Help me understand magnesium cases 2.2, 2.4, 2.7

[Showdown]

Hi Folks,

So before I get started I need to provide this small disclaimer: while I've been drinking the Porsche kool-aid since my father indoctrinated me at the ripe age of 9 months, I know very little about the mechanical aspect of these cars... I'm learning quickly (thanks to all of you) but I'm at the beginning of my education so bear with me...

When I bought my 77S everyone rolled their eyes and told me I was a fool and that the car was terminal with the mag case. This car had 54k miles, mostly sat garaged with little driving for the past 14 years and the original owner, from what I understand swiftly removed the thermal reactors and disconnected the smog air pump when he bought it.

The car is currently being serviced (deferred maintenance) and the mechanics have found that the case is in perfect condition and without issue.

So here's my question:

As I understand (and again, I'm still learning here so be kind) the early 70's 911 RS and RSR were built with magnesium cases (2.2, 2.4, 2.7...) and that Porsche used the mag case for all of it's race cars up until and including some 3.0s

These RS and RSR mag cased engines sell for more than most 911 cars on BaT.

So can someone explain to me why the regular 911 mag case is so loathed when the RS and RSR mag cases are so lusted after (I know there's more to it than just the case, but it is the CASE)

Would a 911 mag case that never saw the thermal reactors and smog systems be viewed differently than one that had all of that evil stuff?

Is is simply the addition of the case-killing reactors that makes these engines so feared in comparison to their brethren RS and RSR cases that never had that crap?

I guess I'm trying to understand if the mag case itself was fatal or it was just the way it was abused to comply with California?

Oh, and as a bonus, could a healthy base 911S mag case with low miles be converted to RS spec when it ultimately does need a rebuild... Not that I'm planning on it, but not that I'm not...

Thanks in advance for the knowledge and patience!

[Charles Freeborn]

The later 7R cases are the best of the bunch. They all tend to pull the head studs out after numerous heat cycles. That is repairable with Timecerts (threaded inserts). Requires a complete teardown and machine shop installation. As soon as you split open a 45 plus year old engine the case halves will warp, so it's usually done in conjunction with a complete rebuild. Some of them warp so badly they're useless. That's the crapshoot. Was worse on the smog cars (75 and on).
RS and RSR cars / engines are more valuable than others on account of rarity. The basic castings are largely the same, but there were various machining differences according to engine specs. The sand cast 3.0 RSR is in a league of it's own and is rightfully unobtanium.

As for your '77, depending on the smog laws in your state, they can be perfectly fine engines / cars. If it has AC get rid of it. Backdate to '74 type exhaust system, have the engine looked at for stud certs. Don't bother removing the CIS injection and installing carbs if you aren't ready ($$) to change pistons, cylinders and cams. From a stock 2.7 in S trim ('74 specs) you'll get 175 hp tops. The way to make the car fun is reduce weight. Get it down to 21-2200 lbs and it's a fun car with good tires, brakes, euro height and a nimble touch on the controls. They're momentum cars. Oh, and learn how to drive it. Check out PCA or other driving schools. You'll need an instructor familiar with rear engined / 911 "pendulum" effect cars.

Yes you can convert to RS specs, but that's a MFI engine. The pump, stacks and associated plumbing alone will run you $7k plus. Then you'll have to do the rest of the top end (P/C, cams, head work porting, etc.).In round numbers it's a $25k project if you don't do the work yourself.. Plus all the proper dyno tuning to get the MFI dialed, but when done it's one of the most fun engines ever.

[spyerx]

my 71T has a 77S motor in it. Runs great. It has early SSi exhaust, and weber carbs.

As described the issues with the mag cases are related to heat-induced warping and studs pulling. Once you address the stud issue, get the case line bored/etc, you have a good solid motor. The real downside is the costs + time to have that work done on rebuild time.

The later 7R have all the bracing and oiling mods (yours has a later 7R)

Here's some crazy: I bought a nice core 77S motor, and am building it into a hot-rod beast... and yeah, the Ollies case work to get it perfect wasn't cheap. Why mag? Well, I wanted to! :-)

[keith28]

@Showdown,you make an interesting point qualified replies notwithstanding and I suspect that if I wanted to sell my ‘72 2.4T case it would fetch considerably less than my ‘72 2.4S case would fetch despite being I assume identical in every way other than the number stamp!

[kent olsen]

I wanted to stay with my 7R mag case mainly for the lower weight. Since I wanted more power I was told to strengthen the case could get expensive. Soo I found a 3.0L case and built my engine on that. Twin plugs, but only 9.5:1 pistons, webers, and we built it to turn 8000rpm, bullet proof. 245 hp and 227 ft/lbs torque.