Air vs liquid cooled

[doakley]

So....I was wondering this afternoon, WHY have dirt bikes, at least the race bikes, gone to liquid cooling. I know some of the official reasons, more even temperature control, tighter tolerances, etc. I know today’s engines produce more power for the same displacement which must mean more heat generated too. But is it really worth the extra weight and complexity of liquid cooling? My old Kawi 175, Honda 250 Elsinore and Suzuki PE250 all had more power than I could really use. I never had an overheating problem with any if these bikes whether trail riding or racing. I was just thinking how much lighter and more compact our bikes would be and wonder if we would really miss the extra power. Makes me wonder how good a hybrid would be if it used an older air cooled engine in a more modern frame and suspension?

[kdxdazz]

race bikes and any high output engine generate lots of heat especially with high compression hence why they use water cooling. Forged pistons used for race engines would continue to expand with the higher temperatures. The only full size dirtbikes available here in Thailand is the crf250l and the Klx250 which produce less power than an xr250 of 20 years ago so no idea of why they would use liquid cooling. The most popular dirtbike here is the Klx140 with the 200 big bore kit and that is air-cooled and never misses a beat. I agree with your point in regards to low powered bikes and why the need for liquid cooling. I think some of it has to do with marketing and public perception, upside-down forks would be an example. There is absolutely nothing wrong with conventional forks, the drz is a good example, but upside down forks are shiny and create a modern look

[kdxsully]

It could’ve been at the recommendation of professional riders. It’s harder to get more power out of an air cooled than liquid from what I understand. I’m thinking in the context of old air cooled Porsche engines though

[KDXGarage]

I think reliability and wear. Also, I read something about amount of power made when the air-cooled engine was really hot.

[doakley]

Maybe it gets down to the "trickle down" theory. Only the top pros are likely to actually be able to use the extra power offered by liquid cooling. It helps the top level racers so, marketing being marketing, it becomes "necessary" for production bikes too. As far as power loss of a really hot (nearly overheated) engine, I'd like to see a dyno study done of engine temp vs power/torque output. I was always under the impression engines actually produce more power as temp goes up, right up to the point of seizure. Maybe liquid cooling actually allows the engineers to design the engine to run hotter without worrying about seizing. I know they can run tighter clearance tolerances with more controlled cooling.
Interesting discussion.

[rtbmrgl]

Quite awhile back a friend had a early air cooled KDX, he told be a story about riding a hare scrambles, every time he would go through the water puddle area, cooled the engine for a moment, there was a significant increase in power. Water cooling keeps a consistent cylinder temp, hopefully. I have heard storing of cold seizing, not letting the water temp come up before going full throttle.
But the way I ride, air cooled would be fine. I do like my vintage bike, no water to mess with.

[bufftester]

IN the case of USD forks it has to do with moving unsprung weight to sprung weight which increases handling. Liquid cooling allows the manufacturer to make smaller castings for the same displacement which saves lots of money, enough to offset the complexity of water cooling I'd wager. Plus the stuff about tighter tolerances, more effective cooling, etc. Just my thoughts.

[KDXGarage]

USD forks are more rigid. For example, 2018 Team KTM's Marvin Musquin's 52 mm forks.

DSCN744040percent.jpg (522.72 KiB) Viewed 5997 times

[bufftester]

Man, those are beefy, beautiful forks

[Goat]

Does anyone have both an air-cooled and liquid cooled KDX? I’d be interested in seat of the pants comparison if power is different.
My buddy’s 1983(?) is what got me into this game, before I even knew the differences in the models. He sold it to “some kid”.

[KDXGarage]

Man, those are beefy, beautiful forks

2018 Atlanta Supercross

[KDXGarage]

Goat, I have an '87 and a '94 (plus others in various states of condition). The E series is a big leap forward in pretty much every sense.

[Goat]

I feel a quantum leap from my DT250 and the KDX200.
But I was never tempted to flog the DT like this, even on the highway, so am not sure what it had on top.
Makes me think there is replacement for displacement.

[lizardfungus200]

colder metal is more resistant to wear, thus longer service intervals and lifespan. Not that it answers your question, just my 2 cents.

[lucy]

I owned multiple '83-85 (pre-KIPS). Even had them bored to 220cc.

The stock KIPS valved water cooled 200cc bikes had stronger acceleration than my '83-85 220s.

[kdxsully]

Yeah my yz125 would wreck my 84 kdx in a drag race. Apparently there is a replacement for displacement.

[Goat]

This is another great rabbit hole for me. I wonder how much top end dif there would be between powervalve engines and non-power valve.
Four months ago...before getting the sh** scared out of me by the beloved KIPS rattle....I did not even know powervalves existed.
I guess I would need to hang out on the RD350 site to really understand better how they affected overall performance that turned the such nice little commuter bikes to 750 slayers.
The powervalves are such an amazing difference.
Makes me wonder if all powervalve 2T motors are water cooled.

[KDXGarage]

Goat, 1986 - 1988 KDX200 is air-cooled with power valves.

[sarrant]

So....I was wondering this afternoon, WHY have dirt bikes, at least the race bikes, gone to liquid cooling. I know some of the official reasons, more even temperature control, tighter tolerances, etc. I know today’s engines produce more power for the same displacement which must mean more heat generated too. But is it really worth the extra weight and complexity of liquid cooling? My old Kawi 175, Honda 250 Elsinore and Suzuki PE250 all had more power than I could really use. I never had an overheating problem with any if these bikes whether trail riding or racing. I was just thinking how much lighter and more compact our bikes would be and wonder if we would really miss the extra power. Makes me wonder how good a hybrid would be if it used an older air cooled engine in a more modern frame and suspension?

Hmmmm. I had a PE and currently still own an IT175, and both were fairly prone to power fade/pinging when run hard in heat. You can jet them rich enough to avoid that, but then they get more blubbery. If you can narrow down that tuning range, all of a sudden there's lots of ways to find power.

We saw a big jump in stock compression ratios follow along with liquid cooling, which makes perfect sense given the above issue. An air cooled PE/IT/KDX are about 7:1, where as 10:1+ is common on liquid cooled bikes (KDXs are 9.4:1). That's probably where the big jump in overall performance from those bikes to the liquid cooled KDX sits, and I don't think you reasonably make that jump on an air cooled head without it becoming fussy, or losing reliability.

Power valves (despite the name) weren't really about creating more power, what they did allow is two overlapping port tunings for not necessarily MORE power, but a broader torque peak area.

That said, both my PE and IT were fine bikes (as was my wife's RT180) and I do really like the simplicity of an air cooled bike for mild trail riding.

[kdxsully]

Correct, power valves basically are altering porting, giving you two cylinders in one essentially.