Miata Mailing List: August 1993, Message #100
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   From: sgrossma@lemans.stortek.com (Steve Grossman)
Subject: Re: Rev limiter vs. Floating valves
   Date: Fri, 13 Aug 1993 17:21:23 -0400



> From: MANSON@fsd.com
> To: Multiple recipients of list 
> Subject: Rev limiter vs. Floating valves
> Date: 	Fri, 13 Aug 1993 16:29:11 -0400


> In a recent mesage on the board, there was reference made to a "Rev Limiter."
> A friend of mine recently told me that he had "floated the valves" (an old
> attribute probAbly more common in the days of non-overhead cam engines).  I
> am curious now whether my friend, unaware of this rev limiter might have
> incorrectly assessed his condition.  Have any others of you out there in
> Miataland had experiences of either type with your Miatas?  Can any of you give
> me a bit more technical info oeither subject with regard to the Miata?

Yes, I have hit the rev limiter in the Miata, and no, I've never floated the valves
in the Miata, but have in other cars.

Floating the valves involves the valve actuator (rocker arm or camshaft) attempting
to open the valves again faster than the valve spring can close them.  Consequently,
the valves never fully close, resulting in poor performance due to low compression
and backfiring.  In pre rev limiter cars, this was a fairly common occurance when
the engine was revved above redline.

The rev limiter is much more abrupt than valve float.  Where valve float causes
a sag in performance, the rev limiter cuts off the spark until the rpms have
dropped below the limit, causing the engine to shut off and back on, rather
jerkily.  When the rev limiter is encountered, the engine will cut out at 7250 rpm
and stay cut out until the rpms have dropped, and then cut back in until the
rpms again exceed 7250.  If the gas pedal remains floored, the engine will
cycle on and off right around the redline.  Valve float will not cause the engine
to cut out, but performance is usually so reduced that unless the engine is
freewheeling, it will be unable to turn any faster and will usually slow down
to below the point where the valves float.

I do not believe it is possible (assuming the rev limiter is intact and
functioning) to float the valves in the Miata.

Steve

Steve_Grossman@stortek.com