Does the clearance between the camshaft and the top of valve tappets (AKA lifters AKA shims) affect fuel consumption? I am asking because it seems to me like the lesser the spacing, the more the cam pushes the valve down into the cylinder, which could affect the fuel intake in the case of intake valve. What are some other implications of camshaft-valve spacing that is out of the manufacturer recommended range?
asked Dec 11, 2021 at 23:13
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too small a valve clearance is bad juju. When heated, the valve stem increases in length due to metal expansion. If that consumes the available clearance, then the valve doesn't seal and the blowby will erode the valve and seat. If the clearance is too large, then the valve stem and valve train get hammered and may eventually fail. Measuring with the engine cold or hot will give different results.So follow the manufacturer recommended clearances and measurement technique.– KartmanDec 30, 2021 at 6:22
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1 Answer
I’m sure there are a number of considerations.
- You never, ever, want a dimensional stackup that precludes full valve closure and sealing.
- Greater valve opening = improved air / fuel flow and exhaust flow.
- Reduce power drags where possible.
- Improve quality, reliability and robustness wherever possible.
- What’s the goal, great fuel economy or awesome power?
I’ve done lots of engineering work on engine cooling and system airflow, not so much in specific valve design. I suspect that #1 above really drives the size of the clearance on a shim design system. Perfect zero would be awesome, but I doubt realistic. And big gaps increase impact loads, and adversely affect reliability and robustness.
You also have to analyze cam pressure angles. Too big of a lift affects power losses as well as reliability.
As you can tell these goals are not in perfect alignment. But I’m sure there is a lot of data out there that gets folks pretty close.
