My friend has installed PMO ITB carbs on his rally/targa 911SC.

He now has rapid throttle response, the engine responds instantly to the gas pedal similar to a motor bike (japanese not HD!!) .

The CIS system is/was very good for the era as it balanced out various requirements such as emissions and economy and the demands of everyday suburban driving in all climates. My friends PMO car takes a minute or so to settle down after a cold start, its petrol economy is way less than with CIS, but it is now very very sporty.

I also agree with Fred Cook regarding the 100mm pistons and cylinders. Especially if they don't cost any more and will give you a bit more displacement.

Peter 78 3.0 SC

why has he lost fuel economy with ITB's?
i've never noticed any loss in economy with ITB's, especially with a good tune.
though given the throttle response i understand if it's because it's more addictive to mash the throttle pedal to the floor :D

I can tune my Bitz EFI for pure power, pure economy or a nice balance. (Where I run mine)
My mileage has improved 10-20% compared to the early CIS my '78 came with rated at 16 mpg.
It is in the tuning map you are running when it comes to mileage, as long as you keep your foot out of it. :)


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you've exceeded his $10k budget in one hit :D

Just some generic thoughts from my re-build years ago.

I would try to stay with Bosch and not go to MSD or other engine/spark management system. If you have Permatune (Perm-a-Junk) I would replace it!!!

I stayed as close to stock 3.0 as possible.

Also had Alusil, cleaned them really, really well. Spent lots of time cleaning everything associated with them. Ordered Gertz rings and just re-ringed. No issues, no blow, less than a quart every 1,000 miles. Might be cheaper to gamble on what you already have, not much of a gamble if you can measure the cylinders in fifths to ensure they are still within spec. Mine had almost no measureable wear. Or precision cut a soft thin material to normal minimum spec and slip that into the cylinders to see how much space remains on the sides.

I kept CIS as there was still much support years ago and parts were readily available. Today I would be looking at my carb options. I like the banks, multiple throttles but would probably end up on the less expensive end. Do not be too scared of re-building older carbs to save some money.

Have the valves done. Send them to someone who can do it correctly and replace all springs, guides, valves. This will give you some nice new life.

Check with the experts on your rods and such. Mine were OK and we did nothing to them.

DO NOT trust just the clicker style torque wrenches. I now use only T-Bar style as they are more accurate and do not lose or gain tension over time.

While you are in there, replace the shifter bushings! They are cheap, simple, and make such a HUGE difference to your driving experience. Can even clean up what might feel like a sloppy transmission.

While everything is out and the rear shocks are easy to access replace them.

Replace the old rubber while your engine is out.

Go ahead and replace the oil sender on the top of the engine, it is not that expensive and they do fail and leak. Easier to do now.

There are so many options and combinations, it makes it real difficult to decide :eek:

I am now inclined to get the 100mm JE pistons with LNE cylinders, and switch to EFI (bitz?) instead of carbs. I'd rather not compromise with used carbs at this point and do this properly, and then down the line go for ITBs. Whatever I was going to spend on refreshing the CIS will go to the EFI kit.

Thoughts?

I like it. My only concern would be the small intake ports and CIS intake runners of the '83 CIS. With the increase in displacement, you'll be leaving power on the table. It would be nice and torquey down low though, depending on the cam and comp ratio you go with.

Since you are going Tbitz, consider finding the '78-'79 or '78-'83 ROW CIS airbox and runners and have the intake ports in your heads opened up to match.

Forgot to include the 964 cams in there. I am talking to Steve@Rennsport about working on my heads.

What is the difference between my current airbox and the older ones or ROW? I have seen several setups with bitz, some use a large K&N filter in the center and some keep the CIS look. Haven't seen any good looking bitz setups TBH.

3.0 rebuild - what would you do?
 

The difference in the intakes for 78-78 and all row 3.0 is the size of the runners. They are larger and allow for more flow.
Here is a shot of my '78 intake prepped for Bitz.

http://tapatalk.imageshack.com/v2/14...bb35d11fa3.jpg


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'80-'83 US CIS used a smaller 34 mm intake port and intake runner. The '78-'79 US and '78-'83 ROW used a 39 mm intake port and intake runner. The ports on the air box match the intake runners. Intake and exhaust valve sizes were all the same.

With the bigger displacement you are going for, the 34 mm intake port and CIS will be a restriction and limit top end power I'm guessing.

I have no experience with building Porsche engined, but what I would do is make sure I have a solid foundation for power and future upgrades. I would go for the largest p&c available (100mm) twin plugged and ported heads. Then I would have the possibility to ad bitz or vems at a later date, as well as efi converted mfi or Jenvey's.

If building an EFI engine and having your heads worked over by Steve, get a Carrera intake and have Steve work the intake ports to work with the Carrera intake. It will flow better than the CIS intake.

The guys with 100mm P&Cs, does the engine case need to be machined for these to fit?

100 mm cylinders
 

Not if you use the LN Engineering 100mm "slip fit" cylinders in a 3.0 liter block. LN also makes a cylinder that requires spigot machining. If I remember correctly, they have cylinders that go up to 105mm bore!

Cylinder heads
 

I used a set of Carrera heads and an early large port CIS air box and runners when I built my 3.3SS engine. The runners only had to be flared about 3mm to match the heads. The later CIS small port air box and runners are much smaller and would be more restrictive. In either case, the main restriction on a CIS system is the air plate.

The Carrera intake has even larger runners and is less likely to have a vaccum leak because there are less pieces to the intake track. To facilitate a modern fuel injector, the injector groove needs to be opened up on an SC head or you get an adapter that spaces the intake up and bridges the gap between the SC injector groove and the Carrera intake fuel injector opening.

Intakes.........
 

I agree, the Carrera intake is better in all respects. However, I wanted to see what I could do with a CIS system. The difference between the large bore CIS runners and the Carrera head intake ports is only about 3mm, not a huge difference. To match up the Carrera head intake ports and the CIS runners, I used a set of Carrera spacers that I modified to match both the intake runner side and the head side configuration. This was done by first cutting a retaining notch (much like a dentist would do when putting in a filling) and then building up the injector notch with JB Weld epoxy. Once dry, I used a file and a Dremel tool to shape the epoxy to match the surfaces. It took a bit of work, but now there is a nice smooth air flow and no vacuum leaks between the head and intake. Another reason to use the CIS was that I wired the cold start valve to act as a 7th injector at full throttle. I did this by mounting a spring loaded contact switch where the throttle arm would make contact only at full throttle. This allows 12v to the cold start valve, injecting extra fuel thru the CS manifold that is built into the air box. At full throttle, the air/fuel mixture drops from about 14:1 to around 13:1 (measured on an a/f gauge) to minimize the possibility of detonation or burning valves. You can feel the kick when the CS valve opens and it is completely automatic only being in use at full throttle. The rest of the time it just sits there and acts like a regular CS valve.

If you needed a 7th injector to accomplish this your CIS was not running properly ( unless you are using forced induction).

Maybe I didn't "splane" this right............
 

The purpose of the 7th injector is to allow the engine to run lean (stochiometric) in normal driving which allows for better fuel mileage. However, when pushing an engine near or at redline, it is advisable to richen the mixture. Yes, the CIS system will do some of the enrichening, but not enough for an engine built for higher performance. So, now I have an engine that runs at around 14.5:1 under normal conditions but will go to about 13:1 at full chat. Considering today's fuels, I would rather be safe than sorry!

Sounds cool.

I'm not dissing the CIS intake. The original poster stated that he's decided to convert to EFI and have Steve rework his heads. I was just saying that he should consider upgrading to a Carrera intake. If he's getting the heads already reworked, it wouldn't cost much more to get them modified to idealize them to the Carrera intake.