stv951

I am in the process of rebuilding a 911 SC engine which I put brand new high compression (10,3:1) Mahle pistons. These are 3,200 cc dispacement. I have just ordered PMO carbs and the question is which cams should be better used for this setup. (single spark ignition)
The car will be used mainly on the street and fast mountain drives, and occasionally on driver eds. It is NOT a race car so I dont want something extreme. Mid range torque is what we are looking for

Shall i go for 964 cams ? Would these have enough clearance ?
I have not got much experience over these so I need some good advise please

TIA

Steve

theiceman

I have an old Thread that Steve W commented on a few years ago i am keeping as I had similar questions. I am sure Steve will be along Shortly to answer your question . From what I have read i am going to do a 3.2 cylinder and pistons and 964 CAM conversion but keep stock CIS.

I am sure the 964 CAMS will fit and clear.

NineMeister

964 cams are the perfect choice, huge mid range torque and peak power well below 7000rpm. Yes, they will clear the pistons.

Steve Weiner-Rennsport Systems

Colin is spot-on here.

With stock pistons, you don't have much room to install anything more aggressive so the 964 grind cams will work very nicely.

NineMeister

A set of higher comp 3.2 pistons, small port heads, 964 cams, SSI and PMO's gave these results on the 9m Racing dyno. Interesting plot for two reasons, one is the 270hp from a relatively cost effective engine, the other is that the customer installed it himself and "ran it in" on Mobil 1 by driving it at 2-3000rpm for 500 miles. The plot shows the power results as it arrived back, then again after we stripped, deglazed and rebuilt it, ran it in on mineral oil and tested it again. Big difference!

Jonathon Rolstin

Wow nice to see some real world numbers to put a magnitude on the mistake of using the wrong break-in oil...! Thanks for sharing. This sounds like a popular build and something I would be considering but would want to go modern injection/crankfire ignition.

My question would be, for the build at hand, is it not necessary to twin plug to deal with the added compression? Would twin plug give you more power by allowing you to advance the timing more, or is it over-rated? Curious on the opinions of the pros

Thanks!

Steve Weiner-Rennsport Systems

Hi Jonathan,

Twin-ignition is necessary when compression ratios go above 9.8:1 on street gasolines.

Twin-ignition allows less advance for more torque and less heat in the cylinder heads.

Read more here: http://rennsportsystems.com/letstalk...twin-ignition/

Jonathon Rolstin

Right less advance! That's what I meant... Thanks for the info

hkspwrsche

Are 46 PMO the right size for all else considered?

TT Oversteer

Maybe too late if you've already bought pistons but the beauty of carbs is that you can run more aggressive cams with some real overlap. This makes these engines really breathe and changes their character completely. 66 solex is a great grind with carbs but you may have to machine valve pockets in the pistons. I question the value of running low overlap cams that are designed for motronic or CIS injection with carbs. 10.3:1 is too high for single plug on pump gas. You'll have to retard the timing to avoid detonation which negates the power gain from higher compression.

Steve Weiner-Rennsport Systems

For a 3.2, yessir.

stv951

Gr8 Responses. Thank you all first of all.

So Colin you suggest that the engine should be "run in" using plain mineral oil (10w50 ??) and after roughly 500 miles switch to synthetic Mobil 1 (0w-40 ?)

stv951

I also understand that 964 cams are the way to go. These are not so aggressive so should I also go for twin plugs ? The car is for street use on premium pump gas. In Europe this is 100 octane (Shell Racing)

stv951

Although there are plenty of twinplug threads, what would be a nice set up for this 911 SC 3,2 on 46mm PMOs ?

NineMeister

All rebuilt 911 engines should be run in on mineral oil. We use a Millers product (CRO) and a pre-determined procedure to break engines in during the first dyno session; if the session is short the oil is left in for another 100-200 miles then changed out for semi-synthetic. Only after another 2-3000 miles of hard use would we consider then running a full synthetic.