How relevant is BHP per tonne
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Soldato
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This is very important point:
Imagine a 3.0 v6 engine (for arguements sake) with 200hp. This doesn't seem a huge amount but it pulls nicely all the way through the revs.
The new version of this car may have 260hp but it is rubbish at low revs and the power only really peaks at the red line.
The 260hp one to market the car as being 'faster' as the last.
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but the likeliness of the new 260bhp one being faster is quite high though. even if it had rubbish bottom end. you are presuming that the manufacturer made it gear quite well and add a competant driver?
sure ingear times might vary but outright performance/accelertation should be better for the 260 vs the 200 taking the weight to be the same in both models.
Obviously this precludes a manufacturer building a car with a front end shaped like a brick wall with just 1 gear and 3 tyres.
but then why would we be comparing a brick with a performance car?
its about as good a general yardstick as you can get, without adding in umpteen other factors, imo.
sure ingear times might vary but outright performance/accelertation should be better for the 260 vs the 200 taking the weight to be the same in both models.
Obviously this precludes a manufacturer building a car with a front end shaped like a brick wall with just 1 gear and 3 tyres.
but then why would we be comparing a brick with a performance car?
its about as good a general yardstick as you can get, without adding in umpteen other factors, imo.
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Geared correctly for what? Different cars are geared from the factory for all different purposes.
Long / Tall gearing will have an effect in all of the gears.....
Gear changes are slow though, so my MR2 with only 160bhp is actually quite quick to 60 due to:
Anything over 100 and it really starts to slow down drastically as wind resistance and gearing start to tell against it.
- Rear Wheel Drive (Good traction off the line)
- 2nd Gear is good for 75mph (only 1 change required)
- Relatively light weight (sub 1200kg)
Anything over 100 and it really starts to slow down drastically as wind resistance and gearing start to tell against it.
You completely misunderstood my post. Long gearing often means your in a lower gear than a shorter geared car. Which means the shorter geared car has no advantage.
Of cour though, the gearing needs to match the engine characteristic to some extent
BHP/Tonne is very relevant IMHO.
Obviously Aero and Gearing is going to affect the top speed, but if I was only using one number in a comparison it would definitely be the BHP/Tonne.
In a track scenario, a high BHP/Tonne car is going to accelerate faster. Might not have the best top speed (but how long is the straight anyway?). Then turns up a corner and the high BHP/Tonne car weighs less so stops a lot quicker and corners faster, then accelerates out faster.
I was passenger in Dad's Westfield at Castle Combe the week before last (it was only a public day so no timing and stupid amounts of traffic). Yes the high power cars start creeping up on the straights, but the 500BHP/Tonne Westfield just laps so much quicker than anything that was there. Still being foot flat-to-the-floor for an extra second or two coming into the corners compared with the lardy Scoobs and Evos is just hilarious!
High power to weight doesn't necessarily mean low weight, you could have awful lot of power to go with the weight.
Does you Westfield really have well over 300bhp, as I suspect with a driver and passenger your vehicle weight is going to somewhat over 600kg.
Soldato
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Firstly, it's not mine. It's my Dad's. (In Motors we're very pedantic about that these days, after the amount of liars we've had!).
I've seen it on the scales. All the fluids, half a tank of fuel and sandbags matched to my dad's weight came in at 500KG
. I keep telling him that perhaps if the driver was swapped with me being a few stone lighter than him.... 
Some Westfield guys have managed to get below that!
. But Titanium Bolts and replacing everything with Carbon doesn't come cheap!if you had a 2.0 200bhp engine, for it to match the 3.0 the 2.0 would have to have the "right" gearing otherwise it would loose due to the torque and delivery deficit
F = m * a
So the more F and the less m, the more a you get. Although this does not take friction into account, so above about 50mph when air effects become dominant it will no longer hold true.
It also helps if you consider F to be the actual driving force at the wheels contact with the road - this takes the gearing, tyres, power curve etc into account.
So the more F and the less m, the more a you get. Although this does not take friction into account, so above about 50mph when air effects become dominant it will no longer hold true.
It also helps if you consider F to be the actual driving force at the wheels contact with the road - this takes the gearing, tyres, power curve etc into account.
Yeh the maclaren was waaaaaay quicker off the line, the veyron only caught up when the speeds got up into the higher figures.
It took the veyron a lot longer to pass the F1 than I thought it would.
BHP is a figure used to sell cars, torque is a figure used to move them according to bhp per tonne figures I should be able to beat a Camaro Z28 in a drag race, yeah lol, in a straight line drag bhp/t is pretty relevant but torque is still more important, on a track the suspension/aerodynamics/chassis/tyres/etc all play their part
Torque isn't really that good an indicator. BHP/tonne is a better indicator in any way than torque is, as BHP is a product of torque and a key factor - the rate of work. 25000 Nm at 50 RPM (~175 HP) is not as useful in a road car as 200 HP at 5500 RPM (~255 Nm).
As others have said, approximately:
For top speed, just BHP relative to drag (which is frontal area * coefficient).
For acceleration, BHP relative to weight.
There are other factors for both, but those are the most influential factors. You can plot 0-60mph and BHP/tonne on a graph, and you'll get a very neat arrangement of data, similar to BHP and max speed.
Differences come in to play for different drive trains below about 7 seconds to 60, and come in to play for top speed pretty quickly too.
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Soldato
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At normal road speeds then you can pretty much eliminate air resistance and pretty much most things that are outside effects like tyres etc.
Gearing will make a big diff yes, but again assuming similar cars for similar purposes. An interesting one is a Scoob RA vs a normal Scoob similar specced (i am talking old world models here) the RA had shorter gearing as it was designed to be a rally spec. In real world dragging it made little diff that it had the shorter gears as the faster acceleration was pretty much cancelled by more gear changes, but if you add some twisties into the equation its easier to be in a good place on the power band than in a longer geared car. Same power, faster car in some situations.
Similar BHP/tonne in similar cars will give very similar real world performance. Assuming it can be used in both cars.
A great example is if you can find some santapod slips, used to have loads of PDFs but not got them now unfortunately. Take say a 250ish BHP scooby and a 250BHP cosworth RWD, the scoob will slaughter the cosworth initially but you will see similar 1/4 mile times. The Cos will have a much higher terminal velocity so if you went another 1/4 mile you would see a bigger gap. This is due to initial ability to grip and use the power, then drivetrain resistance.
If you take power at the wheels / weight you get a better indication of rolling performance than flywheel. Getting genuine at wheel numbers is harder to find than flywheel which most manufacturers give. A rolling road will normally give at wheels then they do a standard calc to take back to at flywheel figs. Only true way to get flywheel is on a bench dyno afaik but then at wheels is what really matters...
Old motorsport saying "BHP sells cars, Torque wins races". A great fast road car will have a fairly flat torque line in the rev range you use, a poor one may have a massive spike at higher revs.
Some engines dont rev that well despite producing high BHP, often when people talk about "sorting the breathing" thats what they are illuding too. They are giving the engine the ability to rev better not really adding any power, but it feels like more power.
Eg take a decent turbo lump with a large blower, its likely to be a bit laggy and hence will take a bit of time to go fully on boost and produce the power, some tricks are available to do things to that response such as cut blades etc, but the crudest and most effective way is to keep the turbo spinning even off the throttle, the turbo is on song and will give instant power, its no more powerful with or without this, but gives better real speed. Very fast way to knacker a turbo tho so have deep pockets on a road car
Gearing will make a big diff yes, but again assuming similar cars for similar purposes. An interesting one is a Scoob RA vs a normal Scoob similar specced (i am talking old world models here) the RA had shorter gearing as it was designed to be a rally spec. In real world dragging it made little diff that it had the shorter gears as the faster acceleration was pretty much cancelled by more gear changes, but if you add some twisties into the equation its easier to be in a good place on the power band than in a longer geared car. Same power, faster car in some situations.
Similar BHP/tonne in similar cars will give very similar real world performance. Assuming it can be used in both cars.
A great example is if you can find some santapod slips, used to have loads of PDFs but not got them now unfortunately. Take say a 250ish BHP scooby and a 250BHP cosworth RWD, the scoob will slaughter the cosworth initially but you will see similar 1/4 mile times. The Cos will have a much higher terminal velocity so if you went another 1/4 mile you would see a bigger gap. This is due to initial ability to grip and use the power, then drivetrain resistance.
If you take power at the wheels / weight you get a better indication of rolling performance than flywheel. Getting genuine at wheel numbers is harder to find than flywheel which most manufacturers give. A rolling road will normally give at wheels then they do a standard calc to take back to at flywheel figs. Only true way to get flywheel is on a bench dyno afaik but then at wheels is what really matters...
Old motorsport saying "BHP sells cars, Torque wins races". A great fast road car will have a fairly flat torque line in the rev range you use, a poor one may have a massive spike at higher revs.
Some engines dont rev that well despite producing high BHP, often when people talk about "sorting the breathing" thats what they are illuding too. They are giving the engine the ability to rev better not really adding any power, but it feels like more power.
Eg take a decent turbo lump with a large blower, its likely to be a bit laggy and hence will take a bit of time to go fully on boost and produce the power, some tricks are available to do things to that response such as cut blades etc, but the crudest and most effective way is to keep the turbo spinning even off the throttle, the turbo is on song and will give instant power, its no more powerful with or without this, but gives better real speed. Very fast way to knacker a turbo tho so have deep pockets on a road car
I read something where they drag raced a facelift R53 and an R56 Cooper S (similar BHP/weight figures) and the R56 decimated it. I don't understand that but the guy who wrote the article said it's more about HOW the car deploys the BHP on the road than the actual figure.