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Valve Clearances

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smegballs
World Chat Champion



Joined: 28 Oct 2007
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PostPosted: 16:55 - 12 Nov 2009    Post subject: Valve Clearances Reply with quote
I just found this
Quote:
One thing we can do to help power with a stock cam is to INCREASE the valves running clearance slightly.

This works because camshafts are designed to run quietly in your road car, and have what are termed quietening ramps on both the opening point and closing point of the camshaft. The problem is that for a good few degrees of engine rotation, the valve is S L O W L Y lifting off the seat, and the same in reverse as it returns. This means that its not sealing so the effective compression, power, induction stroke for example is shortened, while at the same time the valve is not open enough to allow any meaningful gasflow to take place. Setting the valve clearances to the larger end of the allowable range therefore gives more power rather than the expected less power at the possible expense of some valve noise.


Here :https://www.tuning.wanadoo.co.uk/valve-clearances.htm

What do people think of this?

I think this is probably inclined more at cars than bikes but it seemed interesting enough for discussion.... Smile
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the grim reaper
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Joined: 29 Jun 2005
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PostPosted: 17:10 - 12 Nov 2009    Post subject: Reply with quote
Bloody stupid idea if you ask me.

Yes, cams are rounded in profile to save impacting on the followers and you can get more power with high lift cams but to increase the valve clearances just means that the cams impact the followers slightly later in the cam rotation, the valve may open more quickly but the wear on the cams will be higher.

The valves will also open less when at the tip of the cam, so the flow is likely to be impaired by the valve opening less. They state that it is within tolerance but it would also mean that you would have to have the clearances checked more frequently.

Car and bike designers spend a fortune getting engines to work efficiently but also be reliable and, yes, quiet. Why bugger about with it for a few more bhp, possibly. Do it properly and get high lift cams.

Cheers

Grim
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MarJay
But it's British!



Joined: 15 Sep 2003
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PostPosted: 17:35 - 12 Nov 2009    Post subject: Reply with quote
the grim reaper wrote:

Car and bike designers spend a fortune getting engines to work efficiently but also be reliable and, yes, quiet. Why bugger about with it for a few more bhp, possibly. Do it properly and get high lift cams.

Cheers

Grim


Yeah I guess if you've got the cam covers off and you're faffing about with shims... you may as well go the whole hog and put a set of Kent Cams in there or something.
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Pernig
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Joined: 24 Jul 2006
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PostPosted: 17:51 - 12 Nov 2009    Post subject: Reply with quote
The only reason why I would set the valve clearances to be at the higher end of the range would be so I could leave it for longer before replacing the shim again. On all the bikes I've had the clearances get tighter over time.

I have heard of people with C90s winding the tappets out further than maximum clearance. Again, this is just so they can leave it longer before adjusting again, not for any performance gain.
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Old Thread Alert!

The last post was made 16 years, 195 days ago. Instead of replying here, would creating a new thread be more useful?
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