RS vs. S-spec cams?

[MountainRoads]

Greetings and I hope everyone is having a good holiday season.

I'm doing some mechanical cleanup on the 2.7L in my -6. One of the great mysteries when I bough the car, was what cams were installed. It was advertised as an RS-spec 2.7. PPI dyno test showed adequate power, but it fell off a little early for an RS engine. Tons of low-end torque, so undersized carb venturis were also a possibility. Unfortunately, no factory markings on the cams. I traced the manufacturer (Delta), and even they had no idea. Finally had the cams measured, and they came out to: Intake Duration: 268, Exhaust Duration: 231, Intake Lift: .444, Exhaust Lift: .392, and Lobe Separation: 90 (deg). These measurements are closer to Solex cams than S or RS. Also, the venturis were indeed slightly undersized (34 mm), which combined with the reduced cams, would explain the good low-end torque and hi-end power drop off.

Q1: I did a little digging and couldn't find a definitive answer: Is there any difference between 'S' and 'RS' spec cams? I've always believed them to be identical. What little info I could find suggests that the RS cams have a slightly higher exhaust lift.

Q2: My mechanic suggested that Elgin 306/288-106, which are similar to Garretson GE-60, would make great cams for this engine, as long as I upsized the carbs appropriately. He really knows his stuff and I trust him. Anyone have experience with these cams or have other suggestions? The car is mostly driven on the open road and I'm not looking for anything radical. Opinions welcomed. Sorry, I can't provide details on CR or any other work that's been done.

Thanks in advance.

- MR

[Chuck Miller]

MR,

The 1973 911S and RS share the same cam shafts with the same part numbers.
901.105.168.00 right
901.105.183.01 left

Hope this helps

[jameshtaylor]

I use the Elgin 296/276 cams, which they called the Modified S, and like them a lot. If your car is going to spend time on the track, the 306/288 would probably be great. Around town, my 296 or something even less would, at least for me, be more fun. Trade off is 5000-7000rpm performance vs 3000-5000, the lattter being much more important and useful day to day. You certainly want bigger carbs and ports.

[MountainRoads]

Thanks Chuck, for confirming my belief. Much appreciated.

- MR

MR,

The 1973 911S and RS share the same cam shafts with the same part numbers.
901.105.168.00 right
901.105.183.01 left

Hope this helps

[MountainRoads]

Thank you James. Will definitely consider your suggestion. To your point - very little track use expected for this car. Not an around town car either, though. Back roads on the dry side of the mountains are my goal. Thanks again.

- MR

I use the Elgin 296/276 cams, which they called the Modified S, and like them a lot. If your car is going to spend time on the track, the 306/288 would probably be great. Around town, my 296 or something even less would, at least for me, be more fun. Trade off is 5000-7000rpm performance vs 3000-5000, the lattter being much more important and useful day to day. You certainly want bigger carbs and ports.

[Richardnew]

Horsepower is something you brag about. You actually drive torque.

Richard Newton
Historic Racing Images

[Flieger]

Horsepower is something you brag about. You actually drive torque.

Richard Newton
Historic Racing Images

If that were the whole story we'd all be swapping in diesels. You make torque from horsepower using gearing. If you've got the gearbox matched to the engine then horsepower will win, especially if you've got a shift mechanism that doesn't interrupt torque transmission or does so only very briefly. With an H pattern then you've got an advantage if you make fewer shifts but what you want is to have a wide enough powerband at whatever rpm that you can stay in it between gears.

[Chuck Miller]

The 1973 911S and RS share the same cam shafts with the same part numbers.

If I'm not mistaken, I believe all S cams share the same profile from '67 through '73. The only difference being the tapes holes on the left cam for the '69 -'73 MFI drive gear.

[MountainRoads]

I found a camshaft reference sheet that I stole from the bird. It shows slightly different Intake lobe heights, for the 67-68 carbureted 'S' vs. the later MFI. Interestingly, the valve lifts are identical. I can't swear this data is accurate.

- MR

If I'm not mistaken, I believe all S cams share the same profile from '67 through '73. The only difference being the tapes holes on the left cam for the '69 -'73 MFI drive gear.