RPM limits of GM computer/ignition module?

[Ludis Langens]

Nick McAlpin wrote:
> 
> Any chance of playing that game of space invaders on the CPU
> you pull out?  =)

Oops, that's what I meant to say!

> on a more serious note: is it true of older ECM's (like the 88 P4 i
> am working on) that they also update hardware and the hardware
> fires the injector/ignition?  and when you say "older" ECM's are
> 255x25rpm - would that also apply to the P4?

Yes, and yes.  I'd venture to say that the 6375 RPM limit applies _if_
the application is an engine that normally doesn't rev that high _and_
it doesn't have an electronic auto transmission _and_ it isn't OBD II
_and_ it doesn't have a 16 bit (or greater) CPU.


Shannen Durphey wrote:
> 
> Also, any suggestions as to the weight of the brick would be helpful...

I think any smog inspection shop has a calibrated brick...


Christopher Bennight wrote:
> 
>      The RPM value that's defaulted in the Data stream does max
> out at 6375, but the car will rev higher than that - and there is a more precise
> value you can poll from the pcm - not sure where that maxes out.
>     How would this "soft rev limiter" exhibit itself?

Two examples:

The 85-86 Fiero 2.5 L4 ECM has a hard rev limit set way high.  But, at
4800 rpm, it stops giving additional timing advance.  Reports are that
2.5's with non-restrictive exhaust will rev up to around 5K but not any
higher.  Neither the fuel nor ignition is ever cut - the engine just
won't spin any faster.

The 88 Fiero 2.8 V6 turned off the rev limit (which was 6000 in previous
years).  Reports are that the 88 2.8 will spin to the low 6K's, but not
any higher.  Perhaps the intake is too restrictive to go higher - the VE
in the tables falls off a cliff approaching 6K.  Or perhaps the 6375
value is limiting.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

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