[nitrous]

For this discussion, u need to go back to the definitions of BHP and Torque.
Search for that thread.

[danlalan]

well it does depend upon the distance travelled by a particular car having the mentioned engines....so accordingly you tune to get the desired rpm...

[Ford Rocam]

Originally Posted by devsus

hello bhpian's,
one technical question :-> suppose there are two vehicle's A & B , both having similar specification ( engine ( V4-4valve), layout, wheelbase.., engine capacity etc..etc..) but different BHP@RPM..........."A" having 250 BHP @ 7000 RPM while "B" having 250 BHP @ 10,500 RPM...........now u have to tell from both of the vehicles which one is better and why?

Engine A
250 BHP @ 7000 rpm= Longer stroke good low end toque.
If both the engine has same C.C/valves the difference will be in bore*stroke & rod to stroke ratio, for engine A it will have longer strokes compared to Engine B,Engine A will have lots of low end torque good for city driving.engine cannot be revved high cause piston speed increases beyond limit

Engine B
250 BHP @ 10,500 rpm= Shortre Stroke will lack low end torque but higher HP
where as engine B will be less torqy at lower rpm cause high rpm engines are having shorter strokes which helps keeping piston speed down allowing higher crankshaft speed,Higher crankshaft speeds=more power strokes/sec

Engine A will be fun to drive in city cause torque is available at lower rpm, for Engine B you will have to rev it higher to get in torque curve.

In Both the cases ,how well it can hold that torque curve to their rev limit is important.
Formula1 engines are having huge bores & short stroke thats the reason they rev so high.
Hope it explains.

[nitrous]

Well explained,jithu.

[tsk1979]

http://vettenet.org/torquehp.html
Please refer to the above web page to understand acceleration w.r.t bhp and torque etc.,

[ess_a]

Originally Posted by tsk1979

http://vettenet.org/torquehp.html
Please refer to the above web page to understand acceleration w.r.t bhp and torque etc.,

A really nice read.... Couldnt cmprehend all of it, but atleast (i guess) my questions have been answered! Thanx tanveer...

[jat]

........"A" having 250 BHP @ 7000 RPM while "B" having 250 BHP @ 10,500 RPM...........

GTO has given the proper explaination.

F1 engines are made to rev higher to get a high power to weight ratio. Higher the rpm, lower the weight for a given power and the designs are taken to extremes to limit of material strength.

Then the question comes. Why commercial engines are designed to operate at lower rpm. The reason is fuel quality. Higher rpm gives you less time to burn the fuel (one reason for diesel engines having lower rpm) and quality of fuel has to very accurately controlled. Lowering the rpm gives more flexiblilty in fuel quality.

If the power and rpm mentioned are the peak ones and they have similar torque characteristics, theoritically there won't be any difference. The higher revving engine has the power to weight advantage while the lower revving will have lesser transmission losses.

[Shan2nu]

250 BHP @ 7000 rpm= Longer stroke good low end toque.
If both the engine has same C.C/valves the difference will be in bore*stroke & rod to stroke ratio, for engine A it will have longer strokes compared to Engine B,Engine A will have lots of low end torque good for city driving.engine cannot be revved high cause piston speed increases beyond limit

Engine B
250 BHP @ 10,500 rpm= Shortre Stroke will lack low end torque but higher HP
where as engine B will be less torqy at lower rpm cause high rpm engines are having shorter strokes which helps keeping piston speed down allowing higher crankshaft speed,Higher crankshaft speeds=more power strokes/sec

Actually it doesn't work out that way.

If both engine have the same CC and are naturally aspirated, it's impossible for one engine produce 250@7000 and the other to produce 250@10500.

Reason being that engine capacity usually decides how much torque that engine is capable of producing. EG: A 1500 cc engine will produce in and around 150nm (provided it's NA).

But in this particular case, producing 250@7000 rpm will require a huge leap in the torque curve over the other engine, which is not possible without forced induction.

Increasing the stroke will produce similar max torque figures but at a much lower rpm.

While the first engine produces around 187lbft(254nm)@7000rpm the other one is producing 125lbft(170nm)@10500. And we all know that torque produced at peak power is always lesser than the max torque fig.

So just imagine the diff between the 2 torques. Where's 254nm and where's 170nm.

So, unless the other engine is turbocharged, supercharged or NOS injected, it cannot have such a huge advantage over another similarly sized engine.

One might argue that these torue figure don't indicate max torque produced by the engine but even so, if you take 2 similar engine with a long and a short stroke, they will not show such vast diff in characteristics.

Shan2nu

[Ram]

Originally Posted by jat

...the question comes. Why commercial engines are designed to operate at lower rpm. The reason is fuel quality...
...Lowering the rpm gives more flexiblilty in fuel quality.

In the engines that must be mass produced in volumes of lakhs for use by the common man, what must be the design considerations? Just think...

In the world's most car-crowded US market, over 10 lakh 1965-model Chevrolet Impala cars were produced for the common man (American) in that one year!


Considerations
The engine should have a long life. Passenger car engines need to last 8-10 years from new to rebuild to rebuild. Not just one or two high-rpm Formula One Races.

It should be easy to maintain by people with low education and expertise. It should not require daily tuning.

Unlike a racing engine, it is not made with exotic light-metal alloys.
Nor can the typical car owner afford the world's most exotic synthetic lubricants at every oil change.

The higher the rpm the more the wear and tear. It therefore stands to reason that to have long life, engines should not be designed for very high rpms, high temperatures nor high pressures.

Ram

[jat]

You are right. But then.......... highest wear down rates has been in slowest moving engines which operate well below 100 rpm!!! And it has given engineers nightmares in designing a good lubricating oil for it. And remember, in commercial section where the profits are counted, synthetics have failed miserably. The reason is that they don't provide that much protection than what they cost over the mineral lubes.

[jat]

Sorry for interuruption.

And those slow moving engines have seperate (and complex) lubrication arrangement for cylinder walls (that is apart from your crankcase lubrication system - some of them even have additional seperate lubrication for camshaft also)

[Steeroid]

Coming back to the topic - test drove a high-torque/low (comparatively) BHP machine today and I must say the incessant Torque Wave made up for any BHP shortage. More on this later!

[revhappy]

Hi Guys,

I am livening up a very old thread, but I realised that all these discussion took place before the multijet diesel engine from Fiat came into picture.

I would like to compare the TDI engine of the Indica vista with the quadrajet engine:

TDI:
Max power 71bhp@4500rpm
Max Torque 135Nm@2500rpm

QJD:
Max power 75bhp@4000rpm
Max Torque 190Nm@1750rpm

The qjd's max torque is much higher compared to the Tdi and both come at low rpms but then the max power is almost the same at high rpm.
Does that mean that qjd accelarates faster than the tdi but in the end it loses steam? Whereas the Tdi is more linear in acceleration?

[harry10]

[quote=revhappy;1161553]Hi Guys,

I am livening up a very old thread, but I realised that all these discussion took place before the multijet diesel engine from Fiat came into picture.

I would like to compare the TDI engine of the Indica vista with the quadrajet engine:

TDI:
Max power 71bhp@4500rpm
Max Torque 135Nm@2500rpm

QJD:
Max power 75bhp@4000rpm
Max Torque 190Nm@2500rpm
quote]

Isn't the torque for QJD @1750rpm?

[revhappy]

Originally Posted by harry10

Isn't the torque for QJD @1750rpm?

Thanks for pointing that out, I have edited the figures.