Hi all,

First post here and unfortunately it's not about a Mazda, but I figure I will be able to pick up some ideas here

I'm planning on tuning a Sachs KM914B and do some vintage racing with it. The trouble is I have never worked on a rotary before and could use some pointers. I've got lots of experience porting 2-strokes but never did anything with 4-strokes or rotary.
The Sachs engine is an air cooled 303cc single rotor engine from the late 60s. Specs vary a bit but it puts out somewhere between 18-24 HP at 5500-6000 rpm in stock form with a tiny carb and probably a very restrictive exhaust. I need to get to at least around 40-45 HP in order to be competitive, no turbos or other supercharging methods allowed.
Spare parts are very hard to come by so it needs to be somewhat reliable too. On the other hand it won't do a lot of miles, it will run for maybe 2-3 hours of running per year and most of that will be at idle. Probably less than half an hour actual race time, we're talking drag racing and hillclimbs so very short races. With maybe a couple of oval races where it would run on and off the throttle for several minutes at a time.

It uses peripheral porting both for intake and exhaust, but timings seem to be very conservative. Here's a pic of the inside. I haven't actually picked up my engines yet (they're 600 miles away) so these are somebody elses pictures.


The intake is interesting to say the least. It uses the 2-stroke principle for lubricating the whole thing (hence no oil seals on the rotor). Basically the intake channel goes through the side housing to the center of the rotor, leaves some fresh oil on the bearings, and then continues in the other side back up to the intake port. The intake tract is probably at least 2 foot long.

That's probably one of the first things that needs fixing. For starters the intake is probably way too long and narrow for higher rpms and also by the time it gets to the intake port the mixture will be boiling hot. I plan on adding a second intake channel that takes the shortest route to the intake port. The carb for this second intake would be a larger one that starts opening at half throttle or something. Of course the engine still needs lubrication so I'll have to make sure the original intake is also used. The plan is to have it end up at the same port but I'm open for ideas! I think it would be possible to put in a side port for the original intake and use the existing p-port only at half throttle or more. I have a bunch of spare sides that I can experiment with.
The benefit of having an air cooled engine is they are easier to modify, no water jackets

Next is the exhaust, again it looks very restrictive. I'll probably have to do some machining to replace the cast first bit of the exhaust header with a larger one, looks like there will be too little material on it.
Here's a closeup of the stock exhaust port.


Now here come the questions..
1) As you can see in the first picture it only has a single plug, would there be a lot to gain by drilling in a second one a bit to the left on the pic?
2) I was going to aim for 8500 rpm for starters, is that a good number, too little, too much?
3) This engine will go on a snowmobile with CVT clutches, meaning the engine will be at a constant rpm and I don't need much of a powerband. Power lower down in the rev range isn't very important, altho it does help with the starts to have a bit more power lower down. How does this affect porting? I assume I'll be able to use higher durations, squarer ports and more overlap than one would on something with a regular gearbox.
4) What would be a good port width? I have no idea if I can port these to 90% of the rotor width or 60%. Is there any general rule? There are no aftermarket seals available for these engine so the seals I will be using are NOS from the early 70s and might be brittle.

Any other ideas or tips for reading material would be much appreciated too.
This is completely new territory for me but I can't wait to get started!

Thanks

 

Hi!: the working surface in your engine looks seriously damaged, perhaps you can contact the Aixro kart engine maker in Germany for the possibility of re-plating this, or do it yourself with chrome; after a layer is deposited by an electrolytic way, reverse the polarity and have DC running for 15 minutes or so, to have some porosity in the layer, needed to keep the lubricating oil film and not destroying the engine soon after start.

Several refurbish for Wankel appear, Kevin Landers, at RotaryResurrection, and several others in ebay.co.uk

The book by McGovern: 'Rotary engine handbook', can be located in old book shops, e.g., abebooks.com it has a good explanation about assembling and care of the Sachs KM-48 and KM-914, also you can purchase manuals for Snowmobile engines and original Sachs documents in ebay.com and ebay.de
Peter Slater, Slaterpumps, in Australia has an old stock of parts for these engines.
Best wishes, + Salut

 

No, you won't gain from a second plug, I guess, as the liquid cooled car engines from Mazda have a different use, and the only plug in Sachs charge cooled rotor, air cooled housing, is placed in the best position.

Some responses to your other queries can be found in this discussion: Aaron's Homepage Forum - Reed valve fitted peripheral intake port Wankel

Norton developed his twin rotor Wankel engines from the KM-914; by increasing the breathing of original Sachs engine, they obtained more than 80% power added, at much higher rpm; in the range of rotation speed of original KM-914, no noticeable improvements seem existed, but from the thermal constraints, after some hours at top power, cracks developed in the rotor, they added internal cooling fins in the rotor corners of their production models, but these Norton rotors are a bit narrower as the KM-914, the Norton had the displacement of the Sachs KC-27 engine in Hercules W-2000, 294 cc, the KM-914 is 303 cc.

Seals of these engines are made of IKA3, same as the piston rings in reciprocating engines, no reason to be brittle after long time storage, as far as I know.
These engines were developed in the times of leaded gasoline, it tolerate unleaded without problems, but as lead has solid lubricant action, perhaps you may need a bit higher oil in gasoline mix, around 5%; from all the oils tested, SAE paper reports best results with Shell Rotella SAE 30, SAE 40 for hot climate, this oil can be found in some vintage tractor parts suppliers. Synthetic 2-Stroke competition oil, as that sold in Walmart, is reported good, some used Automatic Transmission Fluid, castor oil produces too many gums and ashes.
Adding a bit of MoS2 additive, Molykote A (strength unknown to me), LiquiMoly, to mix, 1 cc per liter with LiquiMoly, may improve power and economy, Mazda used organic Molybdenum additive in the oil of Le Mans 24 h endurance race winning car.

The Wikipedia Wankel engine article contains many links and references, an special site exists: rotaryeng.net mainly focused in experimental aviation conversions of Mazda rotaries, that are quite different from your Sachs motor.

For further improvements, some put a Mikuni type 32 carburetor, see YouTube. Electronic Fuel Injection | EFI | Wideband Controller | LSU 4.9 | Engine Management System | Lambda meter | ECU - ECOTRONS sells turbo-compressors and EFI suitable for this engine. The 294 cc, 34 HP, KC-27 used a carburetor identical to the left side carburetor of a BMW.

Snowmobile engines operate under a higher constant load than a motorcycle engine, so, KC-24 and KC-27 differ from KM-914, even if some KC-24 were installed in Snowmobiles.

For top power, a short duct from carburetor to engine intake plate is good, for low rpm torque and light load operation, some 30 cm are good.