I'll debate the merits of boattailing if Henry doesn't make me cry again!

[Dennis Kalma]

Gentlemen, Ladies, Either, Or,

I was starting to play around with fitting my 6 speed, so mounted the crankshaft and flywheel to my new case to start the measurement process. I had always planned on boattailling the crankcase as the Factory said it gained a few horsepower at high rpm....seemed like a good efficiency play, no cost other than machining the case and whatever horsepower gain there is would be essentially free.

Well, when I started looking at the clearances between the crankcase webs and the crankshaft, well, there did not seem to be a whole lot of benefit to it. Basically there is little clearance at all as the crank and its counterweights seem to block off most of the space that would be made most of the time. Below are a series of pictures I took with several crank rotations, with my 98mm Mahle cylinders....



a little further



a bit further...



The only place where one could materially improve airflow out of the cylinders as the piston moved would be by trimming the bottom of the cylinders to have them be flush with the crankcase webs, which I tried to capture using a mirror as below...



...you can just see the edge of the cylinder about 5 mm or so below the crankcase, seems to me that trimming that back would be the greatest and most cost effective means to improve airflow.

Anyway, Henry has been saying this for some time now, I must admit I was skeptical, but it appears that he is correct...the benefit is pretty marginal, but I would be very interested in opinions of others, or, even better yet, quantitative measurements that prove the case.

Certainly I would be happy to save the money if there is no benefit.....

Thanks all....

Dennis Kalma

PS - dontcha just love the look and feel of brand new machined stuff, sort of a quasi-orgasmic experience...

[304065]

Dennis,

The "quantitative" measurement of the boat-tailing, cylinder "mooning" and crankshaft knife-edging is 10 horsepower for a 2,0 liter 901/20 type engine. This is also known as the "Kremer" mod after the famous German brothers.

That being said, these were engines that were intended to be torn down every 20 or so hours for a maintenance rebuild, at which time the cases would be inspected. If there was a crack in one of the main bearing webs that didn't result in catastrophic failure of the engine, that case was thrown in the garbage and another one used off the shelf! Same with every other part in the system, it helps if you are an auto manufacturer!

So I don't believe that there is much debate over whether the modifications are effective, only whether they are worth it. To which question I submit the answer is YES if you are racing in a displacement-limited class like PCA GT5 or one of the many FIA classes in Europe, and have a budget appropriate to competition at that level.

Would I do this to my own aluminum-cased engine? No, I wouldn't, because you are, after all, cutting material from the main bearing support! Like the sticker on the dash of my PCA H-Stock race car says, "The prize money sucks."

Nice photos by the way!

[Eagledriver]

I had a mag case crack due to excessive windage mods. I also think the advantage for later motors is less than the early 2 litre. Those race engines turned upwards of 8000 RPM and the peak power was up there as well. I think the mods are most effective at that high RPM. I do know that one of our engine builders for the race group I'm in says there is no advantage in our 3 litre SC based engines.

If you "trim" the cylinders you should only do it at the sides. If you trim all the way around the pistons will poke out the bottom and cause them to rock/slap.

-Andy

[Henry Schmidt]

There is no horse power gain from boattailing. We did the dyno runs back in the IMSA 2.5 days and could measure no gain. Other more famous race teams saw the exact same results.

If there were HP gains from this simple modification, why would the factory continue to cast the case with flat webs ? Changing the mold would be relatively cost free. In fact the early cases showed some signs of this mod in the casting of aluminum 2.0 but all later engines were cast with flat webs.

I have never seen a Porsche factory race engine delivered with boat tailing. If there was any horse power gain, why are all 996 GT3 RSR delivered flat as can be?

Why weren't 935 or 962 delivered with this horse power enhancement?

Free horse power ? sorry it isn't here.

BTW: knife edging? not on the 962.

[Porschekid962]

Thank you Henry! I know for a fact that the gt3 rsr and daytona prototype motors have no boat tailing on the case or knife edged cranks. Dont ask me where I saw this because I cannot tell you. While the 956/962 crank was not knife edged you can see in Henry's pic that it has been massaged slightly, it has a nice smooth radius. Keep in mind these engines do not operate at very high rpms in racing terms. Your money and time is better spent up top in the valvetrain and heads. The breathing on these motors is excellent, why do you think porsche continues to use them as the basis for their race motors?

Also that is a great picture of the racing piston squirters, where can I get some??? Best of luck on your build. Now its beer time with the buddies, time to discuss engines and such.

[Dennis Kalma]

Thanks Henry, looks like the only thing I need to do is a bit of sanding/filing to smooth the sharp corner, pretty minor I would assume.

Certainly saves a lot of hassle on my end. If I would have proceeded with the boattailing, I would have had to ship the case to a qualified machine shop and pay for the machining.

Better to put the money, as has been suggested, into head work or some other nice parts.

Dennis