A morning on the dyno with the Scott

I really enjoyed taking the Scott to the dyno yesterday. It was the first time I’d ever actually booked a proper set-up rather than a single run. I suppose cost was always an issue but I have realised how useful it is to be able to see the results of the changes you make and a proper session allows you to experiment with many different settings and see how each affects the result. It’s a bloody good way to see what affects your power output, and how. The cost is similar to me going to a trackday, and there’s no way I’d have gained the same information from that.
Of course, you need to have someone who can run the dyno and can interpret results. GT motorcycles of Plymouth have a very interesting division called ‘Performance Engineering‘ that specialise in rebuilding, restoring and tuning Honda racing bikes, and it’s these guys that run the dyno. Steve is highly knowledgeable and experienced in getting the most from an engine on the dyno. I’ve done my previous runs with him, but yesterday he was able to really show me the benefits of a dyno setup. Steves colleague Andy, another two stroke fanatic, was assisting and as the Scott’s engine note became crisper and cleaner they were both obviously enjoying the experience. Ok, so the Scott doesn’t put out the sort of horsepower as an NSR500, but it did sound so very good .. and maybe the 4.5% Castor mixture helps too!

Anyway, I realise that for many people the idea of going on a dyno seems a bit extreme but carburation and ignition set-up problems can persist for years if you don’t seize the opportunity to really get to the bottom of them. I don’t want to spend my time at a race meeting or a track day like the Beezumph constantly messing around with jets , needles and timing just to try and get somewhere close. You’ll always have a certain amount of tweaking.. but at least I feel like I am quite close with what was a completely unknown arrangement.

So, What happened?

I’d fitted the 1″ carbs with 320 main jets for a start. The needle was on the bottom groove, so all the way up. I’d figured that it might be rich but I had no desire to start from the other direction! The timing started at 22BTDC.

It was rich. Our runs followed the following order:

1: down two mainjet sizes to 300
2: down two mainjet sizes to 280
3: drop needle one notch
4: drop needle another notch
5: drop mainjet one size
6: raise mainjet one size
7: change timing to 23.5

You can see the graphs here for the final results compared to the very good curve I produced when I first set up the bike up on methanol.

Torque curve for the Super Squirrel racer.
Torque curve for the Super Squirrel racer.
horsepower curve for Super Squirrel racer
horsepower curve for Super Squirrel racer

The exhaust temperature sensors were really useful and we were able to monitor for significant changes with any modifications to setup that we made. The final high was around 530°C. Always increasing by about 30°C to 40°C the moment you shut the throttle off… interesting stuff.

Type K thermocouples read by simple meter
Type K thermocouples read by simple meter

You can see that the Torque curve starts later and then increases at a very high rate. It may be that this lost section (around 400 revs) at the beginning is retrievable by playing with cutaways, but the gain in midrange torque is very good indeed. That may be down to the improved gas flow in the head. It loses to the methanol later in the rev range, about 4000rpm. The methanol torque stays high all the way to 5000rpm where it suddenly falls off a cliff. The new curve hits a higher torque figure but tails off more noticeably. However, the fact that suddenly I started to have more revs was a bonus (as long as I can get the 400 back at the beginning!). I think Steve’s attitude was that the extra ‘over rev’ was extremely useful as the engine was able to keep pulling longer because of it.
As far as I understand, the fact that I increased the head volume, lowering the compression ratio might have created the conditions to encourage the engine to keep revving.

As the torque was lower late in the new curve the maximum power is less than before, but I’m not too concerned about that at the moment. It might be that I could do with the shape that I have created in the combustion chambers, but with a high compression ratio. It would be worth experimenting with heads sometime.. but not now. It may even be that the carb size is a little small, and I’ve never done any further tests with the exhausts but short of changing the slides to try a higher cutaway I’m now looking at actually using it! It felt really smooth and sounded great.

So, this afternoon I posted off my entry to the British Historic racing clubs second meeting of the year at Anglesey on the weekend of the 16th and 17th May. Roger is going too. I’ve never ridden Anglesey before but I’m really looking forward to it.

2 thoughts on “A morning on the dyno with the Scott”

  1. Richard–
    Sounds super!! On the torque/HP charts the lines are indistinct, but the audio is just fine.

    You probably know that Harley found on their sidevalve 750 model K racers that gasflow was far more important than compression in the overall power curve–the two being ”at odds”. I think that they ended up at around only 6-1

    Good luck to you and Rodger at Anglesey.

    Wishing I could be there–


    1. Hi Herb,
      If you ‘right click’ the graphs, you can ‘save picture as’ to your computer where it’s a lot easier to see. I haven’t got the presentation of them right yet… I’ll work it out.
      It was interesting to see the bike revving after it was previously so reluctant to pass 5000rpm. Steve said the same, that lowering compression would lose peak power but give ‘over rev’ but the needle influence is interesting … see my latest post about the other charts. The thing is that although it has lost peak power, the peak torque is actually greater, it’s just lower in the range.

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