The ideal heat exchanger design and cooling calculations have been a major topic in the Harley-Davidson motorcycle community for some time.
The rumors that Harley-Davidson was eyeing a liquid-cooled motor design have been circling for years and now with the introduction of the 2014 models, it’s the first time (sans V-Rod), that the company jumped into the “water” with their touring models.
There were a number of changes which came out of the RUSHMORE project, but on the new touring models, the core ethos of change seems to start at the motor itself, where Harley-Davidson calls it “Twin-Cooled” cooling. BMW calls their new partially water-cooled boxer engine setup “Precision Cooling.”
This is a little like the rabbit calling the donkey “Big Ears”… what’s in a name?!
Well everything if you’re a marketing jock.
You have to admit that “Precision” cooling implies something better and has a certain performance connotation attached to the name vs. “Twin-Cooled or the alternative “Partial” liquid-cooling.
The fact of the matter is that Harley-Davidson patented the clever way of adding liquid-cooling to its iconic V-twin motor design back in 2009. Some elements of the design date back to 2007 when Erik Buell had a hand in the process.
Some of the patent specifics can be reviewed at:
- US 2011/0114044 A1 – Nov 18, 2009 (File Date) May 19, 2011 (Publish Date) – CYLINDER HEAD COOLING SYSTEM
- US 7654357 — Jul 2, 2007 (File Date) Feb 2, 2010 (Publish Date) – Buell Motorcycle Company Radiator coil mounted on a motorcycle
- US 20090008180 — Jul 2, 2007 (File Date) Jan 8, 2009 (Publish Date) – Erik Buell Resilient mounting arrangement for a motorcycle radiator
At any rate, the “Twin-Cooled” system is thermostatically controlled, and uses an electric pump to circulate coolant. The liquid coolant, is based on glycolethylene, the same coolant blend as in the V-Rod – a 50/50 premix that uses long life coolant. It is routed through the cylinder heads, which is the most thermally stressed parts of the engine and in the area around the exhaust valves. It’s then ducted to heat exchangers located in the left and right fairing lowers. The new dual radiators are compact and the V-twin iconic barrel cooling fins remain.
The expectation of this new setup is that riders won’t feel the crotch-melting temperatures in slow traffic because cylinder head temperatures are lower and the reshaped fairing lowers improve venting of cooler air to the rider and passenger.
Important to note is that the V-twin engines still use air/oil cooling for the barrel assemblies which to a certain “degree” retain that heritage of Harley’s air-cooled technology. And the air cooling still does the heavy lifting. Meaning it remains as a high percentage of the total cooling formula of the system.
Harley-Davidson has yet to provide specs on the increased weight of the new cooling system vs. the previous air/oil cooling system. But, it’s projected to be in the 10+ pound range, however, when has H-D been concerned with weight on the touring bikes given all the chrome dripping off those models.
There are a couple of oddities with the new liquid cooled change. The first being that liquid cooling has no effect on service intervals. Harleys with Twin-Cooled or standard air/oil cooled engines require service after the first 1,000 miles, and 5,000 miles thereafter. And unlike oil and air-cooled engines which adjust timing to avoid spark knock as temperatures increase, Twin-Cooled engines retain the same timing. I’m not sure what’s behind the thinking on this.
In addition, you might think that Harley boosted engine output. They did, between 5 and 7%, but it wasn’t all due to liquid-cooled heads. They also applied new cam profiles with higher lift and duration, which aids overall performance on both the standard and Twin-Cooled engines.
I’ve heard some rumblings from a riding buddy with contacts at the Arizona Test Track, that Harley-Davidson is having some issues with the 110 cu. in. Twin-Cooled ability to truly keep the heads cool(er). This might be a first year implementation issue, but I believe Harley’s Twin-Cooling system is here to stay, and will likely expand throughout their lineup of motorcycles.
Photo of cut-away engine taken by author at 110th Anniversary factory tours, all others courtesy of H-D.