CBR600 dyno graph

I didnt think it was that bad at all, considering it is 10 years old,
has 34,000 miles on the clock, has been caned to **** on numerous
trackdays and desperately needs a full service. Honda claimed 100bhp at
the crank when this model was new, mine still makes 95.13bhp at the
rear wheel.

http://www.flickr.com/photos/78131584@N00/2692383227/sizes/l/

There's some cleaning up of the fuel/air mixture needed in the
midrange, which can be dialled out by dropping the needles down one
notch and cleaning the K+N air filter that's fitted. The fuel/air ratio
in the midrange won't improve the overall bhp figure but it will
improve the driveability and torque available from 6000 upwards.
Dropping the needles is something I can do when I have it apart in a
few days to service it. I'm happy enough with that graph though.

 

I was told 12:1 across the range is ideal for my bike, yours may be
different.

 

The stoichometric ratio is going to be pretty similar for all petrol
engines, surely?

 

I'll ask a very close friend[1], she - yes, she's definitely a she -
knows everything there is to know about this sort of thing.

[1] PhD in Mechanical Engineering, she specialises in automotive
injection systems

 

As I understand it, the ratio is determined by the amount of oxygen
(from air) needed to fully react with an amount of carbon and hydrogen
(from petrol) - this is determined by the chemical structure of the fuel
and so pretty much fixed.

The problem arises from the fact that the amount of air going in varies
by throttle position, inlet area, temperature, altitude, speed etc, so
the tricky bit is setting the right jetting or injection map to throw
the right amount of fuel in - the stoichometric ratio is a constant but
you're fighting against the other variables.

 

As I understand it the engine goes suck, squeeze, bang, blow...if it
doesn't do any of those four things there's a problem. Anything beyond
that requires someone with a brain to understand.

 

This is one of those places where the theory only works out in practice
with a lot of handwaving. "Lean burn" engines are an attempt to get
close to the theoretical ideal mixture. Most bike engines need a much
richer mixture.

Lots of bad things happen at the lean end of the range so designers always
err on the rich side.

 

14.1:1 is the stochiometric ratio which gives the perfect exhaust
conditions for catalytic converters to do their thing, and that's why
lambdas are used to keep the air/fuel ratio as close to that as possible
on catted petrol cars.

It's not "perfect" for peak power - richer is better for that - or
"perfect" for peak economy - leaner is better for that.

If you're mapping a non-catted engine, there's not really an ideal single
number for all rev/load conditions. It's why mapping tends to be an
iterative process, to get the drivability spot on, rather than do-it-once-
and-finished...

 

Yes, but AIUI, as others have said, different engines require
different mixtures. If you think about it even a single engine will
need different mixtures for different conditions, such as starting,
where the fuel may 'de-aerosol' onto the inlet tract & cylinder walls
when the engine's cold, requiring a richer mixture. Accelerating
usually requires a richer mixture too.

I have a vague idea that I read somewhere that my Guzzi richens the
mixture if it's been left idling too long so as to help cool hot bits.

It's also possible that engines are run lean at times to reduce
emissions.