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I had to pull the engine apart to fix a shelled out transmission. Then I got to doing a bunch of nerdy math to see if I could figure out how jacked up the cam timing would get if I decked the cylinders to get the compression bumped up a bit, and would I really need adjustable sprockets.

To get to the point: Counting teeth, I'm getting a 140 degree shift when moving the intake over to the exhaust (14 teeth total, or 6 teeth left of the 1f to draw a 2). So if the APE site's cam specs are accurate (33 BTDC, 66 ABDC, 279 dur) then I'm getting a lobe center of 113.5 on the 2nd gen intake on the exhaust side. That seems like a wonky number.

What am I missing?

Side note: If I'm thinking about this correctly, the cam timing will go in opposite directions when decking cylinders/milling heads since the cam tensioners are on opposite sides of the motor.
 

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It doesn't matter where the chain tensioners are...taking material off of the cylinder puts the cams closer to the crank so their timing will be advanced.

Picture holding the cam and assume that it is in perfect timing with the crank...then slowly drop the cam down rolling along the chain...it moves forward so without moving the crank the cam is now advancing. Advancing the intake timing means the valves will close sooner which can boost effective compression and often will boost the midrange torque, and if you don't go overboard with it the top end power shouldn't be affected. But this will depend on the exhaust system tuning so be aware that ANY changes will affect the system and be prepared to alter other things to compensate.
 

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It doesn't matter where the chain tensioners are...taking material off of the cylinder puts the cams closer to the crank so their timing will be advanced.

Picture holding the cam and assume that it is in perfect timing with the crank...then slowly drop the cam down rolling along the chain...it moves forward so without moving the crank the cam is now advancing. Advancing the intake timing means the valves will close sooner which can boost effective compression and often will boost the midrange torque, and if you don't go overboard with it the top end power shouldn't be affected. But this will depend on the exhaust system tuning so be aware that ANY changes will affect the system and be prepared to alter other things to compensate.
Timing gets retarded
 

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i think its something like every .010 off head is .5 degrees on cam.
 

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Timing gets retarded
Yes...you are right.:( ( It was really early...shouldn't try to think before lunch). The tensioner WILL determine what happens...it's on the slack side while the other side pulls, so shortening the drive side will retard it. Sorry....
 

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One degree cam timing change is roughly = pi * cam sprocket dia /360.

If cam sprocket diameter is 2.5" then 1 deg = 0.022".

The angle the cam chain makes with the head has some affect but is small enough to ignore.
 

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One degree cam timing change is roughly pi * cam sprocket dia /360.

If cam sprocket diameter is 2.5" then 1 deg = 0.022".

The angle the cam chain makes with the head has some affect but is small enough to ignore.
That formula includes 2 crank rotations per one cam rotation?
Like cam has 36 teeth but each one is 20º.
 

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That is degrees of cam rotation relative to milling the head.

Two degrees crank rotation per one degree for the cam.

One degree of crank rotation would be 0.011".
 

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Nothing gets past you, tovar. :)
 

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I had to pull the engine apart to fix a shelled out transmission. Then I got to doing a bunch of nerdy math to see if I could figure out how jacked up the cam timing would get if I decked the cylinders to get the compression bumped up a bit, and would I really need adjustable sprockets.

To get to the point: Counting teeth, I'm getting a 140 degree shift when moving the intake over to the exhaust (14 teeth total, or 6 teeth left of the 1f to draw a 2). So if the APE site's cam specs are accurate (33 BTDC, 66 ABDC, 279 dur) then I'm getting a lobe center of 113.5 on the 2nd gen intake on the exhaust side. That seems like a wonky number.

What am I missing?

Side note: If I'm thinking about this correctly, the cam timing will go in opposite directions when decking cylinders/milling heads since the cam tensioners are on opposite sides of the motor.

Me, formulas and school did not get along, I like to use measuring tools. :)
How did you calculate 7º difference in lobes and sprockets relation between intake and exhaust cam?
 

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Discussion Starter #13
New Ex Open BBDC= 180-(-33+140)
New Ex Close ATDC= 66+140-180

I based that on a sketch of the valve events overlay-ed on crank rotation (attached) and just rotated the intake events around the circle by 140 degrees. Which now that I think about it, isn't correct. The cam is spinning half crank speed.

I'm not sure what 7 degrees you're referring to, but I'm clearly missing it. Unless you're doubling the lobe center difference between the gen 2 intake and exhaust......


Ok, so lots of faulty logic on my part.... sigh, back to the drawing board.


I also went and compared the 3rd gen intake to the 2nd gen, and I'm not seeing a full tooth timing difference between them, even with a very crappy eye balling it with a dowel held on the cam lobe peak. I'm not sure I trust the APE specs in any event.



So, I ordered adjustable sprockets. Some of the parts I wanted to put in while the cases are split are back-ordered. Figured I might as well just go ahead and do the rest while I'm at it....

Would 105/105 be a reasonable starting place for a 3/2 gen cam setup?

Thanks everyone for entertaining my rambling mental exercise.
 

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Discussion Starter #15
When you moved cam you changed numbers by doing so. You can't just add 140º.
Yep, I realized my mistake as I was typing up what I did. I was tired, it was late, my dog ate my homework.

There has to be a way to calculate it, but my brain is fried for now.
 

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Discussion Starter #16
Well whatever it was, the cams are now 104/105, squish is .040, and the bike ran well today. Definitely moved things up the rpm range
 
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