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-   -   Cam Swap Timing Question (https://www.svrider.com/forum/showthread.php?t=395593)

busdriver 10-16-2019 11:46 PM

Cam Swap Timing Question
 
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.

RecoilRob 10-18-2019 03:19 AM

Re: Cam Swap Timing Question
 
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.

tovar 10-18-2019 02:49 PM

Re: Cam Swap Timing Question
 
Quote:

Originally Posted by RecoilRob (Post 5347921)
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

swiest152 10-18-2019 02:59 PM

Re: Cam Swap Timing Question
 
i think its something like every .010 off head is .5 degrees on cam.

tovar 10-18-2019 03:19 PM

Re: Cam Swap Timing Question
 
Something like that, closer to 1 degree for 10 thou.

RecoilRob 10-18-2019 03:44 PM

Re: Cam Swap Timing Question
 
Quote:

Originally Posted by tovar (Post 5347933)
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....

TeeRiver 10-18-2019 05:31 PM

Re: Cam Swap Timing Question
 
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.

tovar 10-18-2019 05:37 PM

Re: Cam Swap Timing Question
 
Quote:

Originally Posted by TeeRiver (Post 5347945)
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 20.

TeeRiver 10-18-2019 05:49 PM

Re: Cam Swap Timing Question
 
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".

tovar 10-18-2019 07:43 PM

Re: Cam Swap Timing Question
 
So 10 thou is about 1 since crank rotation is measured.


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