ALi-B

iTrader: (1)

BHP isn't a measurement. Its a calculation....Taken from measuring Torque and RPM.

A engine HAS to make torque. BHP is the after effect of work done form creating torque over a number of revolutions. The calculation, due to its mathematical formula can be made to look better that how it "feels" in the real world. In other words - the more the engine revs at a given torque....the more BHP it makes. That's a fact.

An engine making a pathetic 100lbft, but doing 100,000rpm will make a amazing 1904 BHP...but it will be shyte at 5000rpm why? well, 95bhp...says it all. How do I know it'll make 95bhp? Because its a calculation from this formula: BHP = (Torque x RPM) / 5252

So the only thing here making the BHP look big is the engine's max rpm....that is why a Vetec or old F1 pre-turbo engine looks so good (for a non-turbo engine), because they rev so high

The old non-turbo F1 engine only made high BHP was because they revved past 16000rpm. And that purely down to the way BHP is calculated - they barely manage to make 300lb/ft peak. Ever seen how a F1 car bogs and stalls if its not revved high enough? Its because its has no torque at low rpm - can you imagine how useless this engine would be if used in a normal (heavy) car chassis on a public road at normal road speeds?...Its got 800bhp though....


Likewise that's why a big torque diesel doesn't make much BHP...nor a Bentley V8....neither rev past 5000rpm. That's because RPM is a multiplier - no engine that revs below 5250rpm will produce any decent BHP - purely due to the way the calculation works. A big diesel may only make 100bhp at 1500rpm but it'll pull like a train..because its making 350lb/ft at the very same rpm. Unfortunately most lose torque past 4000rpm, so what you have is a narrow power/torque band.


If you want a measurement of how good an engine performs...its the torque curve that is important as this will show the engine's useful powerband.. Peak figures are useless (torque and BHP). An engine that makes 90% of its peak torque at 1500rpm and maintains that torque all the way to 7000rpm makes for a very flexible engine...i.e it pulls just as well at 2000rpm as it does at 6000rpm. As long as the torque is high in that power band/curve the BHP isn't relevant - you don't need to know it.

The same argument can be made for amplifier ratings on audio systems: There is a massive difference between RMS watts @ 10%THD and RMS watts @0.001%THD. Peak watts or PMPO is just something to be laughed at...PMPO (peak maximum power output) is equivalent to peak BHP...its means diddley squat in the real world.

john banks

Do you mean torque and RPM of the rollers (which to my mind I am not interested in)? Dynos can show BHP without knowing engine speed, but they only show torque if using RPM pickup or the dyno software is told when RPM is steady at a preset level.

Shaun

Shall I join in?

Maz

iTrader: (34)

:

Shaun

The single most important factor is use. Decide that and then you can look at what appropriate band to aim for.

As has already said, engine dynos can be no more "accurate" than decent chassis dynos.

To expand on what John has said, it's all about how a car performs on the black stuff. In-gear and through gear acceleration times will give you a better benchmark.

Ali,
Peak figures are far from useless, but in isolation they don't mean the be all and end all.

ALi-B

iTrader: (1)

Personally I was viewing it as a engine ran on a test bench.

With a roller dyno, you just need to look at what it actually measures and what variables are used. The rest is maths. Engine rpm doesn't have to be used - it can be calculated if need be if the axle ratio is known and its a manual gearbox (which is why operators prefer to do a run in a direct 1:1 gear - usually 4th).

Some roller dynos use strain gauges to measure torque, others measure the rate of acceleration and apply Newton's 2nd law (Force = Mass x Acceleration). What they measure is always at the wheels and that's a real measured figure - that's pretty accurate. However when that is used to calculate whats happening at the flywheel assumptions have to made in those calculations and that's whats often argued about (the assumed values) as that's what's screws up the calculations and end figures.

I just look at the plot rather than the numbers. Its easy to see where the turbo (if fitted) spools, or when the engine goes "on cam" (for non-turbos), and its easy to see where it start to run out of steam....as soon as the torque starts to drop, whilst BHP may continue to rise, but the car's rate of acceleration will taper off. Regardless of the peak figure achieved it gives a good idea of how performs.

Even then it doesn't truly reflect what happens on the road due to the car's weight, gearing and aerodynamics. For example...I have a 290bhp (on a good day) Jag that weighs 1.7tonnes....and did have a BMW 530d GT with 240bhp (everyday) that weighs 2tonnes. The latter runs rings round it in every area concerning engine performance, despite being heavier and having a much narrower power band. Admittedly, it does have alot more torque (and gears)......

ALi-B

iTrader: (1)

I don't dispute that, I may have exaggerated somewhat to get the point across to those that solely rely on them.

Alot of people (including myself) quote peak figures, manufacturers* don't help either as that's all they quote. And as you pointed out its not really very reflective of how a car really performs on the black stuff.

*Do Porsche still print a graph plot of their engines output in the back of the owner's manual?....they used to in the 1980's.

Shaun

You've got to consider this all using intelligence. If you haven't any intelligence, just use the peak figures.

Carnut

ALI-B, do you think the best representation of real world use is torque and WHP, for example, that car has 200ft lb WHP?

Shaun

Carnut,
You can't have x ftlb of WHP.

All this talk of BHP, WHP, Torque etc is "just" numbers. They don't mean a fat lot unless you start talking about vehicle aero (drag), weight and gearing (to name but a few).

The best representation of the "real world" are in-gear performance times, but many people don't understand these either.

x bhp & y ftlb are relatively easy to "grasp", and in reality the masses won't understand much else.

The US tend to sway towards WHP (certainly when tuning) and we tend to sway towards flywheel figures. It is what it is, and we can rip the backside out of both methods, in stating the dis-benefits.

The use of flywheel and/or wheel figures is an isolated "gauge", which is predominately used to provide a very simplistic context to "performance". Ideal for Fred Bloggs.

If you want to really know how a car performs, get it off the dyno and on to the black stuff. You can then be in a position to see if your "dyno figures" actually relate to in "performance" terms.

Carnut

Yeah I've already mentioned drag etc but we do need some sort of measurement otherwise we would have to say "here is the new Mondeo, it has ok amount of power" which gives us nothing.

Obviously were not going to start all over again but l believe flywheel BHP is out of date. We now have all sorts of different g-boxs with 6/7/8 gears which surly lends itself to WHP, and with diesels, turbos becoming more popular and us moving into elec motors torque would be the best way to measure a cars power. Saying what BHP a car has is less of a indication of what performance it has over torque.

Just because it's a simplistic representation of a cars performance doesn't mean it shouldn't/can't be improved.

Shaun

Carnut,
BHP at the flywheel will remain the standard - because nearly everybody "gets" BHP. Flywheel will remain, quite simply because it's the larger measure (think about it). Being "out of date" has no relevance in the world of sales bull and commercials - unfortunately that's the "real world". Bigger means better when it comes to power - at least that's what we're led to believe by the sales blurb.

This kind of stuff has got worse over the years, so it won't be changing for the better any time soon.

There is really no point pontificating. You're not going to change the world in this respect....... in reality, does it really matter.

Carnut

I know, can a couple of petrolheads not talk about useless shìt these days?

But, you say the bigger figures sell, well some diesels have twice (maybe not quite twice) the torque as it does BHP.

Shaun

I'd prefer to call it "discussion", rather than "useless ****"..... although I am talking about my input, not yours.

People don't really understand torque though....... this is an "industry" that has been brought up on BHP.

c_maguire

This only becomes questionable as regards performance when dealing with modified vehicles, and then seeing the dyno printouts starts to matter as the curves become all important. Some modified vehicles have impressive headline figures but are appalling to drive.


Any standard vehicle will always have its power as a headline figure if it's petrol or torque if it's diesel. The fact that it is as supplied by the manufacturer means only a few vehicles will throw up any surprises when you drive them (such as an EP3 bread van that sounds OK with 200 horses but is flat as hell until you thrash it).

Shaun

I'd disagree with the above.... which sort of makes the point of this thread.

I used to own a MK4 Mondeo TDCI 2ltr Diesel. It's peak figure of torque was 320nm. I replaced that 2yrs ago with a MK3 Focus TDCI 2ltr Diesel. It's peak figure of torque was 340nm.

Which was/is a quick car in every day use?

The Mondeo!

The Mondeo peak BHP was 140bhp, the Focus is 160bhp.

Guess what they did to get the extra 20bhp...... fit a bigger turbo.

The Mondeo had torque from just over 1200 rpm. The Focus is gutless below 2k. On a diesel this is quite an issue from my perspective..... it means I have to nail the revs to get the car to pull.

The Mondeo configuration, even though it had less peak torque and less peak BHP.... was a much better performing car on the road (oh and it's heavier).

c_maguire

I suppose it's down to what you're after. You want something fun for your 'play' car but want something 'easy' for your A-B motor (the Mondeo).
I have a 500BHP petrol in the garage which to be truthful I rarely use, whereas I spend most of my time in a work van. I've never much liked diesels full-stop (maybe something to do with riding motorcycles for longer than I have cars). I hate the way diesels deliver their power/torque, all low down, narrow useful rev range before wheezing out of puff. This doesn't suit the way I drive anywhere. The van I've ended up with is an Astravan 1.9CDTi with a tuning box. Not exciting, but I can put up with it because for a diesel it drives a bit like a petrol. Below 2000 it is a bit gutless for a diesel, but it pulls hardest between 2500 and 4500 and will carry to the 5000 redline.

daveyj

iTrader: (1)

I wouldn't describe my breadvan ep3 as being particularly flat when not being thrashed. With that 6 speed box fitted it doesn't need to be. It makes very good progress without the need to ramp it, although ramping it does add a bit more of a grin.

c_maguire

I hopped out of my 300/300 Impreza probably 8 years ago and then went for a drive in the breadvan. Amusing when it came on song but after the Impreza it felt like an absolute slug until the revs hit 5500. The immediate impression it left on me was 'no way could I put up with this'.