2.8 Race Motor Questions/ Port size etc.
 

So, I've got an older 2.8 RSR type build that has been mothballed for years. ...part of a project I puchased years ago.

I've decided to have it torn down and reassembled as a back up race motor for my 914/6.

The motor shows to be approx 11.5:1 or more... It has 42mm intake ports and close to 39 or maybe 40mm D shaped exhaust ports.

I'm not sure about valve sizes yet.

So, should I simply reassemble the motor as is or are these ports too much for a 2.8?

I'm thinking a 2.8 with that CR and big ports like that is an engine that is on the edge...

Thoughts?

At that comp ratio I sure hope it's twin plugged-piston crown shrouding is bound to be an issue
if not-detonation city. And yeah pretty damn big intake ports too. And maybe way oversized ehaust ports too.

What's your bore and stroke I had a 2.8 with 92 bore and 70.4 stroke with stock valves and similar port size actually bigger intake then yours with ge 80 cams and twin plug made for very nice engine between 5 to 8 k rpms

...OK, yes twin plugged of course. In fact has uber rare Marelli RSR twin plug distributor. Someone did lots of work on ports with the D shaped exhaust ports.

Thanks for that datapoint.


It is the same bore/stroke as yours. 92x70.4 bore and stroke... Built in a 7R mag case so I'm slightly concerned about the high RPM issues associated etc. It has 2.8 RSR pistons/cylinders and base gaskets to lower C.R. Otherwise it would be north of 12:1

I don't know what cams are in it yet.

I would like to do a simple tear down and reassmeble but also don't want to build a motor that is at risk of coming apart.

I wanted to do a simple tear down and reassemble to use as backup while the engine that is in now gets a tear down, inspection and reassemble.

2.8
 

The D-shape exhaust ports work well.Ports are not too big.For another 20 ft.lbs.of torque from 4-5,000 rpm take down the center of the piston crown a few mm,s the width of the exhaust valve.You drop your C.R. a little but it will rev quicker to redline and increase mid range torque.The less the air climbs the mountain peak of the crown the quicker it flows through the exhaust valve.Fred

Somone also spent a lot of time making the pistons light... Not sure if there is enough meat to take down the crowns.

The real 2.8 RSR had 43/43 ports, and its peak HP was at 8,000 RPM, which means you'd rev it at least 500 RPM higher before shifting. But it had the 49/41.5 valves, which you can't do without the real RSR heads or some custom head.

I've got one of these non-RSR 2.8s. I've been wondering what I can do to get slightly larger valves, especially intake, in there. That has to be part of the magic in those factory engines (titanium rods can't hurt either), and part of the high max hp RPM.

The idea of a trade off of CR vs fuel/air flow is interesting. At 10.5 you can run pump gas, and that is all Porsche said the RSR had.

Is the header flange altered to match the D port shape?

If it has J&E pistons from back when I'd be concerned.

Porsche used the mag case on these, so my take there would be (assuming it is time certed) keep it, while maybe scoring a sand cast case as a back up against the day when the mag gives out. Doubtless you have seen the shop guys here saying that when you take one of these apart you most likely need to do the parting line shaving and line boring, plus spigot decking etc. Measuring would tell if that was needed.

Hi Walt,

This one has Mahle pistons and cylinders. The only issue found so far on this tear down is that one of the cylinders is a little out of round. ...trying to determine if we send them all in or just fix the one that is obviously a problem. I think the rest were in spec sizewise etc. but just in spec... That said I think that race motors were built loose anyway. Correct me if I'm wrong???

I don't have headers for it now when I got it the engine had a set of SSis bolted to it and they were not port matched...

I want to rebuild this one install it and pull out the sandcast case, short stroke, long rod Ferguson motor that we chatted about.

Jay

Ferguson motor - aha, that's why your name rang a bell. But that can't be a SS 2.8 - you can't fit the 95mm jugs into a sand cast case. You can into the early 3.0/Turbo which will take the 66mm crank. Short strokes weren't the hot ticket when the Ferguson motor was built in the '80s. But long rods were in the mix. Dave bought it in the early '90s, and I don't think he did anything special with it, as it was a good motor all along. I remember the timing because I bought his Grady Clay/Bob Benight 2.5 when he got the 2.8, and he had to modify some sheet metal to get the long rod motor to fit his 914. Of course, he may have done more later when he moved to California.

I'm not the guy to ask about what amount of ovality is OK for a race motor. Shops vary from one which was known to believe that somewhat out of spec parts were the thing to use, because cheaper and besides it is a race motor and will come apart soon, to others which are meticulous as all get out. Or Grady, who would use close piston/head/valve clearances because he would take his motors apart a lot just to see if a little wear was creeping up on this to catch interference before it happened.

OK, got it. I just assumed it was a short stroke 2.8 that was in the Ferguson car.

If you ever happen accross the case that came out of Ferguson's old 914 let me know. I know a guy that would like to have it. :)

You mean the Clay/Benight race motor case? Dave's green 914 was first made into a race car by Bob Benight, and Grady built the motor, which I ended up with. I have the ps and Cs (EMPI 88s), the twin plug heads, the 6 bolt cam carriers, the 6 bolt finned exhaust valve covers (bored for twin plug), the oil pump, and maybe some other bits. And the sand cast case, but it needs to go to a place like Ollies to have a major prosthetic repair to the slot extending up from one of the spigots to the case parting line or the sort.

But no 914-6 ever came with a sand cast case, did it? Back when, the T motors for 914-6s were not in any demand. And they were mag. But who knows, maybe it is still around if a guy knew the engine SN.

I have the serial number of the original 914/6 engine. That number is 6405179. That is really what I'm looking for... I wish I could pick up the phone and call Grady. He was always a great guy to chat with either phone or web boards and who knows... He may have either had the case or known the whereabouts.

If I ever get serious about vintage racing (doubtful) I will have to build up a spare 2.5 engine so some of your parts could possibly come in handy...

Relating to the D Ports; Walt brought up the quesion of port matching the headers with the D shape exhaust port. Is this a thing? Thanks!

Just rebuild it the way it was built before.

I think you'll be surprised at the results.

That's generally the plan I'm just going to make sure piston/cylinders are relatively within spec. I might send the pistons over to Poly Dyn and get them coated ...and of course I won't be using the SSis since it's going on my 914/6 :D

It would be nice to be able to run on pump gas but I'm not going to sweat it and just run race fuel.

I'm bringing this one back to the top. I'm now thinking of possibly using this engine that we've been discussing in a street car of mine

If we swapped the crank to a 66mm crank and rods do you think the compression ratio would drop to something suitable for pump gas? I've read that C.R. drops 1 - 1.5 points by changing from the 70.4 to 66mm crank when doing these builds. Would the bad harmonics that I read about with the 2.8 Long Stroke engine go away?

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.

pistons for 66mm and 70.4mm cranks have the pin in the same location. Rod length is different, squish doesn't change as piston is in same location. Lower compression is due to shorter stroke only. Usually swapped the other way to get higher compression ie 2.2S pistons in 2.4 engine. 66MM crank would not have harmonic issue and compression would be lower for pump gas.

john

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.

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

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.

Was the space cam modified/changed on MFI build?

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

http://forums.pelicanparts.com/uploa...1533674525.jpg


With pin.

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.

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.

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?

http://forums.pelicanparts.com/uploa...1534027996.jpg

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.

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.

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.

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.

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.

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.

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.... :D:D:D

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.

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?