Acceleration Times On The Dyno For Various Models

This is a somewhat (?!!) abstract piece, which was originally started when I was at home after the last leg operation. With a few complications (unlike the other ops) I spent much longer away from work than anticipated, and got extremely bored. I’m not the type of person who can watch daytime telly, so I just sat in front of the laptop (as it was then) and played. And talked to Fat Cat (we got fat together).

I had lots of the files for it still however, so thought I’d see what I could make of it. I was prompted somewhat by an emailed inquiry I got from a fellow asking questions sort of along this line. My response to him was that there were so many variables, transferring it from theory to reality could be fraught with trouble, as outlined below. If it gets too confusing, or seems a bit pointless, feel free to go surfing elsewhere. It’ll probably confuse me too before it’s up.

One of the graphing options with the Dynojet software is a speed versus time graph, which looks like the graph below. This graph shows the acceleration (increase in speed relative to time) of the rear wheel on the dyno, where it is accelerating a drum of constant mass. The yellow line is my 600M, the red line one of the played with 998S. Both these runs are done in the one gear (as I usually do), in this case being fourth gear. The purple line is a 748 being run through the gears, with the gear changes apparent as the little flat sections (no acceleration). As is the case on the road, the acceleration is greater in the lower gears (and the curve is steeper).

So, using these graphs I worked out the acceleration times for various speed intervals for quite a few bikes. Just for comparison. The data is presented as a roll on acceleration in one gear, as we do on the dyno. If you were riding on the road you wouldn’t do this for maximum acceleration, but it gives an idea of how the mods work in each part of the rev range.

How this acceleration of the rear wheel on the dyno drum actually relates to on-road acceleration, especially between different bikes I’m not too sure (with the addition of wind drag too). Although, it would be about as good as could be made without adding a multitude of assumptions and estimates. I spent four years at university learning how to make allowances for assumptions (educated guesses really) so I have a full understanding of the shortcomings. As they say, if you assume, you make an ass of u and me. Woka woka woka. Classic engineering humour.

I’m sure some will disagree and think up lots of reasons why I’m wrong. I’m not doing this report to start a discussion, so don’t let me know if you disagree. I don’t care.

Anyway, onward we go. First up we’ll look at the 600M, being the slowest of the bunch. I do love my (well, “our” – well, “her” if you want to get picky) 600M. First up is the power curves relating to the info presented. Green is all std, blue is std mufflers with cams dialled, red is Megacycle mufflers with std cam timing, yellow is Megacycle mufflers with dialled cams. Plotted against road speed.

Next is a chart of speed versus time information for a roll on dyno run in fourth gear, beginning at 50 km/h. All the runs have started below 50 km/h, that’s just where we’re starting. As you can see, the blue line (std mufflers, cams dialled) gets the initial jump, only to be reeled in by the combos with the aftermarket mufflers. This is pretty much how it feels to ride. Over the speed range shown, the combination of dialled cams and Megacycle mufflers gives an improvement of just over 5% - not a huge amount. You can see the RPM that corresponds to the road speeds under the chart.

Next, for the same info, a time per speed increment bar graph. This shows the acceleration in each increment for each combination. Useful for seeing which mod does what where, in a rather impressive formant if I do say so myself.

From this, you can see that even though dialing the cams makes less peak rear wheel power, it will make the bike a little quicker in most situations (except consistent high RPM).

For something slightly faster, we’ll move on to the 900SS carb bike with the Vee Two cams. This bike gained quite a bit through the mid range in particular with each mod – firstly Omrae slip on mufflers, then the Vee Two cams. First up is the power versus speed graph, then the speed versus time, then the acceleration time per speed increment. Once again, the colours are constant through all charts. Green is all std, blue is with Omrae slip on mufflers, red is with Omrae slip on mufflers and Vee Two cams.

This last chart in particular shows the real advantage of the Vee Two cams – that meaty mid range (4,000 to 7,000 RPM) which you use on a windy road. Once you get over 160 km/h (just under 8,000 RPM) there is no difference, but that’s not how you’d use this bike anyway.

Now for a comparison between my 851 and 888. Having run these two side by side on the road, I know the 888 will leave the 851 behind fairly easily. At first I thought I might do OK (I was on the 851), but it was soon pretty obvious there was nothing I could do to catch the 888. Looking at the dyno charts, the 888 has an advantage everywhere, especially the top end which you use for through the gears acceleration, so you’d expect it. It doesn’t feel much quicker to ride, but the flat torque curve is rather deceptive – it just goes, no matter where in the RPM range you are. First up is power, then speed versus time, then acceleration times for each speed interval. Purple is std 851, blue is modified 851, yellow is std 888, red is modified 888. The mods are almost the same for both – dialled cams, ST4 exhaust (bigger 50mm DP system on the 888) and mufflers, open airbox lids and custom chips. The 888 also has no base gaskets, giving it an extra _ point of compression over std, whereas the 851 has a DP light flywheel. The 888 has a (very) slight gearing advantage too, running 15/40 compared to the 851’s 15/39.

In std form the 888 is quite a bit quicker than the 851. With the mods however, the gap is much smaller. I think the 851 does pretty well for itself, all things considered.

Next a 2V ie comparison – 800, 900 and 1000. I had the feeling the 800 would pretty much match a 900 on the road, and these results support that. The 1000 has a fairly clear advantage over both. The 1000 runs the same gearbox as the 900, but with taller 15/38 gearing compared to 15/40. The 800’s 6 speed box has a very similar 4th ratio to the 900, with the same final gearing. Green is 800 std, blue is 800 with mods (pipes and air filter kit), red is 900 std, yellow is 900 with mods, purple is 1000 std, pink 1000 with mods.

The last speed range on the graph is 150 – 170, so only 20 km/h as compared to 25 for all the others.

I was a bit surprised that the 800 actually matched the 900 – I thought it would be close, but not that close. The 800 really is a very nice engine – it shows just what’s been hiding in the design for all that time they were building those crappy 750s. All seem to be as fast or faster than the carby 900SS with the Vee Two cams too, which surprises me.

Finally the 998S. The really fast one. This bike is the one used in the cams dialled report. On the dyno it accelerates (in 4th gear) from 75 to 200 km/h about 1 second faster than the 888, which translates to a total whipping on the road. Green is std, played with red.

As an (totally theoretical) indication, by the time the modded 998S has reached 200 km/h, it will have theoretically covered 158m, at which point it will be about 6m ahead of the std 998S, which is doing about 190 km/h. It would be interesting to see how this relates to reality.

So, there you have it. Somewhat interesting, and probably a good comparator between modifications to a particular bike on the same dyno. As for a comparator between bikes on the road, I’m not so convinced. There are lots of variables not considered, the biggest being aerodynamic resistance (and airbox feeding). I’d expect the 998S to out do the 888 even more simply because of that.

But, a good way to waste some time none the less. Some of you may argue that point.

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