Large port vs smal port SC
 

What are the advantages or disadvantages of large vs small port SC's, when running totally stock engines? Performance diferences, both stock? Why did porsche do it? It seems the newer the sc the better the galvanizing, less rust, but you are into the small port engine. Any advice.

well in the 80's for the us spec sc's they used higher compression and smaller ports for lower end tQ so I have been told along with meeting emmsions and the galvinzing (full body) started in 77 I believe. I use small port sc heads on my engine check out my build in my sig

the RoW SC 930/10 was the strongest street 3.0L ever offered, 204hp@5900, 267NM@4300, 9.8cr, small port

the RoW C3 930/02 was the next strongest, 200hp@6000, 255NM@4200, 8.5cr, large port

all power & torque per DIN 70020

smaller ports speed up the gas flow in the intake, it gives stronger vacuum signals and better throttle response, if you can do that while producing the same or more hp & torque you have a better engine.

The same controversy continues today but is not so clear cut, there is a school of thought that preferes the small port 964 heads w/ larger 993 valves over the large port 993 heads,

galvanized body panals were introduced from the bottom up, by '72 the floors and inner panals were galvanized, by the start of '76 all but the roof were galvanized, by the end of '76 the entire chasssis structure was galvanized

Thanks for the insight. I am the owner of an '83 cab, seemingly rust free. I pent better part of a year looking for an SC sunfroof coupe, when i found this one i couldn't resist it, paltinum/cork/blk. It maybe just that they are few years older but i often found the earlier SC'S to have more rust issues, from all parts of the country. Thanks agian for clarifying the large port vs small issue, very interesting.
Anyone have specific perfromance stats for 78 sc vs let's say an 83sc.

these are factory #s for US cars w/ manual trans
'78SC
top 225km/h
0-100km/h 7.0
standing km 27.5
fuel consumption @90km/h L/100km 9.2
fuel consumption @120km/h L/100km 11.2

'83SC
top 225km/h
0-100km/h 7.0
standing km 27.5
fuel consumption @90km/h L/100km 8.0
fuel consumption @120km/h L/100km 9.7

Looks like your dead on, identical performance with better mileage.

One thing I will say is that I would take the numbers that Porsche published during this era with a liberal dose of salt (on a Margarita glass....?) Porsche changed quite a few things during the Carrera 3.0/SC era and didn't change much of the published data. It's been hard for me to understand why they lost 20hp from the Carrera 3.0 to the first SC.

Likewise, they changed the cam timing all over the place, the port sizes, the exhaust and still quoted the same power output for the US cars during the SC production run. My own experiments with cam timing way back in the day yielded noticeable differences in acceleration, so I know the power changed. The weight numbers were always suspect too, especially in 1980 when lots of things that were formerly optional became standard.

When I drive a '78-79 car, it feels vastly different from a later SC.



JR

While I can't disagree about the suspect #s, things like DIN specs are not subject to debate, the performance #s except for fuel consumption which is DIN 70030,part1 spec are suspect as they don't specify the way the suspension is setup, weight of the car or driver, atmospheric conditions or the enthusiasm of the driver

the change from the C3 930/02 200hp to SC 930/03 or /04 180 is easily explainable by the fuel and ignition tuning used to meet more stringent emissions and fuel economy standards, and yes, cam timing has an effect on the shape of the torque curve too.

The biggest change in the exhaust was the use of a cat on US vs less restrictive RoW premuffler, this also makes a difference

Porsche has been conservative, over the years. There is some sort of German law that says ervey car they produce has to meet or exceed the published specs. They sandbagged a little, to give themselves a cushion to deal with production variations. Why they did this I don't understand, because they dynoed every engine that went out the door.

I know the differences between the Carrera 3.0 engine and that of the first SC and I don't see where the 20 hp went.

JR

I am not an expert but no one knows 100% why Porsche made the ports smaller but I am pretty sure it was the increased port velocity as Bill noted.

Porsche put very small ports on the 930 Turbo also but then when they did the 993 Turbo went to a big port approach.

I suspect the reason for the small ports is to increase the intake port velocity as Bill noted. One usually dose this for better low rpm performance which is a likely reason / benefit.

However, I think another reason was because of how the injection system operated. CIS, Continuous Injection System, sprays fuel 100% of the time and the fuel has to sit in the intake port until the valve opens. With the smaller ports and there increase in velocity helps the air and fuel mix better for a better more efficient burn.

This is also why the CIS piston looks like it dose. To help swirl the air and fuel for a better mix and burn.

Thus, I suspect Porsche went to the near the smallest port needed to support the potental of there stock motor.

For a street car the small ports along with the added compression and lack of air pump seems like a better way to go and if tuned right should be able to make more HP than the lower compression big port. Especially the low to mid rpm range.

If I were building a 3.0 I would be tempted to start w the small port and have some magic worked on it by and expert at porting 911 heads.

With an EFI turbo I do not think I would expect much benefit w a small port head.

SC's have CIS and the compromised cams that go with it. 3.0 Carreras were MFI. 'Nuf said...

No, Carrera 3.0 had CIS.

Not the one we are talking about, that had 20 more hp than the SC. The later ones were CIS, but they had no more power than the SC.

Nope both the '76/77 C3 930/02 and the '78'79 SC 930/03 & /04 are CIS, 8.5cr motors

Biggest difference is in the crankshaft, SC has the more robust, heavier 930 crank w/ 59.9mm mains, C3 has the old standard but still sturdy and reliable 2.7 crank w/ 56.9mm mains

Look at the '74's.

You're probably thinking of the '74 Carrera 3.0 RS, which had MFI. The only other 3.0 liter engine with MFI was the '84 911 SC/RS.

JR

yes, the '74 Carrera 3.0RS has MFI, but just exactly what does that have to do w/ the discussion here, which is specifically about the evolution of the CIS 3.0s from '76 thru '83, jeesh:confused:

Also, I heard the disributor rotor rotates counter clockwise on the 78-79 SC's. Has anyone else heard this?

I used the later bottom end for higher compression and the early top end for the large ports. Add some cams and it makes for a pretty solid combination.