Cam Timing

Just about the only bit about motor building that I was apprehensive about is cam timing. So far my duties have been to clean, reinstall and torque things – which is all very basic, and this is coming from a guy who wasn’t raised doing this stuff. But cam timing is pretty damn critical and requires some higher thinking, especially if you’re not using a stock cam. This isn’t to scare anyone, or to make me sound like a genius for figuring it out – in the end I think it took me 4 or 5 days spread out over 2 weeks. Which was good, because after a day or two of screwing around I was able to really sit on the concept and I think that helped me come to a conclusion in the end. And in the end I also realized that this isn’t rocket science either.

The manual is a good place to start with this process, at least that’s where I started. First Is lining up the flywheel to the crank, there is an arrow engraved on the side of the flywheel which needs to be lined up with a specific case nub [about 11 o’clock when facing the rear]. This is the position for left TDC which is the cylinder I focused on to set my cam. So line up the flywheel to the crank and stick your degree wheel on there too, line up 0º with both of these marks.

Degree wheels can be had here for free: http://machinerycleanery.com/DWUniversal.htm
Download, print and glue to a cereal box. DONE.

I used rolled duct tape in several locations to insure the degree wheel wouldn’t slip – this worked very well for me. Next, I used some solder to create pointers to more accurately get a reading. I installed one at 0º TDC. I stuck another one at an arbitrary location, so that at any point I could check that neither of my pointers had slipped.

Took some sharpies and went to work marking up my degree wheel to make things a ton easier on myself.

These marks are for the b10 cam. In case your eye sight is terrible they are:

Intake opens – 29º BTDC
Intake closes – 60º ABDC
Exhaust opens – 58º BBDC
Exhaust closes – 31º ATDC

I set the valve clearance to an arbitrary amount [.032″] and started recording when the exhaust valve was opening and closing, using a piece of cellophane pinched between the rocker and the valve stem. When the valve was opening, I recorded the value exactly when the cellophane was pinched between the rocker and valve stem – when the valve was closing, exactly when the cellophane was freed. Great, well now I had numbers that didn’t match what I was supposed to get, and I really didn’t know where to go.

Eventually, I learned that because there is no spec. valve checking clearance for the b10 it’d be a miracle if your numbers matched up anyhow. So, what you are looking for is opening and closing degrees, that are equally apart from the spec given for the cam. If your valve opens 5º sooner than spec, you want it to close 5º after spec. If it opens 15º before spec, you want it to close 15º after spec. Dig? That took me awhile to figure out. But it’s that easy. The discrepancy between your values and spec are do to what you arbitrarily set your checking clearance at.

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So while I had this method going on for the exhaust valve as prescribed in the manual, I tried a different method on the intake valve. Using the lobe center method, gave me another chance to use a dial gauge which I find is one of my favorite tools. Love it.

First, I guess before we play with tools – we should calculate the lobe center of the intake valve, that is the exact point where the lifter is sitting highest in it’s travel on the lobe of the cam – the equivalent of TDC for a camshaft. Here is a website that will do the math for you http://www.rbracing-rsr.com/camshaft.html. And another website where I learned myself on the subject http://www.muzzys.com/articles/lobe_centers.html. This will give you the number to shoot for, for me and my b10 I found the intake LC should occur at 105.5º ATDC.

Now let’s play.

To secure the dial gauge, I just bolted on a piece of steel I had laying around to the head. Fixed the gauge straight on to the pushrod of the intake, and zeroed it when the rod was at the base of it’s travel. With me so far? Now the valves dwell at the top, so you can’t just spin the cam until your dial gauge reads it’s highest value and declare that your LC, since it will sit at the highest value for a few degrees. So it’s necessary to take a reading X” before the top, and X” after the top and average the two. For example, if your highest reading on your gauge is .283″ – record the degree when .183″ occurs on the opening ramp, and when .183″ occurs on the closing ramp and average the two readings to find the true LC.

My first attempt, I found my LC to be occurring at 111.5º ATDC. I grew to really like this method because it was easier for me to see the difference between my values and spec and formulate a conclusion. In this case, my experimental value of my intake LC of 111.5º is obviously 6º retarded from the 105.5º calculated earlier. So i knew I had to rotate my cam 6º in relation to the crank. Dig?

Maybe perfection is possible, I was able to have my intake LC occur at 106º ATDC, .5º off from spec.

*Special thanks to Greg Bender and Dick and Bobdar from the wildguzzi forum. Couldn’t have done it without you!

~ by rajandhisguzzi on January 30, 2010.