Actuall horsepower difference between YZ and WR timing?

I was wondering does any one know how much horsepower difference is there in the YZ timing and the WR timing in the old 426s? I know the power delivery is different but is different but is there any real power difference that has been measured on the DYNO?

I don't know dyno numbers, but I've owned 2 YZ400's and one WR426. My WR is running YZ timing with all the freemods, O-ring mod, and an '03 WR decomp cam. Both YZs pulled much harder than the WR. The cam timing is the same on both, but the YZ exhaust cams have 1mm taller lobes on them.

To my knowledge the only difference is the yz has the cam lobe retarded one tooth but the lobes are identicle on the 426s im pretty sure. I know the power delevery is very different but im interested in knowing actuall horsepower differences in the wr426 and the yz 426 because im thinking of using this mod on other bikes.

I'm not totally sure about the 400s but am about the 426s. Like ReadyFredy said the exhaust cams are the exact same, the only differnce is the timing. Same lift, duration and all that hoopla.

I honestly don't think there's much difference in HP. What you're probably feeling are the different torque curves. The YZ has a lighter flywheel so it will wind-up faster giving the impression of more HP (ie: snappier throttle response), but will shed the engine inertia faster when you let off. The WR's heavier flywheel will be slower to wind-up but should have more low RPM 'grunt' by storing more of the engine inertia. The timing differences are just to compensate for the different engine accelerations....

Interesting theory, but wrong.

There is roughly a 4~6 hp difference at peak. Perhaps more in your case, since the '07+ models have revised cam profiles for emissions reasons. The cams in earlier models are in fact the same as the same year YZ450 engine except for the timing of the exhaust. The '06 is unique in that it has the '06 YZ intake cam with a "smog" exhaust cam.

Engines NOT using auto decompression can change between these two timings by moving the exhaust cam retarded or advanced by one tooth:

Auto decompression engines cannot do this because re-timing the cam also affects the timing of the decompression pin. Retarding the cam on an auto decomp WR will leave it with too little compression to start the engine.

'ready fredy' is talking about a 426, not a late-model 450 with the revised emission profile on the cams. His original question was about HP differences, and as I said, I think the only real difference is in the torque curve and the weight of the flywheel. Yes, the heavier flywheel will rob I'm sure you know......but the basic block HP #'s should be very similar. If your point is that there is something special with the '06 exhaust cam for smog reasons that can be advanced or retarded to alter HP, then so be it. I didn't read his question that way I quess.....

Both YZs pulled much harder than the WR.

Besides the cam timing, isn't the YZ much lighter than the WR?

Besides the cam timing, isn't the YZ much lighter than the WR?
Depends on the year. Pre-'03 there was about 20 pounds difference. With the '03 model came the electric start, and a reduction in the weight of the YZ models, so the difference went to around 30 pounds. The WR450 is not very much heavier than the WR400/426, interestingly.
'ready fredy' is talking about a 426, not a late-model 450 with the revised emission profile on the cams. His original question was about HP differences, and as I said, I think the only real difference is in the torque curve and the weight of the flywheel. Yes, the heavier flywheel will rob I'm sure you know......but the basic block HP #'s should be very similar. If your point is that there is something special with the '06 exhaust cam for smog reasons that can be advanced or retarded to alter HP, then so be it. I didn't read his question that way I quess.....
Regardless, the '02 YZ426 produces about 46 hp on a dyno in stock trim, and the last sheet I recall seeing on a WR426 was 39.5.

The flywheel did not cause this, and the proof of that is in the WR450 I recently helped a friend with. The bike was an '04 that posted 43hp with a pipe from a YZ, but otherwise stock. His only mod after that was the addition of a YZ exhaust cam and corrected jetting, and the bike went to 48.5hp on the same dyno.

I know another gentleman who recently added a late WR450 flywheel and stator to an '07 YZ450 to power some lights, and he reports no noticeable loss in acceleration with the setup. In fact, the engine pulls harder at very low speeds than it did prior to the mod.

Flywheel weight does not rob horsepower at all. If enough weight is added, it can slow acceleration slightly, but the difference in rotating mass would have to be extremely great to become more obvious than the effect of shifting up two gears. Because most contemporary dynos are inertia types, really big flywheels can sometimes show up as a slight reduction in acceleration, which will be graphed as reduced output torque, and thus lower power figures will be calculated from that. But the truth is that if the reading were to be taken on an older engine brake dyno at a steady state pull, you would find little if any measurable effect on power.

The WR cams were the same profile as the same-year YZF all the way from 1998 to 2005, with the only difference being the exhaust cam timing. Bottom line is this alone makes a 4~6 hp difference.

Back to the original question. Ok if you are talking about a stock yz and stock wr then yeah obviously there is a huge difference in both seat of the pants dyno and a real dyno in power. However I doubt you are asking that. I think you are asking on just the timing alone no matter which bike you have stock or modified right? If that is the case then I can tell you that there is not much seat of the pants feel in power difference from my experience with my wr that is set up like a yz except for the flywheel and gear ratios in the tranny. I went from the stock wr timing to the yz timing with the same mods and setup and could not tell a big difference in overall power. The only real difference that I felt was that the yz timing made the engine rev out quicker which may feel like more power to some but isnt and with the wr timing you can lug more easily. Peak power feels the same to me, the yz timing just gets you there a little faster so it has more of a hit but not much.

The WR cams were the same profile as the same-year YZF all the way from 1998 to 2005, with the only difference being the exhaust cam timing. Bottom line is this alone makes a 4~6 hp difference.

I'm not posting this to argue your experiences but I don't think the cams alone make a 4-6 hp difference. tests on a conservative dyno. Here are their results for an '08 WR 450f and '07 YZ 450f, respectively

Mind you the WR was tested with the stock corked up pipe, stock jetting, and closed airbox and even then it was a only a 5 hp difference. Fix the jetting, put on a decent pipe, and open the airbox and the power on the WR increases exponentially (or so it feels like it).

I've ridden an '08 YZ 450f with a good pipe and while there was a noticable difference in power and response it didn't feel 4-6 hp's worth. Even then thats not taking into account the WR's super wide trans ratio gap. Just sayin'

There are a number of things clouding this issue. One is the true difference between horsepower output and torque vs. what riders commonly perceive as power.

Torque is twisting force. Torque can exist without any motion whatsoever, as with a stalled electric motor, or a torque wrench readout on a bolt that has stopped turning. But a stalled shaft with 100 ft/lbs of torque applied is producing zero horsepower.

Horsepower is a rate at which work is actually being done, and requires something to be moved. It's calculated by comparing the amount of mass that was moved, how far it was moved, and how long it took to do it. The basic definition is the amount of work done to move 33,000 one foot in one minute. Regardless of how long it takes, a force of 33,000 ft/lbs has to be applied, but if that force takes 2 minutes to accomplish the task, it's only half a horsepower. If it moves the foot in 30 seconds, it's 2 HP, even though the force is the same.

In a rotating power assembly like an engine, the force is measured as torque, and hp is calculated as: (Torque x RPM)÷5252

So, when someone makes a remark as was made earlier that the bike doesn't really have less hp, it's just the lower torque curve that makes the bike feel that way, that's a misconception. If engine A produces 30 ft/lb of torque at 6K rpm, that calculates to 34 hp at 6000 rpm. If engine B produces the same 30 ft/lb at 8k, it makes 45 hp at 8000 rpm, an increase of 11 hp with the same torque. So when someone says that all the cam did was make it rev freer, he's misleading himself, because that in itself creates more horsepower.

Another misconception involves what horsepower feels like. Horsepower does not accelerate a vehicle; torque does. Specifically, lots of torque produced across a wide rpm range will make a vehicle accelerate like crazy. Conversely, something like a big diesel that pulls from 1000 to 2500 doesn't accelerate well at all. Note that people say that the 03 YZ450 is monstrously powerful because it accelerates so hard right from 3500 on up, and the '08 is popularly thought of as being soft and sluggish through the mid range. The '03 makes about 47~48 hp, and the '08 produces close to 52.

Horsepower feels like speed, not acceleration, and it shows up as the highest speed that can be produced with the highest gearing the engine will tolerate. Obviously, you can't have horsepower without torque, but neither can you have it without that torque being produced at a high speed.

Later model WR's, which the OP was not asking about, introduce more elements to the comparison. A lot has been learned about using cam profiles to make engines produce power while meeting emissions requirements, and where calling a camshaft a "smog cam" once meant it would yield a gutless toad of an engine, that's no longer true. The earlier WR's smoothed out the rough low speed nature of the YZ partly by simply advancing the exhaust cam, an old and simple trick, but not really an optimal approach. The newer models have cams designed for the application, and give better results, as can be seen with the '07 and '08 models.

And, of course, inertia dynos cannot really be considered measuring instruments. Their best use is in comparing one thing to another on the same dyno, preferably on the same day under identical conditions. The numbers reached can only be called measurements when they can be repeated over and over across multiple dynos. Brake type dynos are more accurate, but more expensive, harder and more time consuming to use, and much harder on engines, since they require the engine to run as much as 20 times longer to produce a meaningful graph.

There are more things different between the YZ and WR engines than cam timing, but cam timing does make a lot of difference in terms of horsepower.

Im getting some very interesting replies here, Some are right and some are off topic, I needed this information to see if this mod is worth doing to a klx 300 im playing with now just for fun, I know the seat of the pants is very different because of the quicker revs but are we talking about a corked up WR or stock against a Yz here. We all know there is quite a bit of power to be found by uncorking these poor bikes from factory set up and those are the numbers im looking for. An uncorked WR against a Yz of the same year. The only problem im looking at is the klx has puney lobes on it compared to the wr which looks to have alot more duration on it.I will only have to change the ACR on the klx to make it work, The number of teeth are the same on the cams since everything is working on 360 degrees this mod will work but im not sure how well yet. Thanks for all of your hard work on this matter. Ive heard alot of people change to Yz timing and ive ridden both bikes and personally i like the wr timming around here in Houston, its all single track around here anyway.

I'm talking about an otherwise stock WR; yes it makes a difference. Whether it will do anything for a KLX 300 is another matter, but know a couple of things as you begin to experiment:

All 4-strokes are based on a 720 degree cycle, not 360. The crank turns twice for each revolution of the cams in ALL four strokes. So, in the case of the YZF, the cam sprockets have 32 teeth. Dividing 360 by 32 gives 11.25 degrees of cam rotation per each tooth. The crank must travel twice that far to move the cam sprocket a single tooth, or 22.5 degrees.

Thus, when changing cam timing one tooth on a YZF, you are moving the valve events on that cam by 22.5 degrees.

Here's an article on the effects of re-centering cam lobes in general:

Im in total agreement here with numbers not being the total focus here as when i ride a Yz timed bike all i notice is alot more wheel spin and less control through corners and TRACTABILITY!!!! But with this KLX there are need alot more help than a Wr to get going. Im just not sure weather i should go ahead and buy the web cams or try this mod that as far as i can tell no one has tried yet, best of all its basiclly a free mod as i see it. I do like to experiment with these things.

I realize this is an old thread, but when this shows up at the top of the list when one googles the subject, it's worth throwing in some concrete data to add some substance to the arguement.

The Kettering University FSAE team is switching from CRF450X to WR450f power for 2012. I've just finished baseline testing of a 2011 WR450f motor on our engine dyno stand before implementing the mandatory intake restrictor and our custom intake and EFI system to cope with it. There is a driveshaft coupling the transmission output sprocket to a Land and Sea snowmobile water brake dyno. The "stock" test was with AIS removed and no air box, filter, or intake tube (but stock cam timing, grey wire, and "pea shooter" exhaust). An adjustable (clip) needle was used in the carb to get it to hold a 13:1 afr at WOT. Not shown on the graph is the same setup but with the "grey wire mod". It has been speculated that the removal of the grey wire disables the ignition controller's retarding of spark past 8000 rpm. I would believe this because the power peak moved from 8200 to 8000 rpm, the peak was maintained 500 rpm longer, and it was making 3 hp more at 9500 rpm where I stopped the test.

The "YZ timing" test features the exhaust cam retarded 22.5 crank degrees (1 cam sprocket tooth), the "grey wire mod", a modified exhaust, and needle clip changes to maintain 13.5:1 afr. After retiming the cam, cranking compression was only 10 psi, down from 50. We took a risk and carefully ground 1mm from the decompression pin. It worked and restored cranking compression to 50 psi--starts at least as well as it did with WR timing. The exhaust used was a Venom TRX450R headpipe (1.625 OD into 1.875 OD) and a Ron Woods TRX450R megaphone/muffler . The tuned length comes out close to 30" and is to thank for the torque peak at 6500rpm.

No, I cannot quantify how much was gained from the exhaust modifications vs. the cam timing, but I know lots of full exhaust system comparison tests exist. With a 67 deg. BBDC EVO and only 20 degrees of overlap, spending big bucks on an exhaust system for a stock WR seems pointless. The same goes for retiming the cam without fixing the exhaust. The two changes are very much complimentary and should be approached holistically.


It's a shame you didn't run the test in isolation from other mods. The exhaust mods are of some value, of course, but even more so when using the YZ timing.

A question I have is whether you've degreed the cam to find out what the actual lobe center is for the retimed WR cam. That would be interesting to compare to the specs of the various YZ450 cams one year to the next.

Keep an eye on the decomp pin and the lifter it runs against for accelerated wear.

matt's coming over tonight with one of the wr heads for me to port.... yamaha has sponsored the team this year with 2 wr450's for formula 1 sae competition...

let me give you guys some dope on cam overlap and such...

for brief period of time at the end of the exhaust stroke / beginning of the intake stroke... both the intake valves and the exhaust valves are open... this is called overlap...

now... during overlap, a bunch of good magic can happen....

if you have sufficient velocity and vacuum in the exhaust system, before the intake stroke even begins (piston still near or at top of bore) ... you can have that vacuum and velocity start to move intake flow before the piston suction does....

chamber velocity is small at this time....this pull is great for "scavenging" the combustion chamber of residual burnt gasses... the exhaust pull combined with the open intake valves makes for fresh charge starting to flow and move the last of the burnt gasses out of the combustion chamber.... it also get a jump start on the momentum / flow of the intake column... also wonderful!

sooooo... intake and exhaust valves open.... velocity and vacuum in exhaust.... all the burnt stuff swept out during scavenging means no bad stuff to dilute the effeciveness of the fresh charge...PLUS more fresh charge to burn via the earlier than piston movement induced flow....all good and no bad (unless you care about emissions)...

the yzf dirt bike cams (2cams , intake and exhaust)are used in the yfz quad and the wr dirt bike... but with a change in timing.... the quad and enduro bike have the exhaust cam moved 1 tooth forward/advanced by one sprocket tooth... 22.5degrees... when you move the cam lobes farther apart from each other, you have less "overlap"...less time when both cams are opening valves simultaneously...

so the "cam mod" is when people retard the exhaust cam by one tooth.... putting the cams to the same timing as the yzf dirt bike...and increasing cam overlap[...

in my testing on yfz's , this has always added power that picked up before midrange and carried to the rev limiter...about 1.5HP through most of the band, just for moving the exhaust cam by one tooth.... that's some easy / cheap power !!

Here you guys go! Pretty cool for a 39mm carb, 12.3:1 c/r, and stock cams, huh? Considering this is running through a transmission (5th gear) and a driveshaft with two tripod housings, 50 rwhp may not be out of the question.

wr450fporting.jpgStay tuned for EFI, a 19mm intake restrictor, and E85 next week.

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