Guest Blog: Mat Oxley - Why are MotoGP V4s faster than inline-4s?

MotoMatters.com is delighted to feature the work of iconic MotoGP writer Mat Oxley. Oxley is a former racer, TT winner and highly respected author of biographies of world champions Mick Doohan and Valentino Rossi, and currently writes for Motor Sport Magazine, where he is MotoGP correspondent. We are featuring sections from Oxley's blogs, which are posted in full on the Motor Sport Magazine website.


Why are MotoGP V4s faster than inline-4s?

Ducati and Honda have ruled MotoGP’s top-speed charts for years and are the dominant race winners. Here’s why

V4-powered MotoGP bikes have won 44 of the last 50 MotoGP races, a victory rate of 88 per cent, and topped the speed charts at 47 of the last 50 races, a success rate of 94 per cent.

How can this dominance be explained? Is it simply the fact that V4 engines make more horsepower? And if they do make more power, how do they do that?

Yes, they do make more horsepower. And mostly, it’s about the crankshaft. A V4’s crankshaft is shorter than an inline-four’s, so it’s stiffer and stronger.

It also creates less friction because it runs on three main bearings, not five, the minimum required by an inline-four. And with adjacent connecting rods on the same big-end journal it creates less rocking couple. Even better, a 90-degree vee has perfect primary balance.

There are also pumping losses, which is the power wasted in pumping air through the engine from intake to exhaust. Up to 300 litres of air per second flows through a 1000cc engine at 18,000rpm, and a V4 crankcase layout deals with that flow much better than an inline-four crankcase, so the V4 loses less power through pumping losses.

So, a stronger crankshaft, less vibration and better balance allow designers to push the engine harder for more revs and more horsepower, with the added advantage of fewer losses through friction and pumping.

Meanwhile, MotoGP inline-four designers must cope with crankshaft flex, crankcase flex, more friction, more vibration and greater pumping losses, which make the engine less able to produce more horsepower without problems. (Yamaha machines its YZR-M1’s crankcases from solid billets to reduce flex.)

The 90-degree configuration – used by Ducati in MotoGP since 2003 and by Honda since 2012 – has a further advantage. It allows designers to use a greater variety of unbalanced firing orders – big bang, long bang and so on – in their attempts to produce a more rider-friendly type of power and torque.

Read the rest of Mat Oxley's blog on the Motor Sport Magazine website.

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Comments

I’m not at all convinced the V4 is the better option.

“ V4-powered MotoGP bikes have won 44 of the last 50 MotoGP races, a victory rate of 88 per cent, and topped the speed charts at 47 of the last 50 races, a success rate of 94 per cent.“

MM is an absolute outlier. I believe he’d win just as often if not more on an inline-4. For better comparaison I’d remove his results from these stats.

There are the pumping issues below the pistons as well. Compressing the gas in the crankcase.

I must admit my prejudice & bias here. I love 90° vee twins, desmo vee fours and V eights.

This was a great read, and has given me a much better understanding (and an acknowlwdgement of how little I actually know). Thanks Mat.

Can someone please explain the "variety of unbalanced firing orders – big bang, long bang and so on" and when or why its used or changed.

Screamers have an even firing order, every 90 degrees there is a piston hitting tdc on compression. In a big bang engine they alter the firing order so that 2 cylinders fire in quick succession - say 90 degrees apart - and then the other two fire 90 degrees apart after a longer interval, maybe 180 degrees after the first two (the numbers are for illustrative purposes and are not exact). In the big bang engine that break between power pulses allows the tire to gain a bit more traction than a screamer which loads the tire with constant power pulses. That's my understanding. If I'm mistaken, someone please correct me.