tabetha

Torque is power.
Capacity is a key to gaining more power as the more air(NOT petrol) that an engine can consume the more power it has the potential to make.
Torque and power are linked, if torque goes up so does power unless the torque occurs at a different speed(rpm).
At 5252 due to the way Torque and BHP are calculated and arrived at they are the same, if you ever see a chart that is different at this exact rpm then it is bollocks.
It is that increasing capacity is sometimes done to generate more torque, meaning more power lower down(rpm shift!!) and then said to be a torquey engine, may not be any more bhp but rpm has shifted.
bhp= 2XPIX torqueXrpm divided by 33,000
The 33,000 is important and was derived at when it was found the "HORSE" could move a weight of 33,000lb 1 mile in 1 minute.
This is where brake HORSE power comes from.
Another simpler way is bhp = torque X rpm divided by 5252,
or torque = bhp X 5252 divided by rpm.
PS is nearly the same as BHP but is a decimilised GERMAN (near)equivalant and stands for PFERDE STARKE, or HORSE STRONG/STRENGTH.
1 PS is 0.986 BHP
tabetha

rapidcossie

Capacity should increase the possible HP.

A 2.5 is 25% bigger than a 2 litre so no reason why it should not make 25% more power.

focusv8

There's no replacement for displacement.

.

rapidcossie

didnt say it was a restriction


Bigger capacity can flow more air so it should make more power with it.

tabetha

See the word "POTENTIAL", I used a 25% increase in capacity will NOT show 25% more power, as frictional losses increase with larger longer heavier pistons, reciprocating mass increases too due to component weight.
Enguine capacity is very carefully chosen to do the job that the car manufacturer wants, the smaller the better, needs to be just big enough, as smaller engiunes due to the above and other reasons have better VE that is why there are turbo cars out there, a 2.0 pinto has less EC than a 2.0 turbo car.
Air speed is another very important consideration, smaller ports increases VE whilst EC can be increased with supercharging ie turbo.
tabetha
ps this is going to be a long one!!

foreigneRS

not really - power = torque x engine speed

yes, torque and power are linked as shown by the formula above but the rest of your statement is not true - only if torque is in lb.ft and power is in (b)hp. if it's Nm and kW, it's completely different

GARETH T

power = P X L X A X N

p=cylinder pressure
L= length of stroke
A= area of piston crown
N= number of power strokes per minute

when we increase capacity we increase both L and A

ECU Monitor Enthusiast

iTrader: (1)

Whats "X"

GARETH T

AlexF

sadly as you increase bore size you increase the friction forces from the piston rings...

so going up 25% in capacity will never get you 25% more power.

The law of deminishing returns.

aLex

GARETH T

your right alex its never that easy

Red16

what about if it was same diameter bore but longer stroke? would the extra moment of inertia of the long stroke crank outweigh the friction of the larger diameter rings?

rapidcossie

Have any of the people commenting on this thread actually done any testing on this theory, or are the just repeating what someone has told them?

I have seen the possible increases with my own eyes over the last few weeks with one of my friends 2.5 litre Subaru.

So it looks like it is that easy

AlexF

then the rings would be creating the same force as before but for more time per stroke.

either way remember 50% of your engines fricition is ring related!

Alex

AlexF

I worked at Ford in R&D on diesel engines for 3 years.....

I now teach people how cars work.

So without sounding like a complete cunt I think I'm right - of course if anyone knows better I will be grateful corrected


As for the scooby don't forget that as you change the cylinder you are effecting (massively) how much air the turbo can blow into that cylinder.

This is where its going to get tricky as pressure and air flow are totally different subjects!

Alex

GARETH T

no one has told me anything

tabetha

Foreign rs we use lbft and bhp here and it was asked here!!
POWER and TORQUE go together which is the point I was trying to make, and used examples to show this, I thought it clear enough.
tabetha

AlexF

but Nick didn't hence his post

Alex

Karl

Hello all,

I'm only on here briefly whilst checking mail, so please dont ask any other questions as I probably won't be here to reply.

Increasing capacity "can" give a power increase but usually does'nt in 95% of cases. This is because it is usually the cylinder head air flow that defines the total air flow through the engine and hence it is the cylinder head that dicates power, NOT capacity.

As a good example take a 1.6cvh fitted with a stock 32/34 weber carb. Makes around 90bhp. Bolt ALL the parts onto a 1800cvh bottom end with the same compression ratio....................guess what..............90bhp!

The difference is that the 1800 will make that same power a few hundred rpm earlier but the maximum power is actually determined by the maximum air flow through the engine. In this example it is contolled by the head flow.(and manifolds of course)

To put it another way on two otherwise identical engines except one being bigger capacity than the other, provided the smaller engine can physically rev that little harder (i.e. mechanically without failure) then the power will be the same.

These rules apply to N/A and turbo.

However what can be misunderstood is that by using a larger capacity engine with a turbo, it is possible to make use of larger turbos without excessive lag that you would encounter with a smaller engine, and hence produce more power because you have a larger turbo. (Unless you are prepared to live with the reduced response from the smaller capacity engine)

And finally for all the doubting thomas's....................

A 2.4 YB fitted with a T34 will make around 380bhp running the turbo flat out. A 2.0 fitted with the same components.....................guess what................ 380bhp.

The 2.4 will simply make it lower in the rpm band, but likewise will run out of puff earlier meaning that the power band is no wider than the 2.0, simply that it occurs at lower rpms. This scenario is only usefull in cases where the engine has reached the end of its mechanical ability to rev, yet more power is required.

rapidcossie

The engine im talking about has been running with no turbo as part of its development....

Im no expert but I know what I have seen.

The engine in question make 25 % more power than the equivilent 2 litre.

rapidcossie

If capcity makes no differance in max HP produced, why are all the big power N/A cars of a very large capacity?

saw a 7.2 litre today , it makes 550 bhp.

No chance you can get that from a 2 litre is there.

Karl

The reason you have to go big capacity with N/A is exactly as I said above. Once you can rev the engine no more then you need to increase the capacity to continue making bhp. If you can keep revving the engine then you do not need to increase capacity.

e.g. 3.0 N/A F1 engine revs to 18,000 makes 1000bhp.

ECU Monitor Enthusiast

iTrader: (1)

Interesting stuff

rapidcossie

So increasing capacity and revs would BOTH increase HP.

GARETH T

yes you could,,,

as karl so rightly says,, its all about airflow,,, and we can only compare like for like engines specs

AlexF

in the 70s F1 cars were 1.5 litres and 1700 bhp.............

whats your point?

Its all about air flow as everyone is now says - Hi Karl btw

Alex

rapidcossie

My point is that a larger capacity version of the same engine should make more power, simple as.

GARETH T

and thats the oposite of karls point

what limits the engines airflow? normally the head spec and not its capacity

rapidcossie

I can only go by my experiences and Karl can only comment on his

A normal 2 litre 16v engine makes 150-160 bhp.

A normal 2.5 litre engine makes 180-200...looks like a 25% increase in power to me.

GARETH T

but there different engines and have been engineered for the extra capacity

rapidcossie

I give up

Stu @ M Developments

iTrader: (12)

I think what your missing is the 2.5litre was designed to flow more air from teh outset. It wasnt teh capacity alone that increased teh power, it was the capacity in conjunction with the hardware that allowed it to flow to its potential. (Manifolds, cams, valves, heads)

Our point, (well, most who has spoken, and i do concur) is that incresing an engines capacity ALONE will not normally improve your BHP reading, it will only allow it to shift its limit of air a little SOONER, hence they normally drop teh power down the rev band and increase torque at the bottom end as a By-product.

As an example, if you took your scoob engine and bored it to 3.0 litres, then put it back together with no changes whatsover, i would predict its low end torque would improve and its peak power stay within 10% of its output at 2.5litres.

GARETH T

i should of said,, but normally we cant keep the same cylinder pressures

Stavros

apart from turbos, revs, superchargers, nitrous, etc