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Horsepower versus Weight
Many people talk about adding more power to their Porsche or perhaps removing a few kilo's to make it quicker. They often make these changes without first asking themselves if they are the best modifications to make. Have you ever really wondered what 30 or 40 extra horsepower might actually 'feel' like in your Porsche? Or perhaps you've thought how removing 50 kilo's would affect your car in terms of how much horsepower you'd have to gain to accomplish the same thing.
These are really important questions, as they can be used to make significantly better modification choices or even help simulate modification scenarios. Is it for example, better to shed your Porsche of a few Kilo's or add some additional raw horsepower?
For example, many people look at carbon fiber or fiber glass body panel replacements. What if you could buy a carbon fiber bonnet that weighed 10 kilo's less than the factory original. It would be interesting to be able to consider that weight saving in terms of horsepower. In other words, how many horsepower would I need to add to my Porsche in order to accomplish the same thing as losing 10 Kilo's?
Or considering if a particular scenario is worthwhile, like adding a sports exhaust system. How much extra power would the exhaust have to produce to offset its additional weight over the factory system.
All of these kinds of questions can be easily answered with some simple math.
Weight to Power
Many people often compare cars performance by looking at their Power to Weight Ratios. This is a perfectly satisfactory way to compare performance, but it doesn't help answer our questions. Weight to Power on the other hand is a better way to get an idea of acceleration performance. For example, if I have a 1,200 kg Porsche 3.2 Carrera with say 230 bhp, I simply divide 1,200 by 230 to get a weight to power ratio of 5.2 kg per horsepower. The same Porsche with 250 bhp would have a weight to power ratio of 4.8 kg per horsepower.
So Weight to Power ratio's show how much weight each horsepower has to carry.
This concept can be used for more than just simple comparisons. For example, what if you want to know how a weight loss of 50 kg from our 3.2 Carrera would translate into effective horsepower gained. In this scenario we would take 1,200 kg and divide by 230 to get 5.2 again . We then take 1,150 kg (the new weight) and divide by 230 to get 5 kg/hp. That is a slightly better ration with the weight reduction, but what would that have mean't in terms of horsepower? Or in other words what horsepower we'd have to have with the original weight to get the same weight-to-power ratio.
Weight Loss and Power Gain Considerations
1,200 / x = 5 (The original weight divided by the as yet unknown horsepower which would give us the same power-to-weight ratio as 1,150 / 230). Then simply solve for X, as below:
Old weight (1,200 kg) / New Weight-to-Power Ratio (5kg/hp) = 240 hp
So that means that by taking 50 kg out of the original 1,200 kg Carrera, we would have had to gain ~ 10 hp to accomplish the same thing without taking out the 50 kg. So in this senario, 50 kg is approximately the same as if we had done something to gain an additional 10 hp. Another way of looking at it, is like an exhaust system that adds +10hp would feel like losing 50 kg in this particular Porsche 911 3.2 Carrera. In another Porsche with different weight and power numbers, this figure would be different but calculated in exactly the same way.
Weight Gain and Power Loss Consideration
You can adopt this same method to calculate how adding weight is reducing performance in terms of theoretical power loss. So if 50 kg of luxury seats are added to our example Porsche 911 3,2 Carrera what power are we loosing.
Old Weight (1,200 kg) / New Weight-to-Power Ratio (5.4) = 220.8 hp
So that 50 kg of seats is like losing a little under 10 hp as those horses will now be dedicated to hauling that extra 50 kg.
Hopefully this has presented an alternative way to think about the weight and power of your Porsche. You can also make the calculation in reverse and see how power gained changes the car in theoretical terms of it's weight, though this is likely to be less useful. To do that, you say a 1,200 kilo Porsche with 230 horsepower has a 5.217 kg/hp weight-to-power. For this example let's assume we add 50 hp to get up to 280 hp. That's a weight-to-power ratio of 4.285 kg/hp. Now it's just a simple Algebraic manoeuvre to calculate out how much weight you would have to lose from this Porsche to make up the same amount of power:
In formula form:
New Weight to Power (4.285kg/hp) * 230hp (old horsepower) = Theoretical Weight (985.7kg).
So 50 horsepower in this particular Porsche would be roughly the same as shaving off 214.28 kg.
Conclusion
As you can see from this math it indicates that losing weight is rarely as useful as gaining horsepower, or at least, horsepower gain appears significantly more practical and cost effective than weight loss in a production car. To validate this conclusion however it is always recommended that a cost benefit analysis been undertaken including the cost of bhp power modifications, including any change in weight from the modification, versus both the practicality and cost of reducing the weight of the Porsche to provide the same theoretical bhp increase.
Of course, regardless of our goals, the least amount of weight necessary is always best.
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The leather elect seats in a 3.2 carerra weigh in at about 50kg. I am gooing to reppace them with some super light weight carbon fibre based seats which shold weigh in at just a few kilo's - so im in for a few more BHP
This is a really interesting read. I like the way of considering weight reduction as an effective way of adding horse power. Im put my 911 on a diet after reading this
That's a great way to look at it. How many BHP would I have to add to achieve the saem from losing weight. Nice article. Cheaper to lose weight me thinks..... Those electric seats in a carrera weigh about 50kg, So some nice light weight ones could give you around 10bhp. Worth thinking about. I guess the next article should be cost be addtional bhp, either by adding the bhp as a power gain or from loosing weight.
I have never thought of looking at power and weight in this way. Some very useful calculations. I'll be using this for my future Porsche diet plans. Thanks Pete,
Im glad you like it. I thought it would be interesting to look at it from a slightly different perspective
I have not personally seen this approach to power and weight before. When you think about it, it makes alot of sense to go with weight-to-power. It not only provides a view of how much weight each horsepower has to carry, but I really like the way you can more effectivly consider each of your weight or power modifications.
This is a great artcile. It makes you stop and think about the benefits versus cost of BHP increases or vitrual BHP gains from weight loss. I think there are some real gains to be made for some of the simple weight losses like seats and bumpers. But I think loosing the last few pounds that mean carbon fiber panels are probably too expensive as compared to power increase mods. Obviously a combination of both is optimal.