HP on a WR426?

25 replies to this topic
  • ballistic

Posted April 16, 2001 - 11:18 AM


Just wondering, what is the real world HP on a piped and jetted WR426? How about the 400?
Thanks in advance.

  • Jemtec

Posted April 16, 2001 - 10:52 PM


From what I understand.. a Stock Wr400 Made 42 Hp. A stock Wr426 Makes About 36 in stock form, and 50HP Uncorked. Why so low out the door? Still in 2001 (and prior years back to 98)..The Wr/Yz 400/426(all for emissions reasons) has a throttle stop that only allows for 50% throttle and a nasty 8Hp Drop in available performance. The Exhaust Baffle Is easily removed)that takes 4 HP away..And the Airbox lid Takes another 2 HP. Total 14HP lost stock. But..It can easily be replaced with a modified throttle stop or replaced with the throttle stop from a YZ 400(98,99 models) or 00,01 426 thottle stop.(I dont think they are the Same?) and by removing the baffle and airbox lid. The Exhaust is pretty loud though. Some of the guys have been recommending The White bros E series exhaust for a good "balance" between Noise and performance.
The Wr 426 Has Titanium valves, and lighter valve springs. It pulls harder on top, and has a bit more mid range over the 400, so it produces more HP. Less rotating mass= greater Rpm. :)
The Wr400 From what others have told me can reach 46 Hp with the YZ exhaust valve timing and ignition timing changed to that spec..and a different free flowing exhaust.
The Wr 426 takes less to go faster, but it also costs a bit more out the door. IF you went free flowing exhaust, changed the Exhaust timing to YZ spec, you would acheive 54 HP with the Same WR426. thats 2 less than A 2001 CR500!

[This message has been edited by Jemtec (edited 04-17-2001).]

[This message has been edited by Jemtec (edited 04-17-2001).]

  • Brad

Posted April 17, 2001 - 02:06 AM


I think 54 hp is a little overly optimistic. If you look at dyno runs from all the mags (not that they're accurate either), with any exhaust sytem the YZ426 doesn't ever make above 50, and I've seen as low as 45 for the YZ 426. There's no reason to think a WR426 will be any better, unless your talking running a higher compression piston, possibly big valves, and race gas. I also doubt an 8 hp difference between a 400 and 426. I think if you rode a well jetted 400 and then jumped on a 426, the difference would almost undetectable.

[This message has been edited by Brad (edited 04-17-2001).]

  • Taffy

Posted April 17, 2001 - 04:42 AM



very good brad. boy is there some smelly stuff being spun out here or what!

a 426 will produce more torque than a 400 & NOT more top end. the ti valves are a gimic & i doubt they make even 1/2 hp difference due to the loss of friction. bigger engines spin SLOWER. they can rev as high but not HIGHER than a 400. the less mass of what? a 426 piston is bigger than a 400 so guess which one has more mass. i think someone ought to go & take "mass" ASAP! pray for forgivness.

50hp tops is what is being made. even the pipes only improve the torque. airbox lid 2hp-bollocks!

difference in the ignition curves? the starting curve has been improved for '01 in the meantime nobody has been able to prove that the yz & wr have a different curve at all!

high comp creates more top end but it appears that the engine won't rev out; don't do it. top fuel in itself might make a marginal difference. big valves react differently on each model & on the wr400 they would create more top end for a loss in torque. this bike DESPERATELY NEEDS MORE TORQUE.

the 426 would benifit from big valves but not the 400. you should be looking for more torque from these engines.

a good pipe, taff cam timing, properly jetted.

i'd love to see the 250 carb (37mm)on the 400, now that's my idea of tuning!


  • revin_kevin

Posted April 17, 2001 - 10:04 AM


Agree with brad,
a case of owners dreamin there wr400's have46/50bhp
got a stock wr400 seat of the pant feel i,d
say 30-35bhp[ at the wheel]
don't feel much quicker than my old honda crm
just ordered a w/bros E series s bend gonna
get timing done too, hope that'all pep her up a bit

  • Jemtec

Posted April 17, 2001 - 10:06 AM


Some things that Dirt bike magazine pointed out in comparison to the 2000 Yz426 and the 2001 YZ 426 is the Difference in top end power.
The 2001 Has more overrev, and stronger power delivery in the midrange. Titanium Valves Might sound like a gimick, but they really arent and heres why.
With the exception of wearing the valve seats out a bit faster (since Titanium is harder than Steel) They really are much lighter. Yamaha went to light springs because they dont need anywhere near as much pressure to keep them closed at higher Rpm unlike Steel Valves. Of course doing the Math, Less Weight=less reciprocating mass=More Rpm. And because its a "weight" thing.. All Rpms would notice the lighter Springs in "revability". If its easier to turn..its easier to spin sooner.
The 400 and 426 have exactly the same Stroke at 60.1 MM. The difference Is bore. 90 MM for the 400 And 95MM for the 426.(both the Yz and Wr use the same engine with slight changes in cam timing as you already know :)) With that, an additional 26 cc Displacement in the Bore size alone would cause a More Violent rush to upper Rpm. Specially since the engine is fairly oversquare. This engine was based off Yamahas Genesis Streetbike Engine which was really impressive for power output. That one was about rev too, but the Dirtbike engine is tuned for more bottom end since it doesnt have 3 more cylinders like the Genesis engine.
In regards to working "torque" Id be willing to bet that the 2 engines offer similar torque spreads like taffy was mentioning. But, They dont have the same personality. Dirtbike was also saying that the way the Power is delivered is very different for 2001 with the titanium valves. Though I havent personally ridden a 400, I can say that the improvements made for 2001 are very real and noticable. :D And that wouldnt be because I own a 2001 Wr 426 and am being biased.
I guess the best thing to do is ask Fellas that have ridden both bikes. What are your opinions guys? How do the powerbands feel? Low end..mid range..Top end?.. Id love to hear your comments from all the fine guys here on thumpertalk. :D

  • Guest_Guest_*

Posted April 17, 2001 - 10:25 AM


You guys need help.

  • Hick

Posted April 17, 2001 - 10:52 AM


My alter ego, Captain Nitpick, insisted I post the correct bore size for the 400 as 92mm.

I don’t know about the WR, but I would think a comparison of YZ to YZ is roughly equivalent, especially if you are talking about uncorking and rejetting the WR. As far as that goes then even a poorly jetted 426 is noticeably quicker than a perfect 400 (I am speaking from personal experience). They both rev well but the 426 is much stronger right off the bottom.

As for the 54 HP, if measured at the crank I think that could be accurate (my KX 500 was rated at ~ 63). I’ve seen published rear-wheel HP figures for the YZ 426 of 45 and 47, so I would say that is what a piped and rejetted WR is capable of. The YZ 400 gives up only two or at the most three ponies to the 426, but total area under the torque curve is what you feel when riding it.

My two cents.

  • Taffy

Posted April 17, 2001 - 01:48 PM


ok jemtec i'll give it one more go but that's it mind. ok!

you put in titanium valves & you have overrev, did someone not say more power!!!!!!! like you for instance.

more midrange from ti valves.LOL!

the bikes ability to breath higher up the revs is dictated by the valve timing & the size of the valves & ports. it's T x A isn't it jemtec? glad to see the first one in says that it "hits off the bottom".


  • Jason_Foslien

Posted April 17, 2001 - 02:02 PM



Would you mind giving me your cam specs? I tried the WR to YZ change and am not very impressed. If yours is "TOP SECRET", I understand. Here's what I'm running at the moment:

01WR426, YZ cam timing, YZ throttle stop, Stock exhaust(uncorked), Vortex Ignition, "Open" Airbox, EMP#5 Needle, 170 MJ, 48 PJ, PAJ Screw 1 1/2 out, 200 MAJ, PS 1 1/4 out.

Thanks for all the useful and interesting info,


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  • Taffy

Posted April 17, 2001 - 02:30 PM



do you know how to, or do you know someone who will have a crack at it for you?

the figures are for the cams at maximum lift.

exhaust is btdc
inlet is atdc

standard the wr comes;
113d on the inlet
123d on the exhaust

to get the yz timing is then a change by one whole tooth which is worth 22.5d;
so yz exhaust is 101d as std.

as you can see 123d becomes 101d & what is needed is 109d. one could call it "1 tooth back, & a 1/3 tooth forward".

you may gain this by forcing the std wheel around on the cam. there is an old post on this. i think if you search under 'cam timing' & 'taffy' you'll get somewhere.


  • Jemtec

Posted April 17, 2001 - 10:09 PM


I agree taffy. Larger ports would improve Breathing. Bigger gulp= more power..But are you forgetting something important here? With Titanium valves..And Faster reciprocation, you still get more revability not just "overrun" as you suggested. It affects ALL the powerband. Bottom, MID and Top by a higher maximum allowable RPM, because of lighter valve springs, and Feather light valves.
Wouldnt that also suggest that now since its maxium revolutions are even farther up on top that the Volume of the intake at that Rpm Has Been Enhanced by higher CFM flow rates due to this increase? Doesnt that mean Additional Top end Power?. Lower reciprocating mass simply translates into more power in every corner of the powerband. And, Since the engine doesnt have to work as hard to overcome "inertial mass of the valve train" and work is done quicker..that automatically means more Horsepower. Am I smoking dope here?
Im not here to have a pissing contest, But I do have good working knowledge of Mechanics. :D
I feel I am right to say that the Mid range is improved.. But really all the power availiable is improved not only in the mid, but the entire powerband.
Thats my mistake of wording. Such as my typo that hick pointed out about my listing of a 90 MM Bore for the 400 when it was actually 92MM. :D (thanks hick). I was rushing off to work and didnt get to proof read as I normally do.
Still, Just how much more weight is a 426 Piston over a 400? 3 mm isnt alot, but what do you think.. maybe 24 grams? (1 oz?).. I would be more than willing to bet the trade off to titanium valves is alot more than you might think. Though I didnt even think to consider that additional mass. It is indeed a slightly bigger piston on the 426. It would indeed reduce reciprocation somewhat. But..In addition.. We are also talking a bigger "gulp" of fuel and air (more CFM). The additional volume due to the enlarged bore would mean more CCs. So that would mean that "additional mass" is reduced by a more powerful downward thrust. Wouldnt that also mean more torque that would overcome the additional mass of the weight of the piston and would equalize the difference? Am I also wrong to think this? :)
But getting back to brass tacks... Its obvious Titanium has a place. I wouldnt call it a gimmick. If it was, why would Formula 1 racers Use them if they were? Its obvious they have technical advantages.

  • Taffy

Posted April 17, 2001 - 11:05 PM



i've got to get you straightened out.

weight going down, stopping & then coming back up is no good to man nor beast so more mass of piston is no good. no help whatsoever.

you SAVE power with titanium, you don't make it.

just because i bike over revs doesn't mean it is more powerful. it's all about fighting friction. friction is the killer coz it creates heat.

power improves everywhere? only if you think the ti goods have improved the power at all. american bike testers are known for their...you know. bit like we ended up in the 50's & 60's. as i said i think it saved at most 1/2hp but it sold 1,000 bikes so...

nobody doubts that ti improves things IN THE RIGHT CONTEXT. but this isn't it. yamaha quite rightly are just leaving the ti valves & goodies in for at least one-year. they want to prove they are reliable to THE PUBLIC not just ty davis.

given time they will improve the midrange performance with longer duration cams, whilst maintaining the same top end & the over rev of which you talk.

this brings you to the above. over rev etc. they will have to port the engine differently, that doesn't necassarily mean make the ports bigger but probably. any old fool can make a port bigger. ti valves help rev higher but only if they used to be the weakest link. revving higher doesn't make more power it's the things you do around it.

but jemtec, while the ports are the same, & the valves & the cams & their timing are as well yamaha aren't taking up the tuning potential that ti has allowed them.

it'll come, but it ain't there yet.

titanium allows power to be made at higher & higher revs. when i was a kid you were meant to produce maximum power at 3,750ft/min piston speed. in the eighties it went to 4,000 & in the early 90's it was 4'250ft/min probably due to lighter 2 ring pistons & the resultant drop in con rod weight.

ducati in the bike world destroyed this figure. smashed it to pieces. my pantah produced max bhp at 4,000ft/min (won four world titles) it had a stroke of 58mm so it was 10,500 revs.

the new dukes have a 64mm stroke so max bhp is at like 9,500 rpm, right?

nope, try 12,000, 13,000 revs. until yamaha alter other bits & pieces to take advantage we haven't been given much.

i have to smile when even a factory technician announces on our web site that the valve timing is way out & yet the factory gives us ti valves & a manual decompressor! some things never change.


  • Jemtec

Posted April 18, 2001 - 10:14 AM


Now I liked that Post :) I can see what you are saying now. I guess when you consider all the "weight savings" in mechanical mass, One would think that additional revability would be the true mark of easier made HP. But Then Duration of cam lift would need to also be improved to further enhance the Additional RPM of that reduced mass. I was under the Impression that Yamaha Did change the head somewhat in the 2001. But, They didnt change the valve duration one Iota as you mentioned. :D The only other thing that Would make me think they wont be so agressive with that maybe the fact that Engines With very high durations have a serious Loping effect. That would make them Funny in lower speed Idling. Which the Wr will likely see in most woods riding. It would be really cool in a Baja, but not for most peoples riding styles. Dont you think?.
Thanks for keeping me thinking. I think I need to clean some of my "performance rust" out of the head, and you have certainly given me some Wd-40. :D

  • Taffy

Posted April 19, 2001 - 04:18 AM


years ago we used to road race the old xr500 engine. they did a mk1 cam & it made good power, the next year they increased the lift by 1mm. the bike never went any quicker.

the bloke that did these profiles was the hottest tuner around. he's got a new profile for everything & is excellent at base regrinds (& my desmodromic rocker arms!).

so he bought a flow bench & learnt all his baseline knowledge on his own bikes then being ridden by robin riley & laurence martell.

even though he'd ported the engine the bike DIDN'T BENIFIT FROM THE 1MM EXTRA LIFT. THE PORTS WEREN'T RIGHT, they couldn't be altered IHHO.

he found that by pulling the valves out of the head COMPLETELY it didn't flow anymore air-unbelievable or what!

so the next season he increased the duration of the cam & so he found an improvement.

truth is always better than friction as i always say jemtec!


[This message has been edited by Taffy (edited 04-19-2001).]

  • Jason_Foslien

Posted April 20, 2001 - 12:05 PM



I did a search for anything about the "Taffy Cam timing". Nothing there that specfies the procedure. I'm willing to try it myself, however I'm not sure exactly how much the cam sprocket needs to be moved. Do you recommend pressing the cam sprocket off, then line it up and press it back on? Or can it be accurately positioned using the "Armstrong method"? Thanks.


  • Taffy

Posted April 20, 2001 - 02:30 PM



i'm sorry mate but by looking up related archive stuff i had recommended that people go back to early october to look for "more torque!!!!". sadly everything from oct '00 back appears to be lost.

unless someone else knows how to find it.

you've got to know your stuff to turn the camwheel on the camshaft. you've got to know how far to = -1 degree.

i think it would still fit tight enough but that's just my opinion. it's the accuracy thats the problem.

maybe bill is compiling a Y2K archive!!!


  • Gilly426

Posted April 24, 2001 - 03:26 AM


I took my WR426 on a dyno last Saturday. It's in supermoto trim, 17" wheels, DRK pipe & a 15/40 gearing

WR timing gave 43.1hp.

YZ timing gave 45.7hp.

Lost some of the bottom end grunt (but only a little) gains lots at the top.


  • Taffy

Posted April 25, 2001 - 03:09 PM



tell us what you found & did on each run. tell us what the rig runner was telling you. you can't just sit there you know & say nowt!


  • Gilly426

Posted April 26, 2001 - 01:31 AM


I've scanned the curves into a jpeg, but I don't know where to put them so others can have a look.

First run, the bike wouldn't do anything above 9000rpm.

Changed timing whilst bike still on Dyno.

secong run, curve much better revved on, speed wise did approx 105mph.

Went for a 158jet, but I managed to snap it when tightening it.(ham fisted twat).

So I put the original back in. Put another 158 in last night but haven't been out on it since.

The HP was average. Said the most gains he see's are WR's with YZ pipes on.

He's had a YZ426 supermoto in thats had a bigger piston, high lift cams & other mods done. Makes bog standard WR's look silly. But then it should the amount of money it costs.



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