Roadracing a WR426 - need tuning advice

Hi everyone I'm new here and thinking about stuffing a WR426 engine in my TZ250 roadracer. I think the WR would be a better choice over the YZ cause of the wider ratio transmission. However, I'm not so sure about power. Since it will be a roadracer, most of the time the engine will be spinning way up in the rpm's. I think I need to tune for horsepower in the upper rpm range. I need advice on CDI, cams, carb, porting, pistons, clutch basket, etc..etc. Roadracing is torture on engines so I need to make it reliable as well as powerful.



Do you have a WR426 motor already? If not, I say get a newer 450 motor. They do make more power and start much easier. They are also lighter. I agree that the WR series would be a better application than the YZ series for road racing. Understand that they are BOTH race motors and can run in the upper realms of around 11,000 rpm. Stock HP is in the 45-48 HP range. It is possible to get these motors even closer to the edge of meltdown HP-wise. I have seen TT member's posts in this site talking about their 60+ HP 426/450's built for dirttrack or supermotard racing. Figure to loose some reliability when you are making that kind of power. Correct gearing is going to be more of an issue for high speed than whether the engine design can take the stress of road-racing. My stock WR450 can do 90mph with factory issued gearing. Imagine what is possible with a taller counter shaft and a few more teeth off of the rear sprocket.

The low oil capacity of these motors could certainly become an issue in longer road races. Keeping the oil changed and new filters installed often should keep things running smoothly. My motor has been bullet-proof for over 2,500 miles of motocross and high-speed trail riding.

To answer one of your questions directly with regard to being unsure of the power of the WR in comparison to the YZ, they are basically the same motor. The WR can be made to run just as strongly as the YZ's with very minor mods(exhaust, jetting, grey wire removal, throttle stop mods). There are a gazillion posts about uncorking the WR's.

I honestly don't think getting a "YZ cam" would be a good move for a road racing bike. Linear power delivery and the resulting better throttle control is usually more conducive to faster lap times than getting a big "hit" off the bottom that the "hot" cams deliver. I am sure that there are some aftermarket companies making cams that would suit the applications you are shooting for than a motocross cam.

There is a ton of information on uncorking the WR series in here (just do a quick search) and getting it jetted correctly. Run fatter jetting if you are going to be running road racing speeds.

If I were you, I would venture into the supermotard and dirttracker forums within this site to look for more complete information on making your motor a top end missle. Those folks will have alot of info on the direction you should go.

It sounds like a good project. Keep the site informed of your progress.

As a start i would go for yz timing a k+n filter a good flowing exhaust jet it up and let it rip!,the wr is fairly handy with high gearing and combind with a tz should make a good racer,i would worry more on jetting and gearing more than changing the cdi to start,just do all the free mods you think you may need first,yeah should be a good un!,good luck with your project :)

Go for the 450 and buy a yz. The wr stands for wide ratio. not good for road racing.Gear your final drive for your speed. Your TZ has a close ratio trans. If money is no problem convert a 450 yz into a 5 speed.

I too come from a roadracing back ground and remember back a while when steve crevia was racing f2 and some guy came from back east with a rotax 600 in a spondan frame and gave him a hard time untill the rotax puked it's guts out on the back straitaway. I think the 4 stroke would do nice on the track. what year is your tz frame? :)

i'd get a wr450 motor yz time it and i think a flat tracker or super motard guy on hte yz forum got 60 hp at the rear wheel with some head mods and everything, but if your looking for top end defenitely get a bigger carb, k and n filter and the goodies... as for oil capacity its actually pretty easy to make your own resovoir by welding it together and attaching to the in and out oil lines... sounds like a sweet bike your going to have when your done :)

I haven't bought the engine yet. Hard to find a used 450 engine. The cc limit of the class (Sound of Singles) is 510cc. So, I have plenty of room for bigger pistons. I'm not 100% sure that I would want a close ratio trans like the YZ. Since the YZ is not a street motor, I feel the it would be too short in 5th and 6th. I'd have to run really tall gearing which would kill my starts. I figure the carb would be swapped for a 41mm FCR or other with no filter. I'd would rig up a ram air box to force feed the engine at high speed. I'd probably have to contact a cam regrinder to change profile to move hp to higher rpm. I didnt know the WR would spin to 11K rpm! wow... thats impressive. Here's the bike now. It's a '90 frame with a '86 TZ250 engine in it now. This model has down tubes to hold the engine. Many different singles have been put in this type of frame, but never seen a YZ or WR in one. If I could get 60 hp, then I'd be a happy racer!

Photo of TZ250 here

Yeah, I know Matt. He went with the honda engine, I want all Yamaha. WR400 huh? I'll need everything with it. Can I put a 426 cylinder and piston in it?

Try looking up over-racing,they used to do all yamaha racers,i would go for a wr motor for sure,unless you do some real short circut tracks.The sound of singles well there you go!,you could always ask yamaha for one of those 500 motors(auto decomp on the 450) :)

I agree with Captain_S. The WR400's and WR426's hold more oil than the new WR450 as well so for continued high speed running that should help.

With my WR426 YZ timed, the thing flys. With a much lighter chassis from the TZ, you should have some decent top end. Get a good YZ exhaust for maximum airflow since noise probably isn't an issue.

The new YZ450 is only a 4-speed. Basically a wide ratio 4-speed so that won't work very well for your application.

Do we get pictures when you are finished?

"I'm not 100% sure that I would want a close ratio trans like the YZ. Since the YZ is not a street motor, I feel the it would be too short in 5th and 6th. I'd have to run really tall gearing which would kill my starts."

Carbonbody, you did say you were new to the site and with these motors, so what is obvious to all of the riders of these bikes may not be apparent to you. I am going to try to clear something up; I hope it helps you in your decisions.

The '03 and '04 YZ450's have 4 speed tranny's and are geared

for motocross. People do use them to ride trails but usually are compromised with gearing in some situations. Yamaha left the fifth gear out and lowered the oil capacity as a weight savings measure.

The '03 and '04 WR450's have 5 speed tranny's and are geared for a wider variety of off-road riding. First gear is able to deal with tight single track(down to walking speed) and 5th is much like an overdrive, usually for wide open desert riding.

When I race motocross with my '03 WR450, I am easily able to do second gear starts and am able to ride the whole track in 2nd and 3rd, rarely tapping into 4th. The broad powerband or torque curve of these motors and the way they make usable power throughout the rev range is remarkable. They can lug down at low rpm's, pull hard off the bottom and rev to the moon. They make really good power on top and have lots of over-rev.

The engine was originally designed for motocross. The main difference between the motors is the transmission. To sum it up, neither of them have a six speed.

I realize it will be very difficult to find a 450 motor and the 426 will probably be a better choice. The older 400 motors had some cooling issues, so do your research.

You have created alot of interest with this idea.

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