Car 1 has 20 BHP more than car2 .. simple, car1 has more power definately its faster.
They weigh approximately the same etc.
if you are an enthusiast that just about sounds like the right recipie.
I too used to compare the BHP figures and think that more power is better.
This not just applies to buying cars but also aftermarket modifications and tuning.
If you are the techie types - you may even look at the manufacturer's or tuner's dyno chart to confirm which car has more power.
Guys there is more to it -much more.
Lets start with the basics :
1] Engines dont make BHP. Period. Engines make torque.
2] The pistons convert the Air/Fuel explosion into downward motion, the crank converts this "kick" into rotational motion. This rotational motion turns your fly wheel, the gearbox connects to flywheel via the clutch and transfers power to the wheels which then rotate.
3] The important point in the above point is rotational force generated by the engine. This is torque in a simplified description.
4] What about BHP ? oh ya forgot about that - its calculated off the torque..
hp = [torque x rpm ] / 5252
The combination of torque and rpm results in power..
5]Simply put - going faster means accelarating harder and quickly. It's this rate of change of speed gives us accelaration = faster car.
6]BHP advertised is always the max power that can be generated by an engine, the question we should be asking is at what rpm ? i.e at what speed the engine must be running at to actually use this power ?
Remember point no 4 , Hp= resultant of rpm and torque.
We could be getting 300 bhp at say 6800 rpm from car1 Vs 240 bhp @ 5500 rpm from car2
Here we need to ask 3 questions:
(a) How long does it take to reach this 6800 rpm as its way up in the rpm range ?
It takes time to build up this rpm, you could end up spending more time waiting to reach this high engine speed only after which you would experience this power..Also during every gear change the engine rpm would drop and again you would spend time building up the speed to reach that range.
Its like climbing a mountain for 8hrs, reaching the peak and spending 2 minutes at the peak before sliding off.. and again having to restart from teh base of he mountain.
HP figures dont give the whole picture.
(b) Even if we manage to reach it , how long can we use this power ?
This means once you manage to reach this engine speed/rpm what next? You have spent some time reaching this rpm how long will it last? For how long can you mantain that rpm to harness this power ?
On road/track is it practical ? You may have to brake, take corners etc causing the rpm to drop again below the number at which we get peak power.
Also if you give too much throttle you may exceed the rpm the engine makes the most power and engine will be only making noise not power therfore not accelarating.
The width of the power band also matters combined with your gearing and final drive. This is what determines how long will you be riding that wave of torque and accelarating hard.
This advertised power is peak power, it doesn't tell us the whole story
Gearing/Final drive/Tyre size +Power/Wt ratio :
Many aftermarket tuners enthusiasts look at dyno numbers and get impressed if they see a 10-20 bhp bump in power. This is not an indicator of how much performance will be realised on the road.
It depends on your gearing ratio/final drive/tyre size combination to lay down that "power(actually is torque)" of engine on the road...
Let me explain this with an example - Car C1 with same engine but with diff gearing and tyres makes hell of a difference. E.g if the power band is near 4000 rpm, having torque of 150 Nm at 4000 rpm and engine makes 130 bhp at say 5500 rpm, the gearing and stock tyre size allows the car to travel at 80Kmh/50Mph at say 3000 rpm. With a smaller tyre size and a lower gear ratio the same car at 4000 rpm now travels at 80Kmh/50 mph. See now the engine is in the power band and will respond to quick blip of the throttle.
Now taking the same car and comparing it to another car say C2 with power of 200 bhp @ 6000 rpm, torque of 180NM at 5000 rpm.
The difference in power is 70 BHP - based on BHP specs we have a winner, car C2 wins on paper
The difference in torque is 30 NM - based on torque specs we have a winner,car C2 wins.on paper
But based on gearing and overall wt of the car C2 is a loser against C1.
C1's gearing and torque has been optimised to make max use of the engines torque and does not lay stress on the peak hp figures.. its just a marketting gimmick.
99.99 % of the people look at the HP and pay extra 10 to 20K$ for this.
Dont beleive me - look at a real world example.
BMW M1 vs BMW M3
M1= 335 BHP, 6 cylinder, 3000 cc, price :46K $
M3= 414 BHP,8 cylinders, 4000 cc, price :58K $
Price of M3 justified : 79 more bhp, extra 1000 cc, extra 2 cylinders..therefore extra 12k $
Track tested by BBC Top gear : results M1 is much faster, track is designed to test the real world accelaration, handling, braking.. a bit of brute force power also.
How did it manage to do that -->
M1 makes 332 NM of torque from 1500 rpm to 4500 rpm, supposedly bigger, powerful, more expensive M3 makes 295 NM of torque at 3900 rpm
The M1 can pull; anytime anywhere.. people just dont get it.
Along same lines no one belives If I said a slightly modified 50 BHP 3 cylinder, 800 cc, 4 valve SOHC, 600 Kg hatchback can mess with 70 to 90 bhp , 4 cylinder, 1100 cc to 1300 cc engines.. :-) or even 2600 cc diesel SUVs :-)
No comments:
Post a Comment