Hey Peter,
I'm not picking on you for your comments, but voicing a different opinion
and explanation. There are benefits to both, and I wanna talk about where
the power is located. I'm going to tie it in to the SU (.5*46mm) v. Solex
(44mm) discussion from last week since that one didn't sit well enough to
digest.
Note: I'm only voicing my own opinions and trying to help folks
understand the difference so they too can voice an opinion.
On 12 Aug 2002, at 14:51, Peter Long wrote:
> The rate of acceleration is greater. These are two extreme
> examples to show you that 240 hp is not equal to 240 hp.... peak
> horsepower ratings sell magazines, torque wins races."
Funny thing though: The BMW Z cars only get to look at S2000 tail lights.
I've been on track in lapping sessions with both. The S2000s are faster
around Texas World Speedway's 2.8 mile road course. I've watched them
compete at AutoX's too, and the S2000s take the nod there too.
Why? Because acceleration from a standstill isn't the only factor when
winning races. I'm going to suggest that you look at the HP v. RPM curve
and compare the total area under the curve - matched to the right driver
the car with the greater total area takes the nod if it is geared
perfectly for the event.
But, the average driver (like me) needs the power (yes power, not torque)
at the RPM they are accelerating from. It is easier for a driver like me
to deal with the power at the top of the RPM range because this fits with
the idea of press the pedal harder to go faster - it isn't always
convenient or safe to shift in competition.
Sufficient torque is required to get the car moving and accelerating, but
it is power that determines how fast the speed (RPM) of the engine can
change. So, yes, a high torque car can break the tires loose with a gas
pedal application, but that does not imply that it is necessarily
accelerating (changing velocity) quickly.
That said, it's probably pretty clear that I like a rev happy motor in a
car. I take the opposite fence on a motorcycle where I go for low rev'g
torque monsters, but I'm no motorcycle racer either.
So, SU v. Solex carbs on our Roadsters:
It was argued that because the SUs are only supplying one cylinder at a
time, they can be compared with Solex's based on inlet diameter. This
suggested that the SUs 46mm were larger than the Solex's 44mm inlets.
The problem here comes from two variables not considered. One is engine
RPM. At 3000 RPMs, each cylinder is firing 1500 times per minute or 25
times per second. That suggests a lot of direction changes for the
airflow in the SUs, slowing the velocity of the air flow on a cylinder by
cylinder basis. Then, since the SUs feed two cylinders, the air flow path
is not straight - further slowing the velocity of the intake air.
So, the Solex's 44mm inlet definately flows a larger volume of air
allowing it to stuff a larger combustible charge in each cylinder than the
SU's shared 46mm inlet. And, I'm betting the intake duration on the Solex
Cam reflects this improved ability to charge the cylinders.
The other difference is the shape of the intake path. The faster flow
through a straigt path (like the Solex) leads better high RPM performance.
The slower flow through a curved path (like the SUs) leads better low RPM
performance.
Personally, I think my SU 2000 is a little anemic above about 4.5k. And,
since it is easy to tach up a car by downshifting and getting into the
power band when needed - I'd like to move it up into the Solex type
territory. So, just some ramblings. Let me know if there are holes in
this reasoning, 'cause the investment is significant. The returns don't
seem cost effective from a 15hp gain perspective - but the change in the
power curve justifies the expense (that is, if I ever get the $) for me.
Top Down in Houston,
Milton and the BeautyQueen
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