Power Losses...

[Loz]

Have it on good authority that the power losses between the crank and the output shafts (ie. wheels) on a front wheel drive manual Gearbox car are between 12 and 15 %.

Does anyone have any evidence to refute this?

I only ask cos I had thought the figure was much much higher. But it seems not.

I found this as well...

Inertia Measurement Systems

The majority of rolling roads used to determine power figures in the UK are inertia dynamometers. Inertia dynos do not directly measure the force on the dyno rollers to determine power figures. Instead, these systems calculate the force on the rollers using the formula:

F = ma where F is force; m is mass; a is acceleration

The mass and system inertia of the dyno rollers is known. In order to calculate the force applied, inertia dynos measure acceleration of the rollers by measuring the increase in current and voltage production when the dyno's eddy-current retarders are used as a power generator instead of a power absorber (as used to hold the dyno load when mapping engines).

Force on the rollers is therefore the roller mass multiplied by the acceleration determined by the voltage output. This force is multiplied by the radius of the roller itself to give torque at the wheels using the following equation:

T = Frwhere T is torque; F is force; r is radius of application

Power is determined by using the formula:

BHP = Torque (ft/Ibs) x RPM/ 5252

This calculation is then used for the power at the wheels measurement. If an ignition pickup on the engine is used, these power figures can be used to plot a power curve. In order to determine power at the flywheel figures, a coastdown procedure is used which measures the deceleration of the rollers, and uses this figure as negative acceleration and the F=ma calculation is used again to obtain the power losses through the transmission.

The major problems with these systems occur when changes are made to any of the rotating masses in the system. This includes items such as the clutch, flywheel, or aftermarket wheels. These items do not change the power of the engine (obviously). However they will change the rate of acceleration of the vehicle. Therefore these changes will change the power output measured on an inertia dyno. This is another reason why a number of dyno manufacturers will not guarantee an accuracy greater than 5% for their dyno systems.

 

 

will keep looking though....

Loz

[Loz]

also found this... not sure how good the source is though...

ROLLING ROADS DESCRIBED

This section is required reading for all accept for the real expert. Rolling roads or Rolling Road dynamometers, to give the their full title, are a method of calculating the horsepower delivered to the wheels of the vehicle. The word dynamometer means a testing facility for measuring power. You can also have the engine dynamometers or dynos for short, that the engine is mounted on to calculate the power. Technically any engine dyno is probably better than Rolling Road, particularly if you are going to try out a lot of modifications, and is more accessible to work on. On the other hand when the engine is in the car you can make a better judgement of how the package works especially regarding cooling, due to bodywork reducing airflows etc. The one disappointing aspect of Rolling Road dynos is the power figure they give in comparison to engine dynos, due to the power losses through the vehicles transmission. The power losses on most vehicles are between 18% and 27%, so that an engine measured on an engine dyno giving a 100 horsepower will only show 73 to 82 on the Rolling Road. This factor is often a great source of disappointment to an owner of vehicle as his manufacturers hand book says his standard cars has 100 horsepower and cannot understand why it has suddenly lost 25 horsepower.

[ColinBarber]

15% to 25% is what I have heard. Autos have the biggest loss as do rear wheel drive cars.

Some rolloing roads guess the transmission loss and try to give you the engine bhp but it's not totally accurate. On a manual gearbox they can take it out of gear whilst running to measure most of the loss. Best to use a rolling road to compare before/after a mod, not to give an accurate bhp measurement.

[Loz]

Yes, had heard the same thing Colin.

Also I was getting to thinking that the only figure that really effects performance is the figure at the wheels. I could have 700bhp at the fly but if I can only get 56bhp to the tarmac what good is that?

Loz

[tdiplc]

15% is about right for front wheel drive, and up to 30 odd % for four wheel drive. There are many different factors that can affect the losses (even the tyre pressures)

We are changing our dyno this year for the type that bolts directly to the hubs which will remove the drama with trying to test the more powerfull cars that are prevalent nowadays.

The new machine is very accurate and can measure very small changes in parasitic losses. The bad news is that is going to cost over £100k plus vat

[Loz]

Does anyone have a good benchmark for rearwheeldrive manual and rear wheel drive, Auto?

I do however think I risk getting caught up in the horsepower pyssing contest. don't want any of us to do that...

Lets just enjoy what we have, be fruitfull and multiply (the horses, preferably!!! ;) )

:zee:

[dawesy]

as far as im aware power loss through transmission losses aren't really a % more a figure eg 50hp as other wise a car with 100 fwhp would lose 15 hp ie 85whp but drop a 1000fwhp engine in and it would loose 150hp even though the transmission is the same?

as for the dynapack dynamometer i think they're really good my only critisism is that they dont allow turbo cars to get full boost. at billing i only got a peak of 0.98bar boost when on the road i get 1.1 bar at the same rpm

[Loz]

as far as im aware power loss through transmission losses aren't really a % more a figure eg 50hp

admittedly its late, but that does not stack up to me. I think a proportional loss sounds more plausible. but I do take your point. I know you know your onions though so I aint calling you out.

:zee: