On Sat, 09 Apr 2016 17:38:45 +0100, Roger Mills wrote:

On 09/04/2016 11:42, Dave Plowman (News) wrote:
In ,
Roger wrote:
What any car's 0-60 time may be is totally irrelevant to where the
maximum acceleration occurs anyway. Unless comparing apples to
oranges.


The fact remains that a car's power to weight ratio (*not* engine
torque to weight ratio) is a pretty good indicator of accelerative
performance.

It's an oft quoted figure for the masses. Same as BHP is all important
to bar room mechanics. But it doesn't tell the full story. Only that
car makers tend to produce roughly similar engines.

A colleague of mine at Rover studied this is some detail, and plotted a
graph of 0 - 30 times (admittedly not 0 - 60) against power to weight
ratio for a wide range of vehicles. He found a very strong correlation.

But I'll ask you a question. Take a high revving bike engine with a
very high specific BHP per litre and put it up against a lightly
stressed but torquey V8 etc in vehicles with the same power to weight
ratio. Which one will accelerate better?


Why compare a bike engine with V8 car engine?

If you take the same car and equip it with a high revving petrol engine,
and then swap the engine for diesel which produces the same power but at
a lower speed (with a suitable matching gearbox in each case) there will
be very little difference in the accelerative performance - even though
the level of drama may be different.

It depends on the amounts of inertia in the engines relative to the
whole vehicle's inertia. The typical diesel equivalent to its petrol
counterpart will have more inertia, reducing the overall acceleration
performance despite providing the same top speed performance. This
assumes the overall mass of the vehicles remain identical. In practice,
the diesel variant tends to weigh a few pounds more than its petrol
engined counterpart.

Of course, the ratios available in the gearbox along with clutch
technique used when maximising acceleration performance will modify this,
with full throttle change up and abuse of of the clutch even reversing
this trend. An automatic box with continuously variable ratio configured
to allow the engine to run at its peak BHP revs will eliminate such a
difference.

--
Johnny B Good