Can you say Windage

[126coupe]

Twin-plugging and other modification to the 2.0L race engine
caused me to take this low hour (18) down this far.
My Piston to valve clearance is too tight (due to going from a DC 60 to a DC80 Cam) so the pistons are at Ollies getting a little machine work done.

Here is a shot of the boattailing, knife-edging and piston skirts moon cut.

I know their are alot of arguments whether or not this increases HP. It sure cant hurt.

[356RS]

VERY NICE JOB....................sweet..........
How much damage to the valves with the DC-80?

[126coupe]

Originally posted by 356RS
VERY NICE JOB....................sweet..........
How much damage to the valves with the DC-80?

No damage, just did all my dry fitting before final assembly

[Steve@Rennsport]

Nicely done!

Boattailed cases and moon-cut cylinders do make more HP depending on displacement and maximum RPM. Since 1976, most racing engines have seen an 8-10 HP improvement on our engine dyno and I consider it very worthwhile on aluminum-cased competition engines.

I will not boattail a mag case as that helps promote cracking if the engine is run above 7300 RPM. Those things need all the material (stiffness) they can get.

[Turbo_pro]

Wow Steve that is a great improvement. Can you post those dyno sheets for us?

[Steve@Rennsport]

Hi Jim:

This was something we tested several years ago and didn't keep the dyno data after a series of back-to-back tests using the same engines. It was simply done to quantify some observations about windage in these engines. This was part of some extensive R&D using vacuum pumps and different ring configurations,...

For example, I think its a great waste of time to do a 2.0S that will not see much 7300+ usage compared to a 3.6 being run to 7K+. One must remember that Porsche used the same case dimensions in ALL of these engines and the volume of air moving around inside the case varies with displacement. Big motors see greater benefits than smaller motors do but all race engines (Al) should be done if one anticipates RPM operations well above 7K.

[Turbo_pro]

Steve, Thanks for your response.
I was at Porsche Motor Sports for an open house this year and not one of the 3.8 GT3 engines had boat tailing on the cases. Not one. Don't those engines turn RPMs in the 9200 range? I have also seen both 935 and 962 engine cases and have never seen boating.
Is it your contention that GT3 engines would not benefit from a 8-10 horse power increase?
That seems like a questionable proposition.
I know an engine builder from the mid eighties ISMA Daytona winning team that claims zero benefit from boat tailing.
This builder was trying to get to 300hp from a 2.5 and only got to 296. They tried a plethora of techniques and boating proved to offer no measurable difference.

I also noticed at Porsche Motorsport. that none of the cranks were knife edged.
Can you explain these observations?

Wanye and Henry talking about moon cutting cylinders and boat tailing:

Originally posted by Henry Schmidt
....................
For my money, both are a tremendous waste of time and will offer no benefit in all but the most extreme racing engines. We have not been able to measure a benefit on the dyno.

Originally posted by Wayne at Pelican Parts
I agree with Henry. Unless you're trying to extract one more 1/2 HP from a high-RPM race motor, it's really not worth it. I basically said this in the Engine Book too. Also included in this is boat-tailing - not a real good return on your time/effort/money.
-Wayne

[Turbo_pro]

Originally posted by Steve@Rennsport
Hi Jim:

Indeed, GT-3 race motors turn 9200+ RPM,...

I too, have seen these motors and they use a clever system of venting crankcase pressure (one-way valves and special seals) that aleviates the need for the boattailing. Porsche has always been VERY conservative about doing such things,.........they were VERY slow about the need for bottom end mods to prevent #2 & #5 rod bearing failures.

The benefits of knife-edged cranks is questionable to me as the jury is still out on that one.

I cannot explain why the IMSA engine builder you spoke with had trouble going beyond 296 HP,......I do not think that had much to do with boattailing or not.

The bottom line is that everyone has their own set of experiences that forms their opinions about what and how to do something. Its not a matter of "right or wrong", we are all the net result of those experiences and some folks have done more things 'outside the box',...

Steve, the point about 300 hp vs 296hp was an attempt to show that they were in search of 4 more horse power and that boat tailing was not capable under their specific circumstances to provide even 1 hp let alone 4 hp. Their dyno was accurate enough to repeat power runs close enough to measure these very small numbers.
I would have to agree that there is no "right" answer just the experience of the specific builder and we all respect your abilities.

In these small, high RPM engines the benefit of knife edging may be more in the weight reduction (less rotating mass) than in reduced rotating resistance.