but this would change depending on your current compression ratio and stroke, and it is not linear. (Compression ratio theoretically goes to infinity as the piston to head clearance goes to zero.)

If you do not know your piston dome volume or you head volume, but do know your old compression ratio and your stroke, you can do a rough calculation of your new compression ratio as follows:

CRnew=

(Stroke X (CRold - 1))
-------------------------------------------------- +1
(Stroke - (DeltaDeckHeight X (CRold - 1)))

For a 2.7 with 70.4 mm stroke, a 8.5:1 CR and a 1mm deck height change, the new CR would be ~9.39:1.

The problem is you may not really know you current compression ratio. The correct way is to do it is with the full calculation in the previous post.

[cstreit]

Actually I was able to reverse calculate the head volume from known values of bore, stroke, compression ratio, etc...

Then took this with the known dome volume (from Andial) and was able to approximate the change in compression ration to be about .05-.1 per .1mm in height difference...

ENd result was what was important, that the small differences in my deck height from piston to piston (due to factory allowed variance in the crank) are not significant enough to affect compression ratio.

I am not able to adjust at all with differing rod lengths since my rods were re-sized and all within .025mm of each other...

[ChrisBennet]

Originally posted by cstreit
Actually I was able to reverse calculate the head volume from known values of bore, stroke, compression ratio, etc...

Then took this with the known dome volume (from Andial) and was able to approximate the change in compression ration to be about .05-.1 per .1mm in height difference...

ENd result was what was important, that the small differences in my deck height from piston to piston (due to factory allowed variance in the crank) are not significant enough to affect compression ratio.

I am not able to adjust at all with differing rod lengths since my rods were re-sized and all within .025mm of each other...

Chris,
It wouldn't effect your results in this case but be aware that stated compression ratios can be quite different than the actual (measured) ones.
-Chris (the other, slower one)

[cstreit]

Chris,

Thanks...

In this case, it might, because I used the factory compression ratio on the original SC pistons to reverse-calc the head volumes. I don't have a CC kit for the heads so I attempted it that way.

[ChrisBennet]

Originally posted by cstreit
Chris,

Thanks...

In this case, it might, because I used the factory compression ratio on the original SC pistons to reverse-calc the head volumes. I don't have a CC kit for the heads so I attempted it that way.

Hi Chris,
Wouldn't the effect on your deck height compression difference calcs be pretty small? My brain is off-line right now.
Here is what I have heard for compression ratios:
Factory (Actual) Motor
9.5:1 (9.2) 3.2L
7.0:1 (6.5) 3.3 Turbo
11.3:1 (10.4) 3.6L
-Chris

[BK911]

Be careful with reverse calculations. I measured various piston dome volumes and head volumes and reverse calculated the deck volume. In some cases I had a negative deck volume!

[911pcars]

Here's another suggestion. You could compute the effective volume of the combustion chamber by pressing the piston crown into a brick of modeling clay, "cc" the impression, then subtract from the volume of the cc'd combustion chamber.

Sherwood Lee
http://members.rennlist.org/911pcars

[ChrisBennet]

You can measure the piston dome volume indirectly for (pistons that have a ridge around the edge) by putting them in a cylinder so the top of the dome is even with the top of the cylinder. Then use a burette to measure the volume around the dome i.e. the volume that isn't the dome. Subtract the volume that isn't the dome from the calculated volume for this piston if it had no dome and you get the volume of the piston dome. I installed the top ring and used some Vaseline to seal the ring gap.
-Chris

[ChrisBennet]

In case that was clear as mud...
-Chris