The Official 356Talk Forum

I'm at that point where you have to estimate and verify CR.

I'm using SC heads with normal pistons/cylinders. I'll check piston to head clearance although it shouldn't be a problem since the pistons have no domes. I've got a 50 cc burette and plan to dump 50 cc via funnel and then drizzle in the rest with the burette to figure the squish cc's.

Question is what fluid to use. Pellow in Secrets says to use automatic transmission fluid. Any other recommendations?

I use alcohol, it has a thinner minuscus, easy to clean up. Look at Ron LaDow's Web site for his tool, makes it much easier.

I use ATF. I also measure/calculate the CR with the heads off: Measure the volume in the head with a plexiglass disk and burette. Measure the piston crown volume with a piston (without rings) and a cylinder using the disk and a digital caliper. Masking tape + grease will seal the piston cylinder gap. Measure the deck height with the cylinder in the engine. Then just calculate the TDC head volume.
Easier and less messy for me, and you want to equalize the head volumes and check the valve job for leaks anyway.

Harlan Halsey wrote: ↑Sun Jul 28, 2019 6:16 am I use ATF. I also measure/calculate the CR with the heads off: Measure the volume in the head with a plexiglass disk and burette. Measure the piston crown volume with a piston (without rings) and a cylinder using the disk and a digital caliper. Masking tape + grease will seal the piston cylinder gap. Measure the deck height with the cylinder in the engine. Then just calculate the TDC head volume.
Easier and less messy for me, and you want to equalize the head volumes and check the valve job for leaks anyway.

I'm not sure about the masking tape but otherwise this is a good method. I have the rings on with a very tiny amount of grease and the piston crown touching a straight edge across the top of the cylinder. Then measure the height of the piston at TDC and subtract that volume difference.

Whatever compression ratio you choose must be matched to the camshaft that you choose. The cam determines where in the rpm range the engine works best. You decide where you want the power knowing that the engine will work less well at other rpm. More duration equals higher rpm before the peak power is made. More duration requires more compression because the intake valve closes later and compression starts later in the stroke. Short duration cams must have less compression or they detonate. Engine builders must have a plan before buying parts.

Thanks for replies.

Pellow goes through some mind boggling analysis that starts with Vh (head volume - the number impressed on the head or measured using Ron LaDow's excellent tool set), Vs (stroke volume - easily calculated), Vd (dome volume which he provides based on some hard to verify math, stock engine data and assumptions), Vc (clearance volume - volume between top of Vs and Vh - although in another ref, calls it the volume between the piston and head when at TDC - Pellow backs into this number after defining all other V's). His calculations provide for intrusion of dome through Vc into Vh. All this he uses to estimate CR and determine a first crack at determining needed shims. Then after doing the clay or solder on head and cranking engine over to determine that there will be no contact, he uses the fill the combustion chamber at TDC with ATF to calculate the actual CR and then decide to reshim as necessary!

He then does some analysis of the effects of fly-cutting heads.

His data, of course, all precedes his passing, in the pre-ethanol days, so leaves modern day CR's up to the bewildered. Any real-world recommendations? My camshaft is the stock C cam that came with engine. So SC heads, C pistons/cylinders, C cam ...

The problem I have with Harry's data regards big-bore dome volumes; he never measured them, simply assumed that they were the same as the 912 parts, since that was what NPR copied. Except the dome changed diameter with the bore size; 82.5mm to 86mm. Pretty sizable difference.
Anyhow, here's a link to my CR article from years back. AFAIK, it is still valid:
https://precisionmatters.biz/pdf/compression-ratio.pdf
BTW, you'll be perfectly safe with 9:1CR and if you are careful to make all of them 9.25:1 (not one at 9.4:1) you should be good also.

Several knowledgeable people already commented, and I am especially interested in Cliff's comment about cams. I have four questions:
QUESTION 1: Why did Porsche use much lower Compression Ratio on the 1600 normals? If they had increased that (e.g. by reshaping the piston head) would they not have increased performance? With the normal cam would that cause a problem (like knock?). Seems like no cost difference, so why settle for lower HP or torque? There must be a reason.

QUESTION 2: I ask because I am rebuilding a B normal with AA 1720 P&C (22 degree) and upon assembly I come up with a rather high compression ratio of 9.2. With the normal cam and carbs will that cause a problem?

Details: Heads beautifully machined and assembled with new valves by Dave Bonbright. Assembled to case with single 1/2mm cu spacer. Measured closest piston to head gap (using soft solder pieces) is 1.2mm. Measured head volume 58cc. Measured taper and pocket volume at piston flush with cylinder top = 9.5cc. Piston height above cylinder top at TDC = .26 cm

Now CR= (swept volume + TDC volume)/TDC volume . And swept volume = 8.6 x 8.6 x pi/4 = 429cc. Now TDC volume = head volume + pocket volume - volume swept in moving piston from flush to TDC. The latter is just cylinder area times 0.26= 15 cc. TDC volume = 58 + 9.5 - 15 = 52.5 and CR = 9.2. If error estimate is +/- 1cc, CR range is 9 to 9.3.

This seems a bit high considering the factory chose 7.5. What do you think?

QUESTION 3: If I add another 1/2 mm washer, it will drop the CR to nominally to 8.7 but of course increase the minimum piston to head gap to 1.7mm. Is that too much gap?

I am going to remeasure the piston-cylinder gap, this time with the valve train in place (major PITA) just to be sure about exhaust valve clearance. Any tips on doing this besides the soft solder technique? (I have trouble measuring the dimensions of squeezed clay, so I use solder.... bad or good idea?) [this is QUESTION 4}

Thanks for your advice guys, I think we are so lucky to have folks with your talents paying attention to our questions.
Bill

Some good questions Bill. Looking forward to hear comments.

Cheers,
Bruce

Bill,
I have nearly zero experience with 'early' engines, but have come to the conclusion that an engine's C/R is far from an independent variable.
Just posted this over on the 912 board to address some questions there. Not sure it helps your questions.
https://www.912bbs.org/forum/threads/wh ... ers.56252/

Bill; question 1: lower compression maybe because of some pretty low octane fuel in Europe?
Where did you find the 1/2mm copper shims? I have only seen the standard 1/4mm and full 1mm available.

Bill Oldham wrote: ↑Sat Nov 30, 2019 9:56 pm QUESTION 1: Why did Porsche use much lower Compression Ratio on the 1600 normals? If they had increased that (e.g. by reshaping the piston head) would they not have increased performance? With the normal cam would that cause a problem (like knock?). Seems like no cost difference, so why settle for lower HP or torque? There must be a reason.

Porsche would have done load tests on their dyno to determine what was "safe" as a CR. When a cam has shorter duration the intake valve closes earlier and the cylinder starts to build pressure earlier which results in more pressure at TDC. At some point the pressure is great enough to cause detonation.

The process of designing an engine starts with determining what rpm range will be used the most. That decision dictates what camshaft must be used. The camshaft choice sets the dynamic CR which is a result of when the intake valve is closed. The static CR, the CR number used in the factory specs, is misleading since the actual compression pressure is a result of the camshaft timing when the static CR is actually producing pressure. The compression pressure is the key to detonation and that is adjusted by adjusting the static CR because if you try to adjust the pressure with the camshaft you change the rpm range at which the engine runs best.

Bill Oldham wrote: ↑Sat Nov 30, 2019 9:56 pm I ask because I am rebuilding a B normal with AA 1720 P&C (22 degree) and upon assembly I come up with a rather high compression ratio of 9.2. With the normal cam and carbs will that cause a problem?

That is too much static compression for a Normal cam. Your pumping pressure will be very high.

Bill Oldham wrote: ↑Sat Nov 30, 2019 9:56 pm QUESTION 3: If I add another 1/2 mm washer, it will drop the CR to nominally to 8.7 but of course increase the minimum piston to head gap to 1.7mm. Is that too much gap?

Life is a compromise. My wife wanted a better husband but had to settle for me. On the other hand, I was very fortunate. You have no choice but to reduce the static CR so you will have to accept the excessive piston to head clearance if you use those pistons. For the very best result you should buy the correct CR pistons to attain the correct .035"-.045" piston to head clearance. Since you have the pistons already I would just shim them and the next tine you build an engine you will buy matching parts. Even 8.7 might be too high, not sure, try it.

Bill Oldham wrote: ↑Sat Nov 30, 2019 9:56 pm (I have trouble measuring the dimensions of squeezed clay, so I use solder.... bad or good idea?) [this is QUESTION 4}

Solder is ok. Do it in a cross pattern to see if the pocket in the piston is parallel to the valve. I use clay because it covers a larger area to catch inconsistencies but solder is much neater.

CJ, could you post a picture of a piston with Sharpie lines showing where solder would be for a clearance check?

How about where the piston would be closest to the head?

I was specifically wondering how many spots need to be checked, like four spots around the circumference, or ten spots? Maybe a picture is not necessary.

I attach a 1" length of 1/16" solder at the top of the piston, and one at the bottom. Less waste of solder, and you can use the un-squished end for your next check.