Rally2 (ex-R5) News

[itix]

If it's simple than tell me why?

Simple logic says it shall be exactly opposite.

I agree with that, but physics with engines aren't always easy.

With a larger bore you also get a larger surface for the pressure to act on (F = p * a) and even if the crank radius will be less that may lead to more torque (t = f * r).

If you look at others designs though, high revving motorcycle engines with a lot of peak power and little torque traditionally has a large bore to stroke ratio, and diesels with a horrible peak power but good torque curve has a large stroke to bore ratio.

...then of course when you add turbo chargers etc you mess up engine physics very easily so I honestly can't say which way it would go on a rally car.

Also, something I still haven't managed to figure out since my physics lessons is that torque is supposed to be t = (n/60) * p (engine speed times power), so why is it that high speed engines gives little torque, and the torquiest engines in the world are diesels and the damn 18th century v8's that the Americans love so much(both of which don't Rev much more than 6 000 rpm...)?

I'm confused and my job involves engines... So yeah.

[Mirek]

Exactly, that's why I asked. Why high-rpm Fabia S2000 has extremely long stroke but Fabia R5 or Polo WRC have very short stroke etc.

[BV92]

Exactly, that's why I asked. Why high-rpm Fabia S2000 has extremely long stroke but Fabia R5 or Polo WRC have very short stroke etc.

Completely agree mate, especially about the Fabia R8. In fact I agree so much have a biscuit from me

[BV92]

Let me know if you enjoyed the biscuit

[dimviii]

Exactly, that's why I asked. Why high-rpm Fabia S2000 has extremely long stroke but Fabia R5 or Polo WRC have very short stroke etc.

as itix said a long stroke will help the power at upper part of rpms, and a short stroke the has advantage at mid part of rpms. Thats a general rule.
An other example is the long stroke mitsubishi evo vs the short stroke subaru.Mitsubishi always had more powerfull engines.

[Mirek]

Then I still don't get it

You said long stroke for peak power, short stroke for middle rpm (torque) but...

Evo IX gr.N has relatively low rpm, the dyno charts I saw had often peak power at 4000-4500 rpm, huge torque and near constant power for several thousand rpm.

S2000 engine as a naturally aspirated one has peak power at very high rpm but Fabia with by far longest stroke of all S2000 had possibly the highest torque and also the largest power bent of all S2000. Why?

Both engines are counterparts, both have long stroke and both are the most powerful in their category despite both having very different characteristic.

Then we have current WRC cars which use short stroke (Fabia R5 too). These are specially designed race engines unlike the previous two. On the other hand production-based Mini WRC or PSA R5 engines with long stroke suffer from not enough torque.

Why do the WRC look like they completely contradict what can be seen in Evo IX or Fabia S2000 (the illusion is that both for gr.N and S2000 engine the long stroke wins but with WRC it's exactly opposite)?

[dimviii]

Then I still don't get it

You said long stroke for peak power, short stroke for middle rpm (torque) but...

Evo IX gr.N has relatively low rpm, the dyno charts I saw had often peak power at 4000-4500 rpm, huge torque and near constant power for several thousand rpm.

Μirek i mean evo vs subi .I compare two engines with same restrictor with different bore/stroke ratio. Of course evo will have at lower rpms top horsepower due to restrictor,but still higher than a short stroke engine.

S2000 engine as a naturally aspirated one has peak power at very high rpm but Fabia with by far longest stroke of all S2000 had possibly the highest torque and also the largest power bent of all S2000. Why?
Both engines are counterparts, both have long stroke and both are the most powerful in their category despite both having very different characteristic.

Because the long rod is not the only reason.There are other reasons too.
Example....last years fiesta r5 was underpowered.With this year evo is almost same as other powerfull r5 cars.Some times is how good work has done each team.
Another example is with psa r5 torque.Last year not so good,this year much better.
Also dont forget that at last years Psa hadnt put much progress at 207,when skoda was working harder and more proffesional.
Fiesta was far away as you know from psa/skoda bhp/torque,and we didnt see from Msport any significant evolution for power increase all these years(s 2000 fiesta)

Then we have current WRC cars which use short stroke (Fabia R5 too). These are specially designed race engines unlike the previous two. On the other hand production-based Mini WRC or PSA R5 engines with long stroke suffer from not enough torque.

Why do the WRC look like they completely contradict what can be seen in Evo IX or Fabia S2000 (the illusion is that both for gr.N and S2000 engine the long stroke wins but with WRC it's exactly opposite)?

each wrc manufacture make their choice.This choice is not always free of cost or block availability from manufacture.Except that we dont know if mini wrc has 30-40 Nm less from wrc ds3 for example.
An other reason is that with wrc ''strong'' antilag there is no problem with enoygh power at low-mid revs.So maybe you can have a compromise with a longer stroke to make the engine breath better at high revs.
Another reason is that each engine can 'breath'' differently from another engine.Better flow at head ports , with better matched cams,much more flow from exhaust ports/manifold.
So if you have a long stroke engine but not excellent flow inside and outside from engine,there can be another engine with shorter stroke with better power band at high revs.
or another manufacture has better software(dont underestimate that,huge gains/losses/engine bangs/reliability from that)
These are points that they are not open free outside.

[Coach 2]

I think maybe we should talk about what is to long stroke, and what is to short (over and under quadratic).
VW and Hyundai kit car had to reduced stroke length from 93 to 90 to avoid too many meters per second on the piston.
I think we can say that a restrictor (how big) in combination with ccm will determine whether power can or must be utilized at different rpms and this determines whether it is best with long or short strokes.
This must surely be the difference between Sub and Mitsu gr N.
Am I wrong?

[dimviii]

I think maybe we should talk about what is to long stroke, and what is to short (over and under quadratic).
VW and Hyundai kit car had to reduced stroke length from 93 to 90 to avoid too many meters per second on the piston.
I think we can say that a restrictor (how big) in combination with ccm will determine whether power can or must be utilized at different rpms and this determines whether it is best with long or short strokes.
This must surely be the difference between Sub and Mitsu gr N.
Am I wrong?

no yoy are not wrong. There are too much parameters that each one,reacts differently with each other.Each small detail counts.
About subi vs evo there are more reasons why evo breaths better at high revs except the long stroke.There are difference due to boxer engine design(longer manifolds-bad for spooling,waste of thermal energy to turbocharger=solved with smaller/faster reacting turbo=problems at high revs)

[Coach 2]

I agree with that, but physics with engines aren't always easy.

With a larger bore you also get a larger surface for the pressure to act on (F = p * a) and even if the crank radius will be less that may lead to more torque (t = f * r).

If you look at others designs though, high revving motorcycle engines with a lot of peak power and little torque traditionally has a large bore to stroke ratio, and diesels with a horrible peak power but good torque curve has a large stroke to bore ratio.

Both the length and weight of the crankshaft (force times arm length) and weight and size of the flywheel must affect torque, or?
For a motorcycle where you want a rapidly accelerating engine much weight will be negative. Nor should we move a lot of weight, no need of torque.

Explosion velocity of a gas made of diesel fuel is lower than one made of gasoline. This will only be exploited if the stroke is long (relatively speaking) or?
And this will give more torque or?

...then of course when you add turbo chargers etc you mess up engine physics very easily so I honestly can't say which way it would go on a rally car.

Also, something I still haven't managed to figure out since my physics lessons is that torque is supposed to be t = (n/60) * p (engine speed times power), so why is it that high speed engines gives little torque, and the torquiest engines in the world are diesels and the damn 18th century v8's that the Americans love so much(both of which don't Rev much more than 6 000 rpm...)?

I'm confused and my job involves engines... So yeah.

I thought I understand some of this until I read everything written here, but now I'm very unsure.