2.8 Engine Build Info

[werke1]

Was hoping one of the engine gurus on the forums here could comment on what parts are needed to build a 2.8 RSR style engine - the donor engine is a 1973 2.4T , with a 7R case and factory CIS. Going back in its original 1973 body. Current engine is a 1988 3.2

What size crank is needed? The stock 70.4 mm from the 2.4 ?
What size and style of P+C ? Buy Mahle 92mm, and bore the case ?
I hear the early CIS heads are not good for flow. What donor heads should I look for ?

Was planning to use the new PMO throttle bodies instead of MFI since I don’t have those parts and the new ones sound really interesting.

Comments appreciated

Thanks !

[Rosco_NZ]

Yes, to would need to bore the spigots on your small bore 7R case .. but that is kinda valuable .. I’d be tempted to use a 2.7 7R case as a starting point.

High horsepower high revving 2.8/2.9’s with a lot of cam and compression built on Mg cases is really pushing your luck. I’d suggest about 230-240hp is about as far as you sensibly want to go .. a RSR spec 2.9 with 300+ hp may only be fun for a short while…

If you’re really set on this high revving engine, build it on an ALU case .. either from from early Turbo, Carrera 3 or SWB car.

[colingreene]

First thing to under stand is that the 2.8 RSR is a big difference from a 2.8L displacement factory road motor.
the heads are different so is the crank rods and flywheel.
the 70.4 crank had a habit of backing flywheel bolts out due to a harmonic vibration.
someone on here however is selling a factory rsr crank and flywheel...

but to build what you are considering.
you would need it to be 92x70.4.

if the case is from a 2.4 i do think you have to cut the case larger to fit the larger cylinders.
You can have any head ported, the valve sizes for the 2.7 are all the same as far as I know.
That being said how much you spend on air flow such as the heads and cams will directly correlate to how much horsepower you have come back out.

Another thought is you can build what's called a "Short stroke 2.8" something that Henry from Supertec considers one of the best motors Porsche never built.

that utilizes a 66mm crank and a 98mm piston in a early 3.0 turbo/Carrera case.
this gets you a more stable case and larger valves natively from the 3.0.
It however is not the cheapest way to do things.

[stownsen914]

Lots of options. Another couple to consider:
Keep the bore to 87-89mm biral cylinders with custom pistons. I think no boring the case? A little less displacement, but less stress on the case. Red loctite on the flywheel bolts keeps them from coming loose.
You can also look into using a 66mm crank. Less displacement of course, but they don’t have the harmonics issue.
If you can come up with an early aluminum case, it does away with concerns about strength.

[Black 993]

I don't get the fixation on 2.8, unless that's the max displacement in a racing class or something. It's a lot of extra trouble to go through when a 2.7 is so much cheaper, easier, and more reliable.

[icarp]

The 2.8 has a habit of braking 70.4 cranks in less than 10,000 miles
If you are not going to drive much then it will work fine for you
The harmonics of bore / stroke and reciprocating weight are the killers
Ian

[werke1]

All excellent info, thank you who responded !

I’m mostly interested to make the car more period correct in looks and sound. The 2.4 core is original to the car and the numbers match the COA. But it’s tired and needs a full rebuild.

The rest of the car is built to a really high standard. Basically all in period parts and highly detailed. So the later engine looks kinda out of place in there, although some backdating of the exterior engine bits has been done, it’s hard to hide that big 3.2 intake and Motronic bits.

If I went ahead with a 2.7 or 2.8 build it would be for street use only and doesn’t have to be crazy high revving or a real screamer. But would be nice if it made great noises and have similar power to the current rock solid 3.2 which is very close to stock specs, other than SSi, chip and exhaust.

Cheers,

[jac1976]

Originally Posted by werke1

All excellent info, thank you who responded !

I’m mostly interested to make the car more period correct in looks and sound. The 2.4 core is original to the car and the numbers match the COA. But it’s tired and needs a full rebuild.

The rest of the car is built to a really high standard. Basically all in period parts and highly detailed. So the later engine looks kinda out of place in there, although some backdating of the exterior engine bits has been done, it’s hard to hide that big 3.2 intake and Motronic bits.

If I went ahead with a 2.7 or 2.8 build it would be for street use only and doesn’t have to be crazy high revving or a real screamer. But would be nice if it made great noises and have similar power to the current rock solid 3.2 which is very close to stock specs, other than SSi, chip and exhaust.

Cheers,

Shelve the original motor and go with EFI/ITBs on the 3.2 if you want a more period correct look.