2.8 Race Motor Questions/ Port size etc.

[jmz]

@aaron, Warren has them at his shop. I will measure and post. These have been worked on to make lighter.

[Neil Harvey]

All engines suffer from harmonic issues. Cranks break when the harmonic frequency gets close or at the cranks natural frequency.

Don't get confused with dynamic balance and harmonic balance. Torque pulses twist the crank back and forth, this is the harmonic. Dynamic balance is when you consider only the weights of the components without any torque pulses. Smaller journal shafts will have less overlap, hence less stiffness which doesn't help.

All these engines should be fitted with a damper. We have proven multiple times engines run better and make more power when fitted. The cam timing stays where you set it as the vibrations are far less and they do not travel up the chains and play havoc with the camshafts. I have been told that a known Porsche engine builder saw 55 extra HP after fitting one of our dampers. This was never proven and seen very high, but I'm sure if the cam timing was not controlled at high engine speeds you could lose 55 HP.

To fit one of our dampers to an early engine, the front main bearing has to be machined to move the front seal back so the damper has the correct amount of overlap onto the crankshaft.

[manbridge 74]

Originally Posted by lvporschepilot

Using the same pistons but shorter stroke crank will ruin not only severely lower static compression but also the squish factor which one typically wants, in these motors, to be around .040 just under the cylinder deck.

If you do use the 70.4mm crank, I can attest to these being outstanding motors. I just completed and am now running my second one of these after the first one turned out so nice. 39mm/38mm ports, Carrillo rods, twin plug (using COPs), 10.5:1 cpr, DC65 cam, upgraded valve train etc. The first one I did with MFI was great so I made one with full sequential EFI and ITB for myself. Just a fantastic flexible engine that revs like the dickens. With these cams I suspect somewhere around 270-280bhp (flywheel) is had with good drivable torque. Lighter the car the better obviously.

DC80/GE80 cams can even be used in these (MFI or EFI and twin plug with higher cpr only, no carbs! not enough vacuum) and one can still get away with it on the street and be ok, they're not like the old Ferrari P6 cam or even RSR cams which are literally an off/on switch. Still, I wouldn't recommend any bigger than Dc60-65 on the street as it's rare one is revving high enough to take advantage.

Was the space cam modified/changed on MFI build?

[lvporschepilot]

Originally Posted by manbridge 74

Was the space cam modified/changed on MFI build?

Yeah. Place in Germany set it up with one of their repro RS space cams.

[jmz]

Originally Posted by BURN-BROS

I'm not a fan of the factory 2.8 pistons. They are very heavy. something like 650 grams with pin and rings...care to measure them and post?

I limit the 2.4/2.7 cranks @ 7000 rpm now and build around that redline. There are guys breaking cranks on smaller motors with redlines of 7500. So be mindful.

If you de-stroke, You will drop your compression .5 along with a little more deck may get you where you need to be. You will also be out of the harmonic issues as well.

With pin.

[BURN-BROS]

Hi Jay, My mistake on the weight. That was for the 3.8RSR.

Not as obese as the 3.8 but still calorically challenged.

I have an 85.7 Mahle ST piston in house. It weighs 477 grams piston and pin. no rings or clips.

The 90mm should be similar weight to the ST piston/pin.

What is does show is a valve change from 46mm to 49mm caused a 60 to 70 gram increase in weight.

[MST0118]

No, the 3.8 rsr pistons weigh 470. These are the 12.5 compression pistons with the large valve reliefs that can fit a 53mm intake.

Actually, it seems that most of the Mahle Motorsport pistons are under 500 grams. It’s the stock street versions that weigh 650 plus.

[jmz]

As I ponder using this engine in a new street build I'm working on I have a few questions/concerns.

As I mentioned, I'm thinking of a 66mm crank to get the C.R. down to something that we can run on pump gas and I also think a short stroke 2.6 might be kind of cool.

I'm a little worried that the 42.5 intake and 39mm exhaust ports simply won't work on a street build even a rev happy short stroke engine. It was suggested to me that I might be able to choke the ports down by using phenolic spacers between the heads and the manifolds. Any of you experts have any thoughts on this?

[Tippy]

Just use a smaller cam. There’s nothing wrong with large ports IMO.

Never in my life, I’d admit, that I’ve ever heard someone being worried about too big of ports. This is a 1st. Me, I’d celebrate!

[Steve@Rennsport]

Originally Posted by Tippy

Just use a smaller cam. There’s nothing wrong with large ports IMO.

Never in my life, I’d admit, that I’ve ever heard someone being worried about too big of ports. This is a 1st. Me, I’d celebrate!

Au contraire, my friend!

Velocity is every bit as important as flow, if not MORE so.

Big ports without velocity kills everything but high-RPM power and that makes a "dog" of an engine that won't accelerate.

Everything must be designed and configured to work together: intake, heads, cams, CR, rod ratio, and the exhaust system.

[BURN-BROS]

Originally Posted by jmz

As I ponder using this engine in a new street build I'm working on I have a few questions/concerns.

As I mentioned, I'm thinking of a 66mm crank to get the C.R. down to something that we can run on pump gas and I also think a short stroke 2.6 might be kind of cool.

I'm a little worried that the 42.5 intake and 39mm exhaust ports simply won't work on a street build even a rev happy short stroke engine. It was suggested to me that I might be able to choke the ports down by using phenolic spacers between the heads and the manifolds. Any of you experts have any thoughts on this?

Hi Jay, for a street build I would be around a 38mm Intake/ stock exhaust port

If you don't want to go another set of heads then you may need to run a smaller than normal cam to get the midrange up and let the big ports improve the top end. Like a Mod Solex grind. (compression would need to be adjusted appropriately to the cam).

Applying a choke only increases velocity @ the choke. The big ports will still be slow.

[jmz]

Originally Posted by BURN-BROS

Hi Jay, for a street build I would be around a 38mm Intake/ stock exhaust port

If you don't want to go another set of heads then you may need to run a smaller than normal cam to get the midrange up and let the big ports improve the top end. Like a Mod Solex grind. (compression would need to be adjusted appropriately to the cam).

Applying a choke only increases velocity @ the choke. The big ports will still be slow.

This is what I was concerned with is well.

[jmz]

Originally Posted by Steve@Rennsport

Au contraire, my friend!

Velocity is every bit as important as flow, if not MORE so.

Big ports without velocity kills everything but high-RPM power and that makes a "dog" of an engine that won't accelerate.

Everything must be designed and configured to work together: intake, heads, cams, CR, rod ratio, and the exhaust system.

From my own experience I agree. I had a 2.7 with RS pistons, weber 40s, solex cams and the very small ports from 1974 normal ...not even S size heads years ago and it ran like a scalded ape.

This is why I am overly concerned about trying to turn this race engine with the large ports into something I can use on a car that will get driven on the street.

I like the idea of a race car for the street but it has to be something that is driveable on the street! I'm very tempted to give it a try but don't really know what I will end up with if I do.

[Tippy]

Originally Posted by Steve@Rennsport

Au contraire, my friend!

Velocity is every bit as important as flow, if not MORE so.

Big ports without velocity kills everything but high-RPM power and that makes a "dog" of an engine that won't accelerate.

Everything must be designed and configured to work together: intake, heads, cams, CR, rod ratio, and the exhaust system.

Wouldn’t you agree, over-cammed is far worse than oversized ports? That was my point.

[Steve@Rennsport]

Originally Posted by Tippy

Wouldn’t you agree, over-cammed is far worse than oversized ports? That was my point.

They are BOTH undesirable in equal amounts since the consequences are almost identical.

[jmz]

So Steve in your opinion could the heads that I'm referencing be at all something I could run on the street? ...71 MFI heads 42.5 intake and approx 39/40 exhaust depending on where you measure. I'm not sure if I go MFI, EFI or carbs yet.


- I need to come up with something for my 68 SWB sports purpose build and I already own this engine.

[lvporschepilot]

The secret (if one can call it that) to getting something like this streetable would be to go some sort of fuel injection, EFI or MFI, but EFI would be easier. DC60 or thereabouts cam with high twin plug compression further helps. The lack of port velocity would make a vacuum signal to carburetors non existent at lower rpm.

[Tippy]

Originally Posted by Steve@Rennsport

They are BOTH undesirable in equal amounts since the consequences are almost identical.

Ok, always thought differently.

But, I remember a lot of 930 guys going from 930 heads to 3.2 heads and not noticing a drop in low end torque.

But yeah, 42.5 mm ports are pretty darn big on a small motor.

Guess the OP just needs to swap with me since I'll put them to good use on a turbo'd 3.4....

[Steve@Rennsport]

Originally Posted by jmz

So Steve in your opinion could the heads that I'm referencing be at all something I could run on the street? ...71 MFI heads 42.5 intake and approx 39/40 exhaust depending on where you measure. I'm not sure if I go MFI, EFI or carbs yet.

I would not use them in any street engine.

I would use something with 36mm-38mm I & 35-36mm E ports with cam of choice, decent CR, & twin-ignition.

[Geneulm]

Can’t for a fairly modest amount a small sleeve be installed in these heads to choke it down to the 37-38mm mark? And make the heads usable?