Recommended/applicable Cams for 2.7L with stock pistons, and associated questions

tommott · 05:38 PM

Been working on a 2.7L motor to put in a little '72 911 project that has been inching along, and I'm trying to ascertain what are the best cams to use with my setup.

Here's some background. The motor is currently out of the car, it originally came out of a '74 911. The CIS fuel injection is already long gone and I have sourced some Webers to go in their place. I am NOT going to rebuild the motor, that'd be another project for another day which would probably ultimately lead to going with a larger displacement, aluminum cased motor. The head studs were within factory torque specs. The heads are going to have to come off however as I ran into numerous issues when I removed an aftermarket heat exchanger/header system. All the exhaust studs were rusted and every single one of them broke off, save for two which turned all the way out themselves, when removing the incorrect heat exchangers. I am now planning on taking off the heads and taking them to machine shop to have the remaining studs drilled out and re-tapped.

Since the heads are coming off, and the fact that I'm going with a Weber setup, it only makes sense to change out the cams. My knowledge base up until this point is all based off of Bruce Anderson's 911 Performance Handbook, where the suggested setup is to run 'E' cams. After searching around the forum here the last couple days however I'm starting to run into some concerns/confusion. It seems that most folks here are running early 'S' cams with this type of setup. I was always under the impression, not sure I where came up with this, that 'S' cams would not work with 2.7s because of valve clearance issues. Is it only possible to run 'S' cams with RS pistons? In Anderson's book he also recommends using the RS pistons with 'E' cams. This all is starting to have me worried that the only cams that will work the stock pistons is stock cams, in double checking Anderson's book it appears that the RS and stock pistons have the same 8.5:1 compression ratio. If so is the only advantage of the piston swap to open up your options regarding cam choices? Again I'm not planning on really digging any deeper into this motor because I'm aware of the extra work needed to completely rebuild a 2.7 magnesium case, and if ever really go to that point I would probably just start shopping for a 3.0L or 3.2L motor.

If I can go with some other cams is the preferred choice these days to run 'S' cams on a street car. The old school thought back in the day was that 'E' cams where much more manageable in everyday street driven applications. I'm aware of the the shift in usable power towards the upper band of the RPM range with 'S' cams, which in all honesty doesn't scare me away from them. I kinda like the idea of a screamer

Any help and/or guidance, or even a slight nudge in the right direction here will be greatly appreciated.

James Brown · James Brown's Garage

whats wrong with the stock cams? cis cams are good for carb engines. good headers and a distributor recurve will make that engine way fun to drive and cheap till you build a 3.2 ss slide valve MFI engine (hint hint).

tommott · 07-06-2012, 06:25 PM

Everything I have ever read about Bosch CIS is that you have to run a very mild cam with little overlap due to fuel metering system of CIS, and that most of the power loss with these systems is not so much equated with fuel induction but said cams themsleves. This is the case with Porsche's, Ferrari's, VW's, ect, ect............

............wait. Are you just trying to talk me into upping the ante?

James Brown · James Brown's Garage

lol no, well, maybe! think about this, cis needs a constant pull of vacuum, carbs also like that. big cams make pulses. Richard at PMO carbs recommends cis cams. This ant a chevy, big cams don't do that much (going to get crap for that), they move the peak hp/torque around to your shift points. You can get more out of timing than from induction systems. The cis is not the best for flow like the carbs. Think about this: the carbs and exhaust and dissy are bolt on, do that first and if your not happy, then tear into it, won't cost any more than a engine drop. see my post "carbs, let the fun begin"

tommott · 10:43 AM

Quote:

Originally Posted by James Brown

lol no, well, maybe! think about this, cis needs a constant pull of vacuum, carbs also like that. big cams make pulses. Richard at PMO carbs recommends cis cams. This ant a chevy, big cams don't do that much (going to get crap for that), they move the peak hp/torque around to your shift points. You can get more out of timing than from induction systems. The cis is not the best for flow like the carbs. Think about this: the carbs and exhaust and dissy are bolt on, do that first and if your not happy, then tear into it, won't cost any more than a engine drop. see my post "carbs, let the fun begin"

I have never heard this before. Do folks agree with these sentiments?

I guess the crux of the matter, which may render the discussion up until this point meaningless, is if there are any decent cam options out there to run with the stock pistons sans any interference issues? After some more searching around it appears that 'S' cams may definitely not work with those pistons. Anybody else care to chime in?