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California98Civic · 08-11-2014, 01:23 PM

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!

Thanks.

08-11-2014, 01:23 PM	#1 (permalink)
California98Civic Cyborg ECU 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) Thanks: 2,373 Thanked 2,174 Times in 1,470 Posts	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! Thanks. __________________ 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.