yz cam swap, hotcam update

I've been playing with the cam swap on my 08 wr450 for a year now. I never felt the 08 pulled like it should and went flat way too early compared to my 06, even after all the mods including a dr.d pipe and jd kit. It never started great warm either, despite a lot of adjustments with pilot and fuel screw, and never in gear. I tried an 08 yz intake cam only mod first because of the e-start problem with the yz exhaust,. It lost a lot of bottom but gained in the mid and upper. I swapped that cam with an 06 yz intake and got some of my bottom back and still had nice gains everywere else. I ran this setup for about a year and was fairly satisfied but I knew I could be better. Starting was about the same. About a month ago I swapped the exhaust cam out for a hotcam and put the 08 yz intake back in. AWESOME!!!!!! I wanted that smooth power delivery of the 08, the 06 cam has a surge in power that can be a hand full in single track. This is how these bikes are supposed to run. I have torque off the bottom without to much, smooth power delivery, and perfect mid and top end power. I can crack it and wheelie over any obsticle, where before once the rpm's rose it semed to loose torque and the ability to wheelie. I was worried the yz setup would be too much in the singe track but it's the opposite. I can lug 2nd down low and still have torque without too much power, if that makes any sense. She always starts in gear now and instantly. I couldn't be more pleased. I love this bike. The starting problem is definately the pin length on the decompression system. the hotcam measured 23.00mm and the stock wr cam measured 23.73mm. I was hard to be accurate measuring 2 rounded surfaces but thats what I got. If the yz cam pin Is the same or longer than the wr then I could see why the yz exhaust cam creates an e-start problem. When I first did the swap I left the wr jetting just to see and had a bog which I almost interpreted as having lost a lot of bottom. Then I changed to the JD yz jetting and Bamm.....she was awesome. Here's my settings before and after.

wr cams: 168m, red-4, 45p, 1 1/8 fuelscrew, 50 leak

yz cams: 170m, red-5, 45p, 1 7/8 fuel screw, 55 leak

The starting problem is definately the pin length on the decompression system. the hotcam measured 23.00mm and the stock wr cam measured 23.73mm.

Awesome! Thanks for doing real measurements on this for the community. When I ground the decomp pin on our WR exhaust cam to run it 1 tooth retarded, all I had to go on was "about 1mm shorter" and trial and error with cranking compression. The HC exhaust profile isn't the same as the YZ and thus the pin lengths shouldn't be the same, but it's good to have concrete numbers.

Yamaha advanced the exhaust cam 22.5 degrees on the WR and YFZ to knock valve overlap down to about 20 deg. to better cope with a restrictive exhaust. The exhaust valves open around 67 BBDC for a more blowdown time into the restrictive system and close around 3 ATDC so the high pressure difference between the intake and exhaust tracts doesn't fill the cylinder with residuals. When you fix the problem that is the stock exhaust, the long blowdown time is no longer necessary and you can take advantage of exhaust scavenging with increased overlap.

Here's the difference going from the stock exhaust, cam timing, and jetting to a retimed (1t retarded) exhaust cam, good exhaust, and rejetting made on our dyno:


Thanks for the technical data to go with my seat of the pants evaluation :thumbsup: . It's nice to see a dyno comparision and really see the differance on paper. The wr is really held back with those stock cams.

Wow. So, anyone in WA want to help me do mine??

It's really quite easy. Check out this detailed how-to: http://www.yfzcentral.com/forum/32-yfz-powertrain/46023-yfz450-cam-mod.html

If you kick your bike, you should be fine following the exact steps listed in the link. If you depend on electric start, you'll need to remove some material from the decompressor pin--between .75 and 1 mm. With no material removal from the decomp pin, our motor only had 10 psi of cold cranking compression and wouldn't fire. It was up to 50 psi with 1mm removed and starts as it did stock.

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