Tuning the C20LET - Breathing

The LET is based on the XE 2liter 4-stroke engine: this means that left on it's own every 2 crank revolutions it will suck 2 liters of air, mix it with fuel (at a weight ratio somewhere between 20:1 and 12:1), ignite it and shove it through the exhaust.

All in an effort to try and turn the wheels (all four of them!).

A sad fact of life is that these engines cannot suck in the full 2 liters, except if we are cranking them by hand (just a few rpm)
At 1000rpm there is only a fraction of a second available for a cylinder to fill with air, through the narrow gaps under the intake valves. There's no chance the full displacement will be used up. At 2000rpm inertia is getting the better of us, and at 4K is much worse. At a 7K redline we don't really expect 2 liters every 2 crank revolutions. Not with the few millimeters the valves open for a few milliseconds.

Getting the burnt stuff out isn't much easier, despite the higher pressures following the combustion. This is another fallacy, that exhaust gases are pushed out under big pressure. They are not - we don't want this energy wasted, we want it to push the piston during the power stroke. Neither do we want the piston to push the refuse on it's way up (exhaust stroke) because this will eat up momentum from the crank. It's a tight ship we're running.

As a result, we accept that Volumetric Efficiency (VE) will be less than 100%

Let's say that a 16valve engine like the LET has a VE=80% (at full throttle and peak torque rpm)
There is only so much power one can make from burning 1.6 (2 liters* 0.8) of air/fuel. If more fuel is injected, the mixture won't burn properly and power will be down. If we want more power (say double) then we need to turn the engine twice as fast, just like the F1 cars do. If we want to double it once again, we have to double the rpm up to 20K rpm. Unfortunately the rotating parts were not designed for this sort of speeds and they will break apart well before we even reach 9K.

Enter the turbocharger.

This tiny little pump in front of the engine uses the exhaust gases to rev well beyond 100.000rpm and in the process it can make the engine think that it operates on a planet where the atmospheric pressure is double that of earth (when showing 1 bar boost). This means that the force that shoves air into the cylinders is double, so (roughly) double the amount of air is inhaled and we expect the power (and fuel consumption) of a 4.0 liter engine.
Running at 2 bar boost, the engine will suck in 6.0 liters of air. At 3 bar boost 8.0 liters.
Not bad.

OK, we've made an assumption or two:

1. the VE remained the same
2. proportionately more fuel was also mixed
3. the air temperature remained the same
4. the internal combustion pressures remained within spec

The good news is that all 4 above assumptions can be eventually relaxed (hence the mythical 1000bhp+ 3.0 turbos)
The bad news is that it takes money and effort to achieve that - in stock form the LET manages to hold half a bar of boost, and not always successfully!

The weak link in this power chain is breathing - these engines are asthmatic.

We have to make it easy for the air molecules to slide through the passages, trap them in the cylinders, squeeze them till it hurts, then show them an easy way out so they can get the hell out of there. Kinda like the London Underground.

Let's follow the pilgrimage of the air molecules lucky enough to be selected for combustion. They have been told that if they make it through the tailpipe they'll go to heaven:

They first encounter the airbox and the assorted intake pipework. Thick silicon pipes lead them to turbo compressor where they're squeezed and get hot like hell. Then they are shown the way to the intercooler where they chill out and hope they'll be the first to pass the gates of the throttle body. If they're unlucky though, the gates are closed and they get thrown out unceremoniously through the Dump Valve. The lucky ones see the inside of the intake manifold and the cylinder head. Most of them socialise and mix with their fuel pals, then they all die with a bang in the torture chambers. Their half-dead bodies are shoved through the spinning turbine wheels, and their remains are shoved through the exhaust...

On to the Intake

An engine creates only one motive force and that's torque, measured, as you know, in pounds of force applied per foot (lbs.-ft.) .Make that force act in rotation, over time and you need to multiply it by the rpm. Divide that by a constant (5252) and you get a number we call horsepower. It's simply torque applied over time and it's based on the amount of weight a horse could be expected to lift ione foot in an hour, which is pegged at 33,000 pounds.
Horsepower is ALWAYS secondary since it's only a calculation made from the available torque.
It is another common misconception that long stroke always = torque. Not true.
This is simply the result of the fact that the design limits of a long stroke engine , namely piston speed "limits" keep it from revving high enough to create any high hp numbers.
(rememeber it's torque x rpm / 5252). Long stroke engines simply can't be built to rev high enough to create any really high hp numbers. (comparitively speaking)

When you build an engine that needs to fit in a certain space, like under the backbone of a motorcycle, you have a maximum stroke that will fit. If you want to increase the amount of power that design can produce without lengthening the storke, you can:
1. make the bore bigger
2. add cylinders
3 add rpm (without breaking the piston speed design limits)
4. pack more air (and fuel) in it.

Torque the enemy of hp?
On the contrary without torque there IS NO horsepower.