engine RPM [was Re: buying an MX5]

[Karl Mowatt-Wilson]

Greetings,

I noticed a difference in max revs between the 2 cars I have
driven so far.( one NZ new , the other an import)

AFAIK there should be no rev difference between the two and
the only major visible difference that I know of is that the
NZ models have a different rear number plate surround area.

From what Fletcher tells me, the difference in the rev counter is merely

one of different marking - the red gauge markings on his car (NZ) start at
6500 (I think) and mine (import) has its first red mark at 7250rpm.
Fletcher's car is obviously meant for pansies who are scared to drive hard
(Fortunately, Fletcher has overcome this problem and shown the car what
it's meant to do)

I've noticed that the rev counters are not that accurate - some
models show a difference of 200 rpm or more indicated at cutout.
There has been some debate on the actual cutout rpm (Karl - your
logger unit would indicate this - correct?)

Rev limit on my car & Fletcher's has been observed to be the same, at
7200rpm. The data is noisy (apparent engine revs vary from one half-rev to
the next by up to about +/- 50rpm), but I'd say the rev limit is pretty
definitely within 50rpm of 7200rpm.

I've not yet checked rev counter calibration vs my RPM Timer readout.

My RPM Timer is recording the IG signal with resolution of about
1microsecond (that's about 0.025% of a half-rev, at redline) so the data
should be accurately representative of what the ECU is doing. In order to
be completely certain of what the engine is up to, it would probably be
best to record the signal from the crankangle sensor. This way we avoid
jitter introduced by however long it takes for the ECU to respond to the
crankangle.

I've been working on a webpage explaining my RPM Timer, but so far it only
has pictures of the hardware (no graphs of engine RPM yet). If you want to
have a look, go to http://www.bigfoot.com/~karlmw and click on the RPMTimer
button at the top of the page. While you're there, you might as well look
under the DataLogging button to see some of my GPS plots from the MX5 club
run to Snells Beach. When I have the RPM Timer page updated I'll send a
note to the list.


Karl.