None of the (numerous) engine builders I've spoken to recently will use slip fits. All opt for machine fit. The expense of the machine work is not great.

I contacted LNEngineering and they said their 100mm Cs with JE 9.5:1 Ps are a direct fit with no modification required to the case.

Still haven't made a decision on how to mover forward, however I am looking at the carrera intake option as well.

Anyone running a stock engine with ITBs?

3.0 rebuild - what would you do?
 

Post 9 in this thread on the first page.
With a link to more details on what Al was working with in post 10


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Got it! missed it the first time

GT2BH I sent you a PM regarding a Carrera manifold.

2 comments on your project & thanks for bringing it up as you have made me rethink a few of the decisions I have made on my upcoming rebuild :

* those LNEngineering P & Cs look great for a 3.0 motor - pity I didnt know about them before I made other purchases. A set of special JE Pistons would, I guess, cost about $1600 anyway?? then $500 each for new special size nic. cyclinders - seems very price competitive with other alternatives. Be aware that the unusual shape of your present pistons are optimised specifically for the CIS system. New JE pistons may not fully suit CIS, if you decide to keep it.

** I have purchased a 3.2 manifold. I did this as I was going to eventually turbo my motor (stage 2!) & the EFI 3.2 manifold is great for that. If you go this way make sure you get new flexible fuel lines as these old lines are the weak link for this system. Len (Boxter GT) supplies a high quality replacement set at a reasonable price.
In hindsight I probably should have gone with an ITB set up like Al K.

Peter 78 3.0 SC

Thanks for the input! I already contacted Len a while ago and I am getting a full set of new lines from him once I decide what I'm going to do. What do you mean when you say the pistons may not suit CIS?

One of the scenarios is now to get the slip fit 100mm 3.0 (or 3.2?) nickies with 9.5:1 JE pistons, bitz EFI, carrera or early CIS manifold and 964 cams. Will I need to twin plug for this setup?

Pistons and cylinders........
 

The Mahle 10.1:1 pistons that I used work quite well with the CIS system. The original piston shape was more for emissions purposes than anything else. The replacement JE pistons should work well. The 100mm pistons with the 70.4mm crank will give you 3.3 liters. Twin plugging will depend on your compression ratio. A piston with a tall crown will tend to block the flame front during combustion. I recommend it as it will allow your engine to run properly with much less timing advance. It is good to have a bit of "cushion" with today's gas. Mine runs well on 91 octane.

Mahle Pistons
 

Since there has been so much discussion about the different pistons (Mahle, Max Moritz and JE) I thought it might be good to post a picture of the Mahle pistons that I used.

Mahle 100mm 10.1:1 Piston
http://forums.pelicanparts.com/uploa...1410535134.jpg

Carrera Cylinder head
http://forums.pelicanparts.com/uploa...1410535287.jpg
http://forums.pelicanparts.com/uploa...1410535306.jpg

Engine during assembly w/plug wires laid out
http://forums.pelicanparts.com/uploa...1410535511.jpg

Contact switch for cold start valve (7th injector)
http://forums.pelicanparts.com/uploa...1410535619.jpg

Shield for bottom plug #3
http://forums.pelicanparts.com/uploa...1410535817.jpg

Very cool set up Fred. I think it is technically out of my reach being an amateur at rebuilding engines. I am thinking of maintaining 9.5:1 and keeping it single plug.

You lost me when you said "A piston with a tall crown will tend to block the flame front during combustion. I recommend it as it will allow your engine to run properly with much less timing advance. It is good to have a bit of "cushion" with today's gas. Mine runs well on 91 octane."

Can you break it down for me? for the record, our gas is better than US gas. We get 95 octane fuel from the pump an additives and race gas is available to bump it to 98 or 100.

The ideal location for the spark plug on a hemispherical combustion chamber is dead center between the valves. This way, when the spark ignites the air/fuel mixture, the flame propagates equally within the combustion chamber and all fuel gets burned in a controlled manner.

You'll notice on the 911 head that the spark plug is off to the side. This is due to the architecture of the head. With a single overhead cam, large valves, and small combustion chamber, there is no room to place the spark plug in the center.

The problem this creates, especially with a pistion with a high dome, or crown, is that the combustion chamber gets essentially split in two by the piston dome. The spark ignites the air/fuel mixture on one side of the cumbustion chamber and it takes time for the flame front to ignite the mixture on the other side. This requires more advanced ignition timing. Problem is, the amount of ignition timing to make this situation work efficiently either requires high octane gas, or results in detonation.

Twin plugging solves this issue with high compression, high dome pistons as you are igniting air/fuel on both sides of the chamber/piston dome. a 9.5:1 piston has less dome, so therefore leaves more room for the flame front to travel across the combustion chamber and requires less ignition timing.

For a single plug street motor, 9.5:1 and the best gas available is a good idea, with timing to suite. ignition timing will depend on things like air/fuel ratio, elevation, ambient temps, gas quality/octane, etc.

Wow, well done for breaking it down so well for me to grasp the concept! That makes a lot of sense, even to an accountant such as myself :)

Due to the location of the original spark plug, would it be more efficient either way to twin plug, even with a 9.5:1 piston?

Twin plugs
 

Josh D did a good job of summing up the problem with high compression hemi engines. With single plug ignition, it will normally require about 36 degrees of advance while a twin plug setup will only need 24-25 degrees of advance. If you consider that the single plug engine is starting to ignite the fuel (and thus building pressure) 36 degrees before the piston gets to top dead center, it (the engine) has to overcome "negative work" before it can go into the power stroke. By comparison, the twin plug engine only has to overcome 24 degrees of ignition before TDC (top dead center). The end result is that with twin plugs, the engine will run cooler, will not be as sensitive to the octane rating of the gas and (normally) make more power. So, if you are replacing the pistons anyway, why not buy the high compression pistons, add the twin plug ignition by means of a 12 wire distributor or a direct fire unit and get a better running, more powerful engine? I used the Electromotive XDi system which was not difficult to install and wire up. It comes with good instructions that are easy to follow. And, with the direct fire unit, you won't have to keep on buying rotors and distributor caps. Just for the record, this was my first Porsche engine build, so don't let inexperience stop you. Get a copy of Wayne's Porsche engine book, read as much as you can online and ask lots of questions on this BB.

So we are talking about LNEngineering slip fit 100mm cylinders, 10:1 JE pistons, older style CIS intake, XDi ignition, 964 cams, and have the machine shop modify the heads for twin plugs. Did I miss anything? No budget left for EFI this time around, so will the slightly bigger older style CIS work well?

I'm guessing this will make my brand new MSD ignition, coil and magnecor wires redundant?

IMHO, after seeing your other thread, I would have your existing parts evaluated first to know what is reuseable, what needs to be replaced and what, if any, machine work will need to be done. You might have to have the crank turned, for example, and I hear oversize bearings can be hard to find/expensive. Seeing the condition of those rod bearings, it would appear to me it wasn't getting oil changes enough. It's possible the oil pump could be worn and out of spec, for example, and need to be replaced ($$$).

Have you established a budget yet? Or is this a no budget limit/no timeframe kind of build? It's easy for us to tell you what to build because it isn't our money!:D

I'm really hoping the crank is fine. I checked the oil pump and it looks fine from the outside, but since I don't know the cars full history I am not sure how or when the rod bearings were damaged. Since I don't have a local porsche machine shop I will probably need to ship most of my stuff to the US. I will ask around and see if I can get the crank measured and polished, but if its out of spec then I'm kind of screwed.

My current budget is $15k for the full rebuild, and part of that will be eaten by the "must replace" parts as well as new oil lines and oil cooler, fuel lines and harness. The rest will go towards the good stuff. I don't have a deadline set for the build yet since I found a storage facility that can take one of my other cars and the SC can keep taking up space in the garage without the missus giving me a hard time :)

Things to do.........
 

Your heads are probably the small port version. If you go to the early CIS air box and runners, you will want to have the intake and exhaust ports opened up or at least matched in size where the intake meets the head. Otherwise, you will create a non-laminar air flow (bad) instead of having nice smooth laminar flow (good). You will want to have the rods redone with new small end bushings and resized large ends. Another old hot rodders trick is to have the rods shot peened to relieve any stresses in the rods. I assume that you are already planning on replacing the Dilivar lower cylinder head studs that are prone to breaking. Check the cam rockers and shafts for wear. If the rockers are worn, you can have them reground and hardened for less than a new set would cost. Make sure the 964 cams don't have any worn surfaces. Keep in mind that if you do all the good stuff inside the engine, it will be much easier and cheaper later to change to carbs or ITBs if you choose to do so. If you don't have one, I recommend getting a parts cleaner tank w/pump and sprayer. It will make it easier to clean up all the bits. Make sure to get one with a filter to keep the crud out of the parts. Summitt Racing or Harbor Freight both sell reasonably priced units. Get one rated for at least 20 gallons. What you are discussing doing is very much like my engine. I am very pleased with this combination and I suspect that you will be also. Good luck!