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Old 10-18-2019, 08:06 PM   #11
TeeRiver
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Re: Cam Swap Timing Question

Nothing gets past you, tovar.
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Old 10-18-2019, 08:36 PM   #12
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Re: Cam Swap Timing Question

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Originally Posted by busdriver View Post
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 7 difference in lobes and sprockets relation between intake and exhaust cam?
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Old 10-18-2019, 10:22 PM   #13
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Re: Cam Swap Timing Question

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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Old 10-19-2019, 12:48 AM   #14
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Re: Cam Swap Timing Question

When you moved cam you changed numbers by doing so. You can't just add 140.
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Old 10-19-2019, 01:16 AM   #15
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Re: Cam Swap Timing Question

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Originally Posted by tovar View Post
When you moved cam you changed numbers by doing so. You can't just add 140.
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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Old 11-09-2019, 09:23 PM   #16
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Re: Cam Swap Timing Question

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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