rpm , bhp , cc

[Srikant Sharma (chiku)]

Hi ,
can any one care to explain me what is the exact relationship
between rpm, bhp , and the capacity ( in litres or cc) of an engine.

Is it true that more the cc more bhp we get ?

Also how do we exactly calculate the bhp of an engine ?
thanks
--
Srikant
please remove .SPAM from e-mail address if contacting by email

[www.datsuns.com]

Formulas
HP = (torque*RPM)/5252
Torque is (HP*5252)/rpm

This means at 5252rpm, the torque # is the same as HP, so
if your car has
110hp @ 5252rpm, then you also have 110ft/lbs of torque at
5252rpm.

Complicated question. Everything else being identical, more
displacement =
more power. There is no exact relationship between
displacement cc's, as there
are too many variables, but there really is "no replacement
for displacement".
Take any 1600cc motor, bore it out to make it a 1660cc
motor, and you'll have
more HP and torque.
There are ways to calculate the HP, based on compression
ratio and other
factors, but they'll be rough estimates.
An engine is a big air pump - the more air you move
through it, the more
power you get. Another note is while you can get a VERY
wide range of HP per cc's
numbers for a given displacement (from 35 to well over
100hp/liter) you don't get that
wide of a range for torque. For higher HP, you just give
the engine the capacity to rev
higher (some motorcycles make 140hp/liter and spin to
14,000rpm), torque increases
are achieved by this (but to a lesser extent), but also
increased displacement.
The above assumes no supercharging, but even at that, it
still applies

Please take at the technical department at WWW.DATSUNS.COM,
and look at the
technical articles, they explain this further.

--
Dave Lum - remove * to e-mail
'71 510
www.datsuns.com

Srikant Sharma (chiku) <srikan...@wipinfo.soft.net>
wrote in article <361DF8C6...@wipinfo.soft.net>...

[Mark Herring]

displacement, measured in ccs or cubic inches, is the volume displaced by
the pistons---i.e. the amount of air/fule mixture pumped in and out.
Displacement = stroke times ( bore squared X pi/4)

Power (typically measured in HP) is the rate at which an engine (or any
machine) does work. Thus is it the product of the force (or torque) exerted
and the rate of motion (or turning).

To convert torque to HP, one must find the distance thru which the force
(torque) acts per unit time. Note a tricky thing, here: torque is measured
in ft-pounds, as is work---but they have very different meanings.

Suppose an engine has a torque output of 200 ft-lbs at 3000 RPM. The torque
will do work according to the formula:
work (ft-lbs) = torque X 2 pi

1 HP = 550 ft-lbs /second

Thus, the formula for horsepower:

power in HP = (2 pi (torque in ft-lbs) (rpm)) / (60 (550))

Our 200 ft-lb example at 3000rpm works out to 114 HP. If the same torque
was there at 6000 RPM, we would be getting 228 HP. This is why 4 -valve
engines are so spunky--they have a broad torque curve and --coincidentally--
high HP at the top end

HP goes up with displacement, but the relationship is very dependent on
engine design. And--note the comment about 4-valve engines. Peak HP is not
the only metric of performance. These engines are spunky because they have
torque over a wide range of RPM.

Hope this helps

-Mark

[Grape Ape]

Torque of an engine is actually measured in lbs. ft., not ft. lbs. Most
people get this mixed up (although it is commonly referred to as ft. lbs.).

Another thing to add is that BHP (Brake Horsepower) is measured by holding
the engine at a specific rpm in steps, usually 250-500 rpm increments,
called a Step Test. The best way to measure power of a performance engine
is to test Acceleration Horsepower. This way you can see more how the
engine will accelerate under load. The engine is held but is allowed to rev
at a certain rpm per sec. (commonly 300 or 600 rpms/sec.). These numbers
are lower than the BHP but give better comparisons. If we have an engine
that has all lightweight parts (cast pistons, aluminum rockers, light rods,
and a light flywheel), than replace all the parts with heavy forged pistons
(same compression), steel rockers, stock rods, and a heavy flywheel, than
the BHP would be very close, but we would loose a lot of acceleration HP
because of the heavy parts requiring more power to accelerate. BHP will
give the WOW factor, but higher AHP is what to really look for.