Engine build ideas. Starting with a 2.7RS motor

RetroSC · 12-08-2009, 01:03 PM

I acquired a 2.7 RS longblock of unknown internal condition. It's a correct 911/83 case, so I'm going to sell that and keep the heads, cams, crank. My plan is to acquire another 7R mag case and build on that with what I have and run Webers.

My intended use is as an autoX motor in my 67S. I'll pull my 2.0s and swap this in for some added fun

That said I'm looking for some torque and not all out HP. I was thinking different cams maybe instead of S cams. Thoughts?

Also, what piston size can easily go in there without machining the case or modifying the heads? I was thinking of 10.5 compression, is that too high for pump gas?

Thanks for any input! Bill

skinnerd · skinnerd's Garage

I actually had an 2.7 MFI RS motor for awhile, autocrossing it and tracking it.
The engine with S cams actually had a surprising amount of torque.
Plus we know what it does when it passes 4500rpm.
In my 73E, 1/4 mile in 13.8 sec.

In your lightwt early car, it would be very exciting (and fast) to drive.

Not sure how your Webers would differ from the MFI that was in my car?
Good luck.

jpnovak · 12-08-2009, 01:44 PM

I would stay at 9.5:1 or less if single plug on pump gas. 10.5:1 would be fine for twin plug.

Mod-E cam from John Dougherty (DRCamshafts). This will spin to 7K rev limit with lots of low rpm grunt.

Weber 40s will be great just make sure to jet them correctly for the new motor.

skinnerd · skinnerd's Garage

Mine only had 8.5:1 compression ratio, but dyno'd at 208hp to the rear wheels.
It was a very healthy 2.7 RS motor.

RetroSC · 12-08-2009, 05:41 PM

Good to know on the compression, thanks. So how do those compare to actual E cams, or Solex? I may stick with the S's since I already have them, but I really want some more low end grunt compared to my 2.0S.

RSTarga · 12-08-2009, 06:02 PM

I wouldn't go more than 2.7 on a mag case. 9.5 is the best. no need to go higher. The difference from my 2.4s motor to my 2.7 was incredible especially torque so going from 2.0 to 2.7 will knock your socks off.

skinnerd · skinnerd's Garage

Guess I wasn't too clear in my answer.

With 2.7 displacement, you'll have plenty of low end grunt with the S cams.

Don't think you notice any more low end pull with Solex, E or even T cams.
S cams were really only peaky with the smaller displacement engines.
By the time they went into the 2.7, you finally had enough displacement.

Nes pa?
Kao jai mai?

RetroSC · 12-10-2009, 05:56 AM

Good to know Doug, I'll stick with the S cams then.

So if I wanted to find 9.5 pistons, is JE my best bet?

dmcummins · 12-10-2009, 07:04 AM

I'd leave the 67S alone and put the motor into a T roller. And build what I wanted around it.

Of coarse I have a couple of T rollers.

Grady Clay · 12-10-2009, 08:10 AM

I guess I’ll add 2¢.

I like these engines for a lightweight car.
Built properly they have very good life and outstanding performance with a relatively lightweight engine. I have a 2.8S MFI in my street 914-6. Its not 300 hp but in a 2000# car, it is hard to beat. I have also had these engines in my 2200# ’68 911 coupe.

There is at least one Pelican who has one of these we built in 1974.

Starting with a basic 2.4S MFI (2341 cc, 8.5:1 CR, 190 hp).
Add Mahle Nikasil 92 mm RSR P&Cs (2808 cc, set to 10.5:1 CR, 12 mm exhaust sparkplugs).
Build properly.
Finish with about 270 hp but more than double the torque of a 2.4S and acts is if it has milder cams. Don’t get me wrong, from 4500 to 7000 it lets you know it is an S.

These are expensive engines to build. The P&Cs are expensive but worth the price. There is a great deal of effort getting the case correct, getting the piston-to-head & valve clearances correct while reducing the CR to ‘only’ 10.5:1.

Built correctly, properly maintained and not abused (no 8300 rpm, run too hot or run low on oil), these give 50K-70K of trouble free high performance use. For long term use, I would do the heads, rings and rod bearings through the case every 60K mi.

Fuel is the big issue. The very high CR contributes more than half to the performance increase. It also improves the throttle response. With twin plugs it just runs better, gets better mileage and lower HC emissions (not NOx).

I’m inclined to raise my CR to the physical limit and just use race fuel (112 octane, $10/gal taxed & delivered in 55 gal drums). If you consider 3000 mi/yr & 10 mi/gal the cost can be acceptable.

There is a lot about building these engines posted here and on the Pelican 911 Engine Forum. These threads have much of what is required to build one of these properly. Many Pelicans have successfully built these and can advise. Pelican parts can get you most of the parts. Trade members here can perform the machine work and measuring necessary.

This not a ‘casual’ project. The slightest mistake can end up as a pile of very expensive junk. It is a Porsche engine and can be built by mortals.

Best,
Grady

T77911S · 12-10-2009, 10:34 AM

grady said it, 2.8 rs P/C

Ducman · 12-10-2009, 12:26 PM

I think a lot of folks surmise the performance of a 2.7/2.8S based on experience with either a smaller S engine or a non-S 2.7 - And I believe most would be shocked by how this engine feels in the lighter cars, with the right drive ratio. Autocross - just keep a few 1000 rpm on it and it will explode out of the corners.