Hotrod XR500 power vs late 450's

I came across a gut that has a ex road race XR500 motor. Stout bottom end, Carillo rod, crank work, lotsa head work, ported, megacycle cam, shortened guides, megacycle springs, reworked and hardened rockers, 565cc, 12.5-1 compression. 


 The motor is solid and reliable. I worked at a Yamaha dealer for a short time and see the Dirt bike and quad 450's build great power but dont last for sht.  I am going to use it for flat track. Cant get an honest opinion from the FT 450 guys and there are not alot of XR500 guys in that realm. Maybe cause they get their asses whipped so there are none? I have most of the stuff to build a descent bike cheap, and will regardless, just cause I want this kind of bike. 


How will this motor stack up against a 450? Im sure I have them in the reliability, even w the 450 stock, but power? Any reason a single cam 4 valve cant compete or smoke a new, and IMHO a very fragile 450?


Thanks for your input. 

Watercooled motors really clean house vs air cooled when it comes to making power.  A liquid motor can do a better job of keeping the cylinder head cool and that lets you easily get away with running the higher compression it takes to make more power.  Where that 500 is running 12.5:1 those 450s built up might be closer to 14:1.  That kinda number will be pretty hairy to try and pull off on that air cooled mill.  BUT, no replacement for displacement, so that extra hundo of cc's might have that covered and get you right along side them.  


The only downfall I see is that it is an old motor, and although it shares parts with the XR600s and 650Ls, some specific parts might not interchange and it could cost you a race weekend.  With the 450 I'm sure you can get anything for it, or perhaps borrow an item from a friendly competitor who shares a similar power plant.  That alone is probably the the main reason you see the more modern and plentiful liquid cooled 450's around the track.

Modern 450 MX bike will out accelerate a XR650. It would kill a air cooled XR500. The old XR500's were not known to make reliable racers either. To much time has passed to seriously consider an XR motor of that vintage for racing.

I used to flat track a RS600 in the early 90's and it felt like it would out pull my kx450f. Chris Carr rotax 600 was rumored to have 80+ hp. With that said your biggest disadvantage is about 40 more lb's and you have to install a restrictor plate to race a big bore against the 450's :( the only problem I remember with the early xr/xl 500's was the crank haves would spread so you had to weld the rod pin in :)

Granted, not the best choice of motors for racing, but it sure would be a cool project.  I'd love to see something about the build when you get it done.

I didnt consider cylinder temp control with a water cooled vs an air cool. Duh, I should know better. And yeah if it breaks, no spares in the neighbors pit. 


Sure a modern 450 will out accelerate a 650, that is just comparing cc's and not what bike has to haul around. 


For flat track I wont have to install a restrictor to run w the 450's. and why would I be 40 more pounds? The motors dont weigh more than 60 or so each in the first place. The chassis and suspension has to be close to the same weight. 


And yeah, cool project for sure. I like old sht, well cause I am old. I can get a descent 50 hp motor for what a good buildable 450 core or tired runner would cost me. Plus the cost of refreshing it. 


I spend tons of money on my road race bike. I want to race other venues and not spend bank like I do with the RR bike. There is one RR track close and I can take a flat track/SM bike to 4 tracks that are closer, less expensive by 1000's and can race in the winter at the indoor track. 


Ok I will be out powered by a 55hp 450, but wouldnt my power deliver be flatter and more manageable? 


Thanks for the replies. I appreciate the info.  

A 600 engine weighs almost double that of a modern 450. I don't know about amateur rules but when I was racing as a professional you had to use a restrictor plate in the 600's  to race against 450's.  I myself would take a 600 in a good tracker frame if I didn't have to use a restrictor. 


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