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cam and rocker questions

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   Ok guys my cam and rocker questions: 1; Is it normal for aftermarket cams to have the base circle ground smaller than the stock diameter? 2; Does measuring the lift by subtracting the base circle from the lobe height give an accurate measurement? or,,,, 3; Is the rocker ratio an exact 1 to 1 (or do you get more lift at the valve than the actual lobe height measurement)?

   The reason I ask is because I have a stock and 2 aftermarket XR200 cams of unknown provenance and I'm trying to figure out what they are. The reason I'm asking on this forum (the 230) is because this is when the knowledge is deepest :thumbsup: (no offense to Chuck and a few others in the 200 forum. I'm hoping you'll see this and chime in)

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The rocker ratio is not 1:1 , don’t know off hand what it is but can be figured mathematically. The cam end is much shorter than the valve end from the fulcrum. When you cut the base circle on the cam it creates more valve lift. The lobe itself is smaller, but the valve opens more cause the rocker was tightened up more on the base circle in relation to the centerline.

I don’t know if that makes any sense.

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   I understand how taking (grinding) down the base circle increases the lift. I'm just wondering why do that and weld up and raise the lobe itself. But I've been thinking..... The tappet does not go straight up and down on the valve tip. Rather, since the end of the rocker moves in an arc, the tappet moves back and forth across the tip of the valve. It seems that adding all the extra lift by just welding up the lobe would cause the tappet to move further out toward the edge of the valve. Adding lift by reducing the base circle and raising the lobe would have the effect of keeping the tappet (and thus, valve actuation) centered over the valve tip. Make sense???

Edited by Doogee57
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   So,,, it looks like I'll have to assemble a head and put an indicator on the tappet to measure the actual lift of these cams I have.:cool:

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Many times you get steeper acceleration ramps when the base circle is ground. That is not necessarily good for longevity unless it’s well worked out. Mostly it’s done on some lesser cams where they don’t want to bother to weld them. Web cams are welded. 

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I would be really having a close inspection of those cams. It is usual for the cam people to inscribe their name on them. I have seen a few with it on the timing sprocket flange. 

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10 hours ago, StevetheSnake said:

Many times you get steeper acceleration ramps when the base circle is ground. That is not necessarily good for longevity unless it’s well worked out.

   Yup. That was my concern. Would make for more noise too. And Greenhuman, one of the cams has indeed been inscribed: WB1013. It is both welded and has a smaller base circle I'm guessing it's a White Brothers but I haven't been able to find any info on their cams.

Edited by Doogee57
grammer

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   Oh yeah, who was it that has that beastly XR265? Maybe he has an old White bros catalog.

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11 hours ago, Doogee57 said:

   I understand how taking (grinding) down the base circle increases the lift. I'm just wondering why do that and weld up and raise the lobe itself. But I've been thinking..... The tappet does not go straight up and down on the valve tip. Rather, since the end of the rocker moves in an arc, the tappet moves back and forth across the tip of the valve. It seems that adding all the extra lift by just welding up the lobe would cause the tappet to move further out toward the edge of the valve. Adding lift by reducing the base circle and raising the lobe would have the effect of keeping the tappet (and thus, valve actuation) centered over the valve tip. Make sense???

You are on the right track with all of your ponderings, Doogee...

Reduced base circle / RBC cams are quite common when you have to supply your oem cam for regrinding...

In general, I am not a fan of reground cams... And especially not when they have Reduced Base Circles...

You can make a cam lift larger and duration longer by simply making the base circle (non lobe portion) smaller in diameter...  So the lobe section would now be bigger...

The downside being that a big part of what makes a cam reliable are the very beginning and endings of the lobes..  acceleration and deceleration ramps... In order to reduce shock and slam of a valve train, they lift the valve gently off the seat before real valve acceleration takes place... And then they catch the valve during it's free fall to closing and gently allow it to seat... 

Grinding the base circle smaller either eliminates or diminishes the effectiveness of those ramps... Louder valve train action attests to that immediately....

 

Blah blah blah... Much more

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41 minutes ago, Doogee57 said:

   Yup. That was my concern. Would make for more noise too. And Greenhuman, one of the cams has indeed been inscribed: WB1013. It is both welded and has a smaller base circle I'm guessing it's a White Brothers but I haven't been able to find any info on their cams.

Yes ...White Brothers...

Made for them by Web Cams

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11 hours ago, StevetheSnake said:

Many times you get steeper acceleration ramps when the base circle is ground. That is not necessarily good for longevity unless it’s well worked out. Mostly it’s done on some lesser cams where they don’t want to bother to weld them. Web cams are welded. 

Spot on Steve...

Web also makes a lot of reduced base circle grinds... Even if they are welded...

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5 hours ago, mixxer said:

You are on the right track with all of your ponderings, Doogee...

Reduced base circle / RBC cams are quite common when you have to supply your oem cam for regrinding...

In general, I am not a fan of reground cams... And especially not when they have Reduced Base Circles...

You can make a cam lift larger and duration longer by simply making the base circle (non lobe portion) smaller in diameter...  So the lobe section would now be bigger...

The downside being that a big part of what makes a cam reliable are the very beginning and endings of the lobes..  acceleration and deceleration ramps... In order to reduce shock and slam of a valve train, they lift the valve gently off the seat before real valve acceleration takes place... And then they catch the valve during it's free fall to closing and gently allow it to seat... 

Grinding the base circle smaller either eliminates or diminishes the effectiveness of those ramps... Louder valve train action attests to that immediately....

 

Blah blah blah... Much more

And this is why many complain about the noise from their cam. But the good is the valve is open longer at mid lift with steeper ramps and same open/close timing events. Thought someone on here checked once and posted up what the rocker ratio was but I forget. I know someone is working on theirs and could easily check!

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It’s damn close to 1:1. Pulled one of the old ones out of the parts box and measured with a quality antique caliper. No MM works in good ole standard measurements but was made in Japan.IMG_2097.JPG
It came out to 1.12:1 ratio. That kinda sounds like 1:1 to me.

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