Ideal intake port size for an SC?

[Gunter]

I remember intake port being 39 mm on the '78-'79 SC's and 34 mm on the '80-'83 but what size are the exhaust ports?

And for that matter, what are the sizes on SSI pipes for SC's '78-'83 ??

All kinds of info about SSI's on Pelican and Stainless Systems Inc. but nothing about pipe sizes. ??

Thx.

[Tyson Schmidt]

Originally Posted by Gunter

I remember intake port being 39 mm on the '78-'79 SC's and 34 mm on the '80-'83 but what size are the exhaust ports?

And for that matter, what are the sizes on SSI pipes for SC's '78-'83 ??

All kinds of info about SSI's on Pelican and Stainless Systems Inc. but nothing about pipe sizes. ??

Thx.

The exhaust ports are 35mm.

SSI's are also 35mm I.D.

[Gunter]

Originally Posted by Tyson Schmidt

The exhaust ports are 35mm.

SSI's are also 35mm I.D.

Thanks Tyson,

Even though the '78-'79 has larger intakes than the later SC's, the exhaust size is the same for all 3.0 liter regardless of intake size?

'78-'79 intake 39 mm
Exhaust 35 mm

'80-'83 intake 34 mm
Exhaus 35 mm

Somehow, that doesn't sound right.

[island911]

I know many have a exceedingly simplistic mental model of an engine being the same as an air-pump; but clearly, much more is going on.

Intake size varies to optimize the mixing of fuel and air for the combustion process, thru the rpm range.

Bigger intake/valves of the earlier SC's had the same peek HP of the later (smaller intake) 3.0's, But the later had better midrange torque/HP. --that's optimization.

[911st]

I suspect to shift to small ports on the SC was probably needed to achive emissions goals.

CIS is an interesting system. It basically operates at a 100% duty cycle. The only way to vary fuel delivery quantity is to vary pressure.

On top of that the intake valve is only open something like 220 deg's with the injectors spraying for 720 deg's and fuel probably just sits there sticking to the wall of the ports resulting in poor atomization.

Thus, Porsche probably went to a smaller intake port to better accelerate the incoming air to get a better mix.

This is the same time Porsche adopted different emission controls shifting to a Lambda type system and away from air injection.


Think about it. they went from a 34mm to a 40mm intake port with EFI. That is a about a 40% larger port for a 5% larger displacement motor.


At least, that is my guess.

[Racerbvd]

[fred cook]

When I built my 3.3SS engine, I decided to go big with the ports and intakes. I used a set of Carrera race prepped heads with 41mm intake ports and 39mm exhaust ports. Valve sizes stayed stock Carrera. Pistons are Mahle at 10.5:1. For an intake, I used an early CIS air box and intake runners that measure 39mm inside. I opened up the intake runners 3mm to match the heads. Cams are 964s and ignition is by Electromotive XDi twinplug. Turns out, this engine has very good low and mid range torque and will pull quite strong all the way up to ignition cutout at 7500 rpms. I think that I set the cams at 1.30 but would have to refer to my notes to be sure. I installed an Innovate air/fuel gauge to use while tuning the CIS. I have it set to run close to stoichometric (14.7) for normal driving but can richen it up with a small turn of the adjustment screw. To make certain that a valve would not run lean and burn, I added a full throttle switch that activates the cold start valve to put a bit more fuel into the engine when it is being pushed. The extra fuel at full throttle lowers the a/f ratio to around 13.5 or so. If I drop the a/f ratio to 13.5 then it goes to around 12.5. This engine idles well, is torquey and is very strong on the top end. Just what I was hoping for! Having the extra displacement certainly helps as does the improved ignition system. Fuel mileage seems to be about what it was as a 3.0 SC. Runs well on 93 octane pump gas!

[Bill Verburg]

No such thing as one ideal spec

smaller ports raise velocity and help lower rpm performance, bigger ports flow velocity is reduced until higher rpm where they can help

same thing w/ manifold lengths longer is better for lower rpm shorter for higher

this applies to intake and exhaust