Boat tailed main webs

[garibaldi]

This is a case that has had the main webs boat tailed

[oneblueyedog]

CNC? Nice.

Lee 78 SC Boattailed by hand

[Henry Schmidt]

I have been building Porsche engines since the 1970s. I have yet to see a dyno sheet that shows any increase in horse power offered from this modification. I understand the theory of increased flow, but we have never seen a single horse power from this modification and we looked... I could be wrong but it seems to me that if there was any (even 1 or two) free horse power offered by this simple modification, that Porsche would cast / mold the main webs with at least a radius. 2 liter sand cast cases have a slight radius but after that all main webs were flat. It have also been my experience that Porsche factory race engines did not have boat tailed mains. Now of course I have not seen all Porsche factory engines, but every unmolested 935 and 962 engine I've seen didn't use this mod. You have to believe that Porsche experimented with this mod and to my knowledge they don't use it.
It is a simple and inexpensive modification so do it if you want, but we have done it since the mid 80s.
Do any of you have dyno sheets showing even 1 horse power increase ??? I think it looks cool so if there's even one horse power I might use it.
We can do he mod in house if you've got to have it.
stock 3.8 RSR case

stock 3.2 Carrera

stock 2.0 sand cast case w/ case squirters added

In house boat tail modification & shuffle pins

[jgparker]

Henry,

Do any factory race cases have shuffle pins? Do you think it is necessary? If so, which case/application combinations?

Thanks for you input,

JP

[Henry Schmidt]

I have never seen the factory shuffle pin a case. Instead they use a special case through bolt that accomplishes the same thing.
For us, the manufacturing of a special case though bolt is not cost effective so we do the next best thing. Shuffle pins.
It is one of the last things we do on the way to full race.
If the plan is endurance racing by all means shuffle pin. If the plan is sustain high RPM , shuffle pin. If your going to add 1.4 bar of boost, shuffle pin.
Mag cases benefit the most from shuffle pin but if you're trying to make reliable high output engine skip the mag case.

[garibaldi]

I have not researched whether or not streamlining the mains helps free up any hp. Actually, the benefits may not necessarily show up on a dyno in terms of power, it can show up as reduced oil aeration, less turbulaence inside the case, who knows, I will not argue that the gains if any may be small. But like you said, some people gotta have it.

The question isnt how much hp does boatailing the mains give, but how much hp is lost by the amount of blowby that is passing into the case so that you have to streamline the backs of the mains to make it easier to make its way through. Thats the joke.

If you could keep everything that is supposed to be on top of the piston up there, then it woudlnt make one bit of difference if the mains were shaped like one huge brick wall.

The issue is the blowby, not the boatailing. Just like the issue is case rigidity, not shuffle pinning. Make a good case and keep it clamped tight, and you wouldnt need to pin every main to keep it from jiggling around. Same thing with heads, peopel install the flame rings thinking that is going to save the motor, when it does nothing more than buy you some time. There is a better way to help the heads out on high output applications. The list goes on and on. In the end, we need to work with what we have, but most of these "Modifcations" are bandaids, and do not take care of the problem at the source.

[Tyson Schmidt]

It's not blow-by that you are streamlining. It's the air beneath the piston as it moves downward in it's bore, as another piston is moving upward in it's bore. The air needs to travel as freely as possible from beneath one cylinder to another.

I honestly don't think it matters at all until you reach around 7K rpm-plus. Porsche claims that it frees up 10 horsepower in it's race motors.

I just don't see how you could make a scientific comparison between boattailed and non-boattailed race engines on the dyno without some seriously controlled environments, and facilities that only the factory has.

I mean, you could dissassemble an engine, then put it back together with no changes at all and get more variation on the dyno than the factory claims to get from boattailing.

I guess we'll never really know!

[Henry Schmidt]

Originally posted by Tyson Schmidt

I honestly don't think it matters at all until you reach around 7K rpm-plus. Porsche claims that it frees up 10 horsepower in it's race motors.

The point is that Porsche factory race engines don't have boat tailed main webs. At least I've never seen one. If it was worth anything they wood do it, or is 10 hp insignificant?
Where does Porsche claim a 10 horse power increase?
After building 30+ 2.5 IMSA GTU engines, trying to get 300 hp and only reaching 296, I believe that there is no HP gain.
That's the point of my original post.
Does anyone have dyno proof that boat tailing does anything?
Urban legend? or just BS ?

[racing97]

Have any of you Gentlemen ever seen one of these Competition engines
running on the dyno with a clear breather hose.

Best Regards

[garibaldi]

No, the percentage of air that you would be attempting to move freely through the case is indeed blowby. The 911s especially with the thin walled aluminum cylinders have a tendencey to have it occurr much more than would lets say a iron block or steel sleeves. You can take a 911 cylinder, squeeze it between your fingers and get it to oval by a few thousandths without much force. How well do you think it holds shape under load, at RPM, and with temp?

The boat tailing may not do much anyhow, since even if whatever is coming down under the piston has to get into the case, it is then met by the crank and rods that are rotating, and have you ever seen a running crank at RPM in an engine while its operating? There is a "hive" of oil that is getting slung around it coming from the mains and rods that would make it look like a peice of cotton candy. SO its not like any of this is happening slowly, or that there is a clear path to allow the air to pass through anyhow.

[racing97]

At about 7,000 your hive (great discription) lets go and plugs your breather up for about a foot in length. It appears you have the ability to do some serious machine work, and it would be difficult in mock up, but if you would try to seperate that oil from the crank I think you would help the parasitic losses considerably. No one to my knowledge has done so.

Best Regards

[Henry Schmidt]

Originally posted by racing97
Have any of you Gentlemen ever seen one of these Competition engines
running on the dyno with a clear breather hose.

Best Regards

Yes. I have. What's the question?

[garibaldi]

this may help a bit with parasitic losses.

[ChrisBennet]

I thought the whole boat tailing the webs was to let air move from cylinder to cylinder. While one piston is coming down, another is going up; compressed air under one piston, vacuum under another...
-Chris

[Tyson Schmidt]

Originally posted by Henry Schmidt
The point is that Porsche factory race engines don't have boat tailed main webs. At least I've never seen one. If it was worth anything they wood do it, or is 10 hp insignificant?
Where does Porsche claim a 10 horse power increase?
After building 30+ 2.5 IMSA GTU engines, trying to get 300 hp and only reaching 296, I believe that there is no HP gain.
That's the point of my original post.
Does anyone have dyno proof that boat tailing does anything?
Urban legend? or just BS ?

My guess there, would be that they probably cared more about case strength when boosting heavily, than about a few HP worth of pumping losses. Maybe it became too much of a reliability liability (say that 5-times fast!) than it was worth in the larger more powerful engines. I seem to remember it was used in the early high rev motors like the 911R and 906, but I could be wrong.

[oneblueyedog]

Heck, I just went by Bruce Anserson's book saying the factory got 10 more HP this way. I was very bored last winter waiting for my heads to come back, so I decided to do it. It was very fun and satisfying. I hope it turns out that the mod. does enable some gain.

Lee78sc with boatailed webs, sport SC Elgin cams and Euro pre-muffler(original used stock).

[Determined]

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[Henry Schmidt]

Boat-tailing is a non-issue for me but on another front, Gilroy was the garlic capital of California, the USA and maybe the world but almost 60% of the garlic sold in the US is grown in China.
If you cook, you know Chinese garlic has a metallic taste.
Maybe if Trump is elected, his China trade policies will save Gilroy garlic.

Carry on with the boat-tailing myth and I'll wait for the new information.

[manbridge 74]

Probably use people poo as fertilizer. I've been suspecting the same with Chinese ginger.

[Mark Henry]

I looked at this and knife edging on my build and decided it wasn't worth the time for a hot street engine.
I just did a quick deburr on the sharp edges.