View Single Post
Old 01-05-2019, 02:57 AM
  #6  
fyrstormer
Tech Elite
 
fyrstormer's Avatar
 
Join Date: Mar 2014
Location: Maryland, Near DC, USA
Posts: 4,467
Default

The baby has been taking up most of my time the past couple months, no surprise, but I finally got around to doing winter maintenance on this car. I had previously modified this engine to work with a resonant exhaust, but since the RS4 Mini can only fit a non-resonant exhaust, the increased exhaust port timing I'd cut into the cylinder sleeve was no longer helpful. So I replaced the piston and sleeve, which were starting to develop a loose fit anyway, and I was more modest with my mods to the new sleeve:



The biggest modification was hollowing-out the portion of the sleeve directly under the main intake port, to produce a channel for the intake charge to travel straight up and into the combustion chamber. There is no channel cut into the engine block for the main intake port; instead, the intake charge is forced to travel the convoluted path of passing through the underside of the piston, out through the hole in the side of the piston, and up-and-around the lip of the piston. This allows the intake charge to cool the piston from the underside, which is helpful in an airplane engine, because airplane engines are tuned very lean to ensure they heat-up sufficiently despite the massive airflow inherent to being installed on a moving aircraft. (the 12R SS was made by Toki, a defunct Japanese RC airplane engine manufacturer.) However, cooling the piston is not especially important for a car engine, because they are tuned richer and aren't running at full-throttle constantly anyway. On the original cylinder sleeve I just cut that portion of the sleeve completely off, but this time I left a thin bridge connecting the two sides so it can resist thermal deformation and avoid pinching the bottom edge of the piston skirt. Is this really necessary? Hell if I know, but it can't hurt.

I also radiused the lower edges of the side intake ports a little, but that's all I did to them; the lower edge of the piston sleeve is already knife-edged for better airflow around the lower edge, and the channels in the engine block connecting the crankcase to the side intake ports are already plenty large enough, so I didn't see any reason to make further modifications to the side intake ports. Likewise, I didn't change the internal dimensions of the exhaust port this time, though I did modify the external dimensions of the exhaust port to port-match it to the exhaust header like how new engines are constructed. Having a lip around the external dimensions of the exhaust port is sometimes useful in 4-stroke engines to restrict backflow into the cylinder, but since 2-stroke engines need backflow to prevent too much intake charge from escaping, I deemed it worthwhile to remove the lip. But again, the internal dimensions of the exhaust port are unmodified, so the internal lip at the bottom of the exhaust port can still prevent excessive outflow during the beginning of the intake cycle to reduce wasted fuel.

- - -

I also modified the transmission a bit:



2-speed transmissions use one-way bearings to allow the low gear(s) to freewheel on the transmission shaft when the high gear is engaged. This has a secondary effect of allowing the engine to idle-down while coasting, which changes the handling of the chassis by eliminating engine-braking, which may or may not be useful depending on your driving conditions, but it has two big advantages as well: 1) it eliminates a minor-but-nonzero source of heat buildup, which makes the engine easier to tune, and 2) it reduces the risk of oil-starving the engine when coasting because it isn't being forced to rev at high RPMs without adequate fuel/oil supply from the carburetor. The few 2-stroke automobiles that have existed throughout the years have always employed one-way bearings in their transmissions to allow freewheeling when coasting for these exact reasons.

I didn't want to install a 2-speed transmission in my RS4 Mini because I don't intend for this to be a really fast car, but I did want it to freewheel when coasting, partly for the reasons mentioned above and partly because I find the sound of a nitro engine that is being dragged along by the wheels at high RPM to be very irritating. So, I dug out my stash of OFNA 2-speed transmission parts (the same ones I've used in all my other HPI RS4 builds), flipped the low-gear carrier around backwards, made a little cylindrical shim from a piece of aluminum tubing to extend the portion of the transmission shaft that the OWB rides on, and make a one-speed freewheeling transmission. I had already done this in my RS4 3 Evo rally car, so I knew it would work.

- - -

Oh, and one last thing: I bought a set of aluminum supports for the intermediate driveshaft just in front of the engine, scuffed them up with Scotch-Brite and scored the edges with a hobby knife, then soaked them in Liquid Plumr alternating with Tarn-X silver cleaner to strip off the purple anodizing, and installed them on the car. You can see them a little bit in the second picture above. This will support the intermediate driveshaft more securely than the stock plastic part, and also help the chassis plate resist bending. I won't actually be able to drive the car until spring, but it should be much nicer to drive now.

Last edited by fyrstormer; 01-05-2019 at 03:13 AM.
fyrstormer is offline  
Reply With Quote