short/long stroke

[Kham]

which is better? as i understand short stroke allows for high rev and is more oval. seems high rev engine is better? no? can anyone clear this up?

[Karaya One]

The key is port velocity.

Short stroke can work well - ZXR750's

Short stroke can fail miserably - GSXR750 (88-89).

It has to be able to breathe and combust properly if short strokes are to work.

[Kham]

just wondering. on 04 models, r1 and 10r have shorter stroke than cbr1000. dont matter to me.

[TLRMan]

Short stroke, larger piston diameter, equals higher RPMs and more HP.
Bigger diameter pistons allow for larger valve diameters, but also need more igniton advance to get things happening.
If Squishband areas are carefully planned, velocity in the chamber is high, and atomization of the fuel can be complete.

Long stroke, smaller diameter pistons equals lower RPM's less HP, but most of the time, you'll see more torque.

What limits RPM's on long stroke engines is piston velocity.
More than 5000 ft/per minute, and things can get weird...

There is a practice of running longer rods farther up into the piston that will simulate a shorter stroke, extreme rod angles are less, and actually reduce drag on the piston skirts.
The piston doesn't reside at the top as long, so you have to have very good combustion.

As for the New R1, and Kaw, they figure that most people won't be living at redline on these machines, so they can get away with a higher piston velocity, thus more HP.

This is how Drag racing engines survive. The Vance and Hines Pro Stock Harley, has piston velocities in the realm of 7000 ft/minute, and it will stay together, because it only runs this way for 8 seconds....

[Kham]

thank you very much my good man.

[Kham]

oh and this brings up another question. so torqu is not directly proportion to HP? where does torqu come from? gear ratio?

[Rye]

hp= torque x rpm / 5252

or

torque= hp X 5252 / rpm

[TLRMan]

I forgot to mention why Honda is doing the long stroke thing...
They feel that if they keep the engine narrower, it will turn better because of the shorter roll moment.

1 horsepower is 33,000 ft/lbs(torque) per minute....

[Kham]

then where does the extra torqu on v-twin come from if hp and rpm are not all that much different?

a v-twin with 100hp can pull over 70ft/lb torqu.

[jsven007]

Short stroke, larger piston diameter, equals higher RPMs and more HP.
Bigger diameter pistons allow for larger valve diameters, but also need more igniton advance to get things happening.
If Squishband areas are carefully planned, velocity in the chamber is high, and atomization of the fuel can be complete.

Long stroke, smaller diameter pistons equals lower RPM's less HP, but most of the time, you'll see more torque.

Well said, TLR knows this stuff!

[jsven007]

hp= torque x rpm / 5252 or torque= hp X 5252 / rpm

Yes, so at 5250RPM the torque and the HP are exactly equal...on every vehicle!

In terms of displacement...smaller displacements sometimes have more peak HP. Ex. The ZX12R with peak 162HP instead of a Hayabusa's 156HP. But, notice how the Hayabusa dominates below 9,000 RPM. The same can be seen with the CBR1100xx vs. CBR954. For low/mid-end HP, this is no replacement for displacement!

[TLRMan]

this is no replacement for displacement!

Yeah ..it's called "Cubic Dollars"....

[TLRMan]

Originally posted by Kham
then where does the extra torqu on v-twin come from if hp and rpm are not all that much different?

a v-twin with 100hp can pull over 70ft/lb torqu.

It's got a lot bigger bang than the inline 4....
Each cylinder displaces almost 500 cc's

[Kham]

"bigger bang"?

[stoinkythepig]

Originally posted by Kham
then where does the extra torqu on v-twin come from if hp and rpm are not all that much different?

a v-twin with 100hp can pull over 70ft/lb torqu.

Since the longer stroke of a V-twin limits it's redline, the engine is tuned for peak volumetric efficiency at a lower RPM. Peak torque and peak volumetric efficiency pretty much happen at the same RPM. Since HP is just a function of torque and engine speed, a lower engine speed with X torque will yeild less horsepower than
the same torque at a higher engine speed.

Lots of people think V-twins stomp same displacement IL4s in the torque department. This is just not true. They may make their peak torque at a lower RPM but they don't typically make more peak torque. V-twins only became popular in sport bikes when Superbike race rules allowed them to have 33% more displacement than in-line 4s. I predict V-twins will slowly fade away and 3, 4 and 5 cylinder bikes will take over, because of GP rules.

[Kham]

i was missing displacement. i think i got it now.


Engine displacement defines the volume in cubic inches that is displaced as the cylinders are moved from top-dead-center to bottom-dead-center.

Engine displacement is calculated with the following formula: ((3.14159*(bore*bore)*stroke) / 4) * # cylinders

[stoinkythepig]

Originally posted by Kham
i think i got it.


Engine displacement defines the volume in cubic inches that is displaced as the cylinders are moved from top-dead-center to bottom-dead-center.

Yep, but just "volume" would be a better description since it does not have to be in cubic inches. All things being equal, displacement is proportional to torque.

Some rules for normally aspirated engines:

Diplacement is, as you described, a function of bore and stroke.

Torque is (for the most part) a function of displacement.

Engine redline is limited by stroke.

HP is a funtion of engine speed and torque.

Equal torque at higher speeds means more HP.

The easiest way to get more HP is to increase bore or increase the number of cylinders so that the stroke does not increase and therefore reduce the engine redline.

Engine designers aim for a specific HP goal through bore and stroke numbers and then design the head, cams, valves, intake system, and exhaust system to work well with bore and stroke limitations. The ignition system can be designed almost perfectly with just the bore and stroke numbers. There is some extra power available through parasitic loss reduction within the engine (one of the reasons the Suzuki GSXR1000 is so powerful compared to it's competition) Peak power can be increased quite a bit with simple changes but usually noise, reliability or driveability needs to be compromised to do so.

Then there's supercharging which, in the simplest terms, increases the engine volumetric efficiency which acts just like added displacement.

[KB]

Personally I like a short quick stroke!!!

[Kham]

Originally posted by stoinkythepig

Then there's supercharging which, in the simplest terms, increases the engine volumetric efficiency which acts just like added displacement.

compression?

[Honclfibr]

Originally posted by Kham
compression?

Yeah, supercharging is intake compression driven off the engine. Turbocharging is intake compression driven off the exhaust gasses.

A coworker has a grand prix GTP with a supercharger. Always wondered if it was less efficient as far as hp/fuel economy to supercharge or bump up displacement?

[LatinRider]

Originally posted by jsven007
Well said, TLR knows this stuff!

I agree

TLRMan...you're my hero

[stoinkythepig]

Originally posted by Kham
compression?

Compression ratio is the difference between the swept volume of the cylinder and the combustion chamber beween BDC and TDC. CR is limited by available fuel octane and spark advance. More spark advance means lower compression. As Mark stated a larger bore requires more spark advance so CR needs to go down with a bigger bore (generally). A smaller bore engine can therefore have a larger CR. More CR usually produces more torque unless the ignition cannot be advanced enough due to fuel limitations. Supercharged engines produce so much excess heat that they need lower compression ratios to run adequate igntion advance to make good power.

[TLRMan]

Originally posted by stoinkythepig
Supercharged engines produce so much excess heat that they need lower compression ratios to run adequate igntion advance to make good power.

true....but with a little twist....
I can't get get fully into this because I'm at work, but think as using intercoolers, and fuel as a cooling medium, can keep corrected comp ratios higher...I played with nitro methane , running 11.5 to 1 comp ratios with 70% nitro, 30% methanol...
Gasoline's latent heat of evaporation is a lot less than methanol, but you still can use the fuel to cool the internals....

Thanks Pat....

[stoinkythepig]

Originally posted by TLRMan
true....but with a little twist....
I can't get get fully into this because I'm at work, but think as using intercoolers, and fuel as a cooling medium, can keep corrected comp ratios higher...I played with nitro methane , running 11.5 to 1 comp ratios with 70% nitro, 30% methanol...
Gasoline's latent heat of evaporation is a lot less than methanol, but you still can use the fuel to cool the internals....

Thanks Pat....

Agreed on the intercooleer but only to a point. An intercooler can only indefinitely drop temps to ambient (in theory). The intake charge is still pressureized and will make a whole lot more heat as it makes more power so CR still needs to be lower on a blown engine vs. a normally aspirated engine, assuming the fuel and engines are otherwise the same.

As far as methanol goes, it's not practical for anything but racing so I left it out. I think it has a theoretical octane rating in the 140s if memory serves so higher compression would be easier with methanol.

Much of any engine's cooling come from the fuel and air charge flowing through it. Mixing the fuel in very close to the intake valves means it will cool the combustion chambers better than it would if injected further upstream. This works quite nicely if you can thermally isolate the intake manifold from the heat of the engine so the incoming air charge stays cool too.

[TLRMan]

As well as very tight squish areas.....
Gasoline sucks as a cooling medium, but still can make a difference if you run a tight squish.

My Mitsubishi (1.6 litre, TD04 Turbocharger/intercooled), was able to keep itself together with 93 octane at an intial 22lbs, regulated to 18lbs of boost..this was done with a 2 stage injector set up. Now, do you want to talk about a very rich A/F ratio...
Made the best power at 11.5 to 1 A/F ratio without detonationg, but the timing curve was reduced by 2.5 degrees/RPM starting at 4000 rpms. The pistons missed the head by .030", and the engine redlined a 8K, producing over 300 hp at the crank. My wife put it to it's death....Timing belt failed, and put the valve heads another 1/4" into their seats. As well as a cracked head, and little roller bearings all over the place from the rocker arms...I really do miss that car....

No one mentioned Nitrous??? This also cools the mixture, as well as demands more fuel......

In most of your "street applications" We are VERY limited because of fuels...

Hey....Lets get that V-Max and do it up!
We can put Kham on it, to test it out...