Bigger jugs? How big is too big?

[lendaddy]

Ok, we'll see how far OT this goes but, I will be tearing down the 930 this winter for a top end etc... maybe more. Anyway, what is the limiting factor in the displacement increases? I realize the stroke is the stroke, but what is the largest bore I can go with on my 3.0L 930? I am not sure I need new jugs but if I do, then why not maximize. I see 3.2L jugs around for me but I wonder why none bigger? Enlighten me please. Thanks.

[atlporsche]

WILL....EXERCISE.....RESTRAINT!!!!!

sjd:P

[Dantilla]

If I remember right, when you go bigger than 95mm, you need to twin plug the heads.

hmmmmm...... Twin plugs.....

[emcon5]

98mm cylinders is usually as big as people go because they will fit without having to modify the case.

Tom

[lendaddy]

Do you need to twin plug because the CC is so much bigger? I will probably twin plug anyway(I am going EFI) so thats fine. Anyway, I realize there are mechanical limits, but what is considered safe thermally? What is readily available?

[lendaddy]

Thanks Tom,

I assume there is nothing "Special" about my case? '77 930. Let me see, 98mm would be?

[HawgRyder]

I have seen 100mm sets for sale....but at that size the wall thickness is the problem.
OK for racing...but the cylinder will not stay round due to heat.
So...blow by, oil consumtion, etc.
Bob

[lendaddy]

Yep, looks like the 98mm units will get me to just under 3.2L (A 6.5% increase). Not too bad. Figure some losses, so 5% which would mean 15 HP gained right there. Hmmmm

[techweenie]

I think Saxis' avatar has the answer.

As for Porsche engines, I believe HawgRyder has it right.

[rdane]

Past 98mm, 3.2/3.3, it is twin plugs if you want to keep your engine in one piece...add $2500.

[lendaddy]

Rdane, $2,500 for what exactly? Twin plugs or the P/C sets? With My engine control system I wont need a double dizzy, just three more coil packs, six more wires and some easy machine work.

[lendaddy]

"I think Saxis' avatar has the answer"

Just got that

[rdane]

Sorry missed than. I am working on a 3.0 case and no engine managment. If twin plugs are cheap for you make it big...no reason not to. Case work to go over 3.2 is cheap in comparison.

[lendaddy]

Rdane,

Interesting. There have to be other issues with going big, no? I mean the cylinder wall thickness must also be an issue. Especially with high heat turbo applications. And now for what I am sure is the dumbest ? of the day. Is there not a Porsche stock set of jugs out there that has a much larger bore that I could find used? I realize they would have to be machined for stroke, is that possible? Hey, you don't ask you don't learn. I.E the 100mm jugs from a 3.6.

[Amac]

100mm on my 3.3 made it a 3.5L.
I went single plug, I have twin plug
heads but the builder thinks single
is the way to go.
I was thinking larger like 4.0, big jump
in $'s, alterations, and short life span.

[jstgermaine]

lendaddy-

I sent you an email.

[lendaddy]

Jerry, you have mail.

At 100mm I would have a very short stroke 3.3L which I think would be pretty cool. Now, am I crazy thinking I could find some low mileage 100MM jugs from a 3.6? Would that even work, can they be machined to work? Or are you required to buy "special made" conversion pieces from the many tuners out there? Thanks again.

[jstgermaine]

I am not an engine guru by anyone's definition, but I am pretty sure you can't just throw some NA pistons and jugs on your case...compression would be way too high. Am I correct here?

[Bill Verburg]

The wall thickness is a big issue. Amac and Hawgryder are corrrect. The 100mm 7.5:1 w/23mm wristpin on a 74.4mm crank gives you 3506cm and is just about the reasonable limit(yes, twin plug is preferred)

The 102mm p/c come in 6 versions none of which can be used on a 930 block. the 6 versions come in 2 families. Thick wall and thin. All the racing and turbo versions are thick walled.

[lendaddy]

Thanks Bill,

But I have a 70.4 stroke so I end up just over 3.3L