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California98Civic 08-11-2014 12:23 PM

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!


Daox 08-29-2014 03:05 PM

A test would be a great way to show if there is any benefit... :)

MetroMPG 08-29-2014 03:12 PM

The economy cam in the Metro XFi was ground for lower lift and duration, shifting the torque peak down.

You'd be effectively doing something similar by going to maximum adjustment, no?

user removed 08-29-2014 04:08 PM

Max gives you less lift and duration as well as max distance to the next adjustment.


California98Civic 08-29-2014 11:41 PM

Well, I wouldn't know how to ABA test such a thing, because it would take most of a day in between runs, allowing too many conditions to change. But the engine sounds differently (I think a little louder). And my FE has not suffered, though I can't tell if the valve setting has anything to do it. My primary goal was emissions test prep. And my smog test went smoothly. A side effect seems to be that it *might* help fuel economy at the same time.

Gasoline Fumes 08-30-2014 12:48 AM

I set my intake and exhaust valves to the thickest of my feeler gauges. I don't remember the size. It rattles more and it might cause excess cam wear, but I felt a slight torque increase.

Egriffy 05-27-2015 03:36 AM

I realize that it has been months since the last post on this thread but I am curious if any results came of this experiment?

California98Civic 05-27-2015 09:34 AM


Originally Posted by Egriffy (Post 481106)
I realize that it has been months since the last post on this thread but I am curious if any results came of this experiment?

I can't tell. If there was a benefit as there might have been jn theory, it would have been very minor. No way to see such a small change through thr background data noise. Sorry.

cosmick 05-27-2015 09:46 AM

0.020" shouldn't hurt, but I'm not sure it'll help. And you really can't go any tighter than factory spec. More lash will cause the intakes to close earlier, trapping more fresh charge in at lower RPM, wwhich should help if your cruise RPM is too low. Likewise on the exhaust, the extra lash will help low RPM torque there again.
But if your cruise RPM is too high, then the tight end of factory spec would be better.
As you improve the aero of the car, it'll like less cruise RPM, or less lash.

Chrysler kid 05-27-2015 10:41 AM

I have not performed a valve adjustment in my civic, most likely has not been done in 150k miles.

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