Power versus speed and fuel economy

Is there a relation between power versus speed? I was thinking that if
a machine was restricted that it'd just take longer to reach its top
speed?

Also would a machine that's been restricted also suffer from poorer
fuel economy? I think so, but would like the opinion of the floor.

 

No. It's top speed is also directly governed by power if you think
about it - there's the force of the engine vs the force of wind
resistance. More engine power = more ability to push (faster) against
a given amount of wind resistance.

 

Are you pissed already, or didn't you get much sleep last night? Try
sticking a 3.5hp Briggs & Stratton engine in your bike & see what the
top speed is.

Assuming the air/fuel ratios are the same, more power needs more fuel.

--
Krusty

'03 Tiger 955i
'02 MV Senna (for sale) '96 Tiger (for sale)
'79 Fantic Hiro 250 (for sale) '81 Corvette (for sale)

 

I refer the honourable gentleman to my sig.

Whilst that makes sense, if the bike is restricted (on the Z1000, it's
a thingumy-jig that restricts full throttle) but no tuning has been
done, surely any optimisation would have been set up for a full power
bike, hence better fuel economy. Strangling it at 106BHP would give
poorer results, no?

 

No. Because you're unable to deliver as much fuel to the engine, it's
just like you're running at maximum half throttle, so the effect on
fuel consumption will be the same. Of course, if you're struggling for
power then you may feel inclined to rev the tits off it all the time,
which will in turn use more, but 106bhp on a Z1000 isn't going to have
you doing that in your normal town use, so in practice it shouldn't be
possible to get worse consumption, and I'd be very susprised if it
wasn't a good deal better.

 

you were thinking wrong.

 

No. Nothing to do with the fuelling, gearing, weight or aerodynamics
have changed, so restricting it to say 3/4 throttle with a lump of
plastic will be no different to restricting it to 3/4 throttle with
your hand.

I think you may be confusing RPM with throttle position. If you block
access to the top n thousand RPM, it /may/ make the bike less efficient
if it had a poor fuel map/carb settings in the first place (i.e. rich
in the mid RPM & weak higher up). That's extrememly unlikely though
unless it's been fucked about with, as factory settings generally make
it weaker in the mid range to pass emmissions tests.

--
Krusty

'03 Tiger 955i
'02 MV Senna (for sale) '96 Tiger (for sale)
'79 Fantic Hiro 250 (for sale) '81 Corvette (for sale)

 

I can confirm that revving the tits off a smaller engine to keep up with
a bigger engined bike does indeed impact on fuel usage. I went from 137
miles per tank to empty in under 75 miles. This happened three times on
the way back from Pithiviers, so is reproducible.

 

Er, yes. Very much.

Nooooooo. It just won't get there.

Ah. That depends on how the restriction is done. If it's a matter (on an
FI computer-controlled engine), of re-chipping it, and effectively
detuning it then, no, it ought to be more economical. But it won't be as
fast (see above).

A lot of crude restriction devices used in the early days, especially on
small 125cc strokers, did result in more inefficient and thirsty
engines, yes, because they weren't letting the engine get to the spot
where it performed most efficently (usually a fairly narrow rev band on
strokers).

The NSR125 I rode to the Bol in the early 1990s only did 45mpg.

As for restrictions that limit the amount the carb slides can open:
these aren't as bad, but as someone else observed, you'll often burn
more fuel as you wring the thing's neck.

 

Pretty good going with you on board, I'd have thought.

 

See my reply. It depends on how the restriction has been done. I'm
surprised, these days, that they'd just restrict how much you can open
the throttle, since this isn't the best way to do it on an FI bike.

Ideally, you'd re-chip it (maybe new throttle bodies and injectors, as
well) and in exchange for brute horsepower you'd increase the bottom end
and mid-range torque.

It's how manufacturers do it when they produce naked or semi-naked
versions of bikes, using sports bike engines. My Triumph is detuned from
125bhp to 107bhp, but they gave it more low down grunt at the same time,
and the very last thing it feels is detuned.

 

<Impressed>

Well done.

 

I was younger and smaller then. You bastard.

 

Pah. My RGV does the same trick and drinks a third of a litre of oil
while it's at it.

 

Yebbut, strokers are a law unto themselves. My KH400 used to do that,
and for a bike that produced only about 30bhp and struggled to hit a
ton, that was somewhat silly.

 

Don't forget the camshaft. That's the most important factor in properly detuning an engine.

 

detuning an engine.

Oh yes, indeed. No point in having a bumpy cam to flow lots of gas at
imponderable revs if it's never going to rev that high.

 

Wolfram is your friend.

 

it comes up with some totally surreal answers sometimes, mind - needs
a bit of tuning.

 

It's no uncyclopedia.