What is this BHP thing about then?

chesterstu

chesterstu

chesterstu

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OK, i know more BHP is better but why? Also, i have a an S60 T5 that has just been chipped to 299bhp but when do i get that, if i keep the revs low, under ~3500, so my turbo isn't running, do i have less BHP? Is there a peak rev when all my power is being delivered, if so is it in every gear? So, in Second at 6500rpm am i getting 299BHP and the same in 3rd, 4th etc or is it only in certain gears/power bands?

Sorry for the n00b questoins but i am clueless
 
Brake Horse Power. In short its the amount of power your car outputs. Usually the BHP rating you are given will be 'peak' BHP, so itll be when you engine is at its optimum rev range. Obviously your BHP should be its highest when the turbo is at full chat.

Im no expert, so ill shut up now and leave it to the pros :)
 
THe car will make its maximum BHP at a certain RPM, my car for example...

2500rpm = 38bhp

3500rpm = 59bhp

4500rpm = 88bhp

5500rpm =113bhp


then finallly making its peak power at 5600rpm = 115bhp

after this horsepower will drop (as shown as a dip in the curve at high end RPM on most cars when run on a dyno).

I.e at max RPM on my car 7200rpm it makes just 90bhp again (ATW).

So its only a figure of it maximum capable performance at a given RPM.
 
Acolyte said:
You will get the same power through the engine at optimum engine RPM, it does not matter what gear your in.
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Yes it does. The gearbox is a torque multiplier, therefore the the car will produce more torque at the wheels the lower the gear is. Thats why cars are dynoed at a 1:1 ratio which is usually fourth gear unless you got a 6 speed box.

edit: re-read what you wrote and think I mis-read it :o
 
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chesterstu said:
OK, i know more BHP is better but why? Also, i have a an S60 T5 that has just been chipped to 299bhp but when do i get that, if i keep the revs low, under ~3500, so my turbo isn't running, do i have less BHP? Is there a peak rev when all my power is being delivered, if so is it in every gear? So, in Second at 6500rpm am i getting 299BHP and the same in 3rd, 4th etc or is it only in certain gears/power bands?

Sorry for the n00b questoins but i am clueless
Click to expand...

On the road more power = better acceleration and less importantly higher top speed.

Chipping a turbocharged car normally involves raising the boost, so off-boost i.e. when the turbo isn't doing anything you are unlikely to have much if any more power than a standard one.

There is an engine speed (RPM) at which power output peaks, and after this it starts falling off. The peak power RPM is normally the same in every gear, though some turbocharged cars limit maximum power in lower gears to lessen the chances of wheelspin or to prevent damage to drivetrain components.

Apart from the above situation, the peak output of the engine is the same in every gear, but the actual force at the wheels which moves the car (torque) is lower in higher gears, which is why you get more acceleration in lower gears.
 
Acolyte said:
THe car will make its maximum BHP at a certain RPM, my car for example...

2500rpm = 38bhp

3500rpm = 59bhp

4500rpm = 88bhp

5500rpm =113bhp


then finallly making its peak power at 5600rpm = 115bhp

after this horsepower will drop (as shown as a dip in the curve at high end RPM on most cars when run on a dyno).

I.e at max RPM on my car 7200rpm it makes just 90bhp again (ATW).

So its only a figure of it maximum capable performance at a given RPM.
Click to expand...


am i right in thinking that you would change gear at 5600rpm to get max acceleration or am i just not thinking ?
 
well the max bhp would kind of be the best place to change gear. because as the bhp falls off the gained speed isnt as beneficial as having a gain in BHP over the revs in a different gear.

who wants to bring torque into the equation here :)
 
utajoker said:
am i right in thinking that you would change gear at 5600rpm to get max acceleration or am i just not thinking ?
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Theoretically yes because if you take the revs any higher the engine won't be developing as much power and you won't be accelerating as hard.

In practice it's down to the gear spacing and the power curve. Example, my old setup in my Anglia there was a ridiculously long jump between second and third so if the revs weren't high enough the engine bogged down because it was off cam and would hardly move the car. In that scenario I had to rev it past it's peak hp point in second so that when it went into third the engine was at the right revs to be building power.

If you have evenly spaced gears this helps because you can change at a consistant rpm in each gear for optimum acceleration.
 
utajoker said:
am i right in thinking that you would change gear at 5600rpm to get max acceleration or am i just not thinking ?
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In a vtec car you would rev beyond peak power in order to stay in the vtec when changing up, if you cant do that you would see a noticable drop in acceleration more so then just holding past the peak.

In mine theres no vtec just a black hole of a flat spot in the middle of the rev range, so if I rev to the absolute limit I can avoid that when changing up and actually get decent performance.
Cars like this are a pain to drive because you constantly have to juggle the gears and have the engine screaming just to get it to go 'fast' whereas a car with a turbo or more even torque/hp distribution can be a pleasure to be in.

Have a look at some s2000 dyno charts and then some focus st dyno charts and you can see how different they might drive


chesterstu said:
if i keep the revs low, under ~3500, so my turbo isn't running, do i have less BHP?
Click to expand...


Yes no turbo means you probably making like 1/3 of your peak figure so 100hp. Cant say when you might get the 300, maybe 5k revs?
 
Bear said:
edit: re-read what you wrote and think I mis-read it :o
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I think he meant the engine itself will always produce the same peak power figure no matter what gear it was in, i.e. "flywheel (or dyno) rating"

This is not always true with a turbocharged car anyway as in some cars (a lot of Saabs, for instance) the ECU will restrict maximum boost in 1st gear so that the engine doesn't produce its full power (in an effort to control torque steer and traction issues). This is why Saabs show poorish 0-60 times, but better "in gear" acceleration.

I imagine this behaiviour is confined to FWD cars though.
 
utajoker said:
am i right in thinking that you would change gear at 5600rpm to get max acceleration or am i just not thinking ?
Click to expand...

The absolute max acceleration will occur when you change up a gear at the point past the maximum (peak) power rpm such that the current (falling) power output is equal to the (rising) power in the higher gear at the same vehicle speed.

This assumes that:
(a) The revs at this changeup point havn't reached a point where engine damage will occur
(b) the torque curve it roughly linear in the rpm range we are talking about - ie. there is no big hole for some reason.
 
volospian said:
This is not always true with a turbocharged car anyway as in some cars (a lot of Saabs, for instance) the ECU will restrict maximum boost in 1st gear so that the engine doesn't produce its full power (in an effort to control torque steer and traction issues). This is why Saabs show poorish 0-60 times, but better "in gear" acceleration.
Click to expand...
That really has little to do with Saab's poorish 0-60 times - they are "poor" as they struggle for traction anyway. Torque limiting a gear that you wheelspin in should make little difference to the 0-60 time as the optimum 0-60 will obviously not see you wheelspinning. It's more to do with reliablity and driveability than all out performance :)
 
utajoker said:
am i right in thinking that you would change gear at 5600rpm to get max acceleration or am i just not thinking ?
Click to expand...

You want to change gears at whatever rpm the area under the graph is of its max through all gears. Plus that may have to change if there is some unequal gearing.
 
That really has little to do with Saab's poorish 0-60 times - they are "poor" as they struggle for traction anyway. Torque limiting a gear that you wheelspin in should make little difference to the 0-60 time as the optimum 0-60 will obviously not see you wheelspinning. It's more to do with reliablity and driveability than all out performance
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Absolutely right. Glad we agree on something PMKeates! :)
 
PMKeates said:
Torque limiting a gear that you wheelspin in should make little difference to the 0-60 time as the optimum 0-60 will obviously not see you wheelspinning. It's more to do with reliablity and driveability than all out performance :)
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Exactly. What I am saying is that the car has a poor 0-60 time because it doesn't have the traction to get a good one. Infact the traction is so bad that, to get to a place where wheelspin from stationary is virtually removed, the power in 1st gear has to actually be reduced.

If the car had a better traction, it could achieve a better 0-60 time, but only because then the engine would then be allowed to produce more power to take advantage of that better grip.

Actually Saabs don't really wheelspin any more than other frontwheel drive cars (at least in my experience of owning several) but that's mainly because the power is curtailed in low gears to ensure that is the case.

However, once the car is rolling traction has less impact and the car is allowed to produce its full power, which makes "in gear" times surprisingly good when compared to the poor 0-60 time (and, to be fair, under 7 seconds 0-60 for a 200hp, heavy, FWD cruiser isn't dreadfully slow anyway).
 
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