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How to decide on valve clearances if you dont want to use factory specs?

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Next random redneck question from me is as the subject says - how do you decide on valve clearances? I'm thinking of being dodgy and closing them up slightly over factory factory specs by 0.01mm - 0.02mm (from from 0.08mm to 0.07mm and from 0.10mm to 0.09mm) as I fluffed up my first valve lash settings and hooo boy the car went way too well for me to think it was safe so i fixed it to proper (but minimum) factory specs - but like any car person on here - more powah is good so i started thinking about it again :D ("free cams w00h00" sorta thinking)

I'm guessing from factory they would be setup so that if you sit on the autobahn at max revs in top gear you wont melt a valve due to it not getting enough seat time and that you wont get a valve stuck open due to it expanding and then not closing fully.

What happens to a valve if it expands and doesnt close fully - will it recover at all and the valve wont hang open or will it do damage? How might it take for the valve to hang open?

What happens if a valve doesn't get enough seat time - will it melt out - how fast would it melt out - yeah - it would have damage but would the damage affect the pistons / head at all?

or - is it just not a good idea at all whatsoever even in the slightest to close up the gaps over factory? (factory head, cams, springs and retainers)

if the engine makes a difference - its a jdm k20a with no "internal" mods - a "street" car not drag or track car - but it gets its rev limiter tested in 1-4th sometimes and has done a top speed run 2 or 3 times - 8500 revcut

Would it be worth fiddling with it or would it just be a potentially expensive experiment with spectacular (but bad) results?

You may wish to check up on some basic camshaft design features, two in particular.

The first goes by several terms, depending on country and application - such as takeup, clearance, quietening, etc. ramps. They are used to reduce the sudden shock loads that would otherwise occur when the mechanical clearance is taken up suddenly.

The second thing is effective duration, which is an indication of where the camshaft opens the valve enough to significantly affect gas flow past the valve. IIRC the Americans use 0.050" valve lift for this and the Europeans use 1mm which, if correct, means they can't be directly related.

By reducing the valve clearance you're increasing the effective duration slightly, which is where the extra performance you noticed comes in. Back in the day, adjusting the valve clearance, smaller or larger, would indicate whether the engine was under, or over, cam'ed. In your case it indicates it's under.

IMO, you really should stick to the correct clearances, especially when they're on the smaller side, to avoid potential issues such as you mentioned.

Hey - thanks for that extra info and advice :)

I'll keep the standard clearances and look at a set of drop in cams for the engine instead. No redneck "free cam upgrade" path for me after that bit about shock loading and takeup etc - I dont know enough about the consequences so its not worth the experiment after you added in the extra risk factors :)

Thanks and cheers!

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