TECH TALK: More revs, more power

Alan Dowds has had his brain fried by 221bhp at 15,250rpm… He explains:

Ducati’s Panigale V4R makes massive power, thanks to a sky-high rev limit. How does it do it? And why are revs so important for power? Let’s find out.

I freely confess that I’m a bit jaded and cynical these days. It takes a lot to impress these old bones, and while I’m all for new bikes, it seems a lot of them lack quite a lot in the inspirational department. One bike that’s certain ducked that trend of late is the fancy-pants V4R. Not for the obvious glitz and glam of its carbon wings, or super-trick Öhlins NPX25/30 forks, but more so because of what can be found on its spec sheet. Namely, its peak power output – and more specifically, the revs at which it makes said power. A brain-frying 221bhp, at 15,250rpm. Yep, fifteen thousand, two hundred and fifty revs. Every minute. That’s in road trim. And the actual redline is even higher! The limiter kicks in at 16,000rpm in gears one-to-five, but in top gear (sixth), it gets another 500 revs, to 16,500rpm. 

To put this in perspective, the Yamaha R6 made our heads fall off when it was launched back in 2008 with peak power made at 14,500rpm. The legendary Honda CBR400RR NC29 was a proper high-revver, although it was launched 25 years ago, and it only hit 14,500rpm. You have to go down to the lunatic 250cc inline-four Japanese domestic market machinery of the early 1990s to find a road bike that revs higher – the MC22 CBR250RR redlined at 19,000rpm, thanks to the teeny 33.8mm stroke. The CBR250 only made about 40bhp though. And its pistons were more like tiny thimbles than the coffee cup-sized slugs in a litre superbike. 

More revs, more power

So, to truly put things in perspective, how does the Panigale V4R redline compare with its peers nowadays? Well, it’s fair to say it makes them all look a bit donkey-like. Its rival litre sportsbikes on the market include BMW’s S1000RR and Kawasaki’s ZX-10R. The Beemer makes peak power at 13,500rpm, with a redline marked on the tacho at 14k. Pfft. Meanwhile, in the green corner, the RR version of the ZX-10 Ninja has a special valvetrain design aimed to raise revs by 600rpm, and makes its peak power at 13,500rpm and the rev limiter calls time at 14,800rpm. That’s good, but it’s still a hefty 1700 revs shy of the V4R. So, the Panigale is the rev boss. But why are high revs such a good thing? 

Well, broadly speaking, revs means power. Literally. The power output from an internal combustion engine is calculated by the force applied at the crank (we call this torque), multiplied by the rpm (the equation is pretty simple: power = torque x rpm / 5,252, using lb-ft for torque and bhp for power). That’s why, when you look at a dyno chart, the power curve kinda follows the shape of the torque curve – if the torque stays steady, the power increases gradually as the revs increase – but if there’s a sudden spike in the torque curve, then you get a corresponding peak in the power curve. 

Increasing the torque you get from an engine can get you so far. In crude terms, the torque is a measure of the force created at the piston top by the combusting fuel/air mix. A bigger bang will, all things being equal, give a bigger shove to the piston. Imagine a Tour de France cyclist. Give him a bigger pair of thighs (say, by injecting him with elephant steroids), and he’ll be able to shove harder on the pedals, pull a higher gear, get up a steeper hill faster – make more power, in other words.

Performance at a price

If you’re limited by the amount of fuel and air you can burn in each cylinder, then there comes a point where you hit the limits of the force produced on each combustion stroke. Improving efficiency is one route to making a bigger bang – making sure all the fuel burns efficiently, increasing compression ratio, having a strong ignition system, ensuring the fuel/air mixture is exactly right. If you can fit a turbocharger, or increase the cylinder bore, you can force more fuel and air in and make a bigger bang that way.

But if you can’t do this (as with a WSBK homologated 999cc motor), then the only way to make more power is to make the same size of bang, but at higher revs. Our massively-thighed cyclist can go even faster if he manages to pump his giant legs up and down faster on the pedals. More revs – or pedal pushes – per minute, with the same force makes you go faster – because you’re producing more power.

There are a few enemies of revs in an engine. As the rpm increases, there is less and less time to get fuel/air into the cylinder, burn it, and get the exhaust gasses out again. Even at a lowly 5000rpm, a four-stroke engine is firing nearly 42 times a second, and the piston moves up and down the bore four times as often – about 170 times a second. So, if the inlet valve opens for the whole of the piston inlet stroke, there’s just 1/170^th of a second to get the whole fuel/air charge into the cylinder before the valve closes. At the redline of the Panigale V4R – 16,500rpm in top gear remember – there are 137.5 combustions per second, in each cylinder. And the piston makes the journey up and down the cylinder 550 times each second. So, there’s 1/550^th of a second to fill the cylinder with fuel and air.

Okay, air can be moved fast – big ports and valves, large throttle bodies, carefully-sized inlet trumpets, cunning exhaust systems all help here. But there’s a downside to all this, too. Big ports and valves work less well at low speeds, so what you gain up top you can lose down low if you’re not careful. And having 220bhp at the top end of a race engine is no good if your competitors are making 50bhp more through the midrange, when you’re pulling out of a hairpin bend. So just making an engine breathe efficiently at super-high revs is difficult enough. But the next enemy is a mechanical one – maximum piston speed. As we pointed out earlier, at high revs, the piston moves up and down the bore a lot. And there comes a point where the speed gets so high, that normal aluminium can’t hold up any more – the forces on the rings, and the friction created by the high speed is bad enough – but you also need to cope with the massive forces at top and bottom dead centre, when the piston and rod have to be stopped, then started moving again in the other direction.

Maxing out

How fast does a piston move then? We work out the *average* speed of the piston by a fairly simple equation: average piston speed = 2 x stroke x (RPM÷60). If you’ve got a spare 10 minutes on Excel, you can work out the piston speeds for some engines, and you’ll quickly see the current limits. On a basic commuter engine like a Honda CB500, which makes peak power at just 8600rpm, with a stroke of 66.8mm, the average piston speed at peak power is 19.15m/s. The S1000 RR makes peak power at 13,500rpm with a stroke of 49.7mm, and at that point its piston speed is 22.36m/s. At the redline of 14,600rpm though, the little Bavarian pistons are whistling along at 24.18m/s. The V4S pistons move at 25.85m/s at its 14,500rpm redline, while the new V4R pushes this up to 26.62m/s at the 16,500rpm top gear redline. Erk.

This explains why the Panigale V4R piston is such an advanced design. Ducati’s used a two-ring forged design rather than the three piston rings used on normal road engines (the only 4-stroke production bike on the market that goes down this route). A three-ring design uses two compression rings and one oil scraper ring, but the V4R has just one compression ring and one oil ring to cut friction. It’s a bold step – and with one less ring, you might expect more in the way of oil consumption.

Ducati has a load of experience here, of course. Its big twin superbikes had to make massive pistons move at big speeds – the 1299 Panigale’s soup-bowl pistons were hitting 25.6m/s at 11,500rpm. And the 1199 Panigale R revved 500rpm higher: its pistons were doing a stunning 26.72m/s at the 12,000rpm redline – the highest piston speed we can find in a ‘road’ bike. So, making smaller pistons that can hold together at these speeds is well within the Bologna skill set. The final problem with high revs is the valves. Remember that while the crank spins once per engine revolution, the camshafts rotate twice as fast, and the valves have to open and close in a vanishingly small period of time at high revs. 

As we worked out earlier, at 16,500rpm there’s 1/550^th of a second for an intake and exhaust stroke, so the intake valves have to open fully, let the charge into the motor, then close again, sealing in the massive combustion pressures in around that time period. Then the exhaust valves have to open and let the red-hot gasses out again in the same kind of time. With conventional engine designs, the valve springs need to be very strong and stiff to bang the valves closed in time – this adds friction. There’s a big advantage in cutting weight here, of course; lighter parts can be accelerated and decelerated much faster, and the forces involved are smaller, so you can use lighter springs. Ducati uses titanium inlet valves and titanium valve retainer collets to cut mass – but the (smaller) exhaust valves are steel, because the weight saving wasn’t worth the effort.

Get the valve control wrong, and the valves can start to ‘float’ – staying open a fraction longer than desired, and that’s when the pistons can hit the valves, wrecking the motor. Ducati has a big advantage here. Its desmodromic valve operation lets it use radical cam profiles, big valves, and high revs, because its engines use two cam profiles for each valve – one to open the valve and one to positively close it again. Using a mechanical cam to slam a valve closed rather than a spring means more exact valve movements, which is a massive benefit at high revs. 

So, in the end, maybe it’s not such a big surprise that Ducati is able to do this. It’s been working for years with massive piston speeds on its big-twin superbikes, and its decades of experience with desmodromic valves lets it apply that technology to the problem, too. Add in the closeness of the MotoGP Desmosedici engine design to the Panigale V4 motor, and perhaps we shouldn’t be so gobsmacked, disconcerted, or dumbfounded at the V4R’s power and revs at all…