BHP loss calculation for extra weight

[TonyBlair]

Just wondered whether there are any formulas for calculating the loss of BHP for adding weight to the car.

I think I found such a formula a long time ago on a website, however I would have thought the formula would depend on a lot of variables and would be quite individual to each model/make of car.

The reason for asking is because the performance of my Rex suffers noticeably when I\'ve got a passenger in the car, and even more when I\'ve squeezed a third person into the back. Those two people would equate to roughly an extra 180kg~.

Of course the general rule is more weight = less speed. However I remember my mates R33 GTs didn\'t seem to suffer much at all with a car full (4 people) - it still managed 150mph with ease. Perhaps this is more to do with torque?

Cheers
Jon

[Kieron]

I don\'t see how weight can affect BHP, even when measuring at the wheels, rolling resistance might though.

I think its more to do with torque, same thing with the red RX, with 1 or more passengers it\'s a lot slower, but my boss had a 1.9 TDi seat and that use to pull like a train even with 5 on board, that had **** loads of torque but not alot of BHP.

[TonyBlair]

Thanks for your comments mate.

I\'m not a technical person so I couldn\'t really agree or dis-agree with you about whether weight affects BHP. Not because I doubt you at all, I just don\'t know.

I do however agree that torque is an important factor. Not sure how to increase torque though, I thought it was all relative to BHP - but then I\'m only guessing.

I think there is a recent thread about increasing torque - might try and find it and do some reading.

Cheers
Jon

[Kieron]

I\'m not 100% sure either, I\'m sure a boffin will be along soon.

On my MR2 I managed to keep it running a stock 225BHP but increased the torque to 260LBFT but I\'m not sure how that happened LOL

[nikp]

Firstly, if weight effects torque, it effects power. Power is a function of torque. For example, if you increase torque at any given rpm, you increase power at that rpm. There\'s quick lesson for you!

So. Now to your point! :p Lets keep this simple for now! The simple answer is, weight won\'t effect you power at all. Extra weight is all it is. If we go back to physics 101 we come to good ol\' Newtons law of F = ma. Or accerlation is equal to force divided by mass (or weight shall we say!). Therefore, the more weight you have, the less your car will accelerate.

Now, if you have a heavy car, say for arguments sake it weighs 20 times you weight that you spoke about. So, this car weighs, with 1 driver, etc, etc 3600 kgs (ok, that\'s a lot of weight, but it\'ll do for this example!). Adding that 180 kgs to the car, will increase it\'s weight by 5%. therefore, the accelration is only 5% worse. Now, if you have a lighter car, say 1180kgs, and you add that weight, you are adding 10% of it\'s total weight. Therefore, again, you are reducing it\'s ability to accelerate by 10%.

I believe the skyline is a heavier car than an FD. It\'s acceleration wasn\'t effected so much by passengers, as the percentage gan in weight wasn\'t as high as it was on the FD.

Hope that helps!

P.S. In fact, adding extra weight will probably increase transmission losses in fact, by a small margin, as there\'s a coefficient of friction. So in fact, you power at the wheels is decreased slightly! But not so much as the added weight increasing the total weight that the engine has to move anyway - so it was easier not to include that!

[TonyBlair]

Thanks for the scientific explanation mate!

All makes perfect sense. Ditch the passengers for quicker acceleration.

Cheers
Jon

[nikp]

That\'s why less weight = better acceleration.

Similar theory to lightened flywheels but the effect is even more dramatic, dut to the moment of inertia of a flywheel. I could go into that if you want to know the theory behind that!

[zevans]

Originally posted by TonyBlair
Thanks for the scientific explanation mate!

All makes perfect sense. Ditch the passengers for quicker acceleration.

Cheers
Jon

And remember kids, if you have a choice of 10% lighter, or 10% more peak BHP, go lighter. Why? In terms of raw straight line performance, you see the benefit all the way from idle to redline.

On top of that, you gain better braking, you gain better cornering, the life of brakes, drivetrain, suspension, and tyres is extended, etc, etc...

If you ditch the passengers really hard out of the back you\'ll get a bit more forward acceleration as they push off as well

[nikp]

This is all very true.

However, which is easier to obtain: 10% less weight or 10% more power!?

And what would you rather brag down the pub: I have X bhp in my car...

Or:

My car weighs X kgs!

:p

:rollin :rollin :rollin

[zevans]

In mine, 10% less weight is easier, most likely!

BHP bragging conversations don\'t last long with me in them, because as soon as you mention you\'ve actually been on a track day, BHP merchants usually go "oh" and that\'s that.

Either that or I start waving my unfeasibly large final drive ratio around