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.

LOL

Yes, 78-83's rotate the other way... it was worth at least 12 maybe even 17.4 hp :D

-Michael

Settle down there, Bill. This really isn't anything to get so worked up about. The question was posed as to why there was a 20 hp drop from 3.0 Carreras to SC's. I simply offered an observation, using the first 3.0 Carrera as an example.

carrera 3.0l RS and Carrera 3.0l are really two different beasts and should not be confused as Bill pointed out.
The carrera 3.0l RS produced 230hp with hi comp pistons, S cams, MFI.... There is really no point of comparison with the first SC and its 180hp.
The relevant question at stake is between the carrera 3.0l 200hp and the first SC 180hp: same heads and ports, same pistons CR, same CIS system. Then the question is interesting. The only differences are:
Lighter crank on the C3
Different cam timing (more aggressive on the C3) but same cam (so an SC owner can very well use the C3 setting)
Some differences in ignition (SC went breakless) and fuel distributor (cheap to change)
btw ROW cars had the same exhaust system (no cat) and show the same output difference between C3 and SC 180hp.

It has always seemed strange that these apparently light differences can account for 20hp but still dyno tests and perf results evidentiate the C3 advantage. I think the difference in the crank mass is probably a more important performance factor that some of us think.

Mathieu

Crank mass would have no effect on horsepower. Porsche measured it with a steady-state type of dyno reading. The US '79-79 SC had the same cam timing as the Carrera 3.0. Only the ROW SC had the advanced cam timing. The quoted power output was the same for all SC engines in 1978, with different cam timing and differences in the exhaust. The US cars got a two-way cat while the ROW cars got a pre-muffler. Not all of this adds up, in my book.

JR

Interesting, so if crank mass does not affect hp, and given that US SC had the same cam timing than C3, the only difference is ignition set up and fuel distributor.
How different is the SC fuel distributor from a C3?

Both the airflow sensor and fuel distributor are different between the C3 and SC. I have no idea what the differences are.

JR

Sub'd ......to become more educated

Just had the fuel distributor on my USA spec engine (83 SC with small port heads and runners) updated with one off a Euro spec early SC with large runners - only the fuel distributor and WUR were changed.

The difference is like chalk and cheese - no dyno results yet to back it up but to give you an idea at the last track day my lap times came down from 90 to 85 seconds - nothing else was changed.

My USA spec SC (std 180hp) is putting in lap times to match other Euro/Australian spec SC's (204hp)

Tim

other differences some mentioned back in post #9 are
1) fan - the 226mm, 11 blade '78-79 cooling fan, vs the 245mm 5 blade '76/77 fan the SC pumps 1380 l/s @6k, the C3 1265 l/s @6k
2) sai - the SC has a parasitic sai pump which the C3 doesn't
3) exhaust C3 has a free flowing premuffler, SC has a more restrictive cat,
4) C3 has a 930 part # for fuel dist and air flow meter, SC has a 911 #
5) yes the SC dist all rotate backward from any other 911 before or since, they have a pointless triggers too, C3 still used the old 911 point type dist.

I don't know the specific differences in f/d or afm either, they both use 63mm throttle bodies, but back in the day the C3 parts were sold as speed parts for SC. Also remember that emissions regs were tightened everywhere between '76 and '78, particularly CO which is proportional to power producing afr ie lean mixtures to reduce CO also reduce hp

I agree that the crank won't make a difference in steady state #s, it does make a diff in transient response

You can see a similar drop in hp when you compare earlier 2.7 cis RoW to US or even worse Calif engines. The drop in hp is in the same ball park, especially when the smaller engine size is factored in.

And no I don't want to hear about the 911/83 mfi engines, they are not relevent here either

Cam timing: Take a given spec for an acceptable range, say 1.4 - 1.7 mm at overlap for a USA, 930/16, '81 SC. If you time your cams at 1.4 you will be on the hp "side," 1.7 will put you on the torque "side." Generally, for a commuter car, finding a happy medium, say 1.5 - 1.55, will give excellent results for daily/weekend driving. Of course, the difference between left and right cams in an engine should be, ideally, zero. By the way, the high hp 930/10 used cam timing (.9 - 1.1 mm) the same as the USA 930/04 large port engine!

FYI: All SC engine ignition distributors turn counter-clockwise...

Wouldn't it be possible to invent a new tensioner that could alter the cam timing, like the on the 968?
It can't be that hard, using long plastic ramps like on the 3.6 tensioning the chain "together" and moving the package up and down.

Maybe it would be a waste of time and effort on a single cam engine.

Don't forget, a DME ECU sends control signals to electromagnetic valves, which, in turn, activate cam adjusters that are part of twin tensioners. The change from hp to torque is done by changing the valve overlap between intake and exhaust valves, so I think that twin cams are required. I'm afraid that our wonderful SCs are a little to antiquated for all this!

Yes, but that would be the easy part to do. It's basically rpm based when to change cam timing.

Porsche did change the timing of the SC-cams between the Carrera 3.0, SC and Carrera.
But as you say, I think twin cams are needed to make it meaningful.

The 81-83 9.8:1 Euro SC engines used the large port intake runners and large port heads. Bruce Anderson's book is incorrect about this. I have rebuilt a few a these engines and all had the large heads/runners. Here is a link to a thread on Pelican about this. http://forums.pelicanparts.com/porsche-911-technical-forum/373264-did-i-get-special-sc-motor-factory.html?highlight=large+port+heads

This is just some guessing but if you look at the difference in total TQ, HP, and the rpms they are quoted at something is up.

I would guess if the motors are identical except for cam timing the total TQ would stay about the same but move up the rpm range.

However, TQ increases about 7% with the Carrera 3.0. indicating a substantial increase in efficiency between the two motors.

And, Porsche quotes TQ peak at the same rpm for both motors.

The 3.0 Carrera quotes 188# @ 4200rpm.
The SC quotes 175 # @4200.

Why would they quote TQ at the same rpm but quote HP peak at 5500 for the SC and 6000 for the Carrera 3.0? If the HP peak looks to have moved up 500rpms. I would think the TQ peak should have moved up about the same.

Something is fishy with the quoting of TQ at the same RPM with both motors.

While TQ increased 7%, HP increased about 10%. This seems to be an indication that the 3.0 Carrera motor is holding its effecency better w increases in rpm.

Something more than cam timing looks to be involved in the difference between the two motors.

Maybe Porsche played w the tune w the SC to meet smog requirements.

Or, Porsche could have been playing with the numbers to a degree for some reason.

Again, the 3.0 Carrera made about 7% more power with each stroke, did so about 500rpm higher in the power band, and the efficiency's did not fall off as fast with increases in rpm.

Besides cam timing this could be due to ignition timing or A/F ratios.

It is intereasting that with Porsche's increase of the CR of the later US SC motors and droping of the air pump should have showed up as an increase or change in the peak TQ and Hp numbers but no change was quoted.

I think Porsche was to at least some degree playing w the quoted numbers.

I have though this as well. Not only with the power and torque numbers, but with the weight.

Also, note that the ROW SC in 1978 had more torque (195 or so) than the Carrera 3.0.

JR

Interesting if true, probably is from the part #s
all I know for sure is
US SC cyl head w/ valves reary for installation
'78 930.104.019.05
'79 930.104.019.08
'80 930.104.018.03
'81-930.104.018.05

looks like a major redesign for 80 up, possibly small intake port

RoW SC
-'80 930.104.019.05
81- 930.104.019.08

looks like all the RoW used the same head as the '79 US

It sure would be interesting to know exactly what changed w/ the minor and major revisions

Very probable. Take the example och the 3.2 and the 3.6 engines.

ROW numbers:
The 3.2 quoted at 231 hp => 72.2 hp/liter
The 3.6 quoted at 250 hp => 69.4 hp/liter

The 3.6 has higher compression, twinplug and a hotter cam....

I think Porsche has a history of sticking convenient #s on their engines.

The bump from 180 to 188 hp in -80, is probably more of an "paper upgrade" than from the small compression increase.

Also droped the air pump in 80 that had to use some HP.

DIN spec, which are the only hp #s I've quoted here is not subjuect to manipulation or interpretation, those are take it to the bank #s.

a cam w/ longer duration and or lift is not necessarily a 'hotter' cam when used in a larger engine.

the initial prblem w/ 964 was high heat in the heads, this really lilmited hp potential. They eventually went to the RR530 alloy used in 930 heads for the 993