help me Henry!!

[nabilious]

Hi Henry - help me with a question that you are well suited to answer. This winter I've decided to put together a 2.8/2.9L twin plug smile maker (half of the reason is I've always wanted to hear one) and I was wondering if you could compare the long stroke and short stroke versions. I understand that you seem to favor the short stroke. Could you offer me some comparision. I know that when I die, I will have only put together 1 or 2 911 engines at best so I'd like to know as much about these two as possible. I have an early aluminum sand cast case and a few $$ and a Minnesota winter to throw at it. thanks in advance!
Nabil

[KobaltBlau]

I'm not Henry but I don't think that will keep me from giving an opinion. obviously you are looking at 66x95 for the SS flavor and 70.4x92 for the 2.8 regular.

As I see it the 2.8SS is for maximum RPM reliability. the case used for this is the carrera 3.0 or 3.0 turbo so you can use the 95s and the early small journal crank. maybe you already knew all of this.

I think that for street use the SS does not have an advantage over the "long" stroke version due to the RPM range. Look at the cams you are using, I would guess that using less than a GE80 you wouldn't need the short stroke to keep the motor together in the desired powerband. If I were building a mostly street/DE 2.8 with GE40 or GE60 cams I would build a regular 70.4x92 with a 7R mag case for lightness, properly built these are very reliable (Grady Clay on this board has built many of these).

On the other hand, If I were looking for extra durability and RPM in a class where a 66x95 was legal, and I was running RSR sprint cams or GE80+, the short stroke would look more attractive.

[Henry Schmidt]

It seems I always find myself in a position to contradict my friends on the board. I guess it just my curse.
Here goes.
There is no comparison between the 2.8 SS and the Mag case 2.7 based 2.8.
First. The case .
There is no better case built by Porsche than the 3.0 Turbo. It has been proven to handle over 700 hp (935). The mag case seems stressed at 200+ Porsche didn't even use the mag case to make 280hp in their 2.8 RSR..
Second. The crank. The 66 mm crank has been proven in racing for 40 years that it could run at 8000 forever, the 2.7 crank needed to be replace in the RSR because it couldn't handle sustained 7400 for any prolonged period. 2.8 RSR crank is great but hard to find. I loaned a VARA racer at 2.7 based engine to run in his 914/6 gears of a short stroke engine. He ran it at 8000 RPM and broke the crank in one weekend. Few people run that hard but if you might you can't.
Third :The heads. the 2.8 SS uses 3.0 heads. Bigger valves, better valve angle (improved flow) Larger volume for easier combustion control. (flatter dome)
Forth: Engine design. The short stroke has better rod to stroke ratio which means longer piston dwell (more efficient fuel burning), less side loading (less wear and reduced friction) and higher potential rpm .
Fifth : Cams, the cams in the 2.8SS are four journal. This means they have better support and the larger journal size allows for higher lift cams with larger base circle.
End result: The engine will run better at all RPMs. The fact that it will run at higher RPM does not mean that it won't run well at lower RPMs. How the engine performs at lower rpms is a function of the cams you choose. Choose a cam the performs at 3500 and the SS engine will perform there as well or better than the long stroke version and still be functioning at 8000.
It is more reasonable to compare the 2.8SS with the long stroke 3.0. These engine are cheaper to build and offer similar performance. The 3.0 even has some benefits over the 2.8SS. 9 bolt crank being the most obvious.
9.5 to 1 compression, idle smoothly, easy to drive on the street and 315 horse power on street gas. What could be better.
BTW: KobaltBlau's avatar is the 315 hp 2.8SS

[Porsche_monkey]

So the performance and reliability have been addressed. Ballpark cost difference?

[Henry Schmidt]

All things being equal you can count on $2500 difference.
2.7 core = $500-$1000
3.0 Turbo core $2500-$3500

Who knows ?

[andrew15]

What cams are you using to get 315hp? The more I read, the more I think I should be building one of these...

Thanks,
Andrew M

[Henry Schmidt]

A modified version of an RSR sprint cam (104 lobe center). Sounds too wild for the street but funny thing is it works. This engine also runs a slide valve and conventional wisdom is that they won't work on the street but it's cool and I wish it was mine.

BTW : Burn-Bros and I are thinking about reproducing this slide valve intake. Anyone interested?

[andrew15]

Truly amazing!
Looks like I have my work cut out for me this winter...

[Bryan Beaumont]

Anyone interested?? Please....You are a big tease Henry. But,
you put out as well!!! That motor is sooo *****in'. And I thought
slide valve was a complete fantasy. I had it all figured out how I was going to have you build my 2.8SS. One more option to consider---Slide Valve... Build it and they will come.

Bryan

[nabilious]

If its remotely reasonably priced I'd be interested in more than one pair. Are the stories true about them getting dirty easy? It would be interesting to see the difference on the dyno, all other things being equal.
Nabil

[KobaltBlau]

well, the contrast between my and Henry's responses shows who is the experienced professional. I had not even thought about the 3rd, 4th, and 5th points. Cheers Henry for a wonderful summary.

Henry, do you think the 3.0 Turbo case is better than the cases made for 9-bolt cranks (not asking about the cranks themselves, just the case)?

I was indeed going to ask you to compare with a similarly specified 3.0 9-bolt engine as well. What advantages does the 2.8SS have? I'm sure it spins faster, but for the street...?

Originally posted by Henry Schmidt

BTW: KobaltBlau's avatar is the 315 hp 2.8SS

Thanks for permission to use it, Henry, it's a brilliantly executed engine as far as I can tell

[KobaltBlau]

Originally posted by nabilious
It would be interesting to see the difference on the dyno, all other things being equal.

Factory 3.0 RSR gained 15hp going to slides (315hp->330hp) for reference. I believe 'all else was equal' or very close.

[Wayne 962]

Slides are only useful for HP gains at Wide Open Throttle, which most of us don't drive at all the time, unless we're on the track.

Ditto with MFI - a finely tuned engine management system will be able to get more HP out of an engine than MFI. However, it won't look period correct.

The mag cases are indeed weak - the short stroke 2.8 is a much more reliable platform. However, I always tell people that it costs just about the same to rebuild any 911 engine - you might as well go with the most HP. That would be a 3.2 or 3.4 with JE pistons and some 'S'-style cams. The most practical engine I can recommend for the early cars is the 3.2 with JEs and 'S' cams. This is a good balance between power (plenty of it), and period-correct looking...

-Wayne

[Henry Schmidt]

Originally posted by Wayne at Pelican Parts
Slides are only useful for HP gains at Wide Open Throttle, which most of us don't drive at all the time, unless we're on the track.

Ask Jerry the next time your at an RGrouppe event and he'll tell you how easy it is to drive the slides.

Originally posted by Wayne at Pelican Parts

Ditto with MFI - a finely tuned engine management system will be able to get more HP out of an engine than MFI.

930/81 IMSA 3.2 935 760 HP MFI 962/71 IMSA 3.2 962 700 HP Motronic Same booth rules same engine config. 962 had more comp. EFI more hp than MFI ?? Modern technology does prevail but when they were both being raced the MFI was clearly superior for flat out horse power.

Originally posted by Wayne at Pelican Parts

..........you might as well go with the most HP. That would be a 3.2 or 3.4 with ...............
-Wayne

The original question was :
This winter I've decided to put together a 2.8/2.9L twin plug smile maker (half of the reason is I've always wanted to hear one) and I was wondering if you could compare the long stroke and short stroke versions.

Wanye he didn't ask about 3.2 or a 3.4 . He asked which 2.8 would be best and the answer to that question is 2.8 Short Stroke because of the reasons listed above.

As he stated " I've always wanted to hear one ..." Me too!!!

[andyjboy]

Any interest in a re-manufactured slide-valve set-up ?

Depending on cost, I'd be interested in at least one set.

[Wayne 962]

Originally posted by Henry Schmidt
[B]Ask Jerry the next time your at an RGrouppe event and he'll tell you how easy it is to drive the slides.

I didn't say that it was harder. There's no performance advantage with using slides versus throttle-bodies, except at WOT.

As for MFI versus engine management systems, there's no way that any MFI system will produce more HP on the same exact engine.

-Wayne

[Henry Schmidt]

Originally posted by Wayne at Pelican Parts
I didn't say that it was harder. There's no performance advantage with using slides versus throttle-bodies, except at WOT.

Sometimes WOW is what these projects are about. How else can we justify these seemingly endless, money pit projects?

Originally posted by Wayne at Pelican Parts

As for MFI versus engine management systems, there's no way that any MFI system will produce more HP on the same exact engine.

-Wayne

I totally agree that modern technology will always prevail but please remember that some of us are stuck in a kinder more gentle time.
With all due respect and keeping in mind that this is not a pissing contest, I listed a place where MFI did make more HP on the same engine.

[Walt Fricke]

Henry - the RSRs came out in '73 (that model year, anyway), did they not? I had always assumed they used the mag 7R case. If not, what case did they use?

The 70.4 (6 bolt at the time) reportedly had two problems: shaking the flywheel bolts loose, and then whatever it was (breaking, I suppose) that led to the greater fillet radius on the (rod?) journals on the RSR crank. The flywheel problem (which was real enough that even tack welding the bolts to the flywheel did not cure it) due to some 4th harmonic vibration node intrinsic to the characteristics of the crank can be cured with 150 lbs/ft of torque on the bolts and a little bit of red Loctite (maybe the torque alone is enough, but I haven't been inclined to experiment). Once I did that thanks to Bruce Anderson's advice my flywheels stayed put.

As to cranks breaking, I had supposed that the mag case had something to do with that. I have been running a stock 70.4 crank on a 2.7 race motor in a sand cast aluminum case for over ten years, and I used to spin it up to 8,000 routinely until I dynoed the engine and realized 7,600 was a better shift point. The crank has been trouble free. Of course, if the torque curve meant shifting at 8,500, maybe it would have broken.

If I were to want a long stroke 2.8 I'd sure want the sand cast case. To me the sticking point is the heads. The factory race heads with their larger valves (which required a change in valve angle, which suggests that recreating one from a stock 2.2-2.7 head would be quite an involved project) are unavailable to the "budget" racer, and I suspect hard to find even if one has unlimited funds. For years SCCA banned "twin plug heads" for the 2.8 Porsches in GT-2, and I always figured that what they really meant to ban were the RSR heads (which were twin plugged), though my speculation could be way off. I have heard (I think I recall correctly) of 3.0/3.2 heads being used on 92mm jugs (sleeved 95s so the 3.0 case and crank can be used), but my measurements (perhaps inaccurate) of 95mm heads showed that this would require notching the top of the cylinder a little to allow the valves to clear, so this was always a puzzle to me.

Mind you, I am delighted by the prospect that the short stroke 2.8 may be the better design, since that's what I am slowly building.

Walt Fricke

[Henry Schmidt]

The difference between 2.7 crank and the RSR crank was the fact that the RSR crank had 911 SC journals (narrow & larger diameter) with a large radius. In essence a 3.0 SC rod with special bearings. The narrow journals allow for wider fly weights making it considerably stronger.
As I said before, there are those people who will say "they rev their 2.7 to 8000 all the time with no problems" but that is not the norm.
If your theory is correct about the mag case being the problem not the crank then why would they redesign the crank even after they had the aluminum RSR case?

[Walt Fricke]

Henry

I didn't say I've had no problems spinning an engine to 8,000 (I sure wish I had not had some of the problems I have had). But fortunately a broken crank hasn't been one of them.

It also occurs to me that since the stated HP for the RSR is given at something like 307 hp at 8,000 that those engines should be shifted at something like 8,500 for best performance. No doubt the stresses don't just rise linearly with RPM.

But I didn't know that the 2.8 RSR had an aluminum case. Did it start with a mag case, then get an aluminum one? Those must be really scarce.

Walt Fricke