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vickiw106

Bhp Levels

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I have just had my 205 rally car on the rolling road

 

The spec is

 

1.9 standard bottom end

Big valve head

grp a piper cam

Ported and polished head

Dellorto twin 40s

Magnex 4 branch manifold

 

It’s producing 145bhp at the flywheel

 

I personally thought I would get a bit more bhp

 

I’m interested if anyone thinks that this is good or bad

 

Thanks

!st question should have been, did you rebuild the bottom end and did you R,R, the car befor you did the work. You should do this befor doing any engine work as it gives you a starting point. Also dont use the fly wheel fig as it is just a calculated guess.

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Batfink
Please forget gearboxes

you can only do this if the engine is on an engine dyno. Otherwise the gearbox greatly affects the calculations depending on the gear used. We are assuming the nearest gear to a 1:1 ratio was used, but we can't be certain.

 

 

Vicki wanted to know if that was good or acceptable based on her mods TO THE ENGINE!! Hence I started saying/suggesting getting hung up on horsepower figures was perhaps not the best measure of the engine,

If bhp is directly related to the torque then it is something that should not be ignored (but obviously to build the best engine should not be used in isolation)

 

Components which alter fuelling, airflow, exhaust gas extraction or tasks related to that such as altering cam timing, are not directly increasing the power of the engine, they are altering the shape of the torque curve, WHICH IN TURN, based on selection of RPM produces a BHP figure, which is not entirely irrelevant as the customer may want to see that and perhaps can see the difference and understand it better. But even Jas said at his work the engineers/technicians are looking at the torque produced, i.e. the real 'work' done by the engine.
but the amount of fuel and air, swirl etc will directly affect the burn, and the resulting ignition in the cylinder chamber will push against the piston. more fuel and air in the chamber produces more torque, hence the use of forced induction.

 

We have digressed somewhat as the only way you will really know is when the car is taken out on the stages. Is it competitive or not. TBH the only thing i'd be interested is the wheels figure. I think flywheel figures tend to disappoint. For example my mi16 came out at 174bhp at the fly, which seemed crap when people were saying that I should be getting 190bhp, but it had 146bhp (roughly) at the wheels and more importantly was bloody rapid on the road

Edited by Batfink

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DrSarty
but the amount of fuel and air, swirl etc will directly affect the burn, and the resulting ignition in the cylinder chamber will push against the piston. more fuel and air in the chamber produces more torque, hence the use of forced induction.

 

I totally agree with all you've said, which also means you're agreeing with what I said. ;)

 

The bit :) up there is exactly what I said, i.e. the BHP figure is 100%, TOTALLY & UTTERLY related to torque, but in the order that you manipulate one to generate the other, in this case and all cases the torque. That's what I've said all along.

 

So if you want a different BHP *from an engine*, you manipulate the torque curve (which means tinkering with things that will create more bang or a different burn) in such a fashion that you match it to an RPM for the power band most desirable for your application, which most times will inevitably involve some form of compromise. Likewise Kev, AFTER we've tuned the engine, we can match and manipulate the gearbox and drivetrain components to do the same thing.

 

This compromise is apparent unless it's a strict track car only e.g. formula 1, where (and here we are going full circle), high BHP figures will be the order of the day, BUT, and please forgive me for saying this again, that BHP figure is purely derived from torque and where it sits in the rev range. It is NOT a separate entity as it simply can't be.

 

The link I posted above from WeeJimmy's link I think sums it up quite nicely - and includes lots of stuff about gearboxes so I don't disagree with you Kev - where the highest average power, which you can describe as torque or BHP if you like because ultimately they're the same thing although one is a true measure of a force, is what actually produces the best performing car overall. This is why I proposed the twin figure rating showing how much torque or BHP was available for what useful percentage of the rev range.

 

And on gearboxes again, the rolling road is a fair measure if cars are tested fairly, i.e. equal tyre pressures and same gear used, providing the cars are pretty similar at worse. This means both/all cars in the 'shoot out' are subject to the same inadequancies to they pretty much cancel out.

Edited by DrSarty

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kyepan
Work = Force * length (N * m)

Power = Work / Time

 

As previously stated

 

Power is a rate at which the force is applied at a specific length or length of lever

 

Plotting both Torque and Power on a graph against RPM allows us to understand mainly where torque rises and falls. The work done by and therefore power developed by that torque will be directly determined by the RPM.

 

The differing levels of torque across the RPM range therefore explain the power delivery characteristics of the engine.

 

If you're looking for answers as to how an engine (note engine in isolation) would perform at any given point, look at the graph.

 

Neither are better, they are both theoretical explanations of applied force and applied force with reference to time. How they manifest themselves determines the behavior of the engine. What people think is the "best" behaviour is subjective, and we could go round in circles until the world stops turning, and the sun runs out of Hydrogen to light up the sky.

 

I'd like to thank GLPoomobile

The letter A

And the number 7

 

for todays post

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B1ack_Mi16

I still think bhp is the isolated factor to look for if you don't know any other information about the engine, car or transmission.

 

For example, 150bhp and 75lbft bike engine vs 150bhp 150lbft car engine.

 

Put them in the same car with suitable gearboxes letting them hit the same top speed at the maximum bhp revs for each engine.

Assume perfectly flat torque curves on both engines and exactly same weight on both vehicles.

 

You'll find both will accelerate the same and have the same top speed, even though one has twice the torque than the other.

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pip470

In my head im unsure they will accelerate the same but with the above analogy im sure they will both reach the same top speed. If you could explain why I would be wrong it would be much appreciated.

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mfield
In my head im unsure they will accelerate the same but with the above analogy im sure they will both reach the same top speed. If you could explain why I would be wrong it would be much appreciated.

 

I presume the difference in gearbox ? :lol:

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B1ack_Mi16

I just have to try to communicate this one a little bit more.

 

Take a look at attached crappy illustration/picture.

 

post-3331-1228855271_thumb.jpg

 

I really think this one proves that the only thing that counts when you want to accelerate as fast as possible (or go as fast as possible) is to have as much engine power as possible.

 

The engine torque is actually irrelevant as you can see from the equations, it's not even used to calculate the Force that will push the car forward, and that is really what is pushing you back in the seat....

 

I would like to see a physical explanation proving the opposite from those who really think torque is what matters.

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DrSarty

I'm kind of at a loss after reading that.

 

You've baffled me as you're doing what I keep repeating in this thread is fundamentally wrong; you're separating BHP (which I believe you're calling 'power') from torque, and it simply cannot happen, as BHP is a product of torque.

 

An engine ROTATES a crankshaft. Whether you connect that to 23 gearboxes or none at all, the only force in the entire equation is the rotational one generated by the turning shaft. That force is explained and measured as torque, that is it, that is all there is.

 

Now before you rant, the derivation of horse power is to equate the amount of torque generated to the rotational speed of the shaft, which now includes the factor of time, which makes BHP a measure of work done, which is power. Therefore, torque cannot under any circumstances be irrelevant, cannot NOT be part of the equation, because it has to be part of every equation relating to the engine generating motion, as all it creates is a rotational force.

 

I'm not saying engine's don't produce power, I'm saying they make torque and that's all. Power is just a description of the way the torque is delivered.

 

Have I got this totally wrong, or are people winding me up now? :)

 

I'm hammering this out to clarify it in my head too BTW, so I really am prepared to be proven in idiot.

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B1ack_Mi16

I'm not good at explaining things I can tell you, but at least I have a Master degree in mechanical design and been working with some systems transferring power through drivetrains etc. earlier.

 

The point I'm trying to prove for you is that the engine power is what will get you to accelerate or go fast.

You can see it from my formulas.. ?

 

Of course if you up the torque of the engine itself the power of it will also go up, but it really is not important what torque the engine makes to make this kind of power!

 

The power produced is what will get you accelerated in the end. As you can see the only "gear" ratio needed in the equation is the Radius of the wheel, and the rotational speed of the wheel (read speed of car).

 

So two cars independent of what gearbox or wheel sizes they have, (as long as they move at the same speed) when they floor the pedal, the car with the most power will start accelerating fastest.

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DrSarty

OK, I'll try it in a one-liner.

 

But BHP or power, IS calculated from torque. No torque, no power, no go nowhere.

 

The ONLY thing I can consider you're saying (and your explanations aren't that bad BTW), is that 2 identical cars travelling at 100mph side by side (in a slightly Newtonian law forgiving environment) with engines at that moment generating the same torque, would, when the loud pedals were simultaneously floored, have one accelerate faster if its engine was already spinning say 2,000rpm faster?

 

E.G Car A & B both at 100mph; engines producing 100lb.ft torque to maintain speed, but car B's engine at 6,000rpm when car A is revving at 4,000rpm. This would mean at that time, car B is generating more power, as POWER (BHP) = 100lb.ft x 6000 / 5252 which is 114bhp compared to car A's which is 76bhp.

 

Whilst I can see that reasoning, first off the torque IS relevant as you couldn't calculate anything without it. Yes, the BHP figure is higher, but fundamentally it's the same engine, and some different gearing has been used. But this is all about grading an engine is isolation, so the gearbox from that POV is academic. My brain tells me naturally that at the same torque, the engine doing more work, engine B, would accelerate faster, BUT IT'S NOT A MORE POWERFUL ENGINE.

 

It seems perfectly logical what you've written, but I don't think it's address the actual issue of comparing engines, which is how this all started.

Edited by DrSarty

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B1ack_Mi16
OK, I'll try it in a one-liner.

 

But BHP or power, IS calculated from torque. No torque, no power, no go nowhere.

 

True enough, no torque no power...

P = T * w (omega)

I agree with you no problem!

 

Still power is the only thing that will make you go fast and accelerate fast... (as explained in my other illustration)

 

My point is just that engine torque does not matter as long as it has the BHP / POWER.

 

Case is .. doesnt really matter if you are strong, if you can't move fast enough.

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kyepan
I still think bhp is the isolated factor to look for if you don't know any other information about the engine, car or transmission.

 

For example, 150bhp and 75lbft bike engine vs 150bhp 150lbft car engine.

 

Put them in the same car with suitable gearboxes letting them hit the same top speed at the maximum bhp revs for each engine.

Assume perfectly flat torque curves on both engines and exactly same weight on both vehicles.

 

You'll find both will accelerate the same and have the same top speed, even though one has twice the torque than the other.

 

Both are achieving a peak power of 150bhp, so at some point the rpm and torque combine to create a 150bhp rate of work.

 

So because power and torque are linked, peak power for the bike engine will appear higher up the rev range, possibly when it's not producing all that much torque. But because it is cycling quickly, its producing that amount of torque many times over, hence producing the same power.

 

Conversely peak power in the car engine is achieved at a much lower rate of work (revs) because if it generated that larger amount of torque at the higher RPM it would create much more power than the bike engine. Again this engine produces more torque but if peak power is the same, it must be doing it when the number of cycles per second is less.

 

So what I'm saying is by definition these two engines could not have flat torque curves, peak torque would be found at different rates of work to achieve identical peak power.

 

Which leads us nicely onto Sarty's old torque under the graph, if the torque gets up quickly and stays up it means that the engine produces good usable torque across the rev range. If it stays up into higher revs, the engine will by definition also produce force at a high rate of work and therefore high power, bhp.

 

bringing wheels and gearboxes into this discussion only makes it complex and obscures the point about work done over time, vs force exerted being two linked equations.

 

You also have to remember engines cannot keep accelerating to infinity, they must be used within a set limited range of cycles per minute. Their torque is within that range and human usage of that range will determine how the car performs.

 

Driving the car is exactly that, making use of the torque where it is found. Torque at any given rpm, if greater than the frictional forces will generate acceleration how much acceleration will depend on how much torque is to spare over the frictional force opposing it.

 

 

 

J

 

ps in the time it's taken me to write this post about ten others had posted.

Edited by kyepan

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DrSarty

P.S. Please see my lengthened/edited post above at top of page.

 

Thankyou.

 

And this is circular reasoning:

True enough, no torque no power...

P = T * w (omega)

I agree with you no problem!

 

Still power is the only thing that will make you go fast and accelerate fast... (as explained in my other illustration)

 

My point is just that engine torque does not matter as long as it has the BHP / POWER.

 

Case is .. doesnt really matter if you are strong, if you can't move fast enough.

 

The items in bold are a contradiction.

Edited by DrSarty

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Mandic

Let me try this way...

 

If we have 2 engines producing same amounts of torque, only one, let's call it engine B, can rev 1000RPM higher. They have same gboxes too.

 

So, when engine A will hit red line and needed to change a gear up (and so torque on wheels will go down in order to gain speed P=T*w), engine B will still be going strong with the same torque as before for another 1000RPM.

 

So engine with more power (assuming more or less flat torque curve) will be faster as You have 2 variables T and w to choose between.

 

Torque is nothing without omega (w), or with other words, power is everything.

 

Problem is that looking at the torque in engine is one thing, You need to look at what happens at the wheels. If a turbo diesel has twice as much torque as equivalent sized NA petrol engine, which can rev to 7000, and diesel can only rev to 3500, they will be equally fast to a certain speed in ONE gear as they have same amounts of power. If any of them had more power it would EITHER! accelerate faster (more torque) or to a higher speed (higher w).

 

 

Cheers

 

Ziga

Edited by Mandic

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tagy

I'll wade in with yet another example :) , I don't claim to know enough about this but..

 

Engine 1: Produces 105lb.ft of torque at 2000rpm +/- 1000rpm

Engine 2: Produces 100lb.ft of torque at 6000rpm +/- 1000rpm

 

At all other rpm both engines produce 50lb.ft of torque.

 

Engine 1 has more torque, Which would be faster?

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pip470

I am of the understanding acceleration will be at its greatest at peak torque,

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DrSarty
I'll wade in with yet another example :) , I don't claim to know enough about this but..

 

Engine 1: Produces 105lb.ft of torque at 2000rpm +/- 1000rpm

Engine 2: Produces 100lb.ft of torque at 6000rpm +/- 1000rpm

 

At all other rpm both engines produce 50lb.ft of torque.

 

Engine 1 has more torque, Which would be faster?

 

Simple, when you plot this on a graph, it'll be the engine with more area under the torque curve, which means it has more power available across the range to use to accelerate.

 

Surely that's it isn't it? :)

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Mandic

One can not only ask, which car will accelerate faster, one must ask which car will accelerate faster to a certain speed!

 

Just as You can't just say...ok I have a diesel and petrol cars doing 90mph, and they start accelerating, which will accelerate faster...OK, BUT! to what speed? Only way to compare them is to give them same gearboxes, and then it becomes obvious ... the one with more power!

 

If You do not have equal gearboxes You have to "standardize" engine torque and power curves.

 

 

Cheers

 

Ziga

Edited by Mandic

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Cameron

Can't really answer that, Engine 1 could go on to make 105lbft at 6000rpm and would be marginally faster. Unless those are the peak torque values, then engine 2 would be faster.

 

This argument will never be solved. :)

 

Nobody is really wrong here, the only place I can find fault is in some of the earlier posts where people have said that they prefer an engine to have more torque. And the fault is only in the use of the word, as the majority of people who say that use "torque" to refer to bottom-end grunt, which isn't necessarily true. An engine can make peak torque of 150lbft at 2000rpm and feel "meaty" or it can make it at 6000rpm and be "gutless" but most definitely be faster.

 

Neither are the only number you should use, they need to be used together or they mean nothing. If you are quoting an engine to make xxxftlb at xxxxrpm then you are inadvertently quoting the power output anyway. :)

Edited by Cameron

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DrSarty

Christ :) !! (And a little mad now!)

 

Please stop talking gearboxes and speed, as we are only rating an engine. Vicki's speeds on her rolling road were never mentioned and are totally irrelevant, as we are not talking about on the road performance. We are purely distinguishing what classifies an engine best.

 

This started because some said 'oh horsepower is best' and I said let's look at Vicki's torque curve rather than just say 145bhp is great, crap or whatever. I was saying that torque is what matters and is all that rates an engine. Power ratings for engines are derived from torque so as such are just a new method of explaining the same thing.

 

This is like debating colour for a birthday cake and someone talking about burn times for different brands of candle. Similar but totally disjointed, when you're looking through an icing catalogue!

 

:)

 

EDIT: Now I've seen Cameron's post - "This argument will never be solved" = yes it can.

& "people have said that they prefer an engine to have more torque." = that's exactly what I'm saying. If someone wants more power they have to manipulate the torque curve, by shift its shape whilst maintaining the area underneath or increasing the area vertically or sideways or both, which...and here we go again, means generating torque or changing the fashion in which it is produced. So we're back again to what I've said all along and is what you've hit on Cam in your last line.

Edited by DrSarty

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Cameron

:) Nice cake analogy!

 

Yeah I understand what you mean, I wasn't saying you were wrong. I just meant that there will always be people who think power will be a better indication, like there will always be those who think torque is. Both are wrong. :)

You need to be able to understand both to get the best understanding of engine performance. A torque curve is handy to show the VE of the engine, amongst other things, but that will need to be compared against rpm to make any sense of it. Which is where the power curve steps in, as the figure shows you the relationship of torque and therefore VE to engine speed.

Edited by Cameron

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B1ack_Mi16

Lord, I agree with Ziga, he seem to understand it.

Fed up now.

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DrSarty
Lord, I agree with Ziga, he seem to understand it.

Fed up now.

 

Don't be fed up; it's not like I'm stupid and am refusing to understand what you say. And you know I respect you as it was your engine and expertise I emulated some time ago to build my engine.

 

What I'd like to ask you is do you or do you not agree with this?

 

If someone wants more power they have to manipulate the torque curve, by shifting its shape whilst either maintaining the area underneath or increasing the area vertically or sideways or both, which...and here we go again, means generating torque or changing the fashion in which it is produced.

 

I have to ask this as you and Ziga keep referring to an engine's power as being totally separate from the engine's torque. I am saying this can't be so, and sometimes you seem to say the same. That's why I said there was a contradiction.

 

So do you agree with my quote above?

Edited by DrSarty

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