Bathtube combustion chambers

[piggdekk]

Hello guys,
I've got a '71 911T that at some point in the next future will get the engine (matching number) rebuilt, and of course I want more kick. I know there are good and well known ways of doing it but I would like to follow a different path, and I'm very interested in Your opinion.
Most of my experience with engines come from air cooled harley and ducati. The heads of these engines are not very different from the 911, except that they tend to have far more squish area. The ducati has got a nice bathtube combustion chamber, and the harley usually have an emisferic one, or a D shaped, but always with quite a wide squish area. With proper porting, the right compression ratio, and good cams, it is possible to get close to 100 HP/liter from these engines, with virtually no knocking and a nice low end torque.
2.2-2.7 heads don't have much squish area, and to increase the compression ratio one has to use the S-pistons, and to increase further the compression ratio one needs twin plugging.
I'm therefore considering modifyng the combustion chambers of the 911 head to give it more squish. The goals are 2: avoid having a combustion chamber split in 2 by S pistons, reach 10.5:1 CR without twin plugging.
I've bought from a fellow Pelican member a set of heads to play with. Yesterday I've borrowed some play-doh from my daughters and I've tried to see how the combustion chamber may look like:

there is no space to make a proper bath tube chamber, but surely it is possible to create a nice squish area without shrouding too much the valves.
The main problem I see is that the amount of material that can be added is not much. Maybe is is possible to add 5-8 cc, but not more than that. This means that I cannot use a flat top piston to boost the CR to 10.5, but I still need to use a E pistons, or soemthing similar.
So since these engines have now been around for 40+ years, my question is: Is it worth going this way? I'm pretty sure someone tried this before.
I've seen on Bruece Anderson's book that air cooled 962 had a kind of bath tube combustion chamber, so the concept isn't new, but that was a Turbo engine (at least I believe), therefore it could have a low CR and use flat top pistons.
Any opinion?
thanks
luca

[BURN-BROS]

Old school racer trick. The engine ran 8.5/1 RS pistons and had a compression ratio of 11/1 IIRC.

[piggdekk]

Thank You Aaron,
these pictures are really useful for me. I think I'll go ahead with my plan then. My idea was to finish the mock-up of the combustion chambers and then have it 3D scanned. Next weld in the extra material, and CNC the 6 heads to be exactly the same.
luca

[BURN-BROS]

No Worries.

Have fun!

[piggdekk]

Still I need to choose the right pistons. I don't want to go to the 90mm 8.5 RS becase I don't want to machine my case. I think the maximum I can use is 86 for a 2.3. In this case I believe T pistons should work fine if I manage to weld in 10cc. Besides, they are close to flat top, so I should get the nice combustion chamber I'm looking for.
luca

[BURN-BROS]

It depends on your intended usage.

A cast T piston will not have sufficient valve pockets nor will it be a forged piece. If you intend to build a sports purpose engine this would be less than acceptable.

I see a custom piston in your future with good valve pockets and an appropriate dome for the given chamber volume you will have.

[piggdekk]

well, what I meant was a piston with the low dome of a T one, but then of course forged.
As for the pockets size it will depend on the cams that I'll be using. Since I want to build an engine that is quite reliable and more intend for street use than track max RPM should be 7000, not more, and a nice torque at low RPM. I guess S cams are not an option, and I should either go for E cams or for a custom grind. Anyway, I see I need to choose the cams to get the size of the pockets, and therefore the volume of the combustion chamber.
luca

[Flieger]

I would suggest JE pistons.

[Walko]

if this works this would be cheap hp

Michael

[piggdekk]

Walco,
it isn't cheap to modify the heads. You need to scan the mock-up, weld the heads, CNC them with the 3D model, and then port them. Chances are when welding You'll ruine the valve seats as well...
Next You'll need custom JE pistons as well.
I do this for the fun of it, and because I have access to machines. But surely it isn't going to be cheap! Of course if I was racing in a class where it is not allowed to twin plug the engine this mod will be difficult to spot. But this is only a street car.
luca

[Walko]

Luca,

I am about to build my Carrera motor for a specific class type racing where we cant change the type of injection or dual plug the heads.

As my motor must remain as a 3litre on CIS i look forward to seeing how much HP you can gain as I may need to look at this option to gain additional HP.

What sort of cost do you think it would cost to do a set of heads??

Michael

[piggdekk]

I'll tell You when I'm done.
I think it'll cost 100-150 euros for the welding, and 5-600 euros for scanning and CNC. the rest is porting, cc'ing, which is a lot of hours since it is all done by hands....
However, I think if You use CIS pistons You're stuck with their shape, so it isn't easy to increase CR and create a squish area.
luca

[ratpiper71T]

Perhaps you'd have less work if you start with these..

Page Title

(No affiliation)

[Walko]

I was on that website last night but they didn't have any prices

Michael

[euro911sc]

mnnnn Extreme... yum! Bill is a very knowledgeable guy! If you scan the shape you want perhaps he can machine the blanks to your spec. He has some really nice equipment. However, IRRC, a finished set of his heads was running about $7k. That was a few years ago when I dropped my heads off to be done. They turned out perfect, BTW. Maybe less $$ now that the 2.0l guys have thinned out. His shop is about 20 minutes from where I live Nice to have such skill next door

-Michael