Changing CAM duration by valve clearance.

[Trev]

Hi

I have a cam with a duration of 272 In and 282 ex @50

I set the valves at 22 thou intake and 24 thou exhaust.

Would dropping the valve clearance down to say 18 and 20 thou give me much more duration.

Anyone have any negatives about doing this

Trev

[DavidHarsay]

...slightly more duration... slightly more lift... slightly less dynamic compression... really depends on just about everything in your combo if this is actually going to gain or loose power. As long as the lash is within the proper range specified by the manufacturer, there is no danger or drawback... too tight relative to the specified range, and valve will warp as it can't dissipate the heat... once it's expanded from the heat.

[la360]

I was under the impression that the lash amount was relative to the lobes openning ramp design, and how it through the valvetrain accelerates the valve off the seat? Or am I was off here?
AL...

[ALKYBURNER]

Running valve lash to loose allows the roller or flat lifter to bounce off the cam when ramping upand down beacuse there is nothing to keep it in tight contact with cam lobes you need to know the high and low side of lash for your cam and engine combination just call and talk to your cam guy they willbw able to give you these numbers.

[Lasher...]

When it comes to lash settings it really pays to talk to the company who ground the cam in question.Unfortunetly not every cam designer goes about doing this the same way.

For example [most] but not all lash ramps have a portion where the velocity is steady @ .0004 - .0005 per camshaft degree.These ramps can vary from say .006 to .022 in height.... at the end of these ramps the tappet can only be going so fast ( velocity ) .Many people use the end of these ramps as their advertised duration points..... IE adv dur @ .014 lifter rise with a lash of .021 .Many designers take that end of the ramp height X's Rocker Ratio = valve lash.... IE .014 X 1.5 =.021 lash...... or say .020 ramp height X 1.5 RR = .030 lash.

But others do not use that method.One of my cams was measured @ .017 and the grinder specified at lash of .018.

Still others will tell you to add .002 to the exhaust lash... ie .024 in take and .026 exhaust.... yet many mfgs will suggest the same lash on both intake and exhaust.

Others will tell you a range.... suggested lash may be .026 but you can use .016 - .030.

If the lash on a typical V8 is very small , Example .010 or .012 then the take up ramps are likely very short and rapid and the range you can vary the lash safely will be only a couple of thou... beyond that could cause damage.

In any case the valve cannot begin to open until the valve lash is all taken up.... by reduceing the lash , example from say .025 down to .018 then the lash will be taken up sooner and the seat duration will increse. If the lash is increased then it will take more time for the lash to be taken up and the seat duration will decrease.

Because of all these different theories and methods of ariving at a lash setting it's a good idea to check with the cam mfg first and let them tell you how much you can vary the valve lash safely.[/i]

[Jay Allen]

Adjusting lash is a diffulcult thing. As was previously said, you NEED to know what the *Designed* lash is/was on the master lobe.

Lash at lobe X rocker ratio = lash at valve

And if you do not know what went into the lobe, it is pretty hard to make any suggestions or opinions on what to do.

Good Luck.