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Old 08-11-2014, 01:23 PM   #1 (permalink)
Cyborg ECU
California98Civic's Avatar
Join Date: Mar 2011
Location: Coastal Southern California
Posts: 6,299

Black and Green - '98 Honda Civic DX Coupe
Team Honda
90 day: 66.42 mpg (US)

Black and Red - '00 Nashbar Custom built eBike
90 day: 3671.43 mpg (US)
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Fuel economy & valve lash (1998 Honda Civic)

I am wondering how valve lash affects fuel economy and emissions. It seems like loosening (within reason) should benefit both.

In essence loosening the valve lash to max spec should slightly reduce the overlap time between intake and exhaust, resulting in at least three things: less fuel escaping unburnt into the CAT, therefore more complete burn in chamber, and more low-end torque.

As ecomodders who drive the low end rpm band all the time this seems like a good thing. Heck my car almost never crosses 2500 rpms (~65-70 mph) even on the freeway.

So on my 1998 Civic the service manual's recommended spec is 0.009-0.011 for exhaust valves and 0.007-0.009 for intake valves. I set them to max spec of 0.011 and 0.009 on advice I trust and after some corroborating reading online.

I'd be interested in a technical discussion, because much of it would still be over my head a bit and therefore give me a chance to learn something!


See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

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