Increasing Horsepower through boost or RPM

[PrecisionBoost]

I made a post talking about Boost and RPM so I thought I'd talk about the subject here in order to ensure that I don't flood this other post with people asking questions.

First off we need to look at the all important formula:


Horsepower = ( torque X RPM ) / 5252

Dyno's don't measure horsepower, they measure torque as that is what is really putting the energy of the motor down onto the pavement.

That is to say a dyno computer takes the torque and uses this formula to plot the horsepower of the vehicle.

If you look at a Dynograph the horsepower and torque will allways cross at 5252 RPM , if it doesn't something is seriously wrong.


Why is this formula so important?

Simple, it lets us see the relationship between horsepower, torque and RPM.


Did you ever wonder why sport bikes and F1 cars run insainly high RPM redlines?

Simple... the higher the RPM the higher the horsepower as they are directly related.

In an ideal engine your torque curve is 100% flat from 1000 RPM to redline.

Lets pretend we have an engine that is making exactly 200 lb ft of torque all the way along the powerband from 1000RPM to our regulated F1 redline of 19,000 RPM

At 1000 RPM that car would make 38 horsepower and 200lb ft of torque
At 3000 RPM that car would make 114 horsepower and 200lb ft of torque
At 5252 RPM that car would make 200 horsepower and 200lb ft of torque
At 6500 RPM that car would make 248 horsepower and 200lb ft of torque
At 8000 RPM that car would make 305 horsepower and 200lb ft of torque
At 10,000 RPM that car would make 381 horsepower and 200lb ft of torque
At 15,000 RPM that car would make 571 horsepower and 200lb ft of torque
At 19,000 RPM that car would make 724 horsepower and 200lb ft of torque

Now we all know that torque curves are not flat..... but that is the ideal model for the Gasoline engine.

In reality issues such as slow port velocity and volumetric efficency play a huge part in how much torque the engine makes.

With respect to port velocity, at low RPM the fuel and air mix poorly and you don't get a nice clean powerfull burn.

With respect to volumetric efficency, quite often it is directly related to port velocity.

Volumetric efficency is simply how much air gets into the cylinder head.

If you are running 10psi of boost quite often only 8psi of that will actually get into the cylinder head making the engine only 80% efficent and dropping torque by 20%

If you enlarge the ports to get nearly 95% of that 10psi into the cylinder you comprimise the port velocity at lower RPM.

In other words larger ports decrease the speed of the air which creates poor mixing at low RPM which decreases torque at lower RPM.

Manufacturers try and comprimise making the ports big enough to make the peak horsepower they are looking for at high RPM while continuing to allow the car to run decently at low RPM.

You can't make gains at high RPM without comprimising and lowering power at low RPM unless you use very specialized techniques.

Quite often things like VTEC and Variable intake geometery are used to try and increase the volumetric efficency at high RPM while maintaining a reasonable power level in the lower RPM levels.

Obviously with an F1 race car they could care less about the port velocity as they wouldn't be running their engines at 3000 RPM.... even at idle they are at a very high rpm compared to you standard car.

In most dynographs you will see the torque start to decrease around 4500 RPM which results in the horsepower leveling out.

In theory a car that makes an exact amount of torque at all times will have a dynograph where the Horsepower increases in a straight line right up to redline.

Obviously this does not happen due to the issues I've allready mentioned.

When designing a race car you figure out the power band you require.... perhaps you design it to run from 14,500 to 19,000.

In this particular case you design every single part of the car around making it most efficent at this range.... odds are if you do this the car won't even run if you tried to drive it at 1000 RPM.

That is to say it might make 200lbft of torque at 17000 RPM but only 12 lbft at 1000RPM because only 10% of the air and fuel is burning and adding power at low RPM

[wolfsreign]

+1 i like reading your post's, very informative.
well done
~wolf

[PrecisionBoost]

The other reason F1 cars and motorcycles use high RPM to create power is the fact that torque is what will kill your gearbox.

That is to say your car can make 2000hp and it won't break the gearbox if your torque never exceeds 250 lbft.

Think of it like electricity.....

1000V at 1 Amp will result in 1000W of power useage

10V at 100 Amps will also result in 1000W of power useage.

The difference is that the wire size required is directly proportional to the amperage.

That is to say you can use a tiny 29 Gauge wire which is 0.287 mm in diameter to push through 1 Amp of current (at 1000V)

To conduct 100 Amps you need 6 Gauge wire which is 4.1mm in diameter


Both wires conduct the same 1000W but the one running low voltage high current has to be 14 times thicker or it will heat up and burn out.


So horsepower and torque are just like voltage and amperage....... Decrease the torque and you can have smaller, lighter gearboxes

Increase the torque and the handling capacity of the gearbox must increase as well.


Now.... just like we multiply voltage by amperage to get watts we can really look at an engine the same way by multiplying the horsepower by the torque.

If we have a car making 200lbft and 200hp at 5252 RPM we get a number of 40,000 Horsepower Lb ft.

At the same time if we have a car making 200 lbft and 724 horsepower at 19,000 RPM we get a number of 144,800 horsepower Lb ft.


That is to say that the motor making the power up into the 19,000 RPM range will have way more "go" than the one down in the 5252 RPM range.


Now..... how do we put this big horsepower, small torque down to the wheels through a tiny gearbox????

Simple.... we share the load between the gearbox and the differencial since both are geared mechanisms designed to increase the torque going to the wheels.

One could make the gearbox very small and the differencial very big or one could make the differencial very small and the gearbox very big.

Given the differencial has just one gear ( final drive ratio ) it's more efficent to use a small gear box ( which may have 6 gears ) and a larger differencial.


Going back to the idea of electricty.... horsepower is converted through gearing just like a transformer converts voltage.


If you have 1000V and 1 amp and you put it through a step down transformer with a ratio of 100:1 you get 10V and 100Amps out of the oposite side of the transformer.

Same with gearing..... you get 1000 hp and 100lbft of torque on one side and after using a 10:1 gear ratio you get 100hp and 1000lbft of torque.


This is why we have gears in the first place.... first gear needs to be something in the range of 12:1 to be able to create enough torque to move your car.... then as you shift upwards the ratios start to get lower and the applied torque drops.

That is also why you can spin out in 1st gear but it's nearly impossible in 5th gear.

Lets use the following gearset as an example....

1 3.545
2 2.048
3 1.346
4 0.971
5 0.763

final ratio 4.176

In this case 1st gear multiplies the torque by 3.545 X 4.176 = 14.80

So if your engine made 100 lbft that means your gearbox and differencial have multiplied that to 1480 lbft going to the wheels.

By the time you hit 5th gear its multiplying the engine torque by 0.763 X 4.176 = 3.19

So again if your making 100 lbft the means your gearbox and differenical have multiplied that to 319 lbft going to the wheels.


Now...... what does that mean.... torque is simply a force multiplied by a distance.

That is to say if you had a motor giving out 319 lbft of torque and it was rotating a bar 1 foot long it would take 319 lbs of force to stop the motor.

That is to say if you put a car up on a stand, had two guys holding a breaker bar and you reved the car up to 5000 RPM and popped the clutch odds are the guys would be able to hold the 1ft long bar and stall the engine if it was in 5th gear.

In first gear you would send those guys flying.

But... due to the redline of the engine you have limitations on the speed you can do in each gear.


Now.... think if your redline was 18,000 RPM instead of 6000 RPM !!!!!

Lets say you manage 40km/h in first gear at 6000 RPM..... that means an engine that was capable of running 18,000 RPM would top out at an amazing 120 km/h !!!!

Can you imagine your 0-60mph times if you could hit 60mph in first gear???

[PrecisionBoost]

So... what are the downfalls and problems with increasing RPM????

acceleration and deceleration of parts within the engine increase with RPM.

As the piston comes to top dead center it stops.... then it accelerates downwards for the first half of the stroke.... then is starts to decelerate and again comes to a stop at bottom dead center.... then again it accelerates for the first half of the stroke up and finally it decelerates again at it comes up to top dead center.

All this acceleration and deceleration cause very significant forces.... think of tying a string to a 1 lb weight and throwing it out away from your body as fast as you can.... then jerk it back towards yourself.

Then do this with a 10 lb weight and watch your arm disconnect from the socket as you scream in pain.


The mass of the components moving up and down ( pistons, connecting rods, valvetrain, crankshaft ) and the acceleration / deceleration of that mass over a particular distance ( the stroke of your piston ) are the primary limiting factors on RPM.

The heavier they are the more forces come into play..... as well.... if your rotating at high rpm and your stroke is increased the pistons/rod/crank will have to accelerate and decelerate quicker to make up that distance.

That is to say a piston in an engine that has a stroke of 86mm will accelerate/decelerate twice as fast as a piston in and engine that has a stroke of only 43mm

In F1 cars and motorcycles they make the rotating parts as light as humanly possible without comprimising the integrity of the parts.

They also try to decrease stroke..... you might find an F1 engine has a bore of 98mm and a stroke of just 40mm


Next we can talk about "valve float"

At a certain point stock heavy components will have enough inertial mass due to weight and RPM that the valve springs are simply overwhelmed.

The forces on the valve springs increases to a point where the forces at work are higher than the valvesprings can counteract the valves won't close.... they will stay open and cause all kinds of problem including the possibility of smacking the piston at high RPM.

There are only two ways to counteract valve float at high RPM...

1) Increase the spring pressure
2) decrease the weight of the valvetrain

Most performance engines do both.

There are few drawbacks to decreasing the weight of the valvetrain as long as your not weakening the strength of the valve through removal of material.

On the other hand increasing the spring pressure puts more force on your lifters/tappets and cams.

If you go too heavy hydraulic lifters/tappets stop to work as the force from the springs will counteract the force of the oil pushed into the hydraulic lifter/tappet.

Quite often people move to solid lifters which require shims to get the right height ( were hydraulic ones self adjust thanks to oil pressure )

Cam life will decrease as will most of the rest of the valvetrain.... but then again with higher RPM you can also expect the other components in the engine to be worn down quicker.


If you run an engine at 19,000 RPM for an hour it would the equivilent of running a 6000 RPM engine for three hours.

If you get 100,000 km from a regular engine you can expect to get something closer to 30,000km from a high rpm engine.

In F1 racing they can only use 8 engines per season as of 2009 which means each engine must last 3 entire race weekends.

That is to say each engine must last several hours before it can be tossed in the garbage and replaced by a new one.

[daewoomofo]

VERY GOOD READ, thanks for the info!!!!!

[exist3nce]

Chris is very accurate with his posts.... This torque vs horsepower issue is what my tuner always tries to maximize. That is to say, trying to maximize your horsepower with the available torque by making the torque curve as flat as possible. A flat torque curve will also make best use of your available tire traction, and minimize the chance of breaking things (like gearboxes).

Just as an example, here are some of my dyno graphs showing the difference between running a flat boost curve vs a rising boost curve to make up for falling VE in higher RPMs with the stock U20SED.


Flat 8psi across and resulting dyno plots





As we can see, torque comes to a "peak" and then starts falling back down, resulting in horsepower leveling off, and then falling after 5500 RPM.




Rising boost curve and resulting dyno plots

Here we gradually kept adding more boost in the higher RPMs to try to prevent the torque from falling (look at the highest line on the plot, ignore the other two)





We can see that horsepower just keeps rising now..... we like to call it "fake cams" lol

You can really feel the difference on the street with a flat torque curve, because the car just wants to keep pulling hard right up until redline, whereas before you can feel it drop off after 5500rpm or so.

[mr_g]

Very nice article, except the analogy that is incorrect...

Current (A) == Torque
Voltage (V) == rpm * constant
Power (W) == Power (HP)