Just how clever is it?

Mechanical engineering professor Jamie Turner assesses Harley’s new V-twin and Harley engineering chiefs Mike Carlin and Alex ‘Boz’ Bozmoski explain the detail

» Is it completely new?

Jamie: One of the interesting things is that it has the same bore and stroke as the V-rod [Harley’s 2001 water-cooled DOHC 60-degree V-twin], which will have saved some time with the engineering. They would have had engine performance prediction models and loads of combustion data for the V-rod which they could have used to tell them what they had to do with the new engine. But apart from that it appears to be a completely new design. The valve train is completely different from the V-rod’s – it’s got finger followers instead of flat face followers and of course it’s got variable valve timing. Boz: I was on the V-rod project so there were learnings there, but with the Pan Am what we did first was decide what kind of performance characteristics we wanted and then worked out how to get there. The modelling and physics led us to the same bore and stroke as the V-rod because we were after similar characteristics.

How clever is the variable valve timing (VVT)?

Jamie: It looks pretty standard – the 40-degree range is normal. Some car engines go up to 60-degrees but bike emissions regs aren’t as tough as cars. The VVT is on both intake and exhaust, so they’re serious about it. All VVT systems have a spring which drives them to ‘park’ in a certain position when you switch off the engine and it’s quite neat that Harley have used this to set the intake cams to full retard and the exhaust cams to full advance. That means the intake valve relieves the compression so it gives the starter motor an easier time. The Superquadro Ducati engine uses a decompressor to do the same thing. The other advantage of Harley’s system is that with the exhaust valve set to full advance, the exhaust gases will carry more heat down the exhaust pipe and fire the catalyser earlier. That’s a normal automotive trick. It will also make more noise. The advantage of having mappable variable valves is you can do these neat tricks. It’s really nice to see VVT coming in on bikes – a lot of people think it’s a car technology that just adds complication but it does have definite advantages in terms of the spread of torque.

VVT on a V-twin is expensive though because you’ve got to have double the pick-ups and control actuators as you have on an inline engine, and those things aren’t cheap. Off-setting the crankpin is usually done to create perfect primary balance in a 60-degree V-twin engine such as the Harley. But is 30-degrees enough? Jamie: No. If they’d gone to a 60-degree crankpin angle instead of 30, they could have done without the primary balancer. It’s not clear why they did that. They obviously believe there’s an advantage in having 90-degrees between the pistons coming to top dead centre – the same as a Ducati twin. Harley’s spec sheet says the firing interval is 90-degrees, which means a 630-degree gap. Normally we try to make the firing much more even than that to reduce shocks through the transmission. Mike: One of the disadvantages of a 60-degree offset crankpin is that you can’t support the structure in a single pin – you have to use a flying web or another bearing, both of which would have added significant weight and width. As for the firing interval, it would be clearer to say that it is equivalent to a 90-degree engine. The front fires, and then the rear fires 270-degrees later, so there is no 630-degree gap. That would have been a challenge.

» Why two balance shafts?

Jamie: They need a second one because of the distance between the main balance shaft and the crankshaft, which will set up a small rocking couple. KTMS used to have a second shaft in the cylinder head for the very same reason. Mike: We wanted to tune the vibration for the rider at all revolutions and throttle openings. The further you get the second balance shaft from the reciprocating masses the better it is, so by putting it in the cylinder head we could minimise its weight but get maximum effect.

Could you run without the second balancer? Yes you certainly could. But the issue with that is you might find that the vibrations irritate you. This isn’t what’s wanted. The balancer is driven off the phaser [used for the variable valve timing], which changes its position so we could tune the balancer so that it’s more in balance at highway speeds than it is at, for example, idling speeds. That means you get character without that irritation.

‘It’s got all the latest design elements. And there’s a lot of clear, logical engineering going on here’