A smaller space between the piston and cylinder wall (for example) keeps more unburned fuel mix in the combustion chamber. Predictable temperatures allows predictable control of combustion via air/fuel/timing.

BTW - Engine speed is a function of component length and weights, valve spring rates and other things, not a water/air cooled thing.

 

Power=heat as was stated before. Air or water cooling has nothing to do with RPM, look at old 2stroke dirt bikes that were air cooled and could turn very high RPMs. Higher combustion chamber temps reduce emissions along with new chamber designs that allow for more complete burning of the fuel charge. I am not a Harley V-twin expert but I believe the offset firing of them limits reliable RPM ranges due to vibration and harmonics, but this is also what gives them the unique sound.

 

Like many have said, power and energy produces heat. That can just about apply to all things. You go to the gym and work out, your body gets hot. You run the computer and it produces heat. So to keep things efficient cooling is required. When you get hot in the gym you sweat and the evaporating process cools you. Your computer has a small fan to pull hot air away and bring cool air through the vents. An air cooled engine NEEDS air moving across the fins to pull hot air away and keep it running cool. So like the others have said, a water cooled engine has the heat controlled much better. The more power and energy it produces the more heat it produces as well.

The EPA wants a cleaner burn and less noise. So the HDs come with restrictive exhaust, retarded timing, lean fuel systems and hotter burning engines to burn up all the stuff. To make up for it the engines keep getting larger, 74, 80, 88, 96 and so on. They also run hotter, hence all the heat related threads. We combat the heat by undoing what the EPA has done, by adding more fuel and changing the timing and changing the exhaust systems. We even do head work and change cams to let it breath better. HD on the other hand wants to keep the engines the same way and sell us heat deflectors and adjustable vents. When it got really bad they designed the engine to shut down under hot or long idling circumstances. Now water cooled bikes make a lot of heat too, but for them the fan just kicks on more frequently, they don't require a rear cylinder shut down. Just place your hand or leg next to the vents when the fan kicks in on a water cooled bike and it will roast you too.

All manufacturers are caught up in this. Kawasaki stopped making one of the BEST engines ever, the KZ1000 because it could no longer meet EPA restrictions. The carbs and air cooled design became a dinosaur and Kawasaki, unlike Harley did not want to get caught up in the "can't change it, keep it the same, but choke it to death" syndrome. So most of them are water cooled. HD was smart to build the V-Rod. At least it's a water cooled platform they can grow with.

The aircooled VW has gone by the way side, and even the Porsche is now water cooled. Modern day cars have worked most of those issues out with variable timing and more sophisticated FI systems too. But they too have bolt on modification limitations. When I built the engine on my truck and super charged it I had to change everything from exhaust to FI and tuning systems.

 

There are some great points made here. I would like to add that a petrol engine runs at about 35 percent efficiency, in other words we get 35 pc from the crankshaft, but the other 65pc goes out through the exhaust and cooling system. As vehicles get more powerful (which they are achieving, despite all the restrictions imposed by EPA etc) the 65 pc also gets bigger. It is getting rid of that which probably causes the greatest problems.

Water cooling allows tighter running clearances, more uniform running temperatures and higher states of tune, as already mentioned. The V-Rod and latest Buells, when compared with the air-cooled Harleys, make the point. Frankly they both make too much power for my liking - I bought my XB12R while the dealer still had one in stock!

As an aside, I live in the UK and the International models of Harleys suffer even worst noise restrictions than US models (thanks to the Swiss, I gather). So we have an even greater incentive to add a little noise!

 

If you're looking for a watercooled Harley, check out the Vrods.
They're all watercooled, radiator is up front behind the front tire.

 

Yeah, yeah, I know that the Vrod is liquid-cooled... Guess I used the wrong words... I meant an HD Touring.

 

Liquid cooled engines allow you to run a leaner mixture(closer to peak) and there fore a more effecient engine, this helps with hp, and epa.

Air cooled engines use the incoming mixture to cool the exhaust valve and intake valves to keep them from burning up, therefore you are running a much richer mixture and you have left over gas in the exhaust. A rich mixture is as bad for hp as a lean one.
You run on the rich side of lean. Some compensation can be made by advancing the timing to the point of detenation and the excess fuel will stabalize the combustion temperatures. Harley had to retard the timing when they leaned out the mix to prevent pre-ignition which is of course is a bad thing. Remember that the mixture must "burn rapidley" and not explode. Exhaust noise is created by the temp difference of out coming gases and the free air temp. The exhaust temp can rise significantly in a air cooled engine because of the excess fuel. The combustion temp can be lowered by running the engine on the lean side of ideal, but this cost hp. and helps epa. And therefore a retarted ignition(bad for hp)

Every thing else already said about clearences is correct also, but this is all to control the fuel/air mix going into the cylinders.

A by-pass fan jet is quiter because cold air is directed over the outside of the engine an therefore cooling the jet exhaust, the old jet engines and engines found in fighter type jets are not by-pass and therefore much noiser.

As far as rpm's go any engine can turn any rpm determined by its design, I've seen engines that idle at 50 rpm and top out at 900rpm and produce thousands of hp. Look in a ship to see one.

 

Wow. Lots of wrong information on this thread. Many people need to get a basic understanding of engine design and how it relates to rpms and power vs torque.

Your air cooled Harley is a long stroke motor, meaning for every revolution the engine makes, the piston has to travel farther, so it must have a higher piston speed than a short stroke motor. Understand a piston at the end of a rod has to come to a complete stop before it starts going the other way, creating huge stress within the engine. But because of the long stroke, it creates torque over horsepower, as the longer connecting rod applys leverage to the rotating crankshaft.
It would make no difference if there was a water jacket on a Harley engine as we know it--it will still self destruct at not much more than 6,000 rpms because of it's design. Now the V-Rod is totally different, but can be compared to a similar sized Harley engine.

Comparison between Harley engines:

Air cooled Sportster: 1200 CC's Bore is 3.5" stroke is a long 3.8"
Water Cooled V-Rod 1250 CC's Bore is 3.9", stroke is only 2.8" That is a huge difference (nearly 30%) allowing a much slower piston speed, and thus a much higher RPM. Then take away the flexing push rod valve train the Sportster has, and replace them on the V-Rod with overhead cams, directly driving 4 lightweight valves per cylinder instead of two that are much heavier, thus creating less stress, again allowing higher RPM without damage.

Comparing to a V-4 1200CC Yamaha water cooled Venture, which has an even shorter 2.6" stroke, but only a 2.99" bore, which means now the four pistons are now much smaller, therefore much lighter as well and having an even lower piston speed, allowing even greater RPM's.

Your traditional Harley engine will run well below 200 degrees on a cool day when watercooling wouldn't have any effect. Try revving it to 9,000 and see what happens.

 

Thanks for the review of bore/stroke. That explains many of the properties of HD engines, but I offer the explanation in an inverted way.

A stroker is better for an air-cooled design simply because it can get more torque at a lower rpm. Designers choose that approach because the heat will limit the maximum rpm allowable in a given design.

Liquid-cooled bikes often have a bigger bore and shorter stroke because it allows the engine to reach the higher rpms allowable due to the liquid-cooling, which also allows for the higher tolerances necessary for smooth, high-rpm operation, and a higher compression ratio. The more the pistons move, the more power comes out.

An interesting comparison would be to look at liquid-cooled and air-cooled engines of the same manufacturer of similar displacement.

The 2008 Ducati Monster S4R has a liquid-cooled L-twin 998cc, 4 valve, 100 x 63.5mm, 11.4:1 = 130 HP @ 9500 rpm, 76.6 ft/lb TQ @ 7500 rpm.

The Monster S2R has an air-cooled L-twin 992cc, 2 valve, 94 x 71.5mm, 10:1 = 95 HP @ 8000 rpm, 69.4 ft/lb TQ @ 6000 rpm.

The Hypermotard has an air-cooled L-twin 1078cc, 2 valve, 98 x 71.5mm 10.5:1 = 90 HP @ 7750 rpm. 76 ft/lb TQ @ 4750 rpm.

The liquid-cooling of the S4R allows for tighter tolerance, and thus the higher-rpm design and higher compression. The S2R has a longer stroke and is torquey, relative to the amount of horsepower.

But the Hypermotard, also air-cooled, with a 4mm wider bore but the same stroke as the S2R, gains torque and loses horsepower, and it peaks at a lower rpm. But on that bike, that's the mix you want.

So, yeah...bore v. stroke. But the choice is driven by the tighter tolerances and higher compression possible from liquid-cooling.

 

I'm guessing my thread is the one MNPGRider said was referring to with lots of wrong information.