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-   -   Power characteristics of long and short stroke engines: Cast in stone? (https://www.team-bhp.com/forum/technical-stuff/80106-power-characteristics-long-short-stroke-engines-cast-stone.html)

Have been discussing stroke lengths in another thread. Which caused me to revisit a thought which has troubled me for a long time.

Received wisdom says that other things being equal, the short stroke is a power producing screamer while the long stroke is a stump pulling lugger. I have trouble with one part of it.

My question:
Can we design a long stroke to produce the same power as the short stroke?
Can we design a short stroke to produce the same torque as the long stroke?
If so, how? Or is it not possible at all?

Regards
Sutripta

Well, with all other things being equal, there is no scope for improvement.

But generally speaking, how much torque or power an engine produces will also depend on its mode of induction and state of tune.

A short stroke engine with a high state of tune can easily produce more torque than a regular long stroke engine.

Honda's 2.4ltr K24 (87mm x 99mm) on the Accord produces 222nm (92.5nm/ltr) but, the performance tuned K20 (86mm x 86mm) on the Civic Type R manages 215nm (107.5nm/ltr).

The same goes for power as well. The 1.6ltr Ford Ikon (82mm x 75.5mm) produces 91bhp (56bhp/ltr) while the 1.5ltr OHC VTEC (75mm x 84.5mm) produces 106bhp (107bhp/ltr). Infact, the long stroke VTEC engine produces its power at 6800rpm as compared to the short stroke Ikon's 5500rpm.

Shan2nu

Interesting question.
That would be the key.

A shorter stroke diesel will produce more torque than a longer stroke petrol. A longer stroke MPFi should produce equal or more power than a shorter stroke carb engine. Again, with carbs, 2 carbs are better than one for power. Other factors like exhaust manifold design, cam and valve design...

Too many factors involved, to alter the numbers attached to bhp/PS and lb-ft/N-m.

Lets keep out of the equation, by equalising, things like displacement, number and configuration of cylinders, valves/ cyl, compression ratio, and material choice which have more than a nominal effect on cost (ie say no titanium con rods and forged exotic alloy pistons/ liners with special coatings). And for simplicity, lets concentrate only on NA Petrols.

That's a pretty strong absolute.

Sutripta

Ofcourse, if you take 2 engines which are identical except for their stroke, how will the shorter stroke engine produce more torque? There is no way it can gain an advantage over the other engine.

It's like asking, "can a car with 195/60 R14 tyres cruise faster than another car with 225/60 R16, at 2000rpm (all other things being equal)". How will you change the gearing, if all other things are to be equal?

Shan2nu

Explained in previous post what should be taken out of the equation. Displacement obviously has to be the same.
And here we come to the nub of the question:- what generates torque? And how?

Non sequitur.

Regards
Sutripta

Yes, but my original reply was to the opening post and i was merely explaining why i said it.

Shan2nu

Yes Sutripta,
Its definitely possible but the bhp and torque curves might make the engine intractable. Your question was based on probably specific rpm's which is easy to do but to make the curves acceptable is the hard part. The engine can be tuned to make more torque or bhp at specific rpm's but to spread it out over a usable power band is the hair splitting stuff.

If you have driven a GTX/S10 you will know what I mean. The engine can be easily tuned by the factory to achieve 120hp but that would mean sacrificing driveabilty in the lower rpm's. Fiat compromised on peak bhp/torque so as to give us usable torque spread right from 1200rpm to 4250rpm with a relatively flat torque curve. Thank FIAT for compromising on peak outputs. The fiat 1.6 engine is a short stroke unit and still the torque is equivalent to or more than longer stroke engines from other stables and the icing on the cake is the torque spread. I badly miss my :thumbs upGTX.

I have kept turbo's out of the picture because then its a different ball game.

True, an engine with a peaky power/torque cruve can lose a lot in driveability. How an engine achieves its max power/torque is as important as the peak figures themselves.

PS : Do you happen to have the GTX's dyno chart? It would be interesting to compare the dyno graphs of diff engines.

For starters, here's Avinash's OHC graph (at the wheels) - http://www.team-bhp.com/forum/attach...-feb-2010-.png

Shan2nu

I am sure this graph belongs to a VTEC engine or engines with variable valve timings. The curve is splendid though and is expected with the amount of gizmos going into the engine's breathing characteristics.

Do note that the GTX/S10 has a simple dual camshaft with a very mild cam setting. In fact 0 degree overlap of the exhaust and inlet valve if my memory serves me right. There was a member here who has stopped posting here some time back who put in custom made piper cams for his GTX for 800 british pounds. He was happy with what it could do after 3500rpm but was pained with the lack of low end torque which was one of the forte's of the 1.6 FIAT engine. I think the configuration had an output of 125 or 135bhp according to the piper website.

Fiat 1.6 TWIN CAM engines sold in Europe already come with 108 bhp from the factory.

Products - Piper Cams - Europe's leading manufacturer of performance camshafts

Yes you can but not at the same rpm.

But there is no point in keeping everything else the same otherwise it will be a poorly tuned engine.

The Fiesta and Verna petrols produce identical specs on paper but anyone who has driven both will know that the Fiesta is a scorcher whereas the Verna is anything but. Does this have anything to do with short stroke vs. long stroke?

How is this engineered? Does gearing play a big part?

When you talk overall acceleration of a vehicle, there are many other factors affecting it other than engine characteristics. Gearing, rotational mass, weight, rolling resistance, , frictional losses, aerodynamics (this is more evident at higher speeds) etc.

Heres a vid showing how a shorter final drive ratio can affect acceleration on similar cars.

YouTube - S2000 vs S2000 w/4.57 gears



Well, gearing cannot actually change engine characteristics, but with the right ratios, it can surely change the way your car performs.

Yet another guide to Torque versus Horsepower versus Acceleration

Shan2nu

I think my "other things being equal" has not been explained properly by me. It is to provide a level playing field, to help compare like to like, apples to apples. It is not meant to used as a rule by stewards at a meet, or to say "gotcha".

ANY engine can be tuned over a fairly wide range, more so at the manufacturing stage, less so at the after market stage. Question is where will one start hitting real limits, and why?

What I mean by characteristic is the overall shape of the HP/ Torque curve. Not value at any one rpm. Also, if one can match (more or less) value at a particular rpm, what prevents (if anything) it from matching the value at rpm +- delta. And then go forward reasoning by induction.

I am not getting why we are discussing peakily tuned engines, and drivability.

Regards
Sutripta

Correction. Its not 107bhp/ltr but 70bhp/ltr.

Shan2nu