Cam timing question

[Beekeeper]

Got a dumb cam timing question. When a cam is advanced four degrees, is it two degrees at the cam and four at the crank or four at the cam and eight at crank?

[Beekeeper]

Ok, I'll try to ask it another way. I degreed my cam straight up using a degree wheel and verified all opening and closing points matched the cam card exactly. I am now advancing it 4 degrees (to shift the power band lower) using the three way crank sprocket but I'd really like to use the degree wheel again to verify I got it right and to verify the crank sprocket was cut correctly.

When installed straight up, the intake opens 4 degrees before top dead center (checked at .050 lift). When I advance the cam and check it again, the intake now opens 12 degrees before top dead center. Since it is opening 8 degrees sooner, I assume this is correct because I'm advancing the cam 4 degrees and the crank moves twice as much in relation to the cam.

Am I right in my thinking or is there something wrong here? Never advanced a cam before and I just assumed that when you advance it 4 degrees, it is only 2 at the cam and measures out at 4 when checked at the crank. If my readings are correct, then I guess the cam really is advanced 4 degrees and moves the degree wheel 8 degrees because it's attached to the crank.

The markings on the crank sprocket are very unclear so I'd rather be certain I got it right rather than guess.

[lulurocks]

All of your degree reading are degres btdc of atdc.

The degree readings are taken from the wheel at the crank, so there  is no math 112 is 112 . You added 8 not 4.


So  is a 2:1 ratio   If you add a 2 degree ofset key at the cam ,your double at the degree wheel(advance/retard, depending which direction you add the offset key.

So you had 4* btdc  and now you have twelve you  want to be at 8 * if you want 4* advance of  cam card specs.

Again this is my opinion do more research on your own or consult your machinist.

This is a good read :
http://www.lunatipower.com/Tech/Cams/HowToDegreeACam.aspx

[734406pk]

  If you want to advance your cam timing spec +4 degrees so the cam comes in a bit lower in the rpm range, that would be a +2 degree advance of the CAM to see a +4 degree advance on the Crankshaft. Visualize that you are measuring valve timing only by crankshaft degrees, so readings will correspond to the cam timing card. Advancing the cam 8 degrees is a major change and likely too much IMO. Don't forget to measure your piston to valve clearance after you have set the cam where you want it. Good Luck! 

[Beekeeper]

Thanks guys. Part of my confusion is that the timing chain set has additional key ways cut into the crank sprocket. One for straight up, one for 4 advanced and one for 4 retarded. It doesn't say if it's 4 degrees at the crank or at the cam.

I have found endless info. On the Internet about degreeing cams but I can not find anything about checking things again after you advance the cam and what you should see when you do check it. The cam card gives valve opening/ closing specs only when straight up so it gets confusing checking stuff outside of recommended specs.

the cam card they provided is actually a sheet of paper showing specs for dozens of their other cams. Several of their milder cams have opening points at the same spot as mine after I advanced the cam so I can't be that far off (if at all). In looking at all the different cams they have, my new modified specs fall well within normal range. I have degreed this damn thing many times now trying all possible variations so I have come to the conclusion that every 1 degree I advance the cam will cause the valves to open 2 degrees sooner. I even checked it retarded to see if the crank keyways were cut consistently. So when the valves open 8 degrees sooner, the cam must be advanced by 4. It makes sense since the crank moves twice as much in relation to the cam.

I will definitely be checking piston to valve clearances!

[734406pk]

  Good Luck!