by Charles Owens on 11/13/2012
Eric has given authorization to post his response to a recent e-mail conversation a Thumper Talk member had regarding his CR250 case reed engine big bore kit. In his message, he discusses the ’02-’07 CR250.
“Last week Donny Emler of FMF called me to talk about 2-strokes. He’s building a hybrid with a late model KTM250XC and a KX500 engine for racing in the Baja 1000. He likes the idea of the steel flexible frame as opposed to the rigid ali frames of contemporary designs. We talked about my 300 conversions and my new KTM 295 which covers all the way back to 1994. He said “you can’t beat a 500 in the desert, you need something bigger than a 300?. Then he said “remember the old KTM380? that was a cool bike that got discontinued because of the 4-strokes but it may be about the right compromise in displacement”. I agreed, that bike had potential but it needed a couple of garage guys like Donny and I to refine it. After I hung up the phone I started thinking, maybe a 400 or 450 2-stroke would be interesting and I get a lot of calls about a big bore for the 2002-07 Honda CR250R now that we make a 295 for the 1992-2001 models. So I got a base gasket from a CR250 and brought it to Millennium Technologies, my plating vendor in Plymouth Wisconsin.
They have thousands of 2-stroke cylinders from all sorts of motorcycles and snowmobiles. I placed the CR base gasket on a bunch of cylinders and found the perfect candidate to be grafted to the CR250 cases, its a Polaris 800cc snowmobile (big twin). With the stock stroke it would make the CR 410cc but I figure on using a 76mm stroke with a KTM300 rod (longer) and change the balance factor for the heavier 85mm piston to make a 436 and just call it a CR450R.
Millennium gave me a few core cylinders and Wiseco is helping me with a custom piston design. Millennium has a dynamic crank balancing machine that we can use to sort out the crankshaft problem. The best thing is that because these cylinders are a cheap commodity in relative terms, I can design an affordable kit by retro-fitting the Polaris cylinders and Millennium has nearly 400 cores in stock.
There are some great companies in America that will make custom cylinders like CPI and JSR in Oklahoma, but those kits are in the $3,000-$4,000 range and thats too expensive for the average dirt biker. For my design work on this new mega-bore 250 I’m working with students from the University of Wisconsin in Madison, my hometown. We have several versions of 2-stroke engine simulator computer programs where we can model different configurations. I sponsor the Formula SAE team at the university www.uwracing.com and we have better engine design and prototype facilities than most motorcycle manufacturers.
Our formula car uses a KTM525 engine built to 610cc, is turbo-charged, runs on E85 spec fuel, and with a 19mm restrictor plate it turns 92hp. We won best powertrain, best fuel economy, and the horsepower contest in world competition so we know a little about engines. I’m thinking about 2 engine configurations for the CR, one for off-road and the other for mx. The off-roader would be a lug and chug stump-puller with a low rpm peak, it wouldn’t even need an expansion chamber just a silencer box design like a trials bike. The mx bike could make as much as 75hp and the crank and tranny of the CR could take it. The advantage of the 250?s stroke and a mega big bore is an rpm peak of 9,000 rpm. For my test I’ll use a shortened CR500 FMF pipe and a taper-bored 41mm Keihin carb from Carb Parts Warehouse because both of those parts will fit the stock chassis.
Starting January 1, 2013 I’ll be running a 2-stroke mods division for Millennium Technologies and we’ll list all of our new products including info on this kit, assuming it works like I hope. Chris Hackl, the owner of MT has had me working on a bunch of new kits including the KTM125 and 250 models dating to the 1990s. We’re also building a 300 kit for the new Honda CBR250R and CRF250L models.” -Eric Gorr