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Stéphane

Big (enough) Bang Engine?

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Any idea what is the firing interval of new Honda VFR 1200F engine with 76 deg block and split angle crank?

It's a lot like 2 offset crankpin 750 shadow motors siamesed together. The firing order is modified to 1-2-4-3. The cylinders fire in pairs 28' apart like 2 v-twins. Honda has a great little animation of how it works on one of the V4 hype sutes... can't remember where.

It's actually the journals that are physically 28' apart to achieve primary balance of the 76' V-angle...

The firing sequence is 104-256-104-256.

000----090----180----270----360----090----180----270----360

-1-----------2-----------------------4-----------3----------------------1-

The animation is on the main Honda site... :wheel:

I thought "crankpins" referred to the big end journals. Have I got my terminology screwed up?

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Guest Baketech

I thought "crankpins" referred to the big end journals. Have I got my terminology screwed up?

Nope you aren't mistaken, crankpins = rod journals....but that's just the physical arrangement of the crank itself...

When stroked through a V-angle of 76 degrees, that yields a 104'-256' firing sequence.

*(76+28=104)

Edited by Baketech

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Nope you aren't mistaken, crankpins = rod journals....but that's just the physical arrangement of the crank itself...

When stroked through a V-angle of 76 degrees, that yields a 104'-256' firing sequence.

*(76+28=104)

Ah ok makes sense now. I can't wait to ride one... I can't remember the exact figures but they were saying it makes something like 90% of its torque by 4000rpm. That'll be one grunty sumbitch.

I bet it'll power wheelie like my dad's St1300... :wheel:

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Guest Baketech

I can't wait to ride one...

Ditto...

After getting the chance to give an RC30 the beans last summer, I'm really looking forward to seeing how the lumpier firing feels on the liter bike... :comp13:

Edited by Baketech

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Doing my usual - resurrecting an old thread, but it is an interesting topic.

As has now been pointed out, a 'screamer' is a more even firing order, whereas 'big bang' means lumped close together. The latter used to mean more than one cylinder firing together, but now as was pointed out, it can mean just close together.

Honda have used 360° AND 180° configurations on their V4s, the former being oft referred to as 'big bang', but I've not heard the latter being called 'screamer'. In either case, they're not totally true to the name, whereas when Ducati entered MotoGP with their Desmosedici, they did try a real 'big bang' with pairs of pistons firing together, so rather like a V-twin, but with each leg of the V actually 2 pistons and cylinders, moving and firing together. So a 360° crank and firing as a real 'big bang'. Ultimately not successful and apparently had a tendency to throw its motors into pieces. I don't think they ever raced it.

It was tried with an R1 in British SuperBikes, but I think Yamaha nixed that when they introduced their 'cross plane' crank.

An easy engine to make into a 'big bang' was the CBX1000. Each camshaft was in 2 halves, split in the middle with an Oldham coupling driving them together. It was of course just a matter of installing the second camshaft halves 180° from how they were supposed to be installed. With wasted spark ignition, nothing else needed to be done. I did hear of someone who did it by mistake and said it ran very rough, but then I also heard that it was tried by some teams in racing. I never tried it, I liked the CBX the way it was intended.

While looking into the crank angle used by Honda over the years, I have come across some information that I consider inaccurate, concerning V engines. One particular video clearly implying that the engine balance of a V8 is down to each bank being balanced in itself, whereas in fact, it can be wildly out of balance without problem since balance is attained by each V pair. Let me explain:-

In a single cylinder engine, the piston dashing up and down would create a terrible balance problem. Just thrust one fist up and down and feel how it shakes the rest of your body. The answer to this is balance weights on the crank. So at the top and bottom of the stroke, the force of the piston changing direction is balanced by the balance weight on the crank pulling in the opposite direction. If those 2 forces are equal (by adjusting the balance weight - i.e. balancing the crank), that is perfect PRIMARY balance and stated as a 'balance factor' of 100%. But there's a problem...

We have perfect balance at TDC and BDC of our single cylinder engine, but what about when the piston is half way up or down the bore. At this point, there is NO force exerted by the piston, but the crank is now at 90° from T/BDC and that balance weight is exerting just as much force as at T/BDC, that cannot change. So all we've done is change the terrible up down vibration due to the piston to a sideways vibration of the same magnitude, because of that counterbalance weight, which at the 90° position of the crank has nothing to balance it out. This is secondary vibration and in this case we have zero secondary balance.

So the balance factor of a simple engine like this is usually chosen to be something like 60% or 70% or something close that gives the best compromise of primary and secondary balance, but however you look at it, it's a shitty result. Put 2 of these cylinders side by side, operating together, with nice smooth evenly spaced power pulses and you've just made the balance situation worse. Well done all the British bike manufacturers who foisted those jackhammers on us for so many years. Honda realised that uneven power pulses was not really a problem and used a 180° crank for their twins so the pistons were not going up and down together and you get a much better balanced engine. Mind you, that didn't have anything to do with them also being oil tight.

Bear in mind that ANY form of 'out of balance' vibration can be mollified or even eliminated with the use of balance shafts or reciprocating weights which can be designed to throw weights around exactly as required to keep a motor smooth. But let's just ignore those for now.

But what about V engines I hear you ask, they must be very difficult to balance. Nope. A 90° V has PERFECT primary and secondary balance. Here's how:-

Go back to the single cylinder example with the counterbalance weight on the crank providing perfect primary balance but no secondary balance at all at 90°. What we need is some form of balance weight whose effect varies as the piston goes up and down, so we get balance at T/BDC, but also half way up. Something that acts just like our piston but 90° away. Hmm, what about another cylinder and piston at 90° from the first one. With a 100% balance factor we have perfect primary balance for each piston, but the secondary imbalance of each piston is now completely balanced by the other piston. So a 90° V provides perfect primary AND secondary balance. It's a great configuration for an engine.

Of course a horizontally opposed (Boxer) engine can provide the same perfect primary and secondary balance without any counterbalance weights on the crank. But we always need some weight there to smooth out the power pulses etc anyway, so both engine configurations are very good for mechanical balance, whereas any in-line engine is in trouble with no way to avoid it completely without using balance shafts.

For both the V and the Boxer, it is each adjacent pair of pistons, that balance each other. The relative position of the other pairs is largely irrelevant. So our V4s might have a 180° crank where each pair is half a rotation away from the other, or 360° where the 2 pairs are side by side. This is usually termed 360° apart but of course could be termed 0° (although due to firing order etc the former is more applicable overall). As far as crank balance is concerned, you could have any angle between the pairs.

So if these engines are perfectly balanced, why do we feel any vibration at all? Well, not everything is balanced. Cams for example are lumps of metal flying around with nothing to balance them out and generally throwing lumps of metal around at such a speed, it would be very hard to eliminate vibration entirely. Think about it, at 10K rpm, the piston is changing direction at 300 times per second. You try and count that fast.

There is also the problem of 'rocking couple'. With each power pulse hitting the crank, unless these are evenly spaced along the crank, it can cause the crank to try and rock slightly. Obviously it cannot actually move so this translates as a vibration. If power pulses could occur simultaneously on either side of the centre of the crank, we could avoid these couples, but for other reasons of balance that's not really practical, so we have to put up with this rocking couple which fortunately is minor compared to the piston and crank balance described above.

So finally, what about about 'cross plane' cranks. As is easy to read elsewhere, these look like a cross when viewed from the end and breaks up the firing pulses into an uneven order. For an in-line 4, it also requires the use of balance weights as decent primary and secondary balance is otherwise impossible to obtain, but for a V8 it doesn't really matter as each crankpin has 2 conrods and pistons attached and they balance each other out as explained above. So each crankpin could be at almost any angle relative to the others, but in practice either flat (like a typical in-line 4) or cross plane (at 90° to each other) are used.

So that's what a cross plane IS, but why? Well there's been almost as much guff talked about this as there has been about radial brakes. There was much talk about a V engine giving better traction due to the longer interval between some power pulses allowing the tyre to regain grip. This always seemed like nonsense to me. We're talking in thousands of a second here and rubber just don't move that fast. What has now become apparent is that it's more to do with vibration. If rubber is vibrating, it is less able to grip and evenly spaced power pulses can set up a harmonic oscillation in the rubber that causes sufficient vibration to reduce grip. But the uneven spacing of the power pulses of e.g. a V engine prevents any such harmonic, which reduces the vibration and allows more grip. Yamaha's cross plane crank in their in-line 4s does this. The uneven power pulses allowing the same grip levels as the V engines.

There are now several examples of cross plane twins (270° cranks), like Yamaha's Super Tenere, Honda's new Africa Twin and Triumph's new 1200s among others, but whether that is for any supposed enhanced grip or just because they sound cool is anyone's guess. But annoyingly Yamaha have started applying the term 'cross plane' to their 3 cylinder engines which just use traditional 120° cranks. So a case of marketing triumphing over the facts I think.

So that's it. Whether your VFR would provide greater grip if it had a 360° crank is a mute point, for 2 reasons. 1, you're stuck with it and 2, you'd never notice on the road. Realistically, the only difference is the sound and all the above configurations with their different power pulse intervals will sound different. To each their own. Personally I like the sound of a V4. Any V4 will do.

Anyway, hope this is of some help. If not, sorry I bored you.

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