Dyno chart analysis

Andy from Performance Engineering has sent me the files from the dyno runs I’ve done from the last couple of years and with software downloaded from Winpep I am able to view all of them to compare.
I’ve been looking forward to this because I wanted to see where I could maybe make gains by modifying the needle. It’s very interesting to see the comparisons and to consider the next steps.

These are the most interesting charts, torque and power:
To be clear, the settings are as follows (a bit mixed up on the graph notes)

Run #17: Twin carb set up (avgas), High comp head (32cc)
Ign set at 22degs, main jet 280, #4 needle jet position (5bottom), 1/4t out on pilot air screw
Run #18: Same as 17 but the second run.
Run# 19: Same as above except needle now raised to #3 position (middle)
Run #28: Same as above except ign set 23.5, needle on middle position #3.

dyno graph comparison chart torque
dyno graph comparison chart power

You can see how 17 and 18 really want to rev further, and that’s with the needle on position #4. You can also see that they must be too rich at the beginning, as there’s no curve taken. I’m guessing Steve must have felt it flat and simply not sampled it at the beginning.
So it looks to me that the needle could be reduced in diameter in the final section to enable a bit more richness at the top end without compromising the bottom. That should give me an extra five hundred revs according to the chart. That should give me the best of the start of #19 and the end of #17/#18. I can work the amount out simply by checking the difference in the needle within the pitch of a needle adjustment notch.
The timing change seems to have a decent effect in increasing peak torque across the range, so I’ll leave that where it is.
It’s interesting that the needle position change seems to have had an effect within an area that you’d expect to be almost beyond its influence, as I understand it. I need to check the needle jet and needle at that position to see whether it’s restricting the main jet. You’d think that it could be, looking at the graph.
As for the beginning and my lost 400revs, I’m not exactly sure. Part of me thinks that it might be weak because the fuel is too low in the needle jet as I’ve set it to not drip out the pilot air screw assembly. As the carb is at an angle, this obviously reduces the level in the needle jet. This theory is slightly supported by the fact that the very first section (around 2750 revs) was a little stronger with the 300 main jet and needle at the highest position.
However, the ‘technical guy’ from the Amal stand at Stafford said that the float level was set at 0.9″ from the top face of the float banjo. Extrapolating that into the carb means that this works out to be within the thread for the needle jet in the jet block. That’s not very far up the needle jet anyway.
Later carbs made for two strokes had the rear part of the emulsion tube cut away as this created a situation where more of a low pressure area was created to lift the fuel from the jet. Apparently (Amal guy) this was done when the higher performance two strokes came along. It may be that the response time was shortened and at higher revs this became important… I just don’t know. The other ‘known unknown’ is that I think the bellmouths aren’t made with the requirements of gas flow as the primary design factor. I suspected that they wouldn’t be perfect but the way they are made leaves quite a significant ridge around the inside against the edge of the carb inlet. Not ideal for laminar flow as I understand. It may be that this, along with the less than ideal brief radius at the end of the bellmouth is responsible for a meaningful restiction to flow. I didn’t take into account in any mathematical way the extra venturi wall area when I originally planned my conversion to the twin 1″ carbs, but the overall increase in aperture is around 10 to 15%. If I’ve got bad turbulence into the carb then that might cause problems..also since the emulsion tube is right at the bottom of the venturi, it could be that disruption to the flow entering the carburettor is effecting the flow over the emulsion tube.
I think I’m going to try to temporarily effect a radiused entry using silicone sealant and see how it works. The radiused bellmouth I’m going to try and do using epoxy resin, just to test. I’m probably not going to know, but I will have the bike back on the dyno at some point.
Longer term, I’m probably going to file both carbs out to 1 1/16″ (around the top and sides) to increase flow, but that’s not for now… oh and make some decent bellmouths.

Stafford Show visit – April 2015

I hadn’t resolved to go to Stafford until the day before, but I knew that Roger was going and that it was also a good chance to catch up with people who I’ve known through my life around Scotts, but who I have seen rarely in the last few years, since I stopped building engines for a living with Roger. I also had a chance to talk to the technical guy on the Amal carburettor stand about the level the fuel should be within the emulsion tube of a type 76.
I’m constantly thinking of how to improve the power delivery right at the bottom end.
The Scott stand looked very good and had a back drop of screens with interesting photographs, including several of Scott’s in competition over the years. Sheelagh Neal had her father Ossie’s famous Scott racing outfit on the stand and there was a Reynolds special, Harry Langman’s TT racing outfit and a Sand racing Scott.

I also had the opportunity to have a look around the Bonham auction. They had a couple of quite iconic Scotts as well as a Silk. One of the Scotts was apparently a 1926 TT entry and was the first known use of the duplex frame. The other Scott is well known to me as it was the actual machine that I always thought to be the most beautiful Scott when I was very young. It is a Sprint Special and belonged to Dennis Howard, and then Glyn Chambers. It is pictured in the Jeff Clew book, ‘The Yowling Two Stroke’. A special bike I think.

My dad also had a small display of engine parts to enable people to see that there was new spares support for the marque. I think it also shows that the bikes are living and that people don’t have to fear using them.

It was a good show. A few pictures below:

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.

Test run observations

I took the Super Squirrel to my friends farm today to run it down the track, as I didn’t really want to take it to the dyno without knowing it even works. I fitted the pair of gauze covered bellmouths as the road is quite stoney and I really didn’t want any unwanted inhalations. Radiator filled, fuel filled and away it went immediately.
It sounded clean but the first thing I noticed was that it was settling to a high tickover, rather than the stop that we need it to have. I need to check the slides and pilot settings.
It measured between 270°C and 300°C on the exhaust temperature sensors, which doesn’t mean anything much at this point, but might mean more as we move toward a better set up tomorrow.
Pulling away up the hill it was getting bogged down, so I imagine that it’s rich on the needles, which are standard 276 needles raised to the top as a starting point. I don’t think I got to the mains, but we’ll check tomorrow.
So tonight I’ll tweak the bits I can and the rest will have to be done tomorrow.
It’s really a tall order to try and get the whole set-up done in a few hours but we’ll see how far we get.

Scott racer developments

One of my main aims for this year for my ’32 Scott racer was to set up the carburation properly for petrol again, having decided to move away from Methanol. Methanol worked really well, and the power characteristics really suited the three speed box. The bike was fast and really responsive to ride with the single overbored Amal type 89, but I worked out that the venturi area was less than optimum and resolved to improve that.
With the single carb, the shape and position of the torque curve was pretty much exactly what I’d expected based on the calculations I’d made using Gordon Jennings formulas relating to port time/area relationships. Basically, the you need a certain area of port available to enable adequate gas flow at a certain revs and that area increases as the revs increase as the gas has less time to pass through it. Nothing I have done over this winter should change that torque curve shape too much, as I’ve not changed port timings. Changes in exhaust pulse characteristics relating to the increase in temperature anticipated with the change to petrol will make a difference, and quite possibly not a beneficial one but we’ll see.

Vee twin manifold
Vee twin manifold
I do expect that the new fuel system, using a pair of 1″ Amal type 76 carbs on a vee type manifold, should keep that curve from tailing off quite when it does at the moment. I spent hours setting up the float heights and chamber positions so I hope it works. The extra breathing coupled with an attempt to tune the inlet lengths to work better with the slightly extended inlet timing duration I have on this engine should in theory pay off. With the single carb, you could see an inch or so ‘stand off’ of mixture blown out (and sucked back) at low revs. The longer tracts will help eliminate that. I have also spent a significant time working on the head profile to try to allow a more direct route for the flame front to move to the extremities of the combustion chamber but there’s a fair bit of finger in the air stuff… with a bit of borrowed knowledge and the rest; ideas formed through slow but incremental observation of what has already happened.

So, with a completely new fuel system, I need testing to get it right. The most straightforward way for me to do this is by getting onto the dyno that I’ve been using for my other tests so far. This time though, not to do a run but to do a full set up.

So this week, I’ve booked a morning at Alan Jeffery’s dyno in Plymouth, run by Steve from GT motorcycles who seems to spend most of his time tuning NSR 500 Honda GP bikes for people all over the place. A good man to have on-side.

I’ve tried to make sure that I’ll be ready to make the most of it. Roger sent me his block of main jets,IMG_5138 so I’ve got changes to make. I’ve fitted the exhaust temperature sensors to enable me to use that information to help establish whether we are too lean. I’ve got a couple of plugs from a different heat range and on top of that… I also picked up some AVGAS as I’ve been convinced of the advantages of this over even the higher octane rated unleaded from the forecourt.
I’ve also ordered a radiator hose connector which allows the fitting of an 1/8″ temperature sensor and at some point I’d like to fit one of these to go with my Scitsu rev counter which (although it probably needs servicing) stopped working altogether when I switched to methanol.

Hopefully, I’ll get a better result that the first dyno run I did last year. That gave a maximum torque figure of around 38ft/lbs at near to 4000 rpm and a maximum horsepower of 33 at 5000rpm.

I really want to get out to do more racing this year and I really want it to be competitive.