It’s too long since the last post but there’s been a lot going on. Racing, crashing and children with chicken pox.
So, I had the Super Squirrel on the dyno, which showed that I was down on power by almost 2 bhp from the best of my methanol runs. I had some evidence that the reduced compression ratio, combined with an ignition retarded slightly beyond optimum was capable of giving me more revs to play with at the top end but with an ever decreasing power output. The beginning of the curve was not as strong and in fact seemed to not show useful power until a few hundred rpm higher than the dope set-up. I had taken a guess at the needle jet size as 108 which seemed to be ok as we settled on the middle needle position as the optimum position, and the final main jet was 280. I’ll come back to all this later.
So I wasn’t whooping with joy, but neither was I crying in my tea. This is what development is.
After another trip to the airfield for yet more AVGAS, I thought I’d attempt to mask the early season financial hemorrhage by seeing if there were any unwanted bits and pieces I could sell as I’m all out of Kidneys. I put a couple of items on ebay and a guy in a nearby town contacted me to ask about one of them and to see whether I’d got anything else. I called him and it turned out that not only did he used to race (he won a Manx Grand Prix), that also he was a two stroke fanatic and a very recent Scott owner. He also used to run a very well regarded bike dyno cell in the south east and was interested in my dyno work. He asked me a lot of questions about the dyno which I couldn’t answer and gave me cause to doubt my assumptions about the graphs. My basic understanding is that here are different types of rolling road dynos and they can be operated in different ways. The difference between those operating methods should guide the way you interpret the results… all of course mixed in with the operators skill and understanding. So, I felt like I had more questions than answers… and still do. That’s good though, It’s when things aren’t working as well as you want them and you’ve run out of questions that you’ve got problems.
With this in mind, I went back to the dyno charts. I realise that I have taken a very literal view of the charts and assumed that the x axis (revs/time etc) was representative of the position of the throttle, and therefore the position of the needle within the jet. It’s not necessarily a completely wrong assumption, and Steve who runs the dyno was very measured in the way he opened the throttle. However, I don’t absolutely know the extent to which the graphs can be interpreted as a clear representation of engine response to throttle opening and I need to gain a better idea of what I’m looking at. As part of this I also need to understand more about how this dyno works and whether I can gather any other information that will make the graphs more useful.
In the meantime though I had my first race meeting coming up imminently and I had to work out what I could do to try and get back the missing chunk of torque at low revs. I decided to map the carbs and the needles to work out the relationship between the intake aperture and the needle aperture, with the eventual switch to the main jet. I can say 1/8 pilot, 1/4 slide cutaway then up to 3/4 needle and then main jet, but I have no idea what is actually the case in any given machine. How does that relationship actually (and measurably) manifest itself? For a start, I’ve never understood why the needle is a constant taper when the rate of increase of aperture area is not constant for any incremental lift. I expect that it’s simply a compromise born a need for manufacturing simplicity but I don’t know.
I started to look into it and then got carried away…maybe a colossal waste of time but certainly interesting!
I saw pretty quickly (after having made a calculation spreadsheet) that the 1/8,1/4,3/4 guides refer to the aperture area, and not the lift. Obvious that it should be, but I can’t remember ever seeing it written down. Working out the area of a segment when you know the radius of the aperture and the vertical height enabled me to start to put a picture together.
I measured the position of the needle (at #3 position) in relation to the top of the needle jet at the throttle closed position and then measured the corresponding point on the needle at the calculated lifts corresponding to the 1/4,1/2,and 3/4 aperture area throttle positions to give results throughout the needle range and show the transition to the main jet. I accept the possibility that there will be some inaccuracies in my results as I’m only using my eye and a vernier, and using a fine-liner to mark the needle (I don’t have an inspection microscope like my dad!) but I think it’s close enough to show something useful.
The following are the results from my investigations showing the information for one of my 1″ bore type 76 Amal carbs detailing the main jet, needle jet and needle position.
I also created a set of results for the Standard Scott type 206 carburettor, but these are based on one main assumption: that the relative positions of the needle jet and needle are the same as a type 76. They use the same needle, I think, so I can’t see how they wouldn’t be.
The Scott type 206 in standard setting has a 1 1/16″ aperture, runs a 106 needle jet and a 170 main jet. Some people seem to be increasing the main jet size up to 200 nowadays but I have not any information about the individual experiences that have led to this. I did the calcs for the standard set-up.
So, with my standard disclaimers in place!
Needle jet fuel info-Standard_Scott-106-pos3-170main
Assuming (dangerous I know) that this is representative of a correct(?) carburation relationship, what’s interesting to me is the comparison between the annular (or probably crescent shaped if the needle is against the side of the jet) aperture of the needle jet with the needle in it and the single round aperture of the main jet. Looking at the figures you can see that on my setup, the ‘dyno assessed to be appropriate’-main jet aperture is far smaller than the needle jet aperture at 3/4 opening. It was my desire to have a control result that led me to profile the standard Scott setup. On this (needle at#3), with a 170 main jet, the needle jet aperture is larger than the main jet even at 1/2 throttle.
One possibility from all this that the renolds number involved with the greater wall surface area of an annular aperture means that it has to be of a greater surface area than a single aperture to flow the same size. Another is that the needle aperture’s job is different to that of the main jet and that the needle jet/needles job of metering to the airflow at(or near) the top of the emulsion tube, requires a range of surface areas over the throttle aperture changes that are linked to the response of that fuel to a given low pressure area over the emulsion tube. This may mean that the needle/needle jet surface area simply has to be larger than the main jet at throttle apertures where you would expect the main jet to exceed the needle/needle jet aperture simply from looking at its cross sectional area.
It’s probably of limited value to compare the carburettor settings of the Standard Scott to my own (other than for interests sake) since most of the conditions are different. A Standard Scott will would have a different gas speed profile over the rev range through its single 1 1/16″ carb than mine through my twin 1″ carbs. As standard it would probably also change far less throughout the rev range being high already at low revs as the carburettor is far smaller than the inlet port area. A standard iron block I have here shows 6 x inlet ports at 19mm x 14.5mm (sorry about metric but I find it easier for sectional area stuff) which gives ~16.5cm². The throat area of a standard 1 1/16″ 206 is 5.72cm². That makes the carburettor aperture just around 35% of the inlet port area. Even taking into account the flow disturbances of the bridges, that’s a big difference. It’s probably good for low engine speed pick up, though not great for breathing at higher revs. That’s why Scott’s respond so well to inlet work to the carb and inlet tract.
My carb area is actually about 95% of my cylinder inlet port. I’ve a smaller but better flowing inlet port(s) with more carburettor aperture. I would therefore expect my gas speed over the emulsion tube to be lower, at lower revs. I may therefore require a bigger needle jet simply to give more fuel surface area to lift at these revs. However, with a straight taper, would that make me rich further up the range when the gas speed is higher?
I feel glad that I’ve gone into this, even though I know that you can tune effectively simply by changing bits until it’s right and not trying to analyse the workings of a carburettor. Certainly I’ve no solid conclusions to draw and I can’t be certain of my absolute accuracy but lots of interesting relationships and patterns have emerged that I think are valid and I hope will enable me to better develop my Scott.
I’m always open to constructive comment!