Megabusa / Megablade etc etc
Discussion
On the other side of the coin I have a GSXR MAC1 and I weigh about 11st. Its very fast with just me and only fast with my fattish brother (13st ish) as passenger. I deffinatly notice the difference. I can wheel spin in 2nd straight line but not with someone else. Remember that people riding bikes notice two up in speed sense of things (obviously with handaling aswell). Also these cars weigh about 450kg. so two 100kg people is a serious weight addition. They are fast dont get me wrong but you will notice the difference if on your own. Also consider extra stresses on clutch and engine. They are designed to carry approx 300kg as stock. The essence of these cars are light weight. MAC1 Worx have thinner fibre glass for starters!!!
Bi22le said:
On the other side of the coin I have a GSXR MAC1 and I weigh about 11st. Its very fast with just me and only fast with my fattish brother (13st ish) as passenger. I deffinatly notice the difference.
Any sort of even slightly sensitive driver will notice the difference on relatively 'heavy' cars like a Lotus Elise, never mind on a BEC.I've quoted these figures before, but for reference, my FW400 (similar weight to the lightest BEC's, but more torque, so less weight-sensitive) was properly timed by Autocar magazine when they road tested it on a couple of occasions: with solo driver, it managed 0-60 in 3.6 seconds and 0-100 in (iirc) 8.1 seconds. 2-up for the 'standard' road-test-at-kerb-weight-with-passenger, it could only manage 4.5 seconds to 60 and 10.4 seconds (again, IIRC) to 100.
In other words, the acceleration was 20% slower.
Spookily enough, the extra weight of passenger and fuel would have represented approximately a 20% increase in total weight. Now how weird is that?
It's very simple to work out if the car will still be effective or not. 500kg is a reasonable assumption for a Busa powered 7 car, and the engine has aproximately 180bhp.
Just calculate the power to weight in each circumstance:
Car alone = 180bhp / 0.5 = 360bhp/tonne
Car + 90kg driver = 180bhp / 0.59 = 305bhp/tonne
Car + 90kg driver and passenger = 180bhp / 0.68 = 264bhp/tonne
So in full working and laden weight with fuel, you'll still have 264bph/tonne. The average Evo or Subaru with 300bhp will only have 189bhp/tonne with the same passengers on board.
Just calculate the power to weight in each circumstance:
Car alone = 180bhp / 0.5 = 360bhp/tonne
Car + 90kg driver = 180bhp / 0.59 = 305bhp/tonne
Car + 90kg driver and passenger = 180bhp / 0.68 = 264bhp/tonne
So in full working and laden weight with fuel, you'll still have 264bph/tonne. The average Evo or Subaru with 300bhp will only have 189bhp/tonne with the same passengers on board.
LaurenceFrost said:
It's very simple...
Simple, but wrong. Trying to make an assessment purely on the basis of power:weight ratio like this is very misleading because it completely fails to take into account aerodynamic drag.
In very basic terms, you've got to subtract the amount of horsepower being absorbed by aerodynamic drag and rolling resistance and the acceleration is then roughly proportionate to what's left over, divided by the weight of the car.
Aerodynamic drag rises exponentially (ie. in proportion to the square of the speed), so this starts to count very heavily (pardon the pun) against cars that achieve a high power:weight ratio by means of light weight, once you get up to speeds where the rise in drag starts to 'bite' (>60mph), in comparison to those that acheive it by means of brute horsepower.
In practice, it means that Laurence's 264 bhp/tonne Seven will have hit Vmax and stopped accelerating altogether before it hits 130mph, whereas the poor old 189bhp/tonne Evo or Subaru will still have enough power left over to still be accelerating hard, on its way to a potential Vmax of 160mph or so.
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Yes, for sure... I just felt that Laurence's pseudo-scientific calculations were a tad misleading.The forte of lightweight cars is carrying speed; by the time you get to the sorts of speeds you can hit even on the relatively short straights of a motor racing circuit (bearing in mind that even the average lap speeds are well into the region where drag is significant), the aerodynamics on any 'Seven' are really beginning to tell.
And - on topic - they are very sensitive to additional weight both in terms of how much of that speed they can carry, and how long it takes to recover it when lost.
Kiwi XTR2 said:
Sorry, I just HAD to quote that before you get savaged by the Lotus Mafia 
In my defence, I've been a Lotus enthusiast since most Elise owners were still in nappies - I became a fan of the marque when 72's were winning Grand Prix, I bought my first example (a Plus 2S/130) in 1987, and I've owned two Elises myself - I think they're fabulous little cars.Mind you, mine were both S1's; before they were softened up to avoid scaring the hairdressers and computer geeks.
Sam_68 said:
LaurenceFrost said:
It's very simple...
Simple, but wrong. Trying to make an assessment purely on the basis of power:weight ratio like this is very misleading because it completely fails to take into account aerodynamic drag.
In very basic terms, you've got to subtract the amount of horsepower being absorbed by aerodynamic drag and rolling resistance and the acceleration is then roughly proportionate to what's left over, divided by the weight of the car.
Aerodynamic drag rises exponentially (ie. in proportion to the square of the speed), so this starts to count very heavily (pardon the pun) against cars that achieve a high power:weight ratio by means of light weight, once you get up to speeds where the rise in drag starts to 'bite' (>60mph), in comparison to those that acheive it by means of brute horsepower.
In practice, it means that Laurence's 264 bhp/tonne Seven will have hit Vmax and stopped accelerating altogether before it hits 130mph, whereas the poor old 189bhp/tonne Evo or Subaru will still have enough power left over to still be accelerating hard, on its way to a potential Vmax of 160mph or so.
It's only after 100-110mph that you would really see your typical 300bhp saloon start to claw any gain back, but on your average circuit (most typical environment for a BEC), you'll be doing this speed once per lap, so I really don't think power vs co-efficient drag and frontal area are factors worth doing the maths over in this example.
I think the power to weight ratio is a very accurate representation of a BEC when comparing straight line speeds on circuits. If most were geared to 150mph then it would be a different story.
LaurenceFrost said:
Most BECs are mechanically limited to about 120mph anyway, so the aerodynamics of the situation really don't come into it to the point it's worth debating.
Wrong.As an example of high power vs low weight as a means of achieving high power:weight ratio, I ran a very rough spreadsheet comparison, using an SEight versus a Megabusa (to give identical aerodynamics), with the SEight's horsepower figures set to give an identical laden (with driver) power:weight ratio.
In case any of you kiddies want to try this at home, the figures I used were:
Kerb weight: 420 kilos (Megabusa)/750 kilos (SEight)
Driver weight: 85 kilos
Power: 185bhp (Megabusa)/306bhp (SEight) ~ giving 366bhp/tonne laden in both cases
CdA: 8.9
Transmission loss : estimated at 15% for both cars.
Rolling resistance: estimated at 0.09bhp/tonne/mph
This gave effective power:weight ratios (ie. the power left over for acceleration, once drag, transmission loss and rolling resistance have been subtracted, divided by the weight of the car+driver) as follows:
| Speed | Megabusa | Seight |
| 50 | 292bhp/tonne | 298bhp/tonne |
| 60 | 280bhp/tonne | 290bhp/tonne |
| 70 | 264bhp/tonne | 280bhp/tonne |
| 80 | 243bhp/tonne | 267bhp/tonne |
| 90 | 216bhp/tonne | 250bhp/tonne |
| 100 | 182bhp/tonne | 230bhp/tonne |
| 110 | 142bhp/tonne | 205bhp/tonne |
| 120 | 93bhp/tonne | 175bhp/tonne |
| 130 | 35.6bhp/tonne | 140bhp/tonne |
| 140 | -31.1bhp/tonne | 99bhp/tonne |
Obviously the negative figure for the 'Busa at 140mph is telling you that it couldn't actually pull that speed. For what it's worth, given perfect gearing, it'd probably be flat out at about 136mph according to the calculations (which corresponds pretty closely to Westfield's claims for the car).
What this table demonstrates quite clearly is that for two cars with identical power:weight ratios, and identical aerodynamics, the car that uses the brute force approach to high power:weight ratio is showing a significant advantage in acceleration at even quite modest road speeds. For example, at 70mph - which is a corner exit speed on many bends on British circuits - it's already showing an effective 16bhp/tonne advantage. By the time you've hit 100mph on the straight, that's risen to an effective 48bhp/tonne advantage, and by the time the BEC is bouncing off Laurence's 120mph rev limiter down the straight (though Busas are usually geared taller), you're looking at an effective 82bhp/tonne advantage for the heavier car.
Same power:weight ratio; yet an 82bhp:tonne difference in the power that's actually available for acceleration.
Now do you understand why I say your suggestion that power:weight ratio alone is a misleading means of comparing performance, Laurence?
And that's for two cars with identical, barn-door aerodynamics. Give one of them a nice, smooth, streamlined bodyshell and the effects become even more pronounced...
Nice post Sam and demonstrates why a BEC is like a scalded cat at low speed and err, slightly boring at 90 mph plus 
Also most busa engines are making about 165-170 bhp in stock form with stock kit car exhausts, even with a power commander, good exhaust and better induction system it's not going to be any better than 180 without modifying the engine. It seems many people start off with the correct figure then as they build their car, the power gradually increases until it's on the road and suddenly they tell their mates it's got 190 bhp
Also most busa engines are making about 165-170 bhp in stock form with stock kit car exhausts, even with a power commander, good exhaust and better induction system it's not going to be any better than 180 without modifying the engine. It seems many people start off with the correct figure then as they build their car, the power gradually increases until it's on the road and suddenly they tell their mates it's got 190 bhp
Of course I should stress - before someone accuses me of being disingenuous - is that the above figures only demonstrate the acceleration capabilities.
As I said above, the great advantage of light cars is their ability to carry speed and, in the above example, the much lighter Megabusa should be able to brake a lot later and corner a lot faster than the SEight, so in practice it ought to be able to lap a circuit quicker.
The caveat being: 'if it can keep it's tyres in contact with the track surface', because the suspension set-up is much more sensitive on very light cars and it won't corner faster and brake better if rubber and tarmac don't meet.
We're drifting off topic, though: I'll run another variation of the spreadsheet figures tonight to show what happens to the effective power:weight ratio when the OP sits his fatb
d mate in the passenger seat of a Megablade...
As I said above, the great advantage of light cars is their ability to carry speed and, in the above example, the much lighter Megabusa should be able to brake a lot later and corner a lot faster than the SEight, so in practice it ought to be able to lap a circuit quicker.
The caveat being: 'if it can keep it's tyres in contact with the track surface', because the suspension set-up is much more sensitive on very light cars and it won't corner faster and brake better if rubber and tarmac don't meet.
We're drifting off topic, though: I'll run another variation of the spreadsheet figures tonight to show what happens to the effective power:weight ratio when the OP sits his fatb
d mate in the passenger seat of a Megablade...Hi Sam, very interesting post and the theory is absolutely bang on and makes good reading. It certainly stacks up with why the car starts to struggle after around 90mph when compared to my 400bhp Evo 9, but I still stick to my original comments that power to weight is the easiest way of calculating how fast a Busa 7 will be in most real world situations.
I have seen it so many times, and it's really not until over 90mph that your average SubarEvo will start to edge (and I mean edge) away. Fairly soon after this point though, you'll be slamming on the brakes for a corner/tree/car, so the power against aero advantage of another car is so short-lived in almost every situation (i.e. above 90mph), that I still don't think it's worth debating.
Also, in your example you are comparing two cars with identical aero resistance. In a Busa 7 vs Subaru debate you are not only calculating against Cd, but also frontal area of the car which will be larger on the Subaru, so more drag even if it has a lower Cd.
If you wanted to work out the exact point that a Busa will start loosing out to a Subaru with 300bhp then you can, but in reality it won't translate to anything less than about 90mph IMO, so power to weight will give a faster and better indication.
So it's theory vs practice really. Your posts are still very interesting, but I just don't think they are that relevant when talking about something which will top out at 120mph.
I have seen it so many times, and it's really not until over 90mph that your average SubarEvo will start to edge (and I mean edge) away. Fairly soon after this point though, you'll be slamming on the brakes for a corner/tree/car, so the power against aero advantage of another car is so short-lived in almost every situation (i.e. above 90mph), that I still don't think it's worth debating.
Also, in your example you are comparing two cars with identical aero resistance. In a Busa 7 vs Subaru debate you are not only calculating against Cd, but also frontal area of the car which will be larger on the Subaru, so more drag even if it has a lower Cd.
If you wanted to work out the exact point that a Busa will start loosing out to a Subaru with 300bhp then you can, but in reality it won't translate to anything less than about 90mph IMO, so power to weight will give a faster and better indication.
So it's theory vs practice really. Your posts are still very interesting, but I just don't think they are that relevant when talking about something which will top out at 120mph.
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