Short or Long stroke

[andrea70]

Hallo to everybody.

I found lot's of infos about engine tuning during 1970-1973 on SWB chassis in Targa Florio race. How many solutions they tried! it was incredible!

Those news opened a big debate here about the following argoument:

starting from an early aluminium case (not mag), in your opinion or experience is it better a 2.5 short stroke or a 2.5 long stroke engine for a race like Targa Florio (somewhere very fast but somewhere else like an hill climb). And for a street/track use?

And do you think that an alu 901 gearbox can resist to all that torque? or it's mandatory to upgrade to a mag 911 gearbox with alu reiforcement plate?

detailed replies are well accepted!

Thanks in advance
Best
Andrea

[Flunder]

Andrea, The shop that is building my car has 901 and 911 transmissions in use with 2,8 engines with close to 300hp. You must use an aftermarket sideplate for strength and avoid hard launches in first gear. For the transmission, they say that an external pump and cooler are unnecessary for sprint races and only become required for endurance events of 4 hrs or longer. For the crank, I think you might like a long stroke configuration for some added torque exiting slow corners at the Targa and limit port sizes to no more than 38mm to keep port velocities at low revs adequate for sharp throttle response. Gear box ratios are no doubt much more important than ultimate horsepower at the Targa and a gear set to "hill climb" or "airport" specs would be the most fun for the Sicilian hills. These comments should be read in the context of "opinion" rather than as a "recommendation." One important consideration is cam selection. So, will you use S camshafts or 906? I think S cams might be best for the planned event. If you are building to 2,5L I assume you will use 10.3 Mahles and twin plug. And remember, everything you modify to suit use at the Targa will be a compromise when you take the car onto the Autostrada. For the targa, I think either long or short configuration is fine, as long as the tranmission is properly geared. Please tell us, what did Bonomelli use? As there are some board members who took their cars to the Targa last year, perhaps they will chime in.

[andrea70]

Thank you Flunder.

1) do you think is better to use the weltmeister billet alu sideplate ALSO with the 901 alu gearbox? i thought it was studied only for the 911 mag case....

2) the problem about short and long stroke is the "speed" you take rpms... I know that the old italian tuner said that the "2.3 short stroke was wonderful" in their memory....

3) About Bonomelli, he used almost every combination you can image! The problem was always money. That's why e.g. Porsche produced in 72 a 2.5 short stroke: only to permit to use the old 66mm cranks...

As i understood, the better torque with 2.5 long is compensated with the better "rpms speed" of 2.5 short...

My friends say that the 2.5 short was a "compromise" engine, not a really engeneered engine... infact in 1972 Porsche was thinking about 2.7/2.8 displacement.... so the best solution is again a Long 2.5... BUT i think it's not correct for a SWB chassis because you loose the wondefoul handling of that car in streets like in Sicily where the character of a short stroke (2.3 or 2.5) is better suited for that chassis...

i hope you can understand .... sorry for my english, of course...

[SandyI]

If you're going to the trouble and expense to rebuild your 901 for that (expensive) HP increase, do yourself a favor and add the cooler. When I was running my 914/3.2 with my rebuilt 901, the cooler was necessary and I never ran close to a 4 hour race. By the way, didn't read what type of Limited Slip you'll be installing, but this will also dramatically increase your tranny temp during race laps.

[andrea70]

Thank you for the advice about 901.

I'm not going to build an engine, i want only to understand if a SS 2,3/2.5 engine "fit" better in a SWB chassis and a LS 2,5/2,7 in a LWB chassis .


Thanks
Andrea

[a911s]

Great advice from Flunder. 2.5L short stroke can provide better spin up, but a long stroke with light weight pistons and rods will do the same, while providing better torque. Long strokes can be hard on rod bearings though, so make sure you address this issue. 901 tranny with beefed up side cover should be fine. Gear ratios are very important. Have fun!

[Jeff Higgins]

I'll chime in on the side of the long stroke motor, for a number of reasons.

First, I'm a big fan of torque. "Area under the curve" is a term we hear when discussing dyno charts. Peak hp numbers are always impressive and fun to discuss, but, if that peak number comes at the expense of a very high rpm, narrow power band, the car will be difficult to drive and feel sluggish "off cam". This sort of power delivery, when viewed on a dyno chart, will show a very sharp peak on the hp curve. Not much "area under the curve", in other words.

Better is the kind of power delivery that may not climb as high, but climbs earlier and displays a flatter top to that dyno curve. A broader power band, in other words; one that shows much greater "area under the curve". This will pull harder out of the corners and be easier to drive. You will be well ahead of the guys waiting for their motors to spool up "on cam". Granted, on really long straights, peak hp wins, but it doesn't sound like driving the old Mulsane is part of your plans.

We hear a lot about how short strokes "spin up" better. Sure, maybe under no load... But the weight of the car, the power available, and its ability to accelerate that car determine how fast any given motor "spins up". The motor with more torque wins this one, given equal gearing and car weight.

Piston speed is not the limiting factor regarding how high we can rev these motors, at least until we are well up over the 8,000 rpm range. The valve train will be the limiting factor. Rocker arm strength and valve spring rate, or their ability to keep the valves from floating, becomes the issue long before a 66mm vs. 70.4mm stroke comes into play.

So, unless the plan is to build some 8,500 rpm screamer of a time bomb (with a very short fuse), I would choose the long stroke. Just my opinion, but I think its advantages in the real world indicate it's the better choice.

[andrea70]

Thank you Jeff. The "under the curve" theory is simple but clever, of course!

As i can see from the replies, nobody cares about the chassis... swb and lwb is the same..... in 1970/1971 i'm not so sure that it was like this....

Remember that, for example, the swb have the steel front auxiliary support, not the aluminium ones (i know it arrived in 1974 but you can put on a lwb chassis without canghe anything...)

[a911s]

SWB and LWB feel very much the same - as has been discussed before.

Aluminum front cross members are way over rated. The difference in weight (less than 13 lb) is not unsprung and is in the front at the lowest possible point. The steel cross member is much stronger. You will never feel the difference in weight, but you will feel something if the aluminum torsion bar retaining pin becomes damaged (pretty easy to do since it is so low).

[Mr9146]

Best motor I ever had:

2.4S stroker motor based on a 2.2S. Higher comp 2.2 pistons with the 2.4 stroke. Spun like a whirlwind with the stroker torque and high compression HP for the kicker.

BEST MOTOR EVER!! Yes, I like it better than 2.7s and 2.8s.