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