Sorry it took me so long to reply to this, life has been busy.
By the way, I did some research on the Clayton dyno and it is an
eddycurrent device. It uses magnetic drag to impose a load on the
rollers. Of course, flywheel effects of the rollers and the driveline
would also factor into it when the car is not running at a constant
speed.
On Fri, Oct 17, 2003 at 11:22:09PM -0400, James Nazarian wrote:
> Larry, I believe that your comments are correct but for different reasons
> then you mention. The higher apparent HP reading is an actual reading.
> Everything in the car that rotates consumes power, engine internals,
> transmission internals, belt driven accessories, driveshaft, axles, wheels
> and tires. If any one component is lightened then the power required to
> rotate that component is reduced, thus giving back horsepower that you were
> already making but were using to turn car parts.
That is not quite true, many of those items act as flywheels, or
energy storage devices. It takes energy to spin them up to a certain
speed, but apart from aerodymanic, or hydraulic drag, don't take any
energy to keep them rotating. I.e. it takes twice as much energy to
spin an 18lbs flywheel up to 4000 RPM as it does a 9 pound flywheel,
(assuming the same weight distributuion) but it doesn't take any more
energy to keep it spinning at 4000 RPM.
This is why lightened parts improve acceleration, but not top end.
>
> If you look at chassis dyno results, it is almost always measured in fourth
> gear because it is 1:1 and is the most efficient gear in the
> transmission.
Yes, it is straight through, and there are no frictional losses from a
gear train.
> Occasionally you will see tests done in other gears and you will find large
> losses in horsepower. Even though physical acceleration is fastest in first
> gear, you are consuming much more power in the more rapid acceleration of
> the reciprocating mass of the parts. Something on the order of a constant
> times the square of acceleration. It is not unheard of to see 50% less
> horsepower to the wheels between first and fourth gears.
I would expect that it has more to do with frictional losses in the
gears. It may also have to do with the fact that an engine turning at
4000 rpm has twice as much internal frictional losses per minute as
one that is turning at 2000 RPM.
> This in one of the major differences between american and european
> transmissions, americans use low gearing and a high overdrive to loaf at
> speed, while european cars use tall gearing and a 1:1 final drive so that
> acceleration at high speeds is improved via more available power.
Do you mean "high overdrive" or do you mean a "tall final drive",
i.e. numerically low rear end? I thought that American cars were just
geared taller because american engines have lots of torque, but don't
rev very high.
> There are two types of dynos one is an eddy current and I forget the name of
> the other, one is designed to measure rate of change of rpm as you suggest,
> because the dyno computer knows how much power it takes to accelerate the
> rotating dyno parts. The other type varies it's resistance and will allow
> you to hold unnatural throttle openings indefinitely. For instance, if you
> wanted to tune at WOT, the dyno will allow you to vary the dyno resistance
> so that you can hold full throttle without accelerating. They both have
> their uses but the first one that I described is the more common
> type.
Yes, the eddy current is the one that I have access to, though the
printouts list the delta-V at the time of reading, so it seems like it
takes into account accelerating the roating mass.
>
> The behavior I described is the math behind why when someone changes to
> larger rims and tires (with same overall diameter) acceleration will
> typically get slower. The new wheel/tire combination is often heavier and
> even though gearing is the same they will take more power to turn, thus
> slowing acceleration, and showing lower numbers on the dyno. If you were to
> put your engine on a dyno and then put the whole car on a chassis dyno, the
> difference in power ratings is the amount of power being consumed by the
> driveline in order to turn all of those parts. Lightening, and thereby
> reducing polar moment, will not make power, but it will give back some of
> what you already paid for when you built your engine.
That's not quite correct. Putting on taller tires will give you a
taller final drive ratio, the car will go further per turn of the
engine. However since the torque available is the same, X foot pounds
divided by more feet give you fewer pounds of thrust and therefore
less acceleration.
Larry
>
> James Nazarian
> 71 MGB Tourer
> 71 MGBGT V8
> 85 Dodge Ram
> ----- Original Message -----
> From: "Larry Colen" <lrcar@red4est.com>
> To: <mgs@autox.team.net>
> Sent: Friday, October 17, 2003 9:49 PM
> Subject: Chassis dyno theory question
>
>
> > Since I've been asking about lightened flywheels, here's a related
> > question:
> >
> > Somewhere, I picked up the information that chasis dynos measure power
> > by measuring the rate of change of RPM. The faster that your car can
> > accelerate the rollers, the more power it makes. If this is so, then I
> > believe that reducing the rotating mass of the driveline will result
> > in an apparant gain in horsepower on the dyno. Lighter flywheel ->
> > more apparant power. Is this true?
> >
> > I suspect that this may only be true on some dynos as the Clayton smog
> > dyno I've used, can tailor resistance to either a specific level, or
> > to represent "real life at that speed".
> >
> > I do suspect that unless you are measuring RPM at a steady speed, that
> > they would need to take the rotating mass into account, and that
> > changing the polar moment of the flywheel would have a small but
> > measurable effect on the readings.
> >
--
Girling is not a verb.
lrc@red4est.com http://www.red4est.com/lrc
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