atb

Today, before taking the last piston out of my soon-to-be stroker motor, I cc'd the piston chamber volume and the head. This is what I came up with.
The piston was brought to top dead center. I did not have a degree wheel, but I did bring the piston up in stroke to a certain distance from the top fo the cylinder (BTDC), and then brought it to the same position ATDC, then turned the crank back half way inbetween. Without the degree wheel I realize I may be off some, but I can tell you that any recipricol motion in the piston where I took my measurement was imperceptible with a depth gauge. The top of the cylinder was lightly coated with grease and sealed off with a plexi cover. A burrett was used and measured 25.4 cc's of volume. I used ATF for my measuring fluid. There was no sign of seepage under the rings after the measurement was taken.

I then lined the combustion chamber on the head with another thin film of grease, and the burrett was again used measuring 41.6 cc's of volume. The ports were checked after the measurement was taken to confirm that there was no seepage past the valves.

Lastly, the head gasket was measured. This is the only really questionable part of the calculations. The fire-ring measured 102mm across. The head gasket was the used one that came out of the motor, and the fire ring had been compressed to 1.02mm. I don't know if a smaller measurement is warrented here because the gasket wasn't compressed at the time the measurement was taken. The formula r2 x pi x h was used to calculate the head gasket volume which resulted in a calculation of 8.3 cc's for the head gasket.

Total chamber volume: 25.4 (piston) + 41.6 (head) + 8.3 = 75.3 cc's

I did not measure the cylinder volume with the piston at BDC. I went with the factory numbers of a 100mm piston and a 78.9mm stroke, and the formula set forth above, which yields a total cylinder volume of 619.7 cc's. Add the 75.3 cc's of total chamber volume to the total cylinder volume and we are left with a total volume of 695 cc's. Divide this number by the 75.3 cc's of chamber volume and you are left with a 9.23:1 C/R.

So where did the compression go? Is there something off in the measurements (particularly measuring the headgasket volume?), the math?, or is the S4 engine not truly a 10:1 C/R motor?

Thanks to Tom Middleton, Rob Fossett, and Thurston Pritchard for helping with the measuring and math for this post, we definitely were scratching our heads when all of the dust settled and we arrived at the final number.

Jon F

Assuming your volume calculations are correct, my math agrees with yours.

BC

Well those CC volumes are GREAT to know when you are getting custom pistons or rods - no matter what.

Lagavulin

If the compression ratio does come out to be 9.23:1, that would be an interesting development!

For example, that means a stock block S4 could 'safely' run 17psi of aftercooled boost without worrying about heat related detonation as the heat produced by the supercharger and compression ratio is 1073.96 versus Corky's 'max' of 1075.

By the way, the horsepower produced by an S4 with a 20 NA hp exhaust system at 17 psi comes out to be 649.43 rwhp!

ViribusUnits

Is this corrilated with the compression checks?

ie, is the pressure of the compression testing about 14.7*CR-14.7? Minus some losed due to cams, valves, and rings.

If the compression test resulted in a CR of higher than 9.something, we'd know your measurements are off.

Jim V

Adam,

A couple things come to mind; First, if you were off TDC slighly; you can "eyeball" this by looking up from the bottom and making sure the rod is centered in the bore at TDC, at this point upward rod travel is at it's max.

Second; and most important, shape of the piston top. In calculating displacement you are getting a perfect cylinder using bore and stroke for diameter and hieght. In actual operation the dome or dish of the piston is only taken into account at ONE end of the cylinder, thus +/- some cc's.

This can be easily measured using the tools you have, basically set the piston down the bore a certain amount (say 10mm), calculate the diplacement here the same as the cylinder, measure actual cc's and add/subtract from the equation.

GoRideSno

Very interesting post. I hope we find a very accurate answer. I'll be watching.

Andy K

TAREK

Lag,

Do you by any chance have an actual figure for the heat produced by the supercharger and compression ratio on your setup? I'm guessing you don't, as I'd imagine it'd be grounds to increase boost until it safely approaches 1075. But only a guess.

How does one accurately measure this temperature anyway?

Lagavulin

No, I don't have an actual recorded heat number although that would be pretty cool if somehow someone did. Of course, the heat produced by the supercharger itself would be very easy to do though.

That would be a tough to do, although I imagine someone out there has done it. I would think that it'd be possible to remove a sparkplug and replace it with some sort of measuring device, and run the engine with the other 7 operational cylinders. One then could log the various data points based upon RPM and boost.

In the meantime though, since air is a gas (..mostly oxygen and nitrogen), it MUST obey the various Gas Laws, and thus one is enabled to calculate how the charge will behave. Some of those laws are Boyles Law, Charles' Law, The Pressure Law, The General Gas Equation (..which uses the previously mentioned Gas Laws), etc.

heinrich

Is it just me or does the volume of the gasket opening seem disproportionately large at ~20% of the bore?

Normy

I wonder if you have measured the cylinder correctly...

then again, I am a layman- not any sort of expert!

One thing: 102 mm bore? I thought an un-molested S4 had a 100 mm bore....

?

N!
'85 S2 5 Speed

heinrich

I talked this over with a Nascar nut I know who has some engine building background, and here's his input:

Because this wasn't done with the engine at operating temp, there will be room for error. When the piston is at operating temperature its diameter will increase and fill the cylinder. When cold, some of the fluid used to measure the volume will fill the gap between the piston and the cylinder. A certain amount of tolerance must be given for this extra fluid. The grease used to seal the top of the cylinder also has the possibility of introducing some degree of error. Even a small coating of grease would be the equivalent of a small fraction of a cc of ATF.

When the ATF was removed from the cylinder, how did you insure that ALL of the fluid was removed? Was a light film left on the cylinder walls? A light film on the cylinder wall multiplied by 8 cylinders introduces another tolerance to be considered.

8.3 cc's is a fairly good guess but it cannot be considered accurate. The ONLY way to get an accurate measurement would be to compress the gasket to it's working thickness and measure from there.

By using the factory numbers rather than making an actual measurement at BDC you are essentially gambling that the engine was built with zero tolerances, which is not necessarily a good assumption. An actual measurement would be needed.

Tom. M

Total Volume of Bore...was calculated at 695cc......the volume of the gasket was calculated at 8.3cc....more like 1.2% of total volume...still surprising though that that small gasket can add 8cc... Imagine if you use a 2mm thick gasket on an engine rebuild...wonder how much that would drop the compression???? ;-)

We measured the gasket bore diameter..and it was 102mm....100mm was used for the bore volume calculation.... Even if we used 100mm for the gasket ..the diff would be 8.01cc vs..8.3cc..so 0.29 cc difference..or about 1/5 of a teaspoon... ;-)

So..any conspiracy theories out there...has some one check the GT engines..which were according to the factory...exactly 10.04:1 ....hmm...should I pull my heads to find out.. ;-)

Later,
Tom
midlman@rennlist.com
89GT
87S4 AT for sale

Tom. M

We actually used a calibrated burette...(remember those titrations from Chem lab..??)....so we fill the burette to the top..put the plate on the head..(small hole in plate)..and dripped the atf into the small hole until the whole area is filled with fluid....chance for error from small bubbles... afterwards...take the read off the burette for volume of atf added....

there was no seepage down past either the valves or the cylinder bore after the test was done..so pretty confident that there were no losses there...

And yes..the next step would be to drop the piston as far down as it would go..and then measure again....with the burette...

sure..there can be added volume with the grease too...but definitely less than 1mm...which we know adds at least 8cc.... anyway..lots of fun going through the process....

later,
Tom

Jim V

You also have to look at engineering tolerances, while by design it's a 10:1 engine, but each engine is going to vary.
So how much is the actual CR allowed to vary, and where?
It could be 10:1 +/- which would put some a little over, some a little under; or it could be up to 10:1 which would put a few at 10:1 but most under that by varying amounts.