Cam Timing

[M. Hausknecht]

Well, my last post didn't provoke a whole bunch of discussion, perhaps this will get a response or two.

I've picked up a used stock engine to use while my race engine is getting its cylinder repaired and replated. I've pulled the head and other than the piston crowns and combustion chambers needing a good cleaning, all looks well. While I'm waiting for my head gaskets to arrive, I'm going to do a little porting to match my bored throttlebodies and exhaust, and I've thought about timing ("degreeing") the stock cams. Has anyone done the latter, either with the  stock cams or with aftermarket cams? If so, did you slot both cam gears or just the intake? Care to share your cam timing and the results you noted?

Edited May 28, 2021 by M. Hausknecht

[Pursuvant]

Following...

In the olden tymes, universal japanese motorcycles (UJM, in-line air cooled 4 cylinder) poor folks (and rich too I think) would weld up the surface of the cams lobes to add material and then cut new lobes. And you could buy "slotted" cam gears from folks like Yoshimura. Age & meds are wiping away my thoughts. Speed shop really was a thing, they had a lot of work to do to be creative. I suppose I'm just saying what lots of people already know, but age tells me to share it b4 there is nothing left to share.

This will be fun, careful, don't bend anything expensive or poke holes where they don't belong

Edited May 28, 2021 by Pursuvant
and plz, lots of fun pics

[Triple Jim]

10 minutes ago, Pursuvant said:

I suppose I'm just saying what lots of people already know, but age tells me to share it b4 there is nothing left to share.

Was that back when it was the style to wear an onion on your belt? 

Back then my only motorcycle was my H2, so no cam(s), but once I did have more lift ground into the cam of a reverse rotation 289 Ford for my boat.  No welding was needed... the shop just took the added lobe height off the base circle.

Edited May 28, 2021 by Triple Jim

[AP996]

I slotted both cam wheels on my engine, I took 14 thou of the head so my timing would have been slightly retarded if I hadn’t, I don’t have my notes to hand right now but I’m pretty sure I set the lobe centres at 110 degrees. If your racing and more interested in top end power it might be worth trying retarding the inlet cam a bit.

14 thou was the most I was comfortable with taking off the head and maintaining sufficient valve clearance with the modified inlet cam I have.

Hope that’s some help

[klx678]

I'd say do some reading on cam lobe centerline angle, which involves the valve overlap, learn what would work best for the 700, then see what is needed to get that overlap (LCA).  If Yamaha did the timing for performance you may already be not just in the ball park, but in the infield to use baseball terms.   If you really want to get ultra-precise the slotted sprockets will do the trick allowing precision timing of each cam.  

If Yamaha is too conservative or radical you may need to do some timing tricks, but if they are close, the gain may not be worth the effort.

I have a KLX250 and another rider in Europe did some research and found the KLX had very conservative overlap, having an LCA of 110°, better power should be able to be had at more like 100° with the lift and duration of the stock cams.  We lucked out, because the double drilled cam sprockets with EX and IN hole sets provided a way to alter timing by 10° by simply switching the holes, intake bolts on in the EX holes and the exhaust bolts on in the IN holes, then moving the cams 2 teeth on the sprockets to get the cams at the 100° LCA.  The bike does use symmetric cam timing, exhaust and intake are at similar degrees from TDC, intake advanced exhaust retarded.  Perfect, doing the change actually added 5 hp to the 20 the 250 had.  Well worth the work, free power.   No slotting of the sprockets needed.   

Here is the article that gave me a better insight if you don't already know a bit about the cam timing and LCA    Camshaft Shootout: Lobe-Separation Angle Tested and Explained -  Hot Rod  2/21/2017  I will say it may seem weird doing car thought with motorcycles,  but from what I've read over the years the generalities apply to any ic engine.  After all, a small block Chevy has about 750cc/cylinder making it about a 1500cc twin.  I know there's a lot more involved and no doubt you do know far more than I do, I just figured maybe throwing in a bit of what I'd seen would help.

Edited May 29, 2021 by klx678

[M. Hausknecht]

Thanks guys. My Webcams call for 110 degree lobe centers. They are the "smaller" of the two camshaft sets offered generally by Webcam, and allow use of uncut pistons and stock valve springs. Interestingly enough, the engine builder (not me and not under my direction) didn't slot the cams when installing the Webcams, so their timing is stock; whatever that is. Anyone know? I can check when I get the head installed again. 

My thinking is to move the power curve up a bit to take advantage of the bored throttlebodies, Hord airbox, and Yosh exhaust by retarding the intake cam; as suggested by AP996. I can leave the exhaust cam where it is, resulting in more overlap, or dodo it as well to maintain the current overlap. 

The article suggested by klx678 is excellent, by the way; thank you. I've done plenty of reading but since I don't have ready access to a dyno, and in any event the stock cams are a short-term situation, my hope is to get some benefit rather than maximum benefit based on the actual efforts of others with the CP2.

[klx678]

My link to the cam article didn't work, I fixed it, but here it is again   click here

I will say you won't change the rpm for which the cams were designed, by any significant amount.   That is noticeable in the article, that the hp peak rpm is within a couple hundred rpm at best.   Whatever the peak rpm for your web cams might be, it will end up there pretty much regardless of your retiming.  What will change will be the character of the power curve.  As shown again in the article and what I experienced with the 250, the narrower LCA, will likely be better power delivery in the mid-range up near the peak.  

I will say if web says 110° then it should be 110°, probably drilled to make it so with the stock cam sprockets.  If you really want to know if going for the narrower LCA will help I'd say contact Web Cams.  They know their product and should know what the change may do to the power delivery.

[M. Hausknecht]

I'm going to leave the Webcams timed as recommended, although that may require slotting the cam sprockets.

Its the stock cams I want to change to take advantage of more available flow, and no need for power below 4500 rpm and no concern for emissions. Incidentally, the '08 and on R6 sprockets are the identical to the 07 intake cam sprocket, and are cheaper slotted from APE than new from Yamaha. The decompression stuff on the 07 requires a unique sprocket gear I'll have to slot myself.