In the overlap of the gods

by Simon Hargreaves

Making one engine fit many roles is all about getting the timing right...

What’s the similarity between Ducati’s 2009 1198 superbike, the Multistrada, Diavel and new 1200 Monster? Yes, they share versions of the 1198cc Testastretta engine. The versatile V-twin has gone from sportsbike to adventure bike to power cruiser to naked in just five years.

You’d think their disparate riding dynamics couldn’t be achieved with the same engine. So how does the 1198 Testastretta fit such different roles?

The clue is in the name: the Multistrada, Diavel and new Monster use the Testastretta 11° engine. The number refers to the degrees of crank rotation for which both inlet and exhaust valves are simultaneously open. This is valve overlap, at end of the exhaust stroke and start of the intake stroke. The original 1198 engine has 41° of overlap, so both valves open for 5.7% of 720° crank rotation. With 11° overlap, it’s a mere 1.53% (overlap isn’t measured in time is because the higher the rpm, the shorter the overlap. And anyway, at 8000rpm overlap is something like 0.001s).

So why do valves overlap? It seems odd to have both open at the same time and risk polluting fresh mixture with exhaust gas, or it going straight out of the exhaust. On the face of it, the ideal is for the exhaust valves to close and inlet valves to open instantly, to full lift, at top dead centre at the start of the intake stroke. Inlets would stay open until bottom dead centre, then snap shut for the compression and power stroke, then the exhaust valves would snap open to full lift precisely at BDC etc.

But if you designed a valvetrain that could (you might get close with pneumatics) your engine wouldn’t thank you. The reason lies in the milliseconds between exhaust and intake strokes. At high revs, where power is made, the engine needs time to get fresh mixture in and spent gas out. And it needs more than can be delivered with valves opening and closing precisely at TDC etc, especially factoring how long a sprung valve takes to reach full lift. And also, just after TDC, the piston is accelerating from standstill. For a tiny fraction of a second it’s travelling slowly. If the inlet snaps open, gas speed would drop and pull the fuel out of the mixture.

So it turns out engines like overlap. Incoming mixture relies on movement, so slowing it down with a shut inlet valve is bad for charging the cylinder. And exhaust gas disappearing into the exhaust ports helps pull new mixture in. This is scavenging, and when the engine is spinning at phenomenal revs it’s vital to producing power. Hence a relatively long 41° overlap in the 1198 sportsbike.

But long overlap is no good at low rpm. Gas speed drops and scavenging isn’t as effective, or stops completely. The engine won’t idle smoothly, is lumpy and has no bottom-end or midrange.

Which is where, among other things (narrowing throttle body and exhaust headers to increase gas speeds), reducing overlap suits bikes like the Multistrada, Diavel and Monster 1200. Reducing it to, say, 11°.

Or, of course, there’s always variable valve timing...

Navy Boy
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Joined: 12/02/2009

As always from Si an interesting article. Of particular interest seeing as Ducati has just launched their DVT system with the revised Multistrada too.

CCM
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Joined: 14/06/2011

I wonder what's the overlap of the LC boxer: if any (?).

Not very long ago, this website was, most probably, the pinnacle of the (tech) motorcycle forums.
I would be very happy if it would be what it used to be.
Think we maybe can do something about it - sure somebody would be quite happy if we do.
Let's just post, and comment.

unconventional rebel
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Joined: 16/01/2010

CCM wrote:

Not very long ago, this website was, most probably, the pinnacle of the (tech) motorcycle forums.
I would be very happy if it would be what it used to be.

+1

Didn't understand everything but I learned a lot!

CCM
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Joined: 14/06/2011

unconventional rebel wrote:
CCM wrote:

Not very long ago, this website was, most probably, the pinnacle of the (tech) motorcycle forums.
I would be very happy if it would be what it used to be.

+1

Didn't understand everything but I learned a lot!

that's because my poor english, sorry.