Speed Secret # 4

[jim_howard_pdx]

Speed Secret # 4

I was going to cover something else on this thread, but someone mentioned in #3 that I am recommending more duration and lift on the exhaust than on the intake.... And that would feed into the first factor of increasing cylinder pressure to generate extra horsepower.

So I am going to go into this in finer detail NOW. Because like I said, if the exhaust system does a poor job, nothing you do until you run NOS or a charged intake can overcome the mess.....

To maximize your torque and your horsepower, FOR ANY ENGINE, you have to minimize intake charge dilution. Control DILUTION better than your competitors and you will make more power than they do. This brings home trophies.....

Some of you may never have heard the term dilution EVER. So what is dilution?

As the fuel is burned during the power stroke of your 4 stroke engine event, the piston is forced down by the ignition of the air and fuel. When the compression ratio of the burning fuel charge collapses due to the pistons moving down the cylinder, you are left with burned, partially burned and unburned exhaust and fuel.

This volume of "stuff" is now your engines biggest enemy.

Your piston now reaches the bottom of the power stroke, and the enemy sits there waiting to hurt you. At the piston begins to rise, the exhaust valve is opening. This initially shows the high pressure in the cylinder a way of escape.

So the first part of the exhaust is actually physics 101. Stuff moves from from high pressure to low pressure until stuff equalizes pressure in the new volume of space.

So as the exhaust valve just begins to open, the exhaust squirts throught the valve/valve seat space at very high velocity as it seeks to equalize into the low pressure area of the exhaust valve pocket and the primaries of your headers. Some of this initial exhaust movement is improved by the scavanging effect or inertia of the other cylinders exhaust flowing down the exhaust tubes and producing a vacume for this cylinder.

Now the first negative thing happens. As the exhaust "stuff" ( burned, partially burned, and unburned fuel) moves into fresh air, it sees oxygen and the super heated exhaust fumes now explode in the valve pockets and primary tubes of the headers. This explosion or concussion front "sees" pressure in front of it, exhaust volume and mass of exhaust working its way down your exhaust pipes. So the energy converts from expanding wave fronts to ringing wave fronts. Literally, the energy slams side to side, instead of out and down the pipe. This ringing energy produces additional backpressure at the point in the exhaust system that most needs to show vacume.

Do you see the physics at work here?

Going to H or X pipes is ALWAYS in your best interests. Going to an exhaust tube size that tunes the scavanging effect to be greatest at your target torque range is extremely crucial. Going too small or too large an exhaust pipe is going to reduce your scavanging and increase your dilution levels.

But there is more really bad stuff that is about to happen.....

As the exhaust valve continues to open, the piston begins to rise and accelerate in the bore. OUCH. Pressure is now building inside the cylinder volume that increases to shrink as the piston moves up. Now the exhaust valve starts to close. D A M N, now the pressure is really up there now. So you are left with a volume of "stuff" at top dead center that is pressurized, but held back by the backpressure of ringing energy, and the backpressure of the volume and weight of the exhaust that is moving down the exhaust tubes.

So the closer your piston comes to the combustion chamber in the exhaust stroke, the slower the "stuff" moves as the pressure increases to build.

AND as RPM increases, exhaust pressure increases, and the DILUTION EFFECT INCREASES.

Now a third bad thing happens.....well both good and bad. The overlap event begins. and OUCH more bad stuff is happening to hurt your power.

Your intake valve opens to a volume of "stuff" that is very hot, pressurized, and sometimes randomly positioned in the combustion chamber.

Don't take that random position thing lightly either.......Because it effects the next ignition event and ANY MISFIRED CYLINDER will contribute to higher dillution for all your other cylinders.

So lets look at this third negative aspect of dilution and see how it effects the horsepower in great detail.

The "stuff" begins to mix with the incoming intake charge during the overlap event. It immediately heats and expands the incoming charge, increasing cylinder pressure before the piston even reaches top dead center. Worst still, the density of the overlap charge is mostly ruined by the stuff. Dilution is a big bad enemy.

Now the piston reaches top dead center of the intake stroke, the diluted overlap intake charge is now augmented by good, fresh, dense air and fuel. But the damage is already done.

Your intake charge density is now negatively effected, and the amount of "stuff" left over rises as your rpm continues upward.

So just when you need more and more intake volume and higher charge density, the dilution chokes off the good stuff our overlap and lift and intake velocity is designed to accomplish.

So RPM, dillution, and overlap dilution pressure build up to reduce the volume and charge density and your torque begins to fall.....
The proportion of the dilution to the fresh charge is critical to how much horspower you can make, or conversely to how much horsepower you LOSE.

Refresh this in your minds. Horsepower is calculated and not measured. We simply take the engines torque at a certain rpm, and use a formula to determine the horsepower. So torque at high rpms is critical to making horsepower.

We cannot build more torque at higher rpms because the dilution effect is increasing in your engine as the rpms rise.

It is a vicious cycle.

A split duration cam is a great way to cheat your engine into reducing the dilution volume, and to shift the dilution event into a higher rpm range. This allows me to have higher, high rpm torque and this calculates to higher horsepower.

I hope this is not getting lost on you......

Why is my short intake of .496 generating 425 horsepower on my current 358, when I had to run .540 lift on my 351 Cleveland engines to make the same horsepower in the early 70's? The intake port volume of a 351 Cleveland intake port is bigger and better flowing than the intake port of my Trick Flow PORTED heads. It should have made way higher numbers of horsepower with those free flowing heads and huge intake valves. But they could not take advantage of the engineering because back in the 70's we did not have commercially made cam shafts that reduced the dilution effect.

So dilution is your enemy always and forever. Any dilution at all is an enemy. It can never be eliminated, but it must be managed and minimized.

So lets summarize what we can do today to reduce or at least delay dilution.

First, you need excellent exhaust ports that allow easy and unhampered exhaust flow.

Second, you need large exhaust valves to minimize pressure during the overlap event. T

Third you should design an exhaust port to produce swirl to minimize the ringing horizontal energy of the exhaust event and turn it into faster exhaust flow DOWN the exhaust pipe.

Fourth, you should run dual pipes of a diameter that provide maximum scavange at your target RPM band.

Fifth you should run an H pipes or an X pipe to improve scavange and show more vacume to the cylinders.

Six, you should use a muffler design that improves the swirl of your exhaust system.

But most importantly, you should be using a dual or split duration camshaft designed to put the torque curve where you want it.

I hope this helps everyone understand power production. This topic, other than preignition and flame propogation, is the most significant thing to understand when building your next engine or improving the power on the one you race.

Your comments as always are what I am after. I do not do this for the exercise of telling you stuff, but to get a dialog going and have fun thinking the physics through.

Have fun this weekend. Now look for the areas I skimmed through, and areas I may have omitted and lets talk about dilution and the ways you eliminate it.