After MDU I was wondering about the relationship between HP figures on a wheel dyno and the final drive ration. For example does a lower final drive make more HP, or does it only do it earlier within the rev range. Perhaps someone with more knowledge could comment.

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1275 LS Brisbane

Should always be done in 4th gear this gives a 1:1 ratio. The dyno is a device that measures torque, it does some maths then comes up with HP. The diff ratio and tyre size all need to be accounted for (if known) to get a true HP @ the wheels.

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http://www.youtube.com/user/Hanras

Here is where roll on dynos are really a social event rather than a scientific one.

Final drive and wheel sizes have everthing to do with your hp reading. Even though in 4th you run on a 1:1 ratio for final drive, a shorter final drive with a 3.647 diff will read better hp figures than the same car with a taller 3.444 diff. A mini with 10inch wheels will run better figures than a mini with 13 inch wheels.

It's simple: Mechanical advantage. The crank has to spin less times for the final drive to turn 1 whole circle when you have a shorter diff. This means the crank is spinning at a slower rate for any given diff speed. It sort of works like your bicycle gear, the gear that you pedal on is bigger and spins slower than the gear on your rear wheel. This means you put in less and give more out.

Similar but different tyre size. THe bigger the tyre the more effort it takes for your engine to turn the wheels. Effectively bigger tyres changes your fianl drive ratio which in turn does the above.

Also bigger tyres have more surface area which means it eats up more horsepower on the dyno trying to overcome the friction. This is good in the wet when you are trying to fight for grip but not good if you are trying to take off as you would in a dyno situation.

Here is a random article from google about this exact thing:

http://www.turbomagazine.com/tech/0106t ... ize_power/

So YES Hanra is right, to get an accurate comparison of what one car is to another you need final drive ratio and tyre size added to the calculations. While we're at it, ambient temperature and humidity need to be factored into the equation. Engine Dyno is the way to go

Thanks for listenning

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Lillee - 1969 Morris Mini K

Thanks guys, explains why cars with the same CC but different tyres and diff ratios vary so much on wheel dynos.

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1275 LS Brisbane

Some errors of physics above...

The dyno measures the power transmitted to the rollers. It is a simple equation of torque at the rollers and the speed the rollers rotate. This is the power figure that is printed. In theory that is the power that the car can transmit to the road and as we know power x time = energy and energy = 0.5 x mass x speed squared. So the dyno figure "should" directly correlate to the acceleration potential of you car.

The only difference with a chassis dyno is that the tyres run on steel, not bitumin and the rollers are two cylindrical contact patches instead of one flat one.

So, acceleration of the vehicle relates to power at the road and vehicle mass (not forgetting rotating inertia, which effectlvely adds mass)
Power at the road is engine power minus losses which include, accessories, transmission, driveline and the wheel to road interface.

Example:
700 kg mini travelling at 100km/h, ignoring rotating inertia, has 270 kilojoules of energy associated with its velocity. To get to that velocity on 10 seconds requires an average power at the road of 27 kW, sustained for the entire 10 seconds. The above holds true regardless of gear ratios, tyre sizes etc.

Changing diff ratio or gear will change the speed of the tyres on the rollers but also proportionally change the torque applied, so only non-linearity of the loss vs speed relationship can affect the losses. The major affect is the ramp up rate where the rotating inertia of the wheels/brakes/hubs has an effect. Out of Shootout mode, on a constant load/speed point, the wheel inertia will not make any difference to dyno reading.

Tyre to dyno losses are high so the smallest contact patch and least sidewall deflection that can be achieved the better. Narrow, hard tyres pumped up high, with the lowest vertical load gives the best result. The hardest, narrowest, smoothest tyres set to the highest pressure and using the lowest tie-down force you can run without getting wheel slip will produce the highest HP number.

If you want to know how changes to your engine have changed its power, you must keep everything else the same between dyno runs. The dyno can compensate for intake air temp and humidity because these have direct & predicatble links to power on a naturally apsirated car. All other factors must be the same.

If you want to know the exact HP figure of your engine, take it to an engine dyno. The number is irrelevant though, it's the power at the road that accelerates the car.

M

Nice explanation

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Lillee - 1969 Morris Mini K

GO the ENGINE DYNO!!!!

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http://www.youtube.com/user/Hanras

Chong, HP is the rate of doing work. it is torque x rpm /a constant.

Putting a shorter diff in will give more torque on the roller but less roller speed for a given engine RPM.
If your synopsis were right, running the dyno test in 2nd gear would give more HP than in 3rd. Sorry mate, it wont...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Only thing, which made, last semester in physics, rather complicated is the formulae involved in this kind of working doesn't take into account other things, mainly wind resistance. So what my physic's teacher refered to as a "fudge factor" would be needed. It's a number that you could multiply your answeres to, be that dyno figures, velocities or the energy required to accelerate at a certain magnitude. It may not amke much sense all of this but I learnt one amjor thing last semester. It's not that important, lol. Testing/experimenting is easier than calculations, as the "fudge factor" is only an estimate to correct your errors but this is not correct either.... Anyways that's my useless force lesson for the moment.

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1964 Morris 850, 1330 Supercharged - 81.8hp atws.
1975 Leyland Mini S 1100S powered - Nice and reliable.
1977 Leyland Mini LS - Project LS-T

Sorry I was trying to explain it without taking out the text book! Hey I did mechanical engineering 1.1, 1.2 and 1.3 in Uni you know! (not that I remember any of it)

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Lillee - 1969 Morris Mini K

Lillie that's rather impressive, well I've done year 11 physics, soon to be year 12....

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1964 Morris 850, 1330 Supercharged - 81.8hp atws.
1975 Leyland Mini S 1100S powered - Nice and reliable.
1977 Leyland Mini LS - Project LS-T

why the term Horsepower

as opossed to

Camelpower

the Camelpower at MDU was extreme

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Horses are more predictable, and they will work without biting...

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

PMSL

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Cause Camel Power is way too big a measurement to use on Minis! GO CAMEL POWER!

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Lillee - 1969 Morris Mini K