Project 911 993

Part 2: The “expensive” gift

December 5, 2013

Also from Issue 216

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Actual CR Calculation

When building a performance engine, one of the need-to-know numbers is the actual compression ratio of the new engine. This is defined as the ratio of the volume of the cylinder at the bottom of the stroke (max volume, Vmax) divided by the volume at the top of the stroke (minimum volume, Vmin). You will remember from the text that Steven ordered custom pistons with a compression ratio of 11.5:1; note that this is an estimated value. But determining the actual volumes can be tricky due to the complex shapes of the piston crown and cylinder head combustion chamber. A precise calculation can be done to calculate this once one has all of the new parts and parts that were at the machine shop back in-house; this is the one time when oil in your combustion chamber is okay (hopefully, only a tiny fraction of the amount ever makes it there again!).
Measuring the volume in cc of oil in the combustion chamber (Vcc) is relatively easy. Be sure the valves are sealed, invert the head, and using a graduated pipette to measure the amount of oil it takes to fill the combustion chamber to the top without spilling a drop. Next, we determine the volume around the crown of the piston (Vapc). This is done by installing a piston with rings (sealed with grease for these measurements only; yes, you have to clean up afterwards!) into its cylinder such that its highest point is level with the top of the cylinder face, and then filling the void left over using the graduated pipette to measure the amount of oil it needs to fill to the top, again without spilling a drop.
Okay, so we have the volume of the combustion chamber, the volume around the piston crown, and we can calculate the additional cylinder volume with a calculator. This volume has a height from the cylinder top face to the top of the highest point on the piston (H) and can be calculated with the volume formula for a cylinder: Vcyl = πR2H where R is the Radius of the cylinder 5.25cm (remember the new bore is 10.5cm or 105mm). Therefore we can add the measured and calculated volumes together for Vmax , where:
Vmax = Vcc + Vapc + Vcyl
Next (and I know you are getting sleepy, but this has to be done!), we need to come up with Vmin . First we need to calculate the actual volume of the piston crown (Vpc), but this will be a cinch since we already know the volume around the piston crown, Vapc. We simply subtract this volume from the volume of a cylinder the height of the piston crown (Hpc).
Vpc = πR2Hpc – Vapc, where R the same as above.
But for the most precise calculation we must also take into account the additional volume of the thin “slice” of cylinder (Vdh) due to the deck height spec whose height is 0.8mm.
Therefore, since the piston crown fits into the combustion chamber and the deck height volume we can use: Vmin = Vcc + Vdh – Vpc
Finally, the actual compression ratio can be calculated by dividing Vmax by Vmin. Of course, if you want to be ultra-thorough, now you need to complete these measurements and calculations for the other five cylinders. Enjoy!

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In the first installment of this project car series, hardcore track enthusiast and project car owner Steven Tory found the source of the “bits in his oil,” but only after a complete engine teardown on his 1995 993 C2.

As it turned out, the “bits” were pieces of the engine’s intermediate shaft (IMS) thrust bearing that had literally just started to break up inside his engine prior to his “well-timed” oil draining. Only four small pieces of shrapnel were found, and the main pieces of the IMS thrust bearing were still in place. Fortunately the rest of his engine was completely unscathed by the ordeal, judging by a thorough inspection after the teardown. The IMS thrust bearing is not an identified trouble issue on air-cooled flat sixes, unlike the IMS issues on Porsche M96 water pumpers, which are well known, so Steven definitely had the good car Gods looking out for him that day.

Now the real fun can begin—with engine rebuild planning and full clean-up at the top of Steven’s “To-Do” list.

Steven was up to the technical task himself (with a little assistance and encouragement from yours truly), and really had four choices on how to go about the repair of his fully disassembled 3.6L Porsche flat six. The first choice was the simplest: He could do an extremely thorough clean-up, renew the two IMS thrust bearing halves, swap out all “known must replace” parts, and put everything back as he found it. His engine was strong and performing perfectly prior to the “incident,” but the process and man hours of disassembly and assembly would be the same as the option number two.

The second choice was he could undertake a complete OEM rebuild, which would include the measurement of all technical clearances (bearings, pistons, rings, etc.), and replacement of any out-of-spec or damaged parts as necessary with the OEM pieces. Several hours of extra time is required for such measurements and determining what else should be replaced. However, the advantage of this could be time and money very well spent, because the engine was already fully opened for such an evaluation and may very likely reveal another inexpensive (or expensive, it does not really matter which) part that might be ready to let go.

The third option was a mild performance rebuild along with additional head work to RS spec and an increase in displacement.

The fourth and final option was the “enthusiast’s choice”: He could fully rebuild and refresh his engine to custom specifications with something very special in mind, and end up with a lot more performance than he had in the first place. Also, he could sell off all of the excellent condition OEM parts that would not be used in the rebuild to partially offset the cost of the new performance parts (at the very least, a great story to tell the spouse, anyway!). As this is a project involving an enthusiast’s car in an enthusiast’s magazine, of course option four was absolutely the only way forward. Yeah baby, yeah!

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