Quote from Plato, ancient Greek philosopher and early Gilera factory rider.*
*that last has long been rumoured but remains unconfirmed by motoring scribes of the day
I'd managed to knock off most of the jobs on that earlier list, installed the PClll in the nick of time, and the dyno tune is booked in for tomorrow. Tomorrow?! The engine had run in my shed, I'd even given it a couple of squirts up and own my 400m driveway causing a few heads to whip around in the adjacent orchards: a Ducati at full noise, even briefly, was a bit of a change from the usual ATV's and tractors puttering around. If nothing else they didn't have to worry about any birds pinching their fruit for the next few minutes, I'd pretty much cleared the surrounding area.
But realistically there were were zero miles on the engine, not exactly ideal putting it through the rigours of a dyno tune...if a Ducati had a hamstring this would be when it went "PING!". The following day was a Monday so if someone had a mind to put a few miles on their unroadworthy race bike on their surrounding public semi-rural back roads, late afternoon/early evening on a Sunday would hypothetically speaking be about as good a time as they'd get. They would also ensure their phone partook of said hypothetical excursion should a very real phone call need to be made for assistance...or bail money.
They would have probably found it to be a pretty cool thing, with bags of mid-range grunt, and although the redline would not really be troubled, in all likelihood it would pull pretty hard towards it. With no speedo and no traffic to follow, the potential rider should be quite circumspect with his/her speed, content to leave it in the lower gears and roll up and down the rev range. It would be preferable to put a few more km's on it but a hypothetical 20 or so minutes around the backroads would strike a reasonable balance of risk vs reward. Once back at their shed I expect the rider would spend a good 30 minutes fair crapping themselves waiting for a heavy handed knock on the door.
For this dyno session I was working with Ant at Corse Motorcycles, he had some association with the guy who built the engine so I figured he could go in to bat if her insides ended up on the outside. I was pretty keen to be present and potentially help sweep up the ensuing shrapnel but Ant ended up juggling a few balls the day of the tune so I had no option but to drop her off for the day.
Nervous? It was approaching 6 years getting to this point, so "nervous" doesn't quite capture the range of emotions involved: relief that we'd actually reached this milestone, pride in how far we had come, a little embarrassed at how long it had taken (even if I had been away at work for half the time), hope that it would perform well, fear it would go horribly wrong, etc etc. It was complicated, and I just wanted to fast forward the day to know where I stood.
But it turns out my nerves had nothing on the poor ol' 851 who was fair wetting herself, literally. A couple of pulls into testing a few drips were noticed leaking from the alloy fuel tank directly above the rear exhaust manifold....soon turning into more of a continuous dribble. Game over. So much for my initial thoughts I'd never have to worry about corrosion with an alloy tank. I'm guessing sitting empty for a few years, with a bit of humidity, condensation periodically built up in what is a small pocket in the base. Being the worst possible time just before heading away for work, I lost nearly a month before sorting the problem and having another crack at it.
But eventually I found myself here:
The (very) good news was she obviously survived ok with no daylight entering the crank cases, so there was massive relief on that front. And it made substantially more than the 90 odd hp you'd see from a stock 851 Strada....but I couldn't help feeling just a tad underwhelmed.
My old 916 built to SP spec had made just on 120rwhp, and I was kinda hoping to at least match that. That didn't seem an unreasonable aim with my girl basically being a big bore 926cc 748R, with a more modern cam design compared to the old school stuff, maybe a bit more compression, and bigger inlet valves.
Sure, there is dynamometer variation to consider, with many a pub barney started trying to compare figures from different dyno's, but all the same I thought it was worth checking/ruling a few things out to see if I'd left a few ponies in the back paddock or that was simply all there was.
Actually, comparing the old versus the new camshaft designs is pretty interesting, SP spec's in black, R in red:
Duration deg (inlet/exhaust): 304/296, 260/280
Overlap deg: 98, 58
Lift mm: 11/10.5, 12.5/10.5
Valve size mm: 34/30, 36/30
The differences aren't small. But it's all about managing air, more air means you can add more fuel and make more hp. The old school idea was to move a lot of air by holding the valves open for a long time, hence the long duration, with lots of overlap. You are kind of snookered once you get to a certain point, with lift limited by the duration (possible valve piston contact), and if you make the ports/valves too big then it's a pig lower down as the gas velocity drops off. You are literally robbing Peter to pay Paul as you can make good hp but the rideability goes down as power becomes concentrated in an ever smaller rev range.
Then, a lightbulb moment. Picture a Ducati engineer muttering away to himself: "Hmmmm, more power, eh? Desmodromics....no valve springs.....Hey Guiseppe! Why are we designing cams to protect valve springs when we don't have any?" Basically they started exploiting the ability of desmodromics to open/close the valves at rates that would have valve springs in a lather. So, higher lift cams, with shorter duration and less overlap, with bigger valves, results in moving more air in a shorter space of time. Not only did it make good if not better hp but it broadened the spread of that power, making for a much less peaky, more rideable, outright faster machine.
The number's above are one thing but here's a more graphical cam comparison: my R cams have exactly the same lift as the 748RS shown but with even shorter duration, so they have the absolute highest lift and amongst the shortest duration:
But back to the issue putting wrinkles in my forehead....
The second thing that struck me with the dyno chart above (who doesn't look at peak hp first?) is the torque peak way down at 7000rpm, when more usual is up around 8000rpm. Torque is great, and as that hypothetical running-in would have shown, this combo would make for a fantastic road engine with the grunt right where you'd want it. But this is a race bike, I need hp.
Why is torque important but hp is king?
Torque is simply rotational force. Hp is the unit of measure for the rate at which work is done.
Hp and torque are related by rpm, with hp basically being torque x rpm. Think about hand cranking an old Model T: when you initially apply force to the crank handle and there is no movement, you are producing torque but no hp. It is only once the crank starts turning and work is actually done that you have hp. Work = Force x Distance, no distance = no work. Geddit?
So when we bolt our bikes onto the dyno we aren't actually measuring hp, we are measuring the amount of work done and the time taken to do it. It is an inferred measurement, the ability of the machine to do work, not an actual force produced.
So while the rotational force at the crank is a "nice to know", I care more about the ability of the engine to do the work of transporting my boney arse from Point A to Point A (think about it...) as quickly as possibly. Think of a sprinter leaping out of the blocks, they don't take big powerful strides to accelerate, they take a lot of smaller steps, applying a smaller force but at a greater rate, and so it is with my bike: I really want that torque further up the rev range where it is applied more often.
There's an old saying: power doesn't corrupt people, people corrupt power....which leads to the inevitable question: what had I fucked up this time?
