Mooning the Cylinders

[Edwin Ek]

What exactly is that technique? I've heard it described as an approach to relieve air pressure inside the case created by piston movement, and that it gives about 10 hp on small displacement motors. Bruce Anderson supposedly talks about it in his book (which I don't have), and I've read somewhere that Porsche itself started doing it with the 2002 996's.

One thing I wonder about is where the hp savings (gain) comes from. As one piston comes down, another is on the way up. Is the gain because the up and down aren't perfectly synchronized, so there is some compressive (horsepower robbing) and then decompressive effect in a high-frequency pulsing fashion? Or is it simply the friction as air moves from one area to another?

What modifications to the cylinders do you make? Is the horsepower gain real? Any downside?

[nick-moss]

I have not heard of mooning but my cases have been "boat tailed" which sounds like the same process.

[Bobs 67S]

Wayne Dempsey has an illustration of this in his 911 engine book. It removes a arch of material from the cylinders skirt.

The inside of the engine is an air pump, any aid to help the air move around will improve the loss of horsepower. This is in the same catagory as boat-tailing ( angling the face of the case webs) and knife edging the crank.

This was one of the most kept secrets of engine builders. Bruce talks about it and Wayne talks about it. But I have yet to see measurements of how much material is removed. Anybody have a picture...especally a 2 ltr.

thanks
Bob

[gruen911]

Ed,
Just to clarify, that's removing material from the cylinder skirt, not the cylinders themselves. Bith authors discuss the idea of picking up significant hp from bench flowing the heads. However, they refer to this as more of a an art or trade secret, and do not go into great detail other than to state that the idea is reshaping the heads around the intake and exhaust ports in order to improve air flow. I paid quite a bit to a well known machinist in South Florida to have my heads bench flowed. I still have not had my engine dyno'd in order to determine if any hp increases resulted. I would like to think it did though.

[ValveFloat]

One thing I wonder about is where the hp savings (gain) comes from. As one piston comes down, another is on the way up. Is the gain because the up and down aren't perfectly synchronized, so there is some compressive (horsepower robbing) and then decompressive effect in a high-frequency pulsing fashion? Or is it simply the friction as air moves from one area to another?

Yes, as one piston moves down and another moves up the air must move from one location to another, anything restricting that movement is robbing some amount of power.

This was one of the most kept secrets of engine builders. Bruce talks about it and Wayne talks about it. But I have yet to see measurements of how much material is removed.

The way I did it was after the boattailing was complete I put a cylinder in the case and marked the area of the cylinder that was blocking what the boattailing had opened up. Does that make any sense? Then I machined out the area I had marked.

[Edwin Ek]

Since I have a mag case and don't want to do boat-tailing, would mooning for the cylinders still be a possibility? (Strange talk about car engines, eh )

[gruen911]

It is important to keep one thing in the back of your mind when mooning the case. That is that the metal that is being removed is whittling away at the webbing and reinforcement metal that the factory used to maintain the strength of those engine cases. The more more metal removed for better airflow, the more comprimised the strength of the engine case. In Waynes book, I somewher recall that he mentioned there was not alot of hp to be gained from this. If you are racing, every little bit helps. If you are using for a street driver, better hp improvements can be found elsewhere.