Air-Cooled vs Liquid-Cooled: The Argument Splitting American V-Twins
Updated: Sep 28
American V-twins are split on a question that has no clean answer: whether the engine should be cooled by the air going past it or by a liquid circuit built into it. Both Harley and Indian currently sell both. This post lays out the trade rather than settling it, because the manufacturers themselves have not settled it.
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What Each System Is Actually Being Asked to Do
An air-cooled engine sheds heat straight from finned metal into passing air. Its temperature is therefore a function of road speed. Move and it works; stop and it has nothing to work with. That is a complete description of both the strength and the limitation.
A liquid-cooled engine circulates coolant through jackets around the cylinders, into a radiator, and back. Temperature becomes something the designer controls rather than something the road decides. The price is components: pump, radiator, hoses, coolant, fans, and the space and weight to package all of it.
Everything else in this argument is downstream of those two paragraphs. Tighter temperature control permits tighter tolerances, higher compression and higher revs. Fewer components permits lower weight, lower cost and a shorter list of things that can leak.
Where the Numbers Show It
Harley's air-cooled Milwaukee-Eight makes 118 lb-ft at 3,250 rpm in 114 form and 130 lb-ft at 3,250 rpm with 105 hp as the 117. Indian's air-cooled Thunderstroke makes 108 lb-ft as the 111 and 120 lb-ft as the 116. Every one of those engines is sold on torque delivered low in the rev range.
Now the liquid-cooled side. Indian's PowerPlus 108 makes 122 hp and 128 lb-ft. The Scout's 1133 made 100 hp at 8,100 rpm and 72 lb-ft at 6,000, and its SpeedPlus 1250 replacement makes 105 hp and 82 lb-ft, both peaks at 6,300 rpm.
The 8,100 rpm figure is the one that shows the difference most clearly. No air-cooled engine in this comparison quotes a peak anywhere near it, because holding cylinder temperature at those revs is precisely what an air stream cannot be relied upon to do. That is not a verdict on which engine is better. It is a description of what each architecture makes available to the engineer.
Harley Runs Both, in Different Motorcycles
Harley kept the air-cooled Milwaukee-Eight in the touring bikes across all six years while introducing the liquid-cooled Revolution Max elsewhere: the Revolution Max 1250 in the Pan America in February 2021, the Revolution Max 1250T in the Sportster S that July, and the Revolution Max 975T in the Nightster in April 2022.
The division is by job rather than by era. Adventure and sport machines got the liquid engine; the touring platform did not. And at the end of 2022, in Harley's own account in its FY2023 release, the legacy air-cooled Sportster 883 and 1200 were discontinued in North America. The Sport and Lightweight shipment column went 33,894 in 2022 to 18,228 in 2023, then 12,335 and 9,638.
Reading that as air cooling losing an argument would be a mistake. The Milwaukee-Eight is air-cooled and it went the other way over the same period, gaining displacement and becoming standard on the volume bike. What was discontinued was a specific old engine family, not a cooling principle. Our 2022 Harley best seller post covers that model year in detail.
Indian Runs Both, on the Same Floor
Indian's version is starker because the choice is presented to the same buyer at the same time. The liquid-cooled PowerPlus 108 arrived in the 2020 Challenger at 1770cc, 796 lb wet and $21,999. The air-cooled Thunderstroke followed in the all-new MY2022 Chief at 670 lb in base form and $14,499 to $20,999 across six variants. The Scout, liquid-cooled throughout, sits underneath both.
So the newer platform got the older cooling technology, deliberately, two model years after the liquid-cooled flagship launched. A company with a settled view would not have done that.
What Owners Report, Which Is the Real Evidence
Heat complaints are the most consistent owner feedback in this whole category, and they follow the physics closely. Engine heat on the right leg at stops is the single most persistent theme in Street Glide reports across the Milwaukee-Eight era, and it is a stationary-traffic complaint rather than a highway one, exactly as an air-cooled architecture predicts. Those reports are collected in our Street Glide 114 owner reports.
On the liquid-cooled side, the recurring line about the Challenger is that it runs cooler than the air-cooled alternatives but is not cool. A radiator relocates heat efficiently and then has to discharge it somewhere, and on a large V-twin that somewhere is often near the rider. Our Indian Challenger owner reports go through what owners actually describe.
Put together, the owner evidence says something more useful than either marketing position. Liquid cooling reduces the heat complaint. It does not remove it. A buyer choosing a radiator specifically to escape summer traffic discomfort is likely to be partly disappointed.
A Note on Layering for Heat
The practical consequence of all this is that riders in this category spend a lot of time managing their own temperature rather than the engine's, and the item that does the most work is whatever comes on and off at a fuel stop. Vests are bought for that and are frequently the wrong vest. Legendary USA's comparison of a riding vest against a fashion vest is a clear read on what actually distinguishes them in use.
What This Means for a Buyer Today
Ride pattern decides this, not preference. Miles that are mostly moving give air cooling the conditions it was designed around, and the reward is lower weight and fewer systems: 670 lb for a base Chief against 796 lb wet for a Challenger is not a rounding difference. Miles that involve traffic, summer and stopping give the liquid circuit something real to do.
Maintenance is a genuine difference and it is small but not zero. A cooling circuit adds coolant changes and a set of components that can fail, on top of everything an air-cooled engine already needs. Weigh that against how you buy and keep motorcycles rather than against a spec sheet.
And treat the rev range as the honest signal about what a bike wants to be. An engine making peak torque at 3,250 rpm and one making peak power at 8,100 rpm are not competing for the same rider, whatever the displacement figures suggest. Cooling is downstream of that intent, not the reason for it.
Sales data cannot arbitrate any of this. Harley-Davidson publishes no model-level unit sales, and neither does Polaris for Indian or Honda for the US market. BMW Motorrad is the only one of the four that does. There is no public evidence about which cooling choice sells better inside either brand, and anyone presenting one has made it up.
Two manufacturers, four engine families, and no verdict from either company after six years. When the people building them keep selling both, a buyer is entitled to decide on their own conditions rather than on someone else's principle.
Cover photo: "Indian Chieftain parked at Hogs for Hospice, Leamington, Ontario, 2025-08-02" by Chris Woodrich, CC BY-SA 4.0, via Wikimedia Commons.
