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.
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.
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:
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. 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.