changes in SC intake runners from 78-83
 

I am in the process of doing a CIS-to-EFI conversion on my 78 911SC 3.0L. I found an airbox on ebay with intake runners that I was going to hackup. These came from a 82 3.0L, so I thought they would fit on my 78.

They don't. The ID of my runners are 39mm and the ones from the 82 are 34mm. The airboxs are also not compatible because the six holes on the 82 airbox are smaller.

Why did Porsche reduce the diameter for the later year SC's? Should I use these small diameter runners/airbox on my 78?

The intake port size was reduced on the SC in '80. The runner ID was reduced accordingly. Smog crap. They also added the O2 sensor in '80. Using later runners on early heads would be step backward. You need the '78/9 runners and box.

Tony,

Go ahead and use them, they'll fit!

Those are exactly what I hacked up for my three liter. Don't let a minor detail hang you up...just do it!!! Your engine will never know your manifolds don't match up.

In case you're wondering, they went to the smaller manifolds to give back the low end torque that the smog took away.

Joe

Joe,

I though about what the implications would be if I used smaller ID runners, and I though exactly what you mention. The low end would get a boost. I also think that the top end will suffer, though. This mod may make it a better street car, low end torque is what I need.

Thanks.

The one thing I noticed about EFI is that you end up with so much performance all around that top end performance becomes a non issue.

Joe

I was tempted to enlarge my intake ports while the heads were off, just in case I run across an early SC air box and runners. I am a normally-aspirated kind of guy who doesn' t mind power and torque. The later SC pistons with their 9.3:1 CR, combined with the earlier intake ports and air box, would make for a powerful engine. If I do run across an early intake system, I'll just port the heads carefully while they're on the engine, using the larger intake runners as a template.

Superman, do you mean you'll actually increase the valve seat from 34 to 39mm? Or just the port? How will you do this? Will the early SC heads go with a later SC?

cheers
-Brad
80SC

I'm not suggesting changing the valve or seat size. That is the same for early and late SCs. What's different is the intake ports. The area air travels through just before it gets to the intake valve head. Yes, I think the early heads will just bolt on the late engines.

Porting heads without removing them is a little dangerous, in that you can get metal filings in the combustion chamber. I would likely remove the engine for this and place it on an engine stand so I can grind away at the ports while the engine is upside down. Still, I'd pack pieces of cloth down against the intake valve head and I'd make sure the valve is closed before I start.

I remember reading in an SAE paper a long time ago that short intake runners move the torque up the rpm range and long ones downward (maybe that's why drag racers use those tall manifolds). Anyway, the indication was that length was the determining factor, not diameter. Can anyone verify this?

Tony,

Where do you stand on your project?

I have recieved my MegaSquirt ECU, built and tested it. I am now awaiting my Carrerra intake and other bits to put on the engine....

I dont know if anyone else on the board got in on the MS purchase but I must say that from what I can tell so far it is top notch!!!

I will try to post some pictures of my completed unit if anyone is interested...

Ben in SC
80 SC Euro

I have not received my Megasquirt ECU yet. I am in the process of building fuel rails from square aluminium tubing. I'll post pictures on my web page when I get a chance.

I think that narrower, longer intake runners will have the effect of increasing the speed of incoming air, as in the 'venturi effect' at the heart of how carburetors work. With the air moving faster, this changes the way air is packed into the combustion chamber while intake valve is open. At slower rpms, this air is packed in pretty good. Unfortunately, the same physics that speed up this air also reduces air volume at high rpms, so you lose high-rpm HP.

Superman, you are correct.
Also, I want to add that the length of the runner is critical in that it provides a measure of tuning, just like tuned exhaust.
When the intake valve closes, a pressure pulse travels back to the plenum and is reflected back to the valve again. By varying the runner length the reflected pulse can be used to pack more air into the combustion chamber at a discreet RPM value, yeilding a mild supercharging effect. Make the runner short and you get the effect at high RPM. Make it long and you'll have it at lower RPM.
Intake tuning, like exhaust tuning, is another way to improve the volumetric efficiency of an engine.

Here's a question I have, as someone who is now considering moding a 3.0L with a 964 intake:

What happens to backfires when the flapper is eliminated by moving to the newer intakes? Does a 964 intake have a flapper built-in?

The backfiring with CIS was due to cold start enrichment injector dumping fuel directly into the intake, which could then explode. With EFI the cold start enrichment is done by increasing the pulse width of the EFI injectors so this is not an issue anymore when converting to EFI.

This 19 year old thread is a blast from the past! I'm still around and my 1978 911 is still running the BitzRacing EFI conversion. I just took it out of winter storage a couple of days ago, and started on the first crank. Runs like a charm.

Thanks Tony. I am one of your followers as a bitz racing customer and I have loved your product for around 8 years now. I still have an occasional backfire, but I guess what you are saying is I should be able to tune that out. Will do.

Note: The occasional backfire may actually be related to a vacuum leak in the 3.0L plastic intake box. We shall see.

Hey Tony, went through the old yahoo archives looking for an MS2 extra V3.57 software post/file that I thought you had created years ago allowing us to use an MS2 for COP and still have the “pink” wire for fuel working the same way.
(Reversing the normal MS FP function.)
Maybe it was just for MS 1 Extra, but an MS2 version would be real nice… :)

Unfortunately I don't have one for MS2, only the MS1 Extra and B&G firmware.

No worries, just thought I’d ask.