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Location:
sunny coast, qld
Registered:
October 2002
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Location:
Brisbane
Registered:
June 2003
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Re: dynos and diff ratios
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Mon, 28 June 2004 06:52
 
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i was sure they just adjust the dyno to compensate for that?
i.e how can a falcon with 2.88 gears compare to a toyota with 4.5 gears?
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Club Member
Location:
sydney
Registered:
May 2002
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Re: dynos and diff ratios
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Mon, 28 June 2004 07:19
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changing ratios makes significant differences to your results.
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Location:
Kita-Ku, Sapporo, Japan
Registered:
January 2003
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Re: dynos and diff ratios
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Mon, 28 June 2004 08:05
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hmm, i'm not so sure anymore 
4.6 vs 4.1 is similar to the 3rd gear 4th gear thing..
with 4.6, you have more torque at wheels, but lower speed, so maybe 'power' is similar?
i vote that we get one car on toymods dyno day to do a 3rd gear and a 4th gear run to end this speculation once and for all 
shall we have a poll?
Cya, Stewart
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Club Member
Location:
sydney
Registered:
May 2002
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Re: dynos and diff ratios
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Mon, 28 June 2004 08:46
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its not the gearbox selection that makes the difference, its the final drive that changes the torque graph.
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Location:
Adelaide
Registered:
May 2002
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Re: dynos and diff ratios
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Mon, 28 June 2004 08:53
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The Dyno at Rex's work (a Dynapack 4WD Hub dyno), gives individual actual wheel speeds and a calculated engine rpm. The vehicles overall gearing ratio is input/adjusted until the Dyno's calculated engine rpm is the same as the tacho.
Effectively this gives all cars an even playing field.
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Club Member
Location:
sydney
Registered:
May 2002
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Re: dynos and diff ratios
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Mon, 28 June 2004 09:24
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yeah but i dont think it adjusts in shootout mode ... hmm maybe Dyno Dave or Darren could find out for sure.. all i know is that with the 4AC it made a difference of more than 10Kw from the last known figures and the only difference was a changed diff.
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I supported Toymods
Location:
Epping, Sydney
Registered:
May 2002
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Re: dynos and diff ratios
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Mon, 28 June 2004 09:41
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I've been pondering rex - is your avatar a reference to machiavellians, or just something cool you thought you'd put there? 
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Location:
Melbourne
Registered:
October 2003
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Re: dynos and diff ratios
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Mon, 28 June 2004 15:21
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you still technically end up with the same power at the wheels.
Often when extra power is 'discovered' it is because you are able to rev higher with a higher diff ratio.
Most engines torque drops off before they reach maximum HP.
As the torque drops off, rpm's are still increasing at a rate
which causes overall power to be higher..
Assuming a very simple way of describing HP = Torque X Rpm, here's what i mean.
RPM 1---2---3----4-----5---6-----7-----8-----9----10
TOR 1---2---3.5--3.5---4---3.8---3.3---2.5---2----1.5
H.P 1---4---10.5-14----20--22.8--23.1--20----18---15
.............max torque.^.........^max HP...........
As you can see, torque drops off at just after 5rpm, but
maximum power is at 7rpm, because of the RPM factor.
On a dyno the dyno's has a certain resisting torque,
sometimes this resisting torque can now allow the engine
from being able to get to the higher HP figue. As in
say the dyno was applying a resitive torque of 4n/m,
this engine would not be able to rev past 5rpm. And
thus power would read 20hp. But with a higer diff ratio,
the toque is multiplied and it can get past that
'hurdle' Changing diff ratio's can help with this, but in what
way and how much depends on too many factors to mention.
One other point is the drag effect.
Power is power, based off the above simple HP thing,
1nm of torque X 100 rpm = 100kw OR
100nm of torque X 1 rpm = 100kw or
50nm of torque at 2 rpm = 100kw.
If your rear wheels are spinning really slow per given engine rpm, as in a high diff ratio (4:1 lets say), that means your diff is spinning slower, while transmittin still the same amount of power. That means friction losses are much less (as in diff oil is quite thick, and can add a lot of drag) -- not to mention
wheel bearings, etc etc bla bla.. mainly the diff oil though
Similary a car with a low diff ratio, say 2:1 is literaly got 4 times as 'diff drag' (since drag increases with v^2)
shit iv'e gotta go, but i hope i made sense
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Location:
sunny coast, qld
Registered:
October 2002
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Re: dynos and diff ratios
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Mon, 28 June 2004 22:12
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thanks everyone!
Rex: what do you do if the car doesn't have a tacho?
and after all of this, does anyone know if the 4.6:1 would read higher or lower than a 3.5:1 or is there too many factors to take into consideration?
Chris
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Location:
Adelaide
Registered:
May 2002
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Re: dynos and diff ratios
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Tue, 29 June 2004 08:54
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Connect one.
Be it an actual tacho, or a multimeter type analyser or even a timing light with inbuilt display. They are quite common now.
As for the Avatar, Rex was just scribbling one day and he merely drew what came to mind. Thought it would make a good avatar.
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I supported Toymods
Location:
Plumpton/sydney
Registered:
November 2003
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Re: dynos and diff ratios
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Tue, 29 June 2004 10:07
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It is not the tacho they look at as much as the spedo all runs are set to the spedo this is so it doesn't matter what gear ratio you have the dyno reads them the same, and yes dave does set them for every car that goes on the rollers.
anyway your car will make the same power regardless of you diff ratio the only diff will be when it makes peak power so with a 3.5:1 your peak power will be at 5500rpm were as with a 4.6:1 you peak power will be at 4500rpm (this is just a guide)
Cheers
BEN
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Location:
sunny coast, qld
Registered:
October 2002
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Re: dynos and diff ratios
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Tue, 29 June 2004 23:51
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so do you think they would have trouble getting an accurate reading for my car as it has no tacho connected and the speedo is out?
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Location:
Brisbane
Registered:
May 2002
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Re: dynos and diff ratios
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Wed, 30 June 2004 00:58
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The purpose of a chassis dyno is to measure power at the wheels. The dyno doesn't care about your diff ratio, or whether the diff even exists, it only knows what's happening at the wheels! And when you think about it, the power at the wheels is the only figure relevant to you anyway.
I wouldn't worry about getting an accurate reading. They'll use their own equipment to measure engine speed and roller speed, so you don't need a speedo or a tacho.
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Location:
sunny coast, qld
Registered:
October 2002
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Re: dynos and diff ratios
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Wed, 30 June 2004 02:22
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cool, thanks mate
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