What are the performance figures of 2.6 short-stroke?
 

Mag case
66mm stroke
2.2 connecting rods
92mm 2.8 RSR p+c’s (are there any differences between the original and the repro from Andial?)
Original S cams
Standard 2.7RS heads - valves 46/40, ports 36/35
Standard distributor of 2.7RS
Standard MFI of 2.7RS
Standard flywheel and clutch of 2.7RS
Standard transmission of 2.7RS

What are the ‘official’ performance figures?
1. Compression ratio
2. Power output @ rpm
3. Torque @ rpm
4. Rpm redline limit
5. 0-60mph
6. Top speed

TIA

An engine of this spec was never made by the factory. As such I think that you'll need to do a little bit of work before you start to put things together. Specifically I'd cc both the heads and the P&C's to confirm the CR.

As far as the performance, that engine is pretty much half way between a 2.4 S and a 2.7 RS. A good guess is that the performance would behalf way between the two. So I'd say about 200 HP at 6500 RPM. The peak torque will be about 173 at 5200 RPM.

2.4S = 2,341cc
2.6 short = 2,634cc
------------------------
dif = 11.12%


2.6 short = 2,634cc
2.7RS = 2,687cc
------------------------
dif = 1.97%


2.4S = 2,341cc
2.7RS = 2,687cc
------------------------
dif = 12.87%

I don't think it's just cubic inches anyway... Do short-stroke produce a similar or different power band?

that sounds like a great engine.

Im real interested to know an actual measured compression ratio using those pistons and the shorter 66mm crank & rods? I think they were rated at 10.3:1 in the 2.8 RSR.

Yes with 2.8 heads, which had a volume of 76cc each, whereas a stock head had a volume of 68cc. If stock heads were put on the RSR, CR will be increased to something like 11.5:1! So, 11.5:1 minus shorter stroke equals to ??? Can somebody do the math for me? :confused:

Within the context of a 911 motor with ample port flow (like these 3 examples have), I haven't found any data to date that would suggest that there is a different power band. Mind you that I've only been searching for a few years and only have detailed data on about 70 different configurations of 911 motor, and less specific data about another 200 different motors. It's not to say that it doesn't happen -- I just haven't been able to find any data that supports it. :rolleyes:

Actually, this is not quite right. The 2.4 is really 2,341 CC. The 2.7 is really 2,687 CC. The 2.6 is 2,632 CC, so it is only 55 CC smaller than the 2.7. The 2.6 is almost 300CC larger than the 2.4.

The 2.6 will also have higher compression (more on this later), so a better comparison is the 2.2S motor. The 2.2S is 2,195 CC and rated at 180 HP. The 2.6 is 20% larger and thus calcs out at 216 HP. This jives with the estimation reached by taking a 2.7RS motor (210 HP) only slightly reducing its power for the 50CC deficit, but then bumping it 10 HP for the increased compression.

I have run the numbers (I have a set of 92mm RSR pistons waiting to go into my 2.2S), but I do not have them in front of me right now so I'm going from memory now, but I'll edit my post if my recollection is wrong. I deduced the dome volume from the graph in Bruce Anderson's book comparing the compression ratios of these pistons in RSR heads vs. 2.7 heads. With deck heights in the standard range (.030 - .050 inches) the compression ends up being in the 10.0 - 10.7 range when used with a 66mm crank. THIS IS A ROUGH ESTIMATION WITH NO ACTUAL MEASUREMENTS TAKEN. With this compression I think you'll see HP #'s in the 220 - 230 range with the setup like you describe.

IF YOU BUILD IT, IT WILL SPIN
 

We built an engine similar to what you describe 15 years ago.
The hp was impressive and I highly recommend building this combination.
Some things to remember:
The 2.7 RS dist has too much advance for the compression you're producing.
Anything over 10.0 to 1 is too much compression for the street gas.
The power band will be closer to that of a 2.2 "S" than a 2.7.
This engine will want to spin, so gearing will want to be shorter than with the 2.7.
This engine would be happier with a 2.2 "S" pump than a 2.7 RS pump.
Because it will want to spin higher RPMs 38 mm intake ports make more sense.

Henry,

Glad to hear you have had good experience with this combo. Really glad to hear you think the 2.2S pump would work the best with it, as that is what I have:) .

As to the advance curve, would the 2.2S distributor have a more appropriate advance curve? I really do not know what the differences in advance curves are between the 2.2S, 2.4S, and 2.7RS distributors. How about once modified to twin plug? I was thinking about shooting for 10.3 - 10.5 CR and twin plugging it. Would that still be too high for street gas in your opinion?

A guy in Portland, Or is running a motor like this, but twin plugged with GE60 cams. He says he's getting about 235 HP out of it.

Thanks.

The 2.2 dist will not fit a 2.7 7R case.
Street gas is tricky. You are never certain what you are getting. If you're running on street gas, error towards less compression even with twin plugging.
235 hp seems reasonable for this configuration. It may even be conservative.

Henry,

I know the 7R case is the strongest of the mags and has most of the neede updates. I also know the ultimate bulletproof 2.6 would use the early sand cast aluminum case as the starting point (at additional cost and with a weight penalty though). I have had some people tell me that my stock 2.2 (3R) case with spigots bored to 97mm, case inserts, shuffle pinned, case squirters, oil bypass, and oil pump upgrade would be up to the task. I guess the short stroke would put less stress on the case than a 2.7 or 2.8, but do you think it could survive spinning to 8,000 RPM?

Brooke

Henry;
You've built one and I haven't, but given the mix of parts described I'd be hard pressed to see how you'd be doing that much better then a 2.7 with slightly less capacity -- not without a good dose of "dyno marketing".

Looking at it piece by piece it just doesn't add up...

1) Capacity: The capacity is less in the 2.6 then the 2.7, so that certainly isn't helping to make more HP.

2) Camshaft. He described a standard S camshaft. I guess that this bound the rev range between that of the high CR 2.2S (5200-6500 RPM) and the 2.7RS (5100-6300 RPM). I don't see any reason that this camshaft would perform differently in the 2.6.

3) CR: Sure the CR that he's describing would be higher then a 2.7RS's, but 911 motors just don't seem to be that sensitive to CR when when it comes to HP. On one hand you could say that the CR is increasing by 23%. But the 2.2S had a CR that was 15% higher then a 2.7RS's -- did that engine perform significantly better then the 2.7? Compare the BMEP (in PSI) numbers...
................Peak Torque.........Peak HP
2.7RS...........171.....................161
2.2S.............163.....................164 (Note this suggests peak torque RPM = peak HP RPM which doesn't sound right)
2.0S.............164.....................163

So a 15% increase in CR resulted in less torque/liter and slightly more HP? (As I know that you're aware, the high domed pistons on the early S's and more significantly the race engines actually got in the way of the flame travel and inhibited good combustion -- thus Porsche's use of twin plugs for racing applications). Anyway, It's not clear to me how the increased CR would trade off against the increased dome size in the 2.6 configuration.

4) Airflow -- This engine is using the same valves and port dimensions as the 2.2S, 2.4S and 2.7RS, so I don't see any reason for it to flow more air for the range defined by those engines.

Let's play "what if" and say that they engine could make 235 HP, what would the BMEP look like?

................Peak Torque.........Peak HP
2.6..........................................179
...assuming 235 HP at 6500 RPM. If the peak HP rev's were to magically increase to 7100 RPM, the BMEP would be back down to 163. But why would the peak HP RPM's increase over the 2.2S when it uses the same cam, valve and port dimensions while drawing more air per cycle??? At 7100 RPM the 2.6 would be drawing more air per second then the 2.7RS through the same sized ports. How is that possible especially when you consider that the 2.7 already had the highest gas speed range of any of the S cam'd engines (and I maintain as a result the lowest peak HP engine speed)?

Assuming that you're measuring the output consistantly with Porsche's factory spec's, I'd expect the 2.6 to make about 218 HP at 6500 and 178 lb/ft of torque at 5200 RPM. Note that the peak HP BMEP of the S cam'd engines only dropped by 2-3 PSI when the CR dropped from 9.8-9.9 down to 8.5. So I've generously assumed that it will increase by about half that amount when you go from 9.9 up to 10.3 (generally increasing CR with no other changes results in a declining benefit).

http://forums.pelicanparts.com/uploa...1106783057.jpg

I'm not saying that you won't dyno 235 HP. What I am saying is that I doubt that you'll see a 235 HP - 210 HP = 25 HP improvement over a stock 2.7 RS on the same dyno on the same day. I'd really expect maybe a 4% increase in HP over a 2.7RS tested on the same dyno on the same day for your 2.6 configuration.

Assuming Porsche factory specs are accurate is your fatal error.
I believe that Porsche factory specs are inaccurate and that they quote a number somewhat less than the actual hp output. That said compression does make a difference (930/03 3.0 SC 8.5 to 1 180 hp. 930/10 3.0 SC 9.8 to 1 204) the only difference is comp. Short stroke engines of similar size and description historically make more hp.
Short stroke engine have better rod length to stroke ratio and therefor less piston side loading which results in less friction. Proper rod length to stroke ratio {less rod angularity} also allows the piston to remain at TDC and BDC longer which improves flow and combustion..

would there be any point to building a 2.5 or a 2.6 short stroke engine for a street car? Can you get the compression to reasonable enough level to run pump gas and single plug with Mahles or do you have to use custom J&Es at that point. Or should you just build a 2.7 or a 2.8 if you want a street motor?

Thanks for your patient answer Henry. My objective was not to slam you, but just to deconstruct the hows and why to making HP. I'm sure that you'd agree that there are a lot of bogus numbers being thrown around when it comes to HP and I'm merely trying to cross validate and improve the "signal to noise" ratio of some of these HP discusssions.

BTW, I'd agree that Porsche was generally conservative in their HP reporting for many reasons, most have nothing devious about them but rather an honest desire by the engineers at Porsche to only report on what they can honestly delivery (as opposed to some other small volume manufacturers from the other side of the Alps ;) ). The reason that I base a lot of my calculations on their numbers is because they generally appears to be consistantly conservative and they are certainly internally consistant within the set of observations.

As far as the difference in HP between the two 3.0SC models, I believe that the CIS system has a lot to do with that jump in performance. Basically, since the CIS is restricting the intake flow, that the engine is drawing something below ambiant at WOT and high RPM's -- much like a restricter plate motor. Increasing the CR props up the cylinder pressures for a few more RPM's thus allowing more HP. Since I don't have a CIS engine to test this on, it remains a theory of mine for the time being.

If one built the same engine as described in the beginning of the thread but used 93mm P&C,s, I assume the performance would be approx the same as a high comp 2.7 RS motor? Would the benefits of using the short stroke crank outweigh the potential problems because of the thin cylinder walls on the 93mm cylinders or would one be better off building a standard (70.4*90)
2.7 combination?

Henry, in an engine with a larger rodlength/stroke ratio the piston dwells longer in the vacinity of TDC and SHORTER in the vacinity of BDC. This gives a short rod engine some advantage as to camshaft lobe centerline spread. You be careful not to have the intake closing point too late in a long rod engine.

We are talking about a ratio very close to 2 to 1 and this ratio seems to be near perfect at least for the Porsche engine.
Any longer and I can see where problems may develop.

So, which dist do you recommend?

You will need a custom built distributor or a crank fire system.

So whats the best way to a 2.6?? De-stroke a base 2.4T or a early 2.7CIS motor with 66mm crank and 2.2 rods or look for a 2.2 motor and do all of the case mods, etc?

I know the 7R and early aluminum cases are probably more desirable for the high revving engines like these due to their extra strength, but can a 2.2 mag case or even a 2.4 5R case be shuffle pinned etc so that it will hold up?

I would think so yes because the shorter throw crankshaft does not flex the case as much.


Henry,
What's a crank fire system?
Secondly, will a Bosch RSR twin dist fit a 7R case and is its advance OK?

http://www.clewett.com/
- click the catalog link, and then the iignition system link. search here for 'HPX' or 'HPV-1' for old threads about crank fired ignitions.


Im real curious about a comparison between a 2.5 and the 2.6 thats being discussed here. I didnt want to hijack this thread, so I started a new one here.

Short stroking a Porsche is an ideal situation. Why? Because Porsche engines can breathe, breathe very well into the very very high RPM ranges and short stroking helps them rev higher into their more efficient operating range. Porsche heads will support 10,000 to 12,000 RPM operation, the stroke will not unless its shortened a lot. But wait you loose cubic inches and the cylinder to cylinder placement limits the bore. But alas theres another Porsche downfall, at least until recently, air cooling. Air cooling does not support over 300HP reliably, thats according to Porsche engineers and agrees with practice, ie people who do not want to deposit piston material on the exhaust system.

Should throttle bodies be from a 2.2S as well? How about 2.8 high butterfly injection?

Should both ports be 38mm or just the intakes?
TIA