Lose weight or gain power??? An interesting article(?)

[Facial Hair Optional]

I found this earlier this week. It originally was all in lbs (being written by a Yank) so I have converted the figures and rounded them up/down as needed. So if you ask 'why 1590 kgs' now you know!!

It can be a bit confusing but I think is worth getting your head around if you are looking to gain performance.

The guy made one or two mistakes (starting calling it power to weight ratio half way through when he made it clear at the start he was talking about weight to power ratio for one thing!). I have corrected those errors in this version. I have added my two penneth worth at the very end!

I dont believe in plagiarism so here is his original article.. http://www.tuneruniversity.com/blog/2012/03/power-to-weight-ratio/

Here it is then.............................. in kgs!!   



How to Compare Weight Savings to Horsepower Gains


Have you ever wondered what 10 or 20 extra horsepower might "feel" like in your car?

Maybe you've wondered how removing 50kgs will affect your car in terms of how much horsepower you'd have to gain to accomplish the same thing.

These questions are wise to ask because they can be used to make significantly better modification choices and frankly it can be fun to "simulate" different modification scenarios.

For example, if I could buy a carbon fiber hood that weighs 10kgs less than factory, it'd be nice to be able to view that weight loss in terms of horsepower. In other words, how many horsepower would I need to gain in order to accomplish the same thing as losing 10kgs? (Hint: it's pathetically little)

What if I wanted to determine if a dual exhaust system is worth while? How much more power would I have to make to offset the extra 10kgs? (in a 1590kgs car with 215 hp, not even 1.25hp, so probably do-able)

Can someone on the forum claiming to feel a 1-2hp gain on their 'bum dyno' really do so? Well, using this formula you'd see that they'd have to be able to feel the difference between having groceries in the car vs not having groceries in the car to "feel" that supposed gain.

What about if I wanted to see how much weight I'd have to lose to compete with the same car with 50 extra horsepower?

All of these kinds of questions can be answered with the simple math in today's article.



Weight to Power


You've probably heard of power-to-weight more than weight to power, they're the same thing but one has nicer numbers and a better visual representation so I'll be using weight to power for this discussion.

Weight to power is one way to get a general idea of acceleration performance. For example, if I have a 1590kg car with 215 horsepower, I simply divide 1590 by 215 to get a weight to power ratio of 7.39kgs per horsepower. The same car with 250 hp would have a weight to power ratio of 6.36kgs per horsepower.

To give you a really over simplified visual, imagine that each "horsepower" is actually a horse. The fewer pounds the horse has to drag with it, the easier it is for it to run.

The Bugatti Veyron with 987 bhp and a curb weight of 1892kgs (American figures before someone tells me it has 1000+bhp!) has only 1.92kg/hp and thus is significantly faster than our example car. (By comparison my Tiger has 4.04kgs/hp and the Fury has 1.97kgs/hp... allegedly!)

Great, that's all very simple. So how is this useful beyond comparing vehicles to one another?

Well, let's go back to the 1590kg car with 215 horsepower example. Let's say I want to know how losing 50kgs would translate into horsepower gained, because let's face it, most of us think in terms of horsepower gains.

So we take 1590kgs (the original weight) and divide by 215 to get 7.39kgs/hp again (rounding for simplicity, you'll want to use the full number to get accurate results).

We take 1540kg (the new weight) and divide by 215 to get 7.16kgs/hp. That is better of course, but what would that mean in terms of horsepower? Well, we do some simple Algebra to see what new horsepower we'd have to have with the original weight to get the same weight-to-power. Stick with me as this is really cool/useful:

1590kg/ Xhp = 7.16kgs/hp (Non-geek translation: original weight divided by some unknown new horsepower would give us the same power-to-weight ratio as 1590kg/215hp did). Solve for X (which I have done for you below)



"Weight loss" to "Horsepower" Formula


Old weight (1590kg) / New Power-to-Weight Ratio (7.16) = 222.06 hp (new hp needed to get the same gain as removing 50kgs would give)

So that means we would have had to gain ~ 7.06 hp (222.06 minus 215 - the new hp minus the original hp) to accomplish the same thing WITHOUT taking out the 50kgs. Thus in this situation, 50kgs is roughly the same as if we had done something to gain 7.06hp. So to some extent, this means that something like an intake that adds +7hp would FEEL kind of like losing 50kgs (or a very skinny passenger) in this particular vehicle. In another car with different weight and power numbers, this figure would be different but calculated in the same way.



"Weight Gain" to "Power Loss" Formula


You can also use this same method to determine how adding weight is hurting you in terms of theoretical power loss. So if I add 20kgs of stereo equipment to the same car, it's

Old Weight (1590) / New Weight-to-Power Ratio (7.49) = 212.28 hp (the new weight to power ratio would be 1610\215 = 7.49)

So that 20kgs of equipment is LIKE losing a little under 3 hp as those horses will now be dedicated to hauling that 20kgs.

This will give you a new way to think about weight and power. You can also go the other way and see how power gained changes the car in theoretical terms of weight, though I find this less useful. To do that, you say a 1590kg car with 215 horsepower has a 7.30kg/hp weight-to-power, and let's say we gained 50hp to get up to 265 hp. That's a weight-to-power of 6.0kg/hp. Now it's simple Algebra again to figure out how much weight I'd have to lose off the car to make up the same amount of power:

In formula form:

New Power to Weight (6.0) * 215 (old horsepower) = Theoretical Weight (1290).

So 50 horsepower in this particular vehicle would be roughly the same as shaving off 300kgs. As you can see from this math, this is why losing weight is rarely as useful as gaining horsepower, or at least, horsepower gain is significantly more practical and cost effective than weight loss in a production car. (Note from Facial Hair Optional... ie in general it is easier to gain an extra 50bhp than to lose 300kgs, although in kit cars it would be more like trying to gain 25bhp versus trying to lose 150kgs!!!!)

Of course, this is also if we only take into consideration the acceleration effects of weight and power. In road racing or in a daily driver, weight loss is more useful than in drag racing. Of course, regardless of our goals, the least amount of weight necessary is best.

Note from FHO.... so if you are looking to gain performance start with more power rather than losing weight, or better still do a bit of both if the weight loss is easy stuff to do!! Trying to lose 1 or 2 kgs will make sod all difference according to this theory, whereas gaining 5 or 10 bhp should be more noticable. I hope.   

If there are any errors in this......... blame the American guy!!! 

[Facial Hair Optional]

Long innit!!!!!!!!!!!!!!! 

[paintman]

zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz    If I lose any more weight i'll be a skeleton.  lol......

[Camber Dave]

I removed the (non working) air con stuff (35Kgs) from my MX 5 and the Weight to power ratio went from 7.15 to dizzying 6.95Kg/Hp.
Cost - sod all.
Benefit - perceptible feel of more acceleration and agility.
Actual benefit - 0.1% of G (sod all!) - measured with data logger.

In theory removing weight benefits  Acceleration, Braking and Cornering so as an upgrade should be initially cheaper than buying extra power.

[Iancider]

Yep, typical US write-up - it is all about straight line acceleration.

I proved the value of light and good balanced on my first track day.  Yes there were some tin-top muscle cars on the track and they were impressively loud.  They were also all over my back-end on the straight.  But when I looked in my mirror they had left the track on the corners.   And when I braked I could decelerate better than them and they still overshot the corners.   

Light means nothing when the acceleration is over.  So while I know I can beat any Aston Martin on acceleration, they will still be able to beat my 130mph top speed and their hair would not even be ruffled.  On the next corner I will catch them again, they will leave the track and they will become the owner of a much lightened pile of scrap metal that would not beat anything!  Smug with a capital SM at that point  .   

In a previous thread I computed that I needed another 40BHP to compensate for my testosterone fuelled passenger.  I could add 40 horses or just compromise.  Something petite in a bikini only needs 20 horses.  Job done! 

[Hairy Santa]

Yep, typical US write-up - it is all about straight line acceleration.

I proved the value of light and good balanced on my first track day.  Yes there were some tin-top muscle cars on the track and they were impressively loud.  They were also all over my back-end on the straight.  But when I looked in my mirror they had left the track on the corners.   And when I braked I could decelerate better than them and they still overshot the corners.   

Light means nothing when the acceleration is over.  So while I know I can beat any Aston Martin on acceleration, they will still be able to beat my 130mph top speed and their hair would not even be ruffled.  On the next corner I will catch them again, they will leave the track and they will become the owner of a much lightened pile of scrap metal that would not beat anything!  Smug with a capital SM at that point  .   

In a previous thread I computed that I needed another 40BHP to compensate for my testosterone fuelled passenger.  I could add 40 horses or just compromise.  Something petite in a bikini only needs 20 horses.  Job done! 

or without the bikini only 19 horses 

there is an old book called "Speed and how to obtain it " well worth a glance at,
always remember the 1st Law  ----  Power over Weight = Speed

[Facial Hair Optional]

I removed the (non working) air con stuff (35Kgs) from my MX 5 and the Weight to power ratio went from 7.15 to dizzying 6.95Kg/Hp.
Cost - sod all.
Benefit - perceptible feel of more acceleration and agility.
Actual benefit - 0.1% of G (sod all!) - measured with data logger.

In theory removing weight benefits  Acceleration, Braking and Cornering so as an upgrade should be initially cheaper than buying extra power.

Very true, especially if you are just removing stuff, but if you are buying expensive lightweight replacement parts maybe not so cheap? Go on a diet is what I plan to do, there's a good 20kg gain there!!!

[Lucky Ed]

Remember - money can't buy you happiness, but it can buy you horsepower -

..... which pretty much amounts to the same thing

[sanzomat]

I missed this thread until now. Interesting  but in many ways flawed because lightness improves many things that influence driving pleasure and real world speed whereas power only influences acceleration and top speed (and, okay the ability to do power slides Weight is involved in the sums for acceleration, cornering, braking and probably many others too. If its top speed you are after aerodynamics comes into play significantly as most of the power at high speed is being used on moving the air out of the way so to get more top speed you need more power or less drag and weight ceases to be a factor. 7's don't score so well here! I would imagine that if the only factors being changed on a given car are weight and power the sums shown may hold true in acceleration but the heavier car with more power would surely have slower lap times on a circuit. Might use a bit more fuel too. Take this article on a 1.0 ford engine in a "road legal" formula ford:

Diddy Ford outpaces Enzo at the 'Ring...

...using a 1.0 litre Ecoboost engine. The future's looking bright

So. Going fast around the Nürburgring is all about big engines and plenty of horsepower, right? Wrong.

Ford has just proved that size isn't everything by letting a one-off, street-legal version of its popular Formula Ford loose around the infamous 'Ring. Powered by the diddy 1.0-litre, three-cylinder Ecoboost engine (like in the new Focus), and a bit of a tailwind, it clocked a time of 7 minutes, 22 seconds. That's faster around the 'Green Hell' than a Lamborghini Aventador, Ferrari Enzo and Pagani Zonda.

Weight is obviously important - or lack of it. This racer weighs around 500kg and is modified to use Ford's 1.0-litre three cylinder in conjunction with the turbo from the standard Formula Ford. Boffins at Ford did tinker with the engine's ECU to squeeze out 202 bhp, 22 bhp more than the 1.6 litre that normally sits in the single-seater, but there were no major engine mods.

And thanks to some indicators, wing mirrors, wheel covers and a horn, you can even drive it to the shops. Yes, it's road legal. And if you can live without your creature comforts, it's even somewhat practical.

Of course, the headline stats of 0-62mph in under four seconds and a top speed of 158.8mph sound fantastic. But where it gets very interesting is fuel consumption. Ford is claiming that in testing, the EcoBoost-powered single-seater returned 118mpg at 35mph and 57mpg at 75mph. Now that's having your cake and eating it.
"This little engine has people rubbing their eyes in disbelief," said Nick Tandy, the man who piloted the car for the
lap. "It's simply astonishing that a three-cylinder, 1.0-litre engine can deliver that kind of performance."

[Iancider]

Agreed,  Ford Ecoboost 1.0

That is a fantastic little engine and it sounds good too - a bit like a six but trying much harder!  Don't anybody knock it until you have tried it - it really is a quart in a pint pot and a lot of fun to drive.

I am actually surprised that superlight kits aren't using it yet.

Ian

[Badger]

I've only heard good things about that engine so far - it seems purpose made for a mini conversion

...Another project to add onto the bucket list 

[Crunchie Gears]

Great article!

I like light weight cars and my brother and I own a Westfield FW400.  It weights 400 Kgs and we have made some modification to it over our ownership.

In considering these changes we have looked at weight reduction as because the car is so light weight reduction give a bigger bang IMO.  However when considering weight reduction one might consider weight reduction of rotating parts as this reduces the Moment of Inertia and so you get a double gain.

For example taking weight out of a flywheel and clutch assembly not only reduces weight but also results in the engine having to spin up less mass.

Most US articles do not look very closely at the Moment as on a drag strip spinning up a heavy flywheel might give an advantage as it is stored energy, we have all had the kinder egg toy car to prove how effective stored energy con work.

I could go on about the weight loss and the weight we added to, as we added more power too.   the most impressive thing about the car is how well it brakes!