Camshafts and Valve timing

[David R]

This is so we don't step on any one else thread.

 

I see discussed here about setting valve clearance other than what is in the spec.

 

I have a few thoughts.

 

By going a few thousandths more, this will make the duration less, opening later and closing earlier both intake and exhaust. What is gained?

 

Those aftermarket sprockets sold to move the timing ahead nine degrees are supposed to take a little off the top and add it to the middle of the power band.

 

Just for information, I have an RT cam on the right side of my bike and a GS cam on the left side because at the time its all I could find and have shipped for a trip I was going on.

 

The RT cams open the intake at 5* before. The GS cam opens at 1* after. That is a 6* difference.

 

The exhaust cam opens at 5* before and the GS cam opens at 9* After. I read this in the service manual I bought for MY bike.

 

I can't tell the difference when riding the bike before or after I changed the left side cam.

 

Gotta go to work.

David

[dirtrider]

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By going a few thousandths more, this will make the duration less, opening later and closing earlier both intake and exhaust. What is gained?

 

-

 

Morning David

 

Not much is gained by trying to change the cam timing that way with just a few thousandths of lash change. You would probably need in the range of .015"-.030" on the intake valve timing to make a difference you could feel. That just isn't practical on the BMW boxer.

Some race classes demand stock cams or hydraulic cams to meet class rules. Some of the racers have found ways to alter valve lash to get a bit more out of the rule required camshafts. That usually involves a large lash difference like .020"-.030+" over spec. Very hard on the valve gear but it only has to last one or two races.

 

 

 

What people don't realize is that the EFFECTIVE air flow through the valve doesn't become useful until the valve is off it's seat a ways (how far on the BMW boxer I don't know) by that time the valve lifter is on the fast lift part of the cam lobe so a couple of thousandths lash difference makes little difference.

 

There can be a noticeable gain on the BMW boxer midrange with enough valve timing change with things like Lennie's sprockets or home made sprocket re-keying but as you mentioned that is 9° on the Lennie's. I did a cam sprocket re-key on an older oilhead I owned (not a full 9° though & it made a difference I could feel in the low to mid range.

 

Way more ridability gain to be had in the mid range by adding more (richening) the fuel than diddling with the valve lash at a couple of thousandths.