The technical difference between WR and YZ exhaust timing

Can someone please explain the technical difference between the exhaust timing on the WR400 and the YZ? I thought the YZ timing was supposed to give better all around throttle response and slightly more power, but today I read on a thread that the WR timing is supposed to give better low-end. Is this the case? What throttle ranges is each setup better for? Thanks

The stock setting for the exhaust cam on the WR is ADVANCED 1 cam tooth (~23 crank angle degrees) in comparison to a YZF. It was done to balance the restrictive stock exhaust not to increase the low end, despite what some well meaning folks may tell you. If you think about it, opening the exhaust valve that early will shorten the expansion/power stroke by 23 degrees and REDUCE torque. The 23 degree reduction in overlap helps low speed carburation some, but the balance with the stock WR timing is a reduction in torque. What most people preceive as better low end is really just a flattening of the high spots in the torque curve and less chance of wheel spin. Too much torque at the wrong time is usually much worse than not enough if traction is limited


Back to the WR timing and the restrictive stock exhaust. I have found only one solution to a restrictive exhaust port and that is more time i.e. exhaust cam duration. It has proven to help the low end as well as the top. In the case of a restrictive exhaust system, there is no cure. When the system backs up, it's all over. All you can do is increase the efficiency up to that RPM or as a band-aid fix you start the blowdown extra early by opening the exhaust valve much earlier than normal to try and minimize the pumping losses associated with an overly restrictive exhaust system.

I realize that cam numbers seem like a black art if you're not used to seeing them. Just for fun I ran a simulation with these numbers in 1998 when I got my first YZF so we could see what would happen to the torque & power

numbers. Keep in mind this is a very simple

simulation but the trends are still valid for the purposes of discussion.

Simulation Data Report: WR400F.SIM

Rpm Torq (FtLbs) Power (HP)

----- ------------ ----------

2000 22.1 08.4

2500 23.5 11.2

3000 25.0 14.3

3500 29.0 19.3

4000 28.7 21.9

4500 29.2 25.0

5000 29.6 28.2

5500 29.9 31.3

6000 30.7 35.1

6500 30.8 38.2

7000 30.3 40.3

7500 29.0 41.4

8000 27.8 42.4

8500 26.5 42.9

9000 25.3 43.3

9500 23.5 42.5

10000 22.5 42.8

10500 21.1 42.2

11000 19.2 40.1

Simulation Data Report: YZ400F.SIM

Rpm Torq (FtLbs) Power (HP)

----- ------------ ----------

2000 26.4 10.1

2500 27.7 13.2

3000 28.8 16.4

3500 31.0 20.7

4000 30.8 23.5

4500 31.7 27.2

5000 32.0 30.5

5500 32.9 34.5

6000 33.3 38.1

6500 33.4 41.3

7000 32.8 43.7

7500 31.1 44.4

8000 30.6 46.6

8500 29.4 47.5

9000 27.4 46.9

9500 25.7 46.6

10000 24.1 45.9

10500 22.2 44.5

11000 20.1 42.1

Even though this is a simple simulation you can see that the WR kills the bottom end, and basically screws up the powerband across the board. Real world dyno numbers reflect similar trends.

I hope this helps some. :thumbsup:

obviously, rich is very knowledgeable in the technical differences in cam timing. i am not. however, i have changed from YZ timing to WR timing, and both make good power. its more about the intented use of the bike.

YZ timing is snappy and really just wants to be revved out, but feels paper thin at idle (to me). WR timing feels stronger at idle and low rpm, and its perfect for when your lugging the engine all day. no hit.

YZ timing is perfect for MX. lots of wheelspin.

WR timing is much better for trails. very little wheelspin.

i always thought that altering cam timing changed the way the power is delivered, not *how* much power is delivered. :thumbsup:

Exactly what Rich said :thumbsup:

I like to think of it as the YZ being timed correctly, and the WR being 'detuned from factory'

Most people with significant engine building experience (i mean real engine building, not dealership style 'line up the dots' assembly) will tell you that 110/120 cam timing isnt ideal


Thanks a bunch Rich! :thumbsup: That proves what I was thinking, that the YZ timing gives more power, but the WR spec gives more "useable" power to the average trail rider.

Rich, that was an awesome post! It's the only one I've come across that makes good sense.

Just to add, I'm under the impression the ACV (automatic coast vavle?) is another thing the WR has due to the restrictive muffler.

I think the ACV is more to keep the engine from popping on decel after high rpms, makes it more "environmentally friendly" or some crap like that. I don't think it is on there because of the super restricted (quiet) exhaust , I think it compliments the exhaust to make the bike "less annoying" to environmantalists.

Just to add, I'm under the impression the ACV (automatic coast vavle?) is another thing the WR has due to the restrictive muffler.

Air Cutoff Valve....

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