Power to weight ratio vs weight distribution vs aerodynamics

[SPKR]

For people who don't know much about cars, they normally think it's all about BHP. A more powerful car will always be faster.

For those who understand some, they know power to weight ratio is what needs to be considered instead of only power.

But there is obviously more to it than that.

For example, take two quite similar cars. At least they have more similarities than differences. The Corvette and a TVR Sagaris.

The Sagaris weighs close to nothing and has an engine as powerful as the base Corvette. Yet, the Sagaris is not as fast. On paper it absolutely should be.

The Sagaris published performance and data is:

Curb weight: 1,078 kg (2,376 lb)
406 BHP
Top speed: 298 km/h (185 mph)
0 to 60 mph: 3.7 seconds

The Corvette Z06 is:

Curb weight: 1,420 kg (3,131 lb)
505 BHP
Top speed: 317 km/h (197 mph)
0 to 60 mph: 3.6 seconds

The Sagaris has superior power to weight ratio and yet is still slower. The Corvette besides heavier is also a bit larger all around. The 0-60 is close. But given the weight advantage the Sagaris should beat the Corvette. In the top speed the Corvette is undoubtedly faster.

Even compared to the base Corvette, it doesn't do bad against the Sagaris considering the huge weight advantage for the Sagaris. The base Corvette is much heavier, produce "less" power but is still faster in the top end:

Curb weight: 1470 kg (3241 lbs)
405 BHP
Top speed: 306 kph (190 mph)
0 to 60 mph (97 km/h):4.5 seconds

0 to 60 the Sagaris is a bit quicker. But then again it's 27% lighter.

Considering that, it's obvious power to weight ratio is not THE deciding factor.

Thinking of why the Sagaris performs worse than the Corvette even though it has a good power to weight advantage, the first thing which came to mind was aerodynamics. I know that at around 320 km/h (200 mph) aerodynamics plays a major role and every extra speed dot takes a humongous amount of work to achieve. It being a TVR, I thought the body was just designed to look cool and nothing else. I doubted they had money for wind tunnel tests. But as it turns out the Sagaris seem to have been the first TVR which actually used wind tunnels tests and the body work aerodynamics is all functional and supposedly good.

Then my next thought was weight distribution. The Corvette has had nearly perfect weight distribution for the last 4 generations. I have no idea what the Sagaris weight distribution is, but maybe it's not that good and it just doesn't put the power down to the ground very efficiently? Because otherwise it's puzzling. The Sagaris got good reviews for its performance. So despite having never driven one, I don't think it's because it sucks.


I know other things like gearing etc as well as driver skill will play a role. But the power to weight ratio is stacked quite considerably towards the Sagaris.

By the way, I'm comparing the Sagaris to its contemporary Corvette. Not the latest generation C7. I'm comparing it to the C6. So the answer can't be just, the Corvette is more technologically advanced.

It's true any Corvette of the modern era has more electronics than the Sagaris, which doesn't even have ABS. But I would think the Sagaris performance numbers were obtained by a professional driver where this would make less difference. I don't know if the Corvette results were obtained with electronic aids off or not. But many magazines and testers turn them off for such tests. And I'm sure they were at least manual transmission cars. Besides the C6 is hardly an electronics marvel. The Corvette and TVRs are very similar concepts. Simple, plain, honest sports cars. GM does have more money. But the Sagaris has the specs stacked in its favor.
I have never seen a track lap comparison between a Sagaris and any Corvette C6s. But I think it's a safe bet the Corvette would lap faster. It lapped faster than a Porsche 911.

So what would you say plays a bigger role? Power to weight ratio, aerodynamics or even weight distribution? I don't even mean in the case of this comparison only, but in general.

Edited by SPKR on Saturday 13th January 00:29

[SPKR]

Huff said:

Perhaps TVRs never made much like the claimed power ... I'd add a knowing-wink smiley, but it's true.


Also, since power required to overcome drag rises with the cube of speed - top-end acceleration and ultimate top speed have very, very much more to do with outright power than weight. At any weight.

Example: I've a 420Kg BEC with c.170hp, which on paper is comparable with either example above.
And so, up to 80-90, and if I can get the power down /a good launch, it could beat either of the above to 60, maybe 90.. ish; certainly I've seen better times.

But above that forget it: even with tiny mass, tiny frontal area and even a very small Cd*A number - I simply don't have the excess power to swamp aero drag in the same way. At 100 all 3 cars will require 55-70hp to overcome their cumulative (aero+frictonal) drag: at which point I have perhaps 80-90bhp in hand to accelerate me+car (500kg); the TVR an optimistic amount more, let's just say 'some': and the Corvette perhaps c.400 (but 3x my all-up mass thus, at ~100mph, perhaps 1.6x my effective P:W ratio).

It's simple maths, really: if you want high speed, high-HP romps away*. 'Static' P:W, and the car's weight balance have very little to do with it. My 2p.

ETA: in small light, low-powered cars therefore, aero is a big deal: one of the reasons Fisher Furys and Sylva Phoenixes dominated RGB racing within two seasons is that - compared with '7's of comparable weight and power - the wheel-enclosing body shells of the former meant as much as +15 to even 20mph difference at the end of under-1/2mile straights: a big deal!

• and that's fine by me, I don't care about that end anyway; my playground is tight -winding B-road, and ultimate fidelity in feedback

Edited by Huff on Saturday 13th January 00:47

What you say makes a lot of sense.

Like I said, I know that at high speeds it takes a lot of power to beat wind.

It would be ok if the Corvette beat the Sagaris only on top speed but not acceleration. That would even make sense. But that's not the case. It is slightly faster at 0-60 than the Sagaris and it just shouldn't be.

So I guess the explanation can only be what you said. The Sagaris is not making as much power as publicized. Or it weighs much more.

[SPKR]

Palmers said:

Through my vast history of cars, ive always found that power to weight is key.

My superturbodiesel on a 300whp tune would marginally beat a V8 M3, because it was a few hundred kilos lighter.

But my old big powered 2 tonne barges (400-500bhp) wouldnt be that far in front of a hot hatch.

So yeah power to weight is the definition of speed or quickness in my book.

However i have noticed that lighter cars that would trance a big barge under 100mph tend to back off at higher speeds. Smaller turbos getting hot? Of just not enough weight to cut through the air? Maybe.

Once you get into fettled C6 RS6 territory (850-900bhp) the 2 tonne i feel is just irrelevant for the most part. 450bhp /tonne in a road car is mental.

Well, the Sagaris has almost that. It weighs just a bit more than a ton and makes just a bit less than 450bhp, supposedly.

[SPKR]

jagnet said:

Perfect weight distribution? For straight line acceleration in a RWD car you want the mass as much to the rear as possible to gain the most traction from the driven wheels, in which case the original Beetle has better weight distribution than the Corvette when it comes to chasing 0-60 times.

The idea that a 50:50 weight distribution for a car is somehow "automotive perfection" was just a marketing tag line dreamed up by BMW, but it stuck and became "fact".

When it comes to balanced weight distribution I will have to disagree with it being just marketing. For straight line it makes sense that having more weight over the driven wheels is better than well balanced for a 2WD car. But for handling and going around a track, good balance is fundamental and 50/50 helps to make weight transfer easier to manage in my opinion.Balance is a good thing for handling in general.

[SPKR]

Bill said:

Grip plays a big part in 0-60 times, so weight can be an advantage. The Sag driver is probably having to feather the throttle in the first three gears. Come to the first corner and he's braking later, cornering faster and accelerating harder out the other side.

Do you mean the TVR would lap faster than the Corvette? I'm not so sure. I was surprised watching a 911 vs C6 test because I thought the Corvette would get the 911 in the drag race and the 911 would lap faster. But actually the 911 got the Corvette in the drag race because it was a dual clutch paddle and the Corvette a stick. Then the Corvette lapped faster than the 911, which I would have never thought. Also reminds me of the 4C vs Corvette challenge on Top Gear. The Corvette no longer fits the usual mantra of U.S. cars being only good for straight lines. For the last couple generations it's hanging with the best.

[SPKR]

All great points.

Sure the claims might not stand up. But some of the data are from magazine tests such as Motor Trend. Not from the manufacturer, if that makes any difference.

I see the results with the Z06 were close. But I would have thought the Sagaris would at least smash the base Corvette with those specs.

Since apparently the aerodynamics are taken care of with the Sagaris, if you are to believe reviews and articles of the time, I think the most reasonable explanation is, as some have said, the Sagaris is not making 406 bhp. Upon further reading I already found sources listing 380 bhp rather than 406 bhp for the Sagaris. Gearing could also be the culprit here, sure.

But I didn't mean to turn this into a Corvette vs Sagaris topic. I wanted to discuss the issue at hand in general.

All the other points are also well made. Powerband, grip, differences in technique, drivers, road surface and all. I guess this is the difficulty of making a road sports car. If you are building something dedicated, such as a dragster, all you have to worry is that.

So with the interest of pushing the conversation away from Corvette vs Sagaris, if you had to build a sports car which did as well as possible on everything, what would you give priority to? From power to weight, weight distribution, power, aerodynamics, gearing etc? I know it's basically impossible to be good at it all. So let's bias it towards handling as I think that is more useful on the road than something like top speed. You can never really use it.

But let's still keep the car fast. An Elise handles like a dream but it's not what I would consider truly fast.

So what's your recipe?

[SPKR]

High top speed would be nice but not at the cost of handling and acceleration. But also not something that will handle great but can barely reach 320kmh-200mph. A compromise would be great. But biased towards handling rather than top speed.

[SPKR]

Equus said:

God forbid that one would have to suffer something that could barely reach 200mph as one's daily driver.

Wait, I didn't say daily driver. I said road car.

He asked what I consider fast enough and I told him. Basically it doesn't need to be Bugatti or Koenigsegg fast. 200mph today is not really that fast. It's basically mid range for supercars.

[SPKR]

grumpy52 said:

One of the factors in acceleration is the gearbox .
Ratios and speed of change can ?eally have an unusual effect on times .
Most testers don't lift while doing gear changes .
Having the engine drop lots of revs between ratios is not ideal for good acceleration. Or good lap progress .

Yeah. Random data is difficult to compare. Much easier if it's all set by the same driver, same day and same conditions. But I needed the data to illustrate the point. Although the point was to start a discussion about the roles played by the different attributes and how they interact and balance with each other. Rather than a discussion only about that specific comparison. Although we have had some very good points being made about the comparison nonetheless.

Edited by SPKR on Saturday 13th January 23:44

[SPKR]

jagnet said:

Without doubt good balance is important, but balance imho has more to do with suspension tuning than weight distribution. With bad suspension tuning even a car with front biased weight distribution can be made to handle like a first gen Corvair.

For a RWD car more rear bias (maybe 40:60 f/r) is preferable - better traction accelerating, better braking, better turn in, better acceleration out of corners. With 50:50 as soon as there's any weight transfer there'll be under utilised axles whether the weight transfer be fore or aft.

The difficulty is, for a front engined rear wheel drive road car getting any sort of rear biased weight distribution is nigh on impossible without severely compromising passenger space, so make it 50:50 and then get the marketing department to claim that's perfection. It's better than 60:40 f/r for sure, but it's not "perfect".

Fair enough and I see where you are coming from now.

And I agree with what you are saying. Although I'm a bit on the fence about better turn in. Could you elaborate on the technical points which make you say that?

I totally ignored the suspension side because I would like to think it is already sorted. I'm starting from the point where it will be already properly set up for the type of car. But absolutely, suspension is very important too.

But I see where you are coming from. The word "perfect" was probably not the best choice to describe it. I should probably have used something like "optimal weight distribution for a front engine car" or "as good as it gets weight distribution for a front engine car". I guess that would be more fair. Because around 50/50 is as good as it normally gets for a front engine car. Do you know of any RWD front mid engine road sports car with a rear weight bias such as 40/60 f/r? It would be interesting to know if there is something out there. But I'm thinking that would be difficult to achieve for a road car.

[SPKR]

PhillipM said:

Even then it means very little - it really is just marketing b*****t - what good is 50:50 weight balance if you've achieved it by hanging very heavy objects over the wheelbase at either end of the car, compared to something like a Lotus with 60:40 weight balance and all that mass concentrated in the centre of the car as close as possible to the yaw axis?

50:50 weight balance is far from ideal in almost anything (how many race cars do you see chasing absolute 50:50 weight balance over minimising MOI? I'd guess almost none) - it's just marketing stuff that the press gobbled up.

Everything in the centre of the car, with 4wd, then 50:50 on top, and you're going somewhere. But in isolation it doesn't really matter.

Edited by PhillipM on Sunday 14th January 14:30

I think jagnet's response to the same post makes more sense than yours. Because we are not talking about race cars here, so there is very little point in worrying about how many race cars chase 50/50 weight balance .

[SPKR]

samoht said:

I interpret 'as well as possible' to be about objective capability.

First priority, as small as possible. Anything bigger than two people and a suitcase is excess bloat and makes a sports car worse at being a sports car.
Second, having got it small, a carbon-fibre monocoque / panels, magnesium wheels etc to make it as light as possible.
Engine - turbo for power density. Either a four-pot or a rotary (because both are small in service of the above, and make enough power if turboed).
Suspension - double wishbones.

This yields a few possible paths:

1) 2.0 turbo four-pot, transversely mounted behind the cabin. Should be able to get about 400hp and good driveability with variable cam lift and one of today's variable geometry turbos. Basically like an extreme version of the new Alpine, maybe get it down to a tonne with the carbon chassis. Would be v quick, although I'm not sure to what extent the transverse layout gives a higher c.g.
2) Front-mid engined with a rotary turbo. Again, 400hp should be very reasonable with v.g. turbo, one tonne with a carbon chassis (the RX-7 is all steel and 1275kg). Should have a lower c.g. than (1), but a less rearward weight bias so slightly less traction.
3) Mid-engined with a longitudinal flat-four like the new 718 Cayman, but smaller and lighter. Take power off both ends of the engine and have another gearbox at the front for 4wd (like the Ferrari FF does). Would be heavier, but the 4wd capability would make it quicker in the wet because you could get the power down.

In all cases, approx 400hp/tonne would make it really very quick, and it would probably top out around 180mph at a guess. Being small, light and fairly well balanced (somewhere between 50/50 and 40/60 f/r distribution) should make it handle, too.

I'm not sure I agree with as lights as possible. Or even as small as possible. Light yes. As light as possible probably not. Too light is not really the best. I think it's about finding a balance. As many have said here and I agree, too light may become counterproductive. If you could strap some sort of high tech engine the size of a coke can, which makes 1000 bhp ,to a car that is the size of a kid's pedal car just large enough to fit and adult, would you really want to?

This again reminds me of the Corvette vs Alfa 4C challenge on Top Gear. For those who haven't seen it, Clarkson bet was that the 4C would lap faster than the Corvette because it was lighter and so would gain time around the curves, while the Corvette would have to slow down too much for the curves, etc, etc, etc you know, the usual cliche bias against U.S. cars. But Hammond won in the Corvette. Even if the 4C could go around the curves faster, in the end was about balance.

Now I know the 4C doesn't have your suggested 400hp/tonne. It has 270. But the Corvette only has 300 too. But the point is balance.

If you want a car to drive just around tight curves all the time and nothing else, then yes, the 4C. Just as if you want a car only to drag race, then a Veyron. But we normally need a balance of the two. So I think as light as possible may be as bad as too heavy. Balance is the key IMO.

[SPKR]

Notanotherturbo said:

I used to use the rolling road of a TVR specialist and he said their factory figures were massively inflated - as much as 20%. You smell a rat if you looked at their old stats , always a perfect rounded up number - 300 - 350 -400. 1/4 terminal speed is the best figure to judge the faster car as it takes traction out of the equation

That's interesting to know. Since I heard the Corvette can actually be the other way around, with some engines making actually making more power than advertised, this combination could well explain the whole thing.

[SPKR]

996TT02 said:

If the figures are really meaningful - the reason the TVR is "slower" to 60 (if we must) - or, not a lot faster, would be down to how much power you can actually put down on the road. Light weight is great, but not proportionally great, because traction also decreases. On a less than perfectly grippy road TC comes in even on my 4WD "just" 420bhp 911 so with just the light rear wheels driven and barely or no driver aids getting even 300bhp to translate into forward motion rather than smoke and noise is a challenge. I don't know about the comparable static weight distribution but the Corvette has what is deemed to be a front-mid engine, which puts more weight on the rear wheels.

I agree. Too light is not the answer.