Category Archives: 1932 Flying Squirrel Racer

Scott Super Squirrel tuning continued- port calculations

It’s been a dispersed couple of weeks on the Scott front but I have been making progress. I’ve been reading the Jennings book on two stroke tuning from the early seventies. It’s quite well respected and has some fairly straightforward explanations of what modifications are likely to do what. The Scott is a bit unusual though and its extraordinary torque at low revs is something I would like to keep.
Let me say that everything from this point on is me trying to work something out, rather than prescribing a best practice!

It’s difficult to know who to listen to but I’ve used Jennings’ recommendations to establish the important points about the inlet, exhaust and transfer ports, those being the time that they are open and the area that is available for gas flow.
He doesn’t use the entire area, but actually has a method for calculating a mean area which is significantly smaller, but apparently more representative.
His assertion is that regardless of the size of a cylinder it will need a certain time/area to achieve optimum power at any given speed of crank rotation and I’ve created a spreadsheet to assist in calculating what’s happening. Doing the maths, combined with looking at the port timings of other engines, have given me some indications of where to go.

This is a useful chart that was once published in Yowl:

Timing info

From having seen many Scott iron block castings, I can say that the port cores were seldom perfectly aligned. Certainly on my detachable head block the port apertures are at angles to each other and vertically misaligned slightly also between the two cylinders. The difference between cylinder timing durations is not insignificant: 4° on the transfer, 3.5° on the exhaust but only 1° on the inlet.

My own as yet standard timings (from one cylinder) started off as follows:

Exhaust: 159.5°
Inlet: 129°
Transfer: 134°

With those timings, I calculated using Jennings method that the combination of available exhaust port area and it’s time open would provide optimum power at 3500rpm, the transfer at 3250rpm and the inlet at about 3100rpm. Of these, the transfer and the exhaust figures are exactly standard, where-as the inlet differs for a couple of reasons.

1: As we have ported pistons to aid the gas flow from the crank chamber on the transfer phase, the rear part of the inlet gallery is blocked up on my engine (otherwise the port in the piston would communicate with the inlet gallery). This means that I have less actual inlet area available than standard. However, since the standard inlet gallery ports are quite small and each bridged, mine has the bridge removed between two and the port raised (as far as possible) to enable more area. My guess would be, that simply adding up the area without taking into account the effects of turbulence around the bridges at higher gas speeds is ignoring an important factor. Whether the gas speed ever gets high enough (given the amount of ports in the gallery) in a standard engine for this to be a truly limiting factor, I don’t know.
2: Our inlet timing is already extended from standard by the removal of about an 1/8″ skirt at the bottom of the piston. According to the tables available, this amounts to a difference of about 17° in total duration over the standard figure.

The standard port timings are really set up for a low engine speed, and the power and torque curves shown on dyno charts bear this out. Here are some charts to see this:

original Scott dyno test sheet
original Scott dyno test sheet

My Super squirrel racer, running methanol, tested on a dynojet rolling road dyno (albeit with RH blown head gasket) in October 2013.

Super Squirrel racer HP Dyno chart (Oct 13)
Super Squirrel racer HP Dyno chart (Oct 13)
Super Squirrel racer torque curve (Oct 13)
Super Squirrel racer torque curve (Oct 13)

Although the fact that the head gasket was blown means that the results themselves are not reliable, the curves are likely to be, and so I can at least see where (with the expansion chamber fitted) the torque and power is being made.

So you can see that the torque curve gives a good spread of consistent peak torque between 3200rpm and 4200rpm. You can also see that everything stops at 5000rpm, although I’m not sure I understand exactly what that shows. I’m assuming it is showing that the output drops off sharply and not just that the bike was only revved to 5000rpm during the test as you expect the curve to just terminate high if that were the case. This tailing off is definitely something to look at.

This torque curve does show that the output roughly corresponds with the Jennings based predictions. The expansion chamber will alter the results according to it’s own harmonics too, and in a perfect world I’d have run a straight pipe to try to remove that effect… but I only had an hour that time. I may do this in the future.

So, I should have a look at how this curve works within my gear ratios. It’s all a bit more important when you’ve only got three. I know it works really well at the moment, so I’m simply hoping that by not trying to do anything too extreme I shouldn’t have any problems.

I have set up the spreadsheet to show the % percentage of the optimum time area figure I am achieving for any given revs. This enables me to change a port duration figure, or a port width and instantly see its effect as a % of the optimum. Theoretically.
Alongside this, I also have some figures from A.Graham Bells book on two stroke tuning from the eighties which gave details of conclusions drawn from the known port timings for different engine configurations. By extrapolating the results to 5000 rpm, it seems 140° would be in sequence for the inlet, the exhaust would be around 165° and the transfer would seem to already be long at 134°.

So, how so these look on the Jennings spreadsheet?

As things are at the moment, the optimum theoretical rev/minute figure corresponding to my port timings (regardless of exhaust influence) is between 3100rpm and 3500rpm. With the above inlet port duration modification plus a little widening of ports, the figures say that I’ll be able to create the optimum time/area conditions for power at 4000rpm. That would be, around 135° for the inlet, 160°° for the exhaust and to leave the transfer alone.

I’ve got this week to finish the ports, I’ve ordered some new main bearings and next weekend I’ll be rebuilding the engine.

Super Squirrel tuning for 2014

Although I’m unlikely to be able to afford to do anything like a full season this year, I’m pushing to try and get the Super Squirrel engine rebuilt to be as competitive as possible. It’s quite heartening to know that it went as well as it did as there had been little in the way of time spent on the detail of gas flow and port timings within the engine. Sometimes you have to stand back and re-evaluate why you are doing what you are and whether the original reasons still exist. I’ve done this and have some thoughts for ways to extract more power.

As it is, the port timings have been unchanged from standard, except for the inlet which has a slightly longer duration due to having relieved the skirt by about 1/8″. I didn’t change them because I didn’t want to lose the tractability and strong torque at low revs that the engine produces. I am still running a three speed box with ‘vintage close’ ratios and it’s important to have as much flexibility in the power band as possible. So, the idea was to increase the power and efficiency without narrowing the range too much.
The power of the engine seems to have been noticeably increased(though I lack proof of this) by using this new exhaust showing, i think, that it is effectively ramming unburnt gases back into the cylinder prior to the closure of the exhaust port. This has been a success which needs building on as I still think that there’s a significant amount of further power to be had by careful development.

I thought I’d start by looking at the obvious impediments to gas flow. I’ve been using Jennings book on two stroke tuning for guidance but essentially the idea is to have a gas flow which is not full of disruptive internal turbulance. The turbulences can be caused by changes in the surface of the ports, either things sticking up or the surface falling away (bumps or hollows). Also flow out of and into ports is facilitated by radii on edges. I’m applying this to all my ports and piston ports as a beginning, though minimally on the top edges of the ports where the timing will be affected. I’ve got a pair of Roger’s ‘high flow transfer ports’ which have no internal bridge and a non symmetrical shape, the idea being to send the transfer gas into the hump on the piston rather than over it. I always intended to spend time matching them exactly to the transfer ports on the crankcase and the block and two weeks ago I decided that the time was upon us.

I started with the crankcase and worked steadily on the left transfer aperture and found that when I’d absolutely matched the aperture to the cover, I’d increased the aperture from 904mm²(1.4″²) to 994mm² (1.54″²). That’s a 10% increase in area and the removal of edges over which eddies can form in the gas to restrict flow even further.

Transfer port work. 10% bigger on left before work commenced on right.
Transfer port work. 10% bigger on left before work commenced on right.

There was a bit of work to the covers themselves in matching to the transfer port openings on the block, but no work on the block on either the top or bottom edge , though a little at the sides to prevent the gas hitting the sides of the port. This gave me fractionally less area as it entered the ports. My understanding is that this is preferable as the gas speed is increased and the tendency to have internal turbulence affecting flow is less.
Once I’d dealt with these, I looked at the transfer ports themselves.
Whilst the engine was still together I’d noticed the height of the top of the skirt at bottom dead centre and found that it was around 1/16″ below the transfer ports both sides. To my mind this gave at least some opportunity to use some of this available space and at present I have elected to radius the bottom edge of the port to assist flow and also deepen the port in the middle adjacent to the bridge by that 1/16″ as well as radius the bridge on the transfer side.

before... (note blocked up inlets)
before… (note blocked up inlets)

I thought that there would be gas displacement when the flow hit the bridge and deepening the port there would give it somewhere to go.

The timings themselves are:

Transfer: 134°
Inlet: 129°
Exhaust: 159.5°

I have to be cautious about messing around with the port timings too much, as Scott barrels are no longer commonly (and cheaply) available should I completely mess it up. There is, however, a factor that has never really featured in Scott tuning before, that we now have got an expansion chamber exhaust which goes some way to (over) compensate for the effects of enforced silencing to 105 db. This means that the inlet gas, which was originally intended only to be subject to a pumped transfer is also possibly assisted by the expansion chamber extracting gases through the transfer, which would leave a negative pressure in the crank chamber. If this is taking place, then there is possibly also merit in extending the inlet duration further as there is possibility that more gas could be introduced without it spitting it back out. This also might be assisted by the use of the twin carb manifold and long inlet tracts contributing some inlet inertia to the situation.

previous engine with twin carb minifold fitted
previous engine with twin carb minifold fitted
Whether in fact the 289 carbs I have for this are too big to allow the gas speed and inertia required for this (and atomisation of the methanol) I don’t know. It’s be a suck it and see. I do have a rolling road dyno down the road and the smart money would be to run the single carb and then try the twin set-up and see the difference. Unfortunately my attempt to Dyno test at the end of last year to provide me with comparison figures didn’t go to plan as the head-gasket was blown from the beginning.

I need to do some port timing calcs (time/area) and continue the flow work to the crankcase and ports.

The evolution of the Super Squirrel racer

I’m doing things backwards here. I realise that I need to give some more of the the original history of the Super squirrel and how Roger’s Flying Squirrel racer came about. As I said in the previous post, the Super Squirrel racer is really only just returning to having the potential of competitiveness that it did in the early 1970s.

Super Squirrel racer, prior to frame modifications (around 1971/2). Note Roger's Laverda SF750 production racer with race kit. An unusual racing stable.
Super Squirrel racer, prior to frame modifications (around 1971/2). Note Roger’s Laverda SF750 production racer with race kit. An unusual racing stable.
Roger on the Scott Super Squirrel racer (around 1971/2) racing at the New Brighton circuit on the Wirral.
Roger on the Scott Super Squirrel racer (around 1971/2) racing at the New Brighton circuit on the Wirral.
My dad, Roger, having found that the Scott was fast and competitive in racing was very much committed to finding the solutions to the bike’s shortcomings, namely in handling, gearbox and engine reliability.

The single down-tube frame of the Super Squirrel had broken once before at the seat post and had been re-inforced substantially. Tie bars had been created to give some tension to the lower engine and undertray (carries the gearbox on a Scott) mounts as the original lower frame ‘rails’ have to be removed to be able to race. Left in, they will dig into the track and have you off.

Later shot of Super Squirrel racer around 1973/4. Note lower rails removed , replaced by tensioned 'tie bars'
Later shot of Super Squirrel racer around 1973/4. Note lower rails removed , replaced by tensioned ‘tie bars’ and no silencing!
This did leave things a bit more flexible in this frame and he resolved that therein lay some of the problem. He was sure a stiffer frame would be a great improvement.
He addressed the handling issue by having a duplex frame made by Bob Stevenson of Spondon to a similar design and geometry to the Flying Squirrel, only using lighter tubing. To be honest, he’s always said that the duplex frame he had made didn’t actually improve the handling, but it did allow a bigger carb (because you didn’t have the single down-tube in the way) and it was quite a bit lighter as it was a welded construction and not lugged.
Years later it was Paul Dobbs, the talented Kiwi rider who suggested that he thought the handling could be improved by moving the riders weight forward.
Paul Dobbs in inimitable action over the mountain on the Scott at Cadwell park, 2005
Paul Dobbs in inimitable action over the mountain on the Scott at Cadwell park, 2005
My dad did this, moving the seat forward, and a big improvement was felt. In about 2010 he had the tank shortened to allow this to be more neatly contrived.
I also moved the saddle forward on the Super Squirrel when I re-built it, and swapped the ‘Brooklands’ style bars that my father favours with a set of wide straights that force your hands wider and make your body weight shift forward. The handling is far better for this, and actually I much prefer the extra leverage too.

The gearbox story is well explained in his story of the affair, here, and the pursuit of power and reliability were definitely linked, as the inevitable longstroke crank breakages inevitably took it’s toll on successive crankcases, prompting a decision to re-cast cases with better material and extra strength. Cases were redesigned to have larger transfer apertures and inlet port areas and cranks were re-designed to use the crankcase doors as an outer main bearing support to overcome the design flaw and material shortcomings of the original overhung crank.

The development of the Super Squirrel racer into the Flying Squirrel was not instantaneous though and it was largely about a substantial focus on re-engineering. In truth, that has consistently been the focus of his very successful Scott racing development work. In the process, he has developed his Scott to the point where some people even dispute that it is one. To me however, the Scott was Alfred Scott’s creation and he was a man of vision and ingenuity. He left the company that bore his name in 1915 and died in 1923. It’s impossible to look at the balance and finesse of those early shortstroke machines and imagine that he would allowed the bikes to have developed as they did, in both weight and fragility, had he stayed with the company.
To me the very spirit of the Scott is strongest in those machines where people have employed their skill and imagination to take the unique qualities of the Scott and develop them.
It is in the DNA of the marque and though I understand of course that there are those who have great enjoyment of their original machines, to me there is no Scott more a Scott than one that has been intelligently modified, and there is no Scott that can lay claim to having been been developed with more ingenuity, determination, focus and success according to its remit, than Roger Moss’s Flying Squirrel racer.

Waiting for the call...
Waiting for the call…

The Super Squirrel racer … where we are.

It’s not been the best couple of weeks for getting on with the Super Squirrel racer’s engine with both my wife and little girl both poorly and work being very busy indeed. A very good friend of mine also passed away, although this did rather contribute toward the quietness and patience required for gas flowing with riffler files.

So what is the plan?

A bit of history… In 2006 I finished re-building the old Super Squirrel racer and into it went a good Scott engine that I’d built with Moss crank. I sold this engine to fund another more radical engine build, and machined up my own head and heavily modified a Scott barrel to suit. I also welded up my own expansion chamber.
The crankcase I used had been a damaged case that we’d had welded, but had some evidence of cracks still remaining beyond the welding.

Anyway, I took it to it’s first BHR meeting at Mallory and it felt really strong for the first two laps, before it died. I didn’t really look at the engine until I’d pushed it back to the van, but when I did I realised that the damage was absolute to the crankcase. It was split in two and completely irreparable.

oh dear.
oh dear.

Upon reflection, the case wasn’t up to the job. I might have had some tiny amount of piston/ head contact too.. I know they were close as I’d had it before during testing and had worked to increase the clearance. The main bearing assembly was experimental and I think that also may have been a weakness. You live and learn and competition sometimes just brings the answers a bit quicker.

I had been working with my dad building the engines for a few years and I think this just happened just as I was going to move on to do a contract working to changeover a cylinder head line to a new head in the casting plant at Nissan in Sunderland. As I was not in a position to build another engine, my dad resolved to machine up one of his crankcase castings to at least provide a sturdy basis for a race engine. Into this he built the internals of the previous engine, and the new engine was badged ‘Phoenix’ in reference to it’s resurrection from the remains of its predecessor.

from the ashes...
from the ashes…

It was a fantastic thing to do for me.. I think because he felt quite sorry as I’d put so much work into the previous engine and also, that he felt that the far stronger crankcase casting was a far better place to start.

So I ran the engine for a couple of years, on petrol, and it kept going, but it wasn’t really competitive. It wasn’t really ‘tuned’, just solid and although I really enjoyed my bike, it wasn’t anything like as good fun to ride as my dad’s Flying Squirrel, which just had a sense of thrilling urgency that mine lacked utterly.

The catalyst for the major improvements that came was when my wife and I received a wonderful wedding present in 2011, in the shape of a new expansion pipe that my dad had made to fit by Gibsons exhausts in the South East.IMG_6831IMG_6828 My wife found this quite amusing. It was somewhat better looking than the one I’d welded up myself.. but did it work?

The first test came early in 2012 when we participated in the Prescott hill climb in aid of the blood bikes. With the same jetting as previously used with my old pipe I accelerated from the line hard and then pulled in the clutch quickly as it seized on the needle as I rolled it off.

We played around with it in the afternoon, changing plugs, altering the timing and jets but it just seemed to be running very hot. The next outing was at Lydden with the BHR club and we put in an extra head gasket to decrease the compression. It was still running a bit hot, but better… at least it finished a race. It really wasn’t quick enough though. I realised that I needed to make a decision.

It may be that the exhaust is not of the optimal shape and there may be a build up of heat because of this and not simply because it’s charging the cylinder so effectively… but we are not running a blank cheque development program (!) and so we needed to try and see if we could get it to work.

I figured I had three obvious choices. The first was to put my pipe back on. I did not want to do that .. It seemed such a retrograde step. The second was to work on getting the heat away. I’ve got a speedway radiator in the bike so a bigger one may well be much more effective. Also my dads bike has an aluminium cylinder barrel which also transfers the heat away from the exhasut port and cylinder head much more quickly than my iron block. Great. A new radiator would be about £1000 and a cylinder several hundred.

The third way was not popular with my dad.’Dope’ I said, that’s what I’m going to do, ‘run it on dope’.
The positives of methanol are that it really cools an engine and allows a far higher compression ration to be run than with regular petrol. Methanol also burns more slowly and that can make for a smoother and more progressive power delivery. On the negative side, it’s comparatively harder to get hold of, more dangerous to deal with and you need a much larger amount to run on. I’ve also experienced lubrication issues since I’ve used it, but it may be that some careful development may improve that. It also doesn’t give much warning in terms of plug colour if you are running lean. It tends to let you know by melting a hole in a piston apparently.

So it was that I invested in a barrel of your finest methanol and talked to a few people who had experience using it. By far the most useful contact was Roger Cramp, who used to race the highly developed Velocette that his son Ian now campaigns with the BHR. He had been involved in the building and development of an ariel leader that he had run on methanol as well as a Greeves. He confirmed the research I’d done about the necessary changes to ignition timing but also said to be aware that methanol was singularly averse to atomisation (at least when bucketing it in) and that high intake gas speed really helped. This encouraged me to stick with my smallish single carb to at least try out and see if it worked.

I decided on a huge 980 main jet and then measured the needle jet with a taper pin. I then put the carb together and drew lines on the needle at 1/4, 3/4 openings at the top of the needle jet and then worked out the dimensions of the needle at those points against the aperture open for air inlet. I ended up with a pretty severe taper on the needle, but it seemed to make sense.

I advanced the ignition by about 7°, closed the plugs right up and pushed. It fired up and it ran, albeit a little roughly, before I put it in the Van to took it to the last BHR meeting of 2012.

It was brilliant. The jetting seemed to work fine and there were no holes in the power delivery according to throttle position. The bike pulled and was so much fun. It felt totally different to my dads bike, but the torque and flexibility of mine suddenly made it feel like a completely different bike. It absolutely hammered the clutch though (as it does tend to with three gears) and I was up to 1am on the Saturday night stripping, releasing, filing plates and and rebuilding it.

2013 came and one of the first jobs I did was to rebuild the clutch with new GFS plates, laser cut. They were perfect really because I had to dremel each one to fit with abut 0.010″ clearance. The less clearance, the less hammering… My dad also had some pressure plate he’d had made out of solid, which didn’t flex like the original ones. IMG_3703These didn’t have the adjustable clutch actuation pins I normally used, which are a pain to set up. I made pins up instead from some silver steel and got them within 0.001″ of each other using a cordless drill as a chuck, a file, a dremel and some emery cloth.

We took it to a couple of track days, and then the last Cadwell park in 2013 and (with new 21″ racing tyres robbed off my dad’s poorly Flying Squirrel) managed a couple of second places and even a fastest lap. It was flying, although not in the league of Mike Farrel on his Rudge, who was really out on his own and un-catchable for us at least. You can lose a lot on the start as it’s difficult to get off the line with three gears when you’ve geared top for a long straight. Except for a CS1 Norton belonging to the famous Lewis family who’ve been campaigning Triumphs and the Norton for many years. I think everyone else runs four gears. It does make a difference.

I ended the weekend having blown three composite head gaskets and with the feeling that there was a bit too much piston slap noise, but apart from that it was the best racing weekend I’ve ever had at Cadwell. I also knew I needed to strip the engine and that I’d do some gas flowing whilst it was apart.

And that’s where we are!