Compression Ratio

[cmcfaul]

So the pistons I got from JE have significantly more metal on top. I suspect this will adversely up my CR.

Based on the following and a head volume of 67 ML (cc's) can anyone do the math and tell me what my CR will likely be. Its an otherwise stock 73 E with 2.4 heads

Many Thanks,

Chris
73 911 E

[cmcfaul]

[HawgRyder]

Easiest way I ever found was to screw in a fluid filled burrette into the plug hole (used coloured alcohol) and run the piston through one complete stroke with the rockers not attached.
This makes the fluid fill/empty in the tube and if it's graduated in cc's ... you subtract the little number from the big one to get the swept volume...and thereby the true compression ratio.
Any other method does not take into account various small irregularities (tulip valves/valve head shape/etc) into the math.
Bob

[Walt Fricke]

What's your deck height? That's the distance the circumference of the piston top is below the cylinder top at TDC. You need to know that (and you can just measure it doing a trial assembly of one cylinder) to calculate CR if you don't have a burette handy (they are more expensive than one might think, at least glass ones are). You should do this measurement with the cylinder base copper gaskets in place, and the cylinder reasonably snugged down, using some cylindrical spacers to nuts on the studs so they can press the cylinder down.

You know your cylinder diameter (84mm nominal).

I believe J&E has given you the volume of its dome, as cut with those pockets: 28.5cc.

You should be able to find the nominal value of the volume of your heads and use that as a reasonable approximation. Our moderator John Cramer once posted this as 68ccs if they are stock '73s.

Then you just do the math. Which you can easily look up - it is in most of the books on engine rebuilding, and not even specific to 911s.

I might try to do it for you (it is really simple), but it needs deck height, which you haven't specified. If you don't know how to measure that, search this forum for it - there are plenty of descriptions and tips out there.

When I have done this with a burette, I used a shortcut. Assemble a piston and cylinder and put at TDC. Put an assembled head on and torque the nuts some. On the engine stand, make the spark plug hole point as straight up as you can make it. To make things easier, I busted the center out of a spark plug, and epoxied a piece of fairly transparent plastic tubing into the plug, and screwed it into the engine. Fill with Marvel Mystery Oil (or some other fluid of your choice) until it starts to rise in the tube. Rotate the engine some to see if it burps some trapped air. Mark the fluid level on the tube. Drain the oil (or just rotate the crank some so you can drain the oil when you take the head off). Block the base of your spark plug, and measure how much fluid it takes to fill to your mark. This you subtract from the total fill volume. That is your combustion volume directly measured.

Then the formula is simple: displacement in ccs plus combustion volume divided by combustion volume = compression ratio.

The textbook method is a bit different. With piston and cylinder installed, lower the piston until the top of the piston is right at the top of the cylinder (or below it) and measure that distance down. Measure the fluid volume needed to fill the cylinder to the top. Calculate the volume down to that point if the cylinder wasn't there, and subtract the measured volume from it. That gives you the volume of the piston dome (the 28.5ccs).

Then put a head upside down with a spark plug in it (and valves, of course) and fill it with fluid up to the mating surface. This is usually done with a clear plastic circle plopped down on the mating surface, and sealed with a little grease before it is pressed down. It has a small hole in its middle where you drip the fluid in, and you can watch for and deal with bubbles if they appear. That gives you your actual cylinder head volume, which should be close to the factory figure, but more accurate.

Of course, if you are anal you will do this for all heads, as they can vary a bit. Not likely to be useful for a street motor, as you aren't going to be adjusting for this by taking metal off of individual heads or pistons to make the volumes exactly equal to the least common denominator, and then having special base gaskets to adjust the CR up to what you want (if needed) by changing the deck heights.

At this point you can use one of the standard formulae for CR which use bore, stroke, deck height, piston dome volume, and cylinder head volume to calculate how much volume there is at BDC, and divide that by how much volume there is left at TDC to give CR.

[MBruns]

Chris, if your head vol. is 67cc, dome is 28.5, 3.307 bore, 2.772 stroke (70.4) with a neg. deck of .030 in 10.13, .035in 9.99, .040in 9.84
Based on your info , Mike Bruns

[cmcfaul]

plugged in the numbers and came up with a CR of 9.86/1. A bit on the high side for a single plug but the cams I have are the DC 30's (E cam with more lift). These have more overlap and thus reduce compression a little while increasing VE. So should be a perfect combination.

Super Unleaded gas only, cold plugs and I am thinking of buying octane booster by the case just to be on the safe side.

Thinking I will make 200HP which will be fun.

Chris

73 911 E

Deck height is 1 mm
Stroke is 70.4

[Eagledriver]

Originally Posted by cmcfaul

plugged in the numbers and came up with a CR of 9.86/1. A bit on the high side for a single plug but the cams I have are the DC 30's (E cam with more lift). These have more overlap and thus reduce compression a little while increasing VE. So should be a perfect combination.

I believe this is a common misconception. If the VE is increased then the cylinder fills more and the compression ratio is increased (over what you would call "dynamic compression ratio"). If an engine achieves a VE of 100 percent, then it's dynamic compression ratio is equal to it's static compression ratio (9.86 in your case). I would use a little more deck height or have a little aluminum machined off the top of the pistons.

-Andy

[HawgRyder]

Some top rings need pressure to seal completely (or close to it).
Dykes type rings are like that.
Also...some pistons have small holes drilled from the top down into the back side of the top compression ring land for this reason.
If you take that into account...I think Andy is correct...the static compression might be within the limits for the octane used...but the running ratio goes higher...and you run the risk of knock or pre-ignition under those circumstances.
Bob

[cmcfaul]

Crap, was hoping the whole VE thing resulted in a net decrease in compression. That is what John Dogourty of Dougherty Racing Cams told me.

"It will decrease the cranking compression a little, and the calculated dynamic compression ratio. But, the VE will go up a little. This means more torque and HP from better cylinder filling. "

[cmcfaul]

[HawgRyder]

I remember an experiment by Bill Elliot (NASCAR) years ago...he had the piece of the piston in between the first and second ring land machined in a sort of saw tooth design.
The idea was to build up a pressure wave to help support the top ring and prevent it from twisting (so that the face of the ring stayed in better contact across its surface).
Also..he bumped the compression ratio up near 17:1 in the engine (incredible!)
The end result according to him (and his engine builder) was...the pressure wave from the mod leaked up around the first ring slightly...and created a rolling? wave front to prevent the charge from reaching the cylinder wall all around (360 deg I guess) and this prevented detonation from bouncing off the wall?

The mod was banned about 1/2 way through the season as being a cheat or something....but it is interesting.
Bob

[PFM]

Chris,

The compression ratio a motor can live with is not directly related to the number of plugs per cylinder. Bob was talking about a motor with 17 to 1 compression ratio, that motor had a single plug per cylinder. For the Porsche Hemi head the second plug was to help light the " other half" of the cylinder. Look at you piston and how it fits the head, a single plug near the intake leaves the exhaust side to fend for it's self. A lower compression motor has a more open chamber and does not benefit as much from a second spark source. If you search peanut chamber you will find that Henry welded up the heads to increase compression while using a single plug.

If octane for the higher compression is an issue consider setting your motor up to run on E85 if available in your area.