How do I get more power from my engine?
Although most of our Rover's aren't in the boy racer class there is still some
impressively fast Rover machinery around. Especially the V8's give any Rover,
even in basic tune, lively performance. But for some people it is never enough
and for these people we have a look in what is involved. First back to the basics
of the internal combustion engine.
As you can see in the formula on the right there are a lot of factors involved in
determining the amount of power of an engine. Let's start with the most important one
The average pressure on the piston.
The bigger the pressure on the piston when
it makes the combustion stroke the bigger the engines power. 100% true!. Power
just means getting in as much air and fuel as you can and then burn it. More air
and more fuel for the same engine size mean higher combustion pressures and thus
more power. Increasing the pressure can be achieved by:
The last two may sound a little bit strange at first.....Reducing inlet and exhaust
resistance?? Well maybe the next figure will clear things up a bit. Every engine only
produces power on the combustion stroke because the combustion forces the
piston down. The inlet stroke needs power because the engine has to suck in the air and
fuel. The compression stroke costs power because the air/fuel mixture has to be
compressed. And the exhaust stroke also costs power because the exhaust gases have to be
expelled out of the cylinder by the piston.
- Turbocharging where the exhaust gases turn a compressor wheel which forces
air into the cylinder
- Supercharging where a compressor driven by the engine forces air into the cylinder
- More efficient burning (higher compression, better spark, mixture distribution, etc)
- Reducing inlet resistance
- Reducing exhaust resistance
So fitting a less restrictive air filter or a bigger exhaust really do help because
they make the engine breathe better and increase the mean pressure in the cylinder.
Also camshafts have a big influence on the mean pressure. More on this later....
Then there is the compression....it costs power to compress the mixture, right?...
well simple solution, reduce the compression ratio! If it only where that simple
I've we have a look at the Efficiency formula for our Otto cycle engine we see that
a higher compression ratio means a more efficient engine. Furthermore on the downwards
stroke the energy used for compression will be returned. So all in all a higher compression
ratio means a more powerful engine. But the maximum compression ratio is limited to prevent the air fuel
mixture to ignite prematurely without our beloved spark-plug, you might as well buy a Diesel
Furthermore above a CR of 9 there really isn't that much of improvement left in efficiency
as the graph shows. Then why did they use high compression ratios in the old days when the fuel
was full of lead additives? The answer is flame speed. A higher compression ratio also
means the air/fuel mixture is quite dense. Now it is a fact that the speed of the combustion
increases with the density of the mixture. Low density means low combustion speed.
This explains why you've got the vacuum advance on your distributor.
A high vacuum means a lean, less dense mixture and a slow combustion speed.
As a result the mixture must be ignited earlier.
With a higher CR you've got a higher average effective pressure on the piston,
this because of the faster combustion speed the available fuel can be burned more efficiently.
Now we arrive at a point where the Americans prove their point, There is no substitute
for cubic inches!! A bigger engine simply means more power because it burns more air and fuel
in the same time than a smaller engine.
However there is always the law of diminishing returns...a bigger engine means heavier
car more weight, heavier suspension,etc..
But you can increase the capacity of an existing engine without much increase in weight...
Voila, the reason why there are not only 3,5 litre Rover engines around but also
3,9/4,2/4,6 and even Buick 4,9 and 5,6 litre versions!
The bore and stroke of the various Rover engines are
|3.50 (88.9 mm)||2.80 (71.1 mm)||P5 / P6 / SD1 /|
MGB V-8 / etc.
|3.70 (94.0 mm)||2.80 (71.1 mm)||Range Rover|
|3.70 (94.0 mm)||3.03 (77.0 mm)||Range Rover|
|3.50 (88.9 mm)||3.50 (88.9 mm)||Australian P76|
|3.70 (94.0 mm)||3.22 (82.0 mm)||Range Rover|
|3.75 (95.2 mm)||3.40 (86.4 mm)||Buick 300|
|3.70 (94.0 mm)||3.54 (90.0 mm)||TVR Chimaera|
|3.75 (95.2 mm)||3.85 (98.0 mm)||Buick 340|
When the stroke is longer than the bore we have a long stroke engine like the Buick 340. The P76 called a square engine with the bore and
stroke being equal. As a rule of thumb the bigger the bore in relation to the stroke the
more willing the engine is to rev. If it's low end torque you want, go for a longer stroke.
The number of cylinders
Increasing the number of cylinders also increases engine capacity off course. But then
it is quite difficult to make a V10 or V12 from your Rover. However BL motorsports have
made a V6 version of our V8 which was used in the B group Metro 6R4 rally car.
The Wing Wonder
The faster an engine runs the more air and fuel can be burned in the same time...thus more power.
By letting an engine breathe more freely and selecting an appropriate camshaft it
will be able to reach higher engine speeds....however with increasing engine speed
also the pistons,crankshaft,etc....must be able to withstand the increasing forces.
More on this later.
Two or four stroke
A two stroke has a combustion stroke every crankshaft rotation. The four stroke only
every two crankshaft rotations. Thus theoretically a two stroke could have twice as
much power. However because of the less efficient in- and outgoing gas flow it produces
generally only 50 to 70% more power instead of 100%.
Internal engine friction
All moving parts in the engine have some friction. The piston going up and down in the bore,
the friction of the conrods on the crankshaft journals, friction of the crankshaft in the
bearings, etc...All this internal friction consumes power. It can be reduced by assembling
the engine with selected matching components this blueprinting will give some extra bhp.
Furthermore also the use of synthetic oil can reduce friction. Other possibilities are
to reduce on the ancillaries the engine has to power, like the steering, airco, oil-pump,
So that are all the factors from the formula which have a direct relationship
with the amount of power an engine can give. In the following pages we will look
into it a bit further......After all these were just the basics..
Thanks to: Adriaan Briene
Engine power can be calculated with
Pe= Power output
pe= mean piston pressure
Vs= Engine displacement
z = no. of cylinders|
n = engine speed
i = 0,5 for 4-stroke
i = 1,0 for 2-stroke
etam= internal engine friction
Theoretical efficiency of the Otto engine
Eff = Engine efficiency
CR = Compression Ratio
k = ratio of specific heats
For a Rover V8 with a CR of 9.35 this would
give a maximum efficiency of 0,591 (e.g. 59,1 %)
Why Otto Rules....|
(Sorry 2400 SD owners.....) Maybe a second question comes up now? A Diesel has better fuel efficiency because of a
higher compression ratio, why then don't they give more power? This is because the average
pressure on the piston during the combustion stroke of a Diesel engine is lower than
with the Otto cycle engine.
Still the Diesel's peak pressure's are higher, hence it's heavier construction, in the end it is
the average pressure on the combustion stroke that counts,
so for power.....goodbye Rudolf Diesel!!
- average combustion pressure Otto engine 850-1000 kPa
- average combustion pressure Diesel engine 600-750 kPa
Oh..just to give the Diesel a theoretical kick in the ass.....
If a Diesel and Otto engine have the same compression ratio
than the theoretical efficiency of the Otto engine will be better.
You can mail me if you want the proof of it.
Buick 340 V8 transplant in a Rover SD1