2.8L Engine

[motogman]

I am starting to look for information and back ground on 2.8L engines. I have the opportunity to get one that could go into an RS (tribute / replica). The motor is said to have a 2.4L crank, 10.5:1 CR and RSR Sprint cams with MFI and twin plug heads.

So I have lots of questions and was surprised that a search for 2.8L did not turn up anything. So any reference to info or other posts is appreciated.

Questions:

When and where did Porsche use 2.8L motors?

What are the specs of RSR Cams... I did not find them in Wayne's book (unless I missed them).

What other things are good to know or look for? I will get the engine type and number shortly as a place to start in knowing more about this motor build.

I appreciate any comments and input.

[Flat6pac]

The 2.8 started to show up in the 73 Carrera RS for the RSR in late 72.
Its a fun engine to build....
Bruce

[KTL]

Henry @ Supertec often refers to the 2.8 as the best engine Porsche never built. That's a pretty good endorsement from a guy who's built his fair share of cool engines.

Short Stroke 2.8

The best engine Porsche never built

Note that there's more than one way to build a 2.8

[BURN-BROS]

The Long stroke motor made good power but suffered from destructive crankshaft harmonics...the same with the 2.5 longstrokes.

If you are building a 6 bolt crank engine that runs past 7500 rpm, a 2.6 or 2.7 shortstroke would be my first choices....as the parts are easier to come by and the 2.2/2.7 combustion chamber is small enough to make good compression without too much piston dome.

2.5 SS DC90 Cams 11/1 comp...made 285hp @8500rpm and 21 mpg highway

[Plavan]

Do not waste your money on a 2.8LSS. Too expensive, and it looks to be a roll of the dice HP wise. If you want a 2.8L, just build a LS and keep it below 8000RPM. You will be fine. Or just buy a 3.0L

My next race motor will be a 2.8L LS carefully built to max HP under 8000rpm.
I currently only take my 2.5L SS to 7800 RPM, sometimes 7900 RPM for longevity. I run my race motors for 100hrs per builder.....Yep- no failures.

[motogman]

Is there a place to find head volumes and piston dome volumes for the production engines and some of these less than standard configurations?

[304065]

2,0 72cc

2,2-2,7 68cc

3,0 90cc

dome volume you have to backsolve and we think that Mahle used Zero deck

[motogman]

Does anyone have some measured values or spec values for various piston dome heights?

[YTNUKLR]

Totally maxed out 3.0 piston dome with stock valve sizes ~ 47cc (Ie 3.0RSR piston)

My recommendation: build a 3.0L on an 3.0SC base, and turn up the cams/compression/ports to the specs. you mentioned (basically RSR). You can find cam info on Elgin, Web Cam websites. The 3.0 is just a way better base, for less $$: bulletproof alu case, 9-bolt crank (much better, not a lot heavier), 4-Bearing large base circle cams, and big valve heads. What's not to like.

It is preferable to reduce the head chamber volume as much as possible on the 3.0, for example with some careful work you can get down to ~85cc from 90-91cc on a stock head...this makes a significant difference to your compression ratio. Any much below 80cc on a 3.0 head ( you can get there by filling in the chamber with some aluminum welding, a la IMSA single plug 962 ) you are probably hurting flow and not benefiting from the compression increase any more..

If you are keen on the 'real deal' 2.8 long stroke like you asked about, as opposed to the 3.0, my recommendation is to use an early sandcast alu case, 2.2-2.7 heads (make sure they *flow*), and all the rest of the specs. basically 2.8RSR. It's a cracking motor. Try to put in some bigger intake valves, like some 48's.

Unless you are in a 2.8L race class and you are routinely spinning 9000 to keep up, forget the 2.8 short stroke, IMHO. OR, unless you are spinning 8k + AND you are using Gruppe B cams AND you have a K29 out back The 2.8 SS, with its tiny stroke, "slow" Rod-length ratio and big head volume, is compression-challenged, unless you use some unreasonably "tight" cams, SO the best way to "fix" that is forced induction. However, with turbos, it's best *not* to spin the motor because they make so much torque at low rpm...so it's kind of a strange beast, this mythical high-revving 2.8 SS Turbo. But that would be cool as F***! Like that 936 video at Le Mans...

Does that help or just confuse you more..lol sorry

[motogman]

Very good information and appreciated. I am learning a lot but have a lot to learn.


So for the 3L motor you are proposing what crank (70.4?), piston diameter (95mm?) and cam (RSR) combination are you really proposing?

Forgive my dumb questions but clarify what a 9 bolt crank and the four bearing large base circle cams mean and where & when these designs/parts were used.

Thanks.

[YTNUKLR]

Ok. Good questions.

Porsche had 4 strokes in air cooled engines: 66mm, 70.4mm, 74.4mm and 76.4.

66mm was '65-71, 2.0/2.2 (changed bore)

70.4mm was '72-83, 2.4/2.7/2.8/3.0 (changed bores). The crank was changed from 6-bolt to 9-bolt flywheel end in '78 w/ the SC motor.

74.4-3.2L Carrera, 3.3 Turbo (changed bores)

76.4-3.6L 964/993

The large base 4B cams came out in the '78 engine as well, along with the stronger late aluminum case.

I dont know what youre doing with the car, but I was proposing essentially a 3.0RSR engine if you were looking to do something like a radical 2.8. 95x70.4 (standard stuff), hot cams, high compression, ports, good hardware (head studs and rod bolts). Essentially start with an SC engine. There are lots more details but this should keep you going !

[Jimmy18E]

Why not what aaron mentioned above....

I do not know what case you have, but a 2.6SS or 2.7SS done right is a screamer and puts out plenty of HP. On two different dynos my 2.7SS is in the mid 260 HP range to the wheels at 8K RPM and it is about to get modded again with 280rwhp as the minimum target.

Now if you are starting with a new case the 3.0 would be a great way to go, no more money than the above with some added obvious benifits as mentioned. I like the early case/SS type motors in the early cars, but that is why Baskin & Robbins has 31 flavors, tuff making a choice sometimes....

It comes down to what you are looking for and how many $$$$ you want to part with.

[YTNUKLR]

I dunno guys, there is no 'right' answer. To be honest I think we are overthinking this a bit. (I'm including myself amidst the guilty.)

A fact though, is the early case motors cost a lot more cash to get and get them up to snuff...$2-5,000 pretty easily to get the expensive sandcast case and do all that machine work that Porsche incorporated into the later 3.0L+ castings. For me, my money, to spend extra cash and get less power,....I would rather not. For cool factor yes maybe the 2.6/2.7SS is worth it. I think they are thoroughly cool and I think the 2.7SS is the ultimate expression of that 66mm crankshaft. But I am not a person that wouldn't miss $5,000 in my checking account.

That's why I mentioned more about application...what you're doing with it is much more important than what it is, IMHO. If I were stuck in traffic in an air-cooled Porsche, I'd like a 3.8 with street cams, bigger is better. At the very least, even if it were a 2.0, the T motors are great for exactly what they were made for - "T" for "Touring". My boss built a neat little motor, a 2.0 with 2.4 "T" CIS cams and high compression; this is the greatest little street motor with abundant torque at slow speeds....connected to a 8:31 915 gearbox it is a pussy cat in traffic and a sweet highway cruiser. The key is that it has working cylinder pressure at slow speeds, so it feels peppy, like a diesel. However, when you really give it the stick like a hotter 911, you realize it's not there in the same way. Different needs. He has plans to use it as his daily driver. Not the case for some of us...

If I were driving a narrow early 911 sportily, I'd keep it simple and build a roughly 2.7RS spec. It's a classic performer. A serious weapon widebody car, a bigger motor, a racy 3.4 or 3.6...why not. Stuck in a narrow vintage racing class running 2.8L and having trouble keeping up @8000 rpm, yes I'd max out a 2.8 SS on a 3.0 Turbo case and wind it to 9500 and rebuild it every other month..(not quite) all about application and taste.. If I was really racing and I could, and the point is nothing else but to go faster (regardless of all else), I would turbocharge anything I drove. More power

However, there are other considerations. Street driving? What kind? With the wife? Track only? Limited by class rules? What about the sound? There are choices that are more 'aesthetic' or 'taste' oriented. Do what speaks to you. Try to try out some different combos if you can..

There is really no 'wrong' answer. You will enjoy yourself any which way you go. That is what this is all about. But there some answers that are much more right than others.

[Raceboy]

Originally Posted by YTNUKLR

SO the best way to "fix" that is forced induction. However, with turbos, it's best *not* to spin the motor because they make so much torque at low rpm...so it's kind of a strange beast, this mythical high-revving 2.8 SS Turbo. But that would be cool as F***! Like that 936 video at Le Mans...

Those are exactly the reasons I started to build this thing: My 2.5SS turbo engine build

[Walt Fricke]

Bruce Anderson has some useful discussion about the 2.8 race motor, and the effects of using 92mm ps and cs with stock 2.7 heads.

The real 2.8s produced 308 or so DIN hp at 8,000 rpm advertised. They used titanium rods. They have really big ports. Most importantly, I think, was the fact that they used 49mm intake valves. To get these into a 92 [edited]mm bore, they changed the valve angle. You can take any of the 2.2-2.7 heads and open up the ports, but changing the valve guide angles and relocating larger valve seats might cost as much as a set of custom aftermarket heads?

I don't know if using 3.0/3.2 heads would cause valve to cylinder interference with a 92mm bore. I do know that it is difficult and expensive to move the head stud holes in these heads in to the spacing used on the 2.7s and earlier. Peter Dawe did this on one of his race motors, I think. Called for cutting and welding here and there to make sure you didn't get some walls too thin.

Using stock valved heads on a 2.8 replica with a variety of cams, you can for sure get 250 rear wheel HP. These are, of course, race motors. Don't know just how well they would drive on the street. Mine doesn't idle through the paddock all that well going over to the grid.

You can have your aftermarket pistons cut and pocketed to avoid too much compression, so that's not a tricky bit.

[tom1394racing]

Here is an example of a 3.0 RSR spec motor.

Another RSR Clone Project - Dyno Day

PM me for more detail

[motogman]

Thanks for all the info. I am learning a bunch.