boat tailing adventure

[oneblueyedog]

It took 6 hours with my Dremel tool and a couple of trips to Home Depot for bits and a sanding belt.

Start with a pristine clean case.

I used various cutters (not carbide). One interesting one was a router bit that rounded the corners nicely. I used a round burr after that . I took the strategy of knocking off the edges to a 45° then trimming off the edges of the first cut Then I did what I could to trim those edges to make it round . I then had the rough form shaped.

I took a sanding belt (thin strip type), medium grit and cut it at it 's joint. I ran it along the roughed out joint and smoothed out the roughness to a smooth blunt-round leading edge. Walt at CE told me that's as radical as you need. I then took a strip of 220 grit wet/dry and smoothed it more.They pretty much match the pictures in Wayne's book.

There were two tools that smoothed off the edges at the corners for the finishing. One was a mini flap sander and the other was a new Dremel tool that looked like a round piece of Scotchbrite sandwitched between a couple of washers . Those lasted for the project. I left the remnant of it in the photo.

I had a flex shaft on the Dremel. I did one side without it and the other with it. I liked the control I had holding the tool. It jumped less than with the flex shaft.

I made sure to avoid the oil squirters. You have to be careful because the bits sometimes jump!

Vacuum everything. Wash the case. Clean out the oil passages.

Here's a bonus picture of a clean transmission. All new seals!

[oneblueyedog]

One clarification. With the sanding belt, you carefully thread it through the spigots on either side of the web, abrasive side to the web. You then pull back and forth on the belt ends as the wrapped belt rides on the rouged out face. It's like the technique shoe shiners use to buff out the tops of shoes. Don't stay in one place.

You use the same technique with the strip of sandpaper.

[Wayne 962]

Bigger pics please?

-Wayne

[oneblueyedog]

[oneblueyedog]

This may help clarify how I made the cuts.

[Plavan]

wow that looks great

[goat]

I was just reading about boat tailing & what it does. Very nice work. And details of how you did it.

[smestas]

Would there be any performance increase in a stock '78SC motor from performing the mentioned boattailing?

[oneblueyedog]

Going slowly and carefully is essential. I cut pass after pass at the same angle, cutting material off a little bit at a time. If the RPM of the tool drops a lot, I figured I was going at it too agressively. Those tools cut by speed. I'd say the main workhorse was the smaller round burr. Just keep the cutting straight down the web -all of the cuts paralell to the next. My hands were sore and tingly for a couple of days after. Wear SAFETY GLASSES-new clean ones. You might want to wear a dust mask too.

Stated in Porshe 911 Performance Handbook - "the factory recommended this modification for 2.0-liter racing engines, saying it would increase the output by about 10 hp."

Everybody else I talked to said it was a worthwhile mod If you didn't go too crazy.

How to Tune and modify also says something about this, along with cylinder mooning, which I'd like to try but I haven't asked how to do it.

Everyone I talked to said rounding them over was the way to go. I blew air over an unaltered web face and felt the air rebound back over my hand. I then did the same over a rounded web and didn't feel blowback. I felt for the airflow coming over the sides of the web from the crank end and could feel the air coming off the front of the rounded web at about a 45° angle. That being paired with the unobstructed wind from a piston traveling down is going to equate to less wind resistance that hitting that flat surface area (brick wall) of an unaltered web.

I doubt if the flow is very laminar at all but the turbulence of the air shooting out the sides from the flat wall and blowing back up and colliding with the air and oil coming down from the downstroke can't be as efficient as it shooting back to the crank and possibly making its way to the opposite piston going back up (slim chance).

Being a glider pilot, I love tinkering around with airfoils. It was fun even if I never realize a scientific outcome of more horsepower in my 78 SC. I imagine others have this info though. I think I read it's really good for Hi Revving race motors.

That would mean less turbulence would be getting in the way of peak HP near redline. That'd be fine with me.

And I trust the books.

Lee 78SC

[goat]

from what I read boat tailing enables the motor oil to flow better and do what oil is supposed to do more effective, lube and cool. I am sure I will be corrected on this but overall perfomance is very little. The boat tailing that I read about it seemed like they tailed many of the parts on the inside.

[TimT]

Porsche found about a 10hp increase on a 2.0L engine with this mod.

A few hours with a die-grinder and sanding belts nets 10hp, thats cheap hp.

Lee, your going to eyebrow the cylinders as well?

[oneblueyedog]

I'd like to eyebrow the cyls. Does anyone have info on how to do it and how much to remove and where? And where not to?

Thanks for all of the kind words.

Lee
78SC

[TimT]

I havent eyebrowed the cylinders myself. I had them done by German Precision a few years ago.

I would perform that mod now myself though. Looks like a band saw/die grinder might do the trick. My engine is all together so I cant look at the skirts. I recall about 1/2" removed though.

[ChrisBennet]

I did this mod on my 3.4 (from a 3.2). The 10 hp improvement was on high reving race motors. Air resistance increases at the square of the velocity, yada, yada. I don't know if will make as much of a difference in a lower reving motor but hey, its free horsepower - the best kind.
I intended to moon the cylinders but at least on my case it made no sense to do so. The 3.2 cylinders only stick into the case about 1/10 of an inch. I suspect it would also ruin them for use as cores should you ever want to upgrade.
As I understand it, the air is being sucked between cylinders; as one piston goes down one of it's neighbors is going up so the air is "pumped" between cylinders. I can see how mooning would assist this intra cylinder airflow.
-Chris