Large port vs smal port SC

[robert walsh]

This is alot info to digest, thanks for all the input, but I am a little confused do the large port engines have lower cmpression and the small port higher compression? I would like clarification but regardless which of the 2 type of engines do you guys prefer and I'm talking strictly "stock" large or small port 3.0 SC engines? The knowledge you fellows have on these engines is truely amazing. Thanks again

[javadog]

In general terms, they all started at 8.5:1 and the later engines got progressively higher compression. The ROW engines ended up with higher compression than the US.

I'd prefer a ROW engine over a US engine, the later the better. 2nd choice would be a '78-'79
US engine, although the later ones drive a little better in daily driver useage.

JR

[Peter Zimmermann]

I agree with JR, the small port engines with their better torque 189/4200 rpm vs 175/4200 rpm makes a better, more responsive car to drive daily. When driven back-to-back the '78/79 cars feel a bit sluggish in town - not bad, but noticeable - while the '80-83 cars feel a little quicker. This feel is probably due in part to higher compression, 9.3:1 vs 8.5:1, but the whole package is what produced the results. I personally like the late engines better in street-use cars.

[911st]

Get the 80-83 SC. With no air pump, higher compression, the Lambda system it will be a bit faster and get better fuel economy.

By the newest and best condition car you can afford.

[fintstone]

Originally Posted by robert walsh

This is alot info to digest, thanks for all the input, but I am a little confused do the large port engines have lower cmpression and the small port higher compression? ...

yes. I think 78-79 was at 8.5:1 80-83 was 9.3:1.

[javadog]

And the '80 ROW was 8.6:1 and the '81-on ROW were 9.8:1.

JR

[Bill Verburg]

Originally Posted by javadog

And the '80 ROW was 8.6:1 and the '81-on ROW were 9.8:1.

JR

I've gotta wonder about the 8.6, I know it's in the spec book but the pistons & heads were the same as used on the '78 & 79. 8.6 is likely a typo, 8.5 is richtig.

it's interesting that the US cars got the higher 9.3 cr before the RoW cars
US 930/07 got 9.3 in '80, that years RoW 930/09 had 8.5(or 8.6 if you prefer)

US cars never went any higher, while the 81-83 RoW 930/10 got 9.8

[Mysterytrain]

But we do like the larger intake runners on the 3.2 SS..correct?

[fintstone]

It seems to me that the best bet if one wants to stick with a 3.0 is to do what I had Supertec do for me...build an ROW of sorts. Use an '83 bottom end for the higher compression and better ignition...but change the pistons for a slight bump-up in compression. Use a '78 top end for better breathing. Add a tweaked, earlier type CIS, no air pump and some cams (webcam 20/21 grind in my case). Top it off with a "74 type exhaust (SSIs with no cat) and end up with a better than ROW engine.

[Bill Verburg]

Now we're getting away from stock motors, if I was going to do a 3.0 it would become a 3.2ss w/ Max Moritz 9.8 98mm pistons, in the interests of keeping costs down, single plug, hollow CS valves(or undercut ss), TWM manifolds, Motec, GE60 cams

But then you might as well spring for a 3.6 conversion

[Mysterytrain]

The reason I brought up the max moritz mod was because it was and I guess still is a common upgrade to these motors. Along with the boost in compression there is also a boost in displacement...so I assume in that case the larger runners are perferred. I got to tell you I love the engine. Gunter should be here any minute.

[Bill Verburg]

The MM 98s also have a better, piston dome w/ room for some decent valve lift, not the swoopy cis squish dome.

as to which cis head is better(small or large), I certainly dunno, you would have to build 2 identical engines w/ the 2 different head choices and compare. Neither will be bad

95mm cis 8.5


95mm cis 9.3


MM MFI/carb 98mm 9.8

[robert walsh]

Going wat out in left field here, but aren't these pistons similar in design to a "hemi" type piston?

[javadog]

Well, a 911 is considered to have a hemisperical combustion chamber, so...

JR