2.4L --> 2.7RS MFI questions
 

I'm going to rebuild my 73E's engine later this year to 2.7RS MFI specs. This is for 70% street / 30% track (DE) use.

I know that I'll have to have some machining done on the case and have the heads rebuilt. I already have a set of new 2.7RS p&c's. I assume that I'll need to upgrade to at least a Carrera oil pump. I'm thinking about having the cams reground to S specs instead of buying used S cams. The MFI injection pump will be sent out to be rebuilt and an RS space cam installed.

I assume that my plastic E stacks can't be bored out out to S specs, so I'll need to acquire S stacks...Is this right?

Can my E throttle bodies be bored to S specs or am I better off getting S throttle bodies?

Thanks for your advice.

Jim,

I have a ‘72E MFI engine that I have had in several 911s and my street 914-6. In ’74 it was converted to 92 mm RSR P&C at 10.3:1. Nothing else was changed and is absolutely stock appearing. It is the best street engine I have ever had. Neck snapping throttle response, great street manners, and outrageous horsepower. The displacement increase from 2341 to 2808 cc (almost 20%) and 8.0:1 to 10.3:1 (huge difference) actually made it a smother street car. The only downside is having to use the best premium fuel for the street and race gas for the track.
Before it goes back in a car, it will get twin plugs. That will reduce the octane requirements somewhat.

I’ll be glad to share the details.

Best,
Grady

Now that's an unexpected turn, Grady. If I didn't already have new 2.7RS p/c's, I'd love to explore that more...heck, I may regardless. Am I right in assuming that this combo provides lots of torque with a nice wide power band but runs out of breath at high RPMs? What case strengthening needs to be done for the engine (i.e., similar stuff that you'd do for a 2.7RS)? I'm also surprised that the E's MFI pump is up to the fuel needs for this engine. Of course I'm easily surprised. :)

Assuming I stick to my original plan, can I bore out the plastic E stacks (anyone done this?) and the E throttle bodies, or should I plan on selling the E parts and buying S parts? Thanks!

Jim-
Your throttle bodies can be converted to S spec. The throttle plates are actually the same. the area below the trottle plates needs to be enlarged to 36mm in order to match the ports. Eurometrix can do this. If your throttle bushings are worn, you could kill two birds with one stone by having them rebuilt along with the enlargement. As for the stacks, using the early magnesium ones makes it possible to get a T or E stack to S spec. I don't think the plastic ones can be enlarged.

Grady-
You ran 32mm ports in your 2.8? Will you continue with that size? Did your motor feel like it was running out of breath at 6000 rpm or thereabouts?
-Scott

Jim,
Sorry to raise more expectations. You can always sell or save your RS pistons and buy a set of 92 mm RSR Mahle Nikasil. They are currently available and I’m sure they are expensive but I would (will) buy them again.

As far as running out of breath, the E still pulls great at 6500, even OK at 7000. Yes, I will continue with the stock ports. Definitely not like an S, but good strong performance and the E will pull an S - zero to 80 MPH. The S is better performing on the track at DE. The E is a better street car. As I said, neck snapping throttle response, great street manners, and outrageous horsepower. I recall it being 230 HP peak @ 5900 RPM. The 20% larger displacement moved the peak HP RPM down because of the intake restriction.

I have two 2.8S MFI 10.3:1 that the peak HP (~270) is at only 6300 RPM. Again for the same reasons and are far more streetable than the original 2341s.

These are single plug engines but they will be twin plug when they next run. In my next life I want to try the S intakes with the E cams and pump, only matching the ports and not. The dyno tells.

Whatever, Jim, don’t sell your OE 911E parts. Just save them in a cool, quiet part of your basement. I can offer some preservation sugestions.

Connect with Pelican rs911t (in Virginia) who has a 2.8S carb built on a 2.4T we built in ’74-’75. The case has never been split even after almost 30 years. He ran it fast in DE all last summer.

Best,
Grady
gradyclay@hotmail.com

That's interesting Grady;
So you're saying that you've dyno'd a 2.8 using 32 mm ports and it was still pulling hard at 6500 RPM plus and put out 230 HP at 5800. At first I thought that this would "send me back to the drawing boards", but assuming that the 32 mm ports are pulling 200 CFM, at 5800 RPM and an E cam they would flow just about .47 liters of air per cylinder -- for an engine with cylinders of .47 liter capacity. So that works. But 230 HP at 5900 works out to a BMEP of about 180 PSI - which puts it about 40 higher then all of the other E's and up in the 908 and GT3 teritory. :eek: Wow! For comparison, a 2.2S had a BMEP of 164 PSI (2 more then the low CR 2.4S), and the RSR had a BMEP of 178 psi.

I'm sure that the later E style cam with the reduced overlap combined with the 10.3:1 CR ratio helped a lot to prop up the cylinder pressures above 5900 RPM, but I'm surprised by how much.

I disagree with the notion that S-cams are too wild for a street engine. Especially in a 2.7 (or 2.8), they are almost mild, and are fine in city traffic. And the best part is they really come alive at the high RPMs, and isn't that why we drive these cars? Spirit and personality. If you want manners, go CIS. I do have both E and S-Cam engines, so I do have a very good side-by-side comparison. BTW, My E-cam S-head engine peaks at 5900 rpm, and my S-cam CIS-head engine peaks at 6400 rpm. You do the math.

Good luck,

JP

Oh Grady, I'd have been happy with hot dogs but you go and throw steak on the table! I need to mull this over.

I know Greg real well. As a matter of fact, I just helped him pull out his 2.8L on Saturday so he can address some oil leaks. You're right, it is fast!

John,
You are right, the BMEP is up there with RSR and even 908. The best S-cam engines had around 10.5:1 CR Which is difficult with the Mahle RSR piston. That piston is designed for the RSR head which is shallower than the 2.4 head. The piston is cut out for the 49 and 41.5 mm valves. Had the pistons been designed for the modified 2.4 head and had valve cutouts for the 46 and 40 mm valves then CR in the range of 11:1 and more would be possible. I think I’ve heard of non-Mahle setups in the range of 13:1 but that is not pump gas or street use.

I think the BMEP numbers are high because of the high CR and the close clearances we ran. Of course the RSR and 908 peak pressures were at substantially higher RPM with appropriate mixture flow and consequently more specific HP.
I have never dynoed a 908 but have driven one.
I dynoed several 2.8 RSRs and with a fresh valve job and rings they tend to put out about 10% more power than the rated 300HP.@ 8000 (so long as the flywheel would stay on.)

JP,
I didn’t mean to imply that S cams weren’t suitable for the street, they are great. You are right, the fun of having the engine “come on the cam” is great. The 20% larger displacement and much higher compression tends to tame the S cam somewhat.

Jim,
This isn’t any o’l steak – it’s Fillet Mignon. Have you driven Greg’s 2.8S?

Best,
Grady

does the 2.4S 7R case need work if changing from 2.4s to 2.7rs spec, or will the new 2.7 barrels and pistons fit straight on?

Thanks

Manipou,
You will have to do some machine work to get the 90mm cylinders to fit. The spigots need to be bored out so that they fit. On the plus side, you can do a whole slew of reliability upgrades while the case is split. better oil pump, oil bypass mod, bring the main bore back to spec etc.

Manipou, I was lucky to find a 2.7 7R case that was in good shape (thanks Tim Walsh!!!) that saved me from having the spigots machined. It still needed the oil bypass mod. I was also able to acquire a Carrera oil pump.

Some would also suggest shuffle pinning the main bearing webs and boatailing them, too. I've heard a lot of respected sources on both sides of this issue (do it vs. don't bother) and I ultimately decided to do both of these case mods. One excellent source of advice stated that shuffle pinning was really only needed when your engine was generating more than (IIRC) 250HP.

I'm looking forward to getting all my engine parts back from the machine shop soon. Spring and the upcoming DE season is almost upon us.

Grady, sent you a PM. Please don't keep me in suspense. :)

Is there any way to test a MFI pump? My 2.4S is running lumpy, timing is good as is plugs spark etc. It is almost like its running on 5 cylinders. Seems like its something to do with induction? Help please....

Jim, have you consulted '724doorE' (Dennis)??? He comes by here and on GruppeB. I beleive he did the E to 2.7RS conversion and didn't touch the MFI. Just new P/C's, and req'd case upgrades. He said it's a blast. I was considering the same with my T, but decided to lower my engine budget, and put a little more into a road-worthy vehicle. Dennis was very convicing, and I almost took the plunge.

T&E plastic stacks can be enlarged to S spec. I have a cutting tool I made that does it . Exactly to S specs. 150.00 to do the S mod. on both stacks.
Jeff

Jay, these were 72-73T MFI stacks and throttle bodies after Eurometrix bored them to S specs and refurbished them.

http://www.dorkiphus.com/porsche/mod...dies.sized.jpg

HOLY CRAP THAT'S BEAUTIFUL JIM!!!

(yes, I'm yelling :D)

:D

WOW!

(contemplates send his perfectly good t-bodies and stacks out)