Speed Secret # 3

[jim_howard_pdx]

I think we have picqued some interest out there in learning speed secrets.

So far we have talked about SQUISH and quench factors in ignition events, and reducing parasitic power losses to improve et times.

I have had just a couple of you who seem incredulous at first, then someone with experience chimes in and they see the light.

No single approach to speed is ever the best or only way. It is only important to know these secrets and apply them as the rules of the class allow you.

But you had better know these, and learn how they can be applied to your engine. I guarantee that the winners taking home the trophies know it and apply it.

Now on to speed secret number three.

Why did one pass net a decent time slip and next week we shaved almost 4 tenths off the ET? More Cam you say, Intake change, different Carb, NOS.....

Just one change..... the Compression Ratio.

We were running a wild Pinto with a full space frame, FMX automatic, 4.88 rear end, and the tallest slicks we could buy at the time. Times were ultra sweet unless we blew the launch. The engine had all the good stuff, a stroked 351 Cleveland with $2,000 of custom head work to raise the exhaust port, reduced and lightened valves, and run some really light rods and pistons.

We spent hundreds of hours on the bench to develope hurricane like swirl, which the cantelevered heads made possible. Ultra cool titanium rods that were so beautiful it was a shame to hide them in the block. Custom pistons with all the tricks I outlined in speed secret one.

A custom forged steel crank rated for 10,000 RPM that we knife edged and filled with mallory metal inserts to maintain balance.

But the thing that the engine responded to most was small changes in compression ratio.

So what is my speed secret???? After all every engine builder worth a d a m n knows that compression ratio effects horsepower. So what is so secret about this? Well how do you increase your compression ratio while keeping the static compression constant?

This will be the subject of today's secret.

First, you really have to build the engineering around the bracket you plan to race. Ours dictated we had to run a static compression ratio not to exceed 12 to 1. The class we ran at required all winners to subject to complete engine disassembly. And at the time, coatings were not allowed in this bracket. So guys we could not cheat. Measurements are measurements. But cheat we did, in a legal fashion.

Our engine ran at 14.5 to 1 compression ratio......wanna know how?

Engineering.

An engine is really nothing more than an air pump. You have to do everything you can to make it inhale and exhale properly at the rpm you run to win your races. So what I am alluding to is you need to compute your rpm from 1/8 mile to the speed trap and figure how to build maximum torque during this period. On our engine, RPM at third gear started at 5,700 RPM (the stall of the converter and we crossed the finish line at 8,600 RPM.

So the goal was to increase the compression ratio between 5,700 RPM and 8,600 RPM. See why we were using a Cleveland Head. This is the RPM range where this engine sings and dances.

Now how did we get 14.5 compression out of a 11-1 static ratio?

Well lets go back to physics 101. I take 50 cubic inches of stroked area, and compress it into 5 cubic inches of combustion chamber, we get 55 cubic inches of area divided by the 5 cubic inches of combustion chamber and voila....we get 11 to 1 static compression ratio. Now measure the actual cylinder pressure obtained at the rpm you want with the cam you are using. Now bump up the static compression to 14 to 1 and look at the cylinder pressure. If you want your 11-1 engine to run like a 14-1 compression engine you simply need to produce the same cylinder pressures at the proper rpm range of the 14.5 to 1 engine. This is what we call dynamic compression ratio.

Understanding how to build maximum dynamic compression at the rpm you need to win, is why 100 guys go out to race and 1 guy wins. 1 guy figures out how to do this best with his 12 to 1 bracket class engine.

Now there are 5 ways I will tell you about right now to build dynamic compression ratio. But there are alot more.

Please comment to this thread by remembering what things you did to improve ET's without adding stuff like blowers, Nitrous, nitrogylerine and the like.

First way to improve cylinder pressure. You have got to get all the exhaust out, or flowing at a pace that allows the air fuel mixture to pack the cylinder at the rpms where you want to build power.

Unless you really scavange well, you might as well forget this whole speed secret. The whole thing starts with the exhaust flow.

What we did is run 3 inch primary exhaust tubes out the raised D port heads for about 6 inches. The D ports were specially shaped to create swirl out of the exhaust port as it entered the round primary tube. Then we stepped the primary to 3.5 inch tubes for the next stretch to the collector. Then we ran 4 inch collectors for 22 inches with vanes to keep the exhaust spinning instead of ringing. The vanes were an insert we built and placed into the collector. We ran 3.5 inch H tubes to the spin tech mufflers, and 4 inch pipes from the mufflers on out.

Almost everyone in our class ran Hooker Competition headers, 4 or 5 inch open exhaust (no mufflers). They laughed when they saw mufflers on our car. Ugly and square looking ones too. They laughed when they saw the primaries snaking all over the place so we could phase their exhaust pulses into the collectors. They laughed until we wiped them up.

Second way to increase cylinder pressure: Takes some time to understand what is happening to the intake charge at the rpm you need to make maximum torque.

Ever wonder why your street cam runs so good at 4,000 to 5,000 rpm and then falls on its a s s at 6,000+???? Simply, as your RPM increases, exhaust pressure increases. The valve event is insufficient to maintain the earlier flow it produced at the lower rpm. Air and fuel are actually a LIQUID in engineering terms, and as your RPM climbs, they cannot keep up with the faster and faster valve events.

Sorry to oversimplify here. Someone will probably address this in a response. I just want to get you thinking here.

So cylinder pressure drops as your rpm increases.

In the 60's and 70's, we would correct this drop in cylinder pressure at high rpms by increasing valve overlap. Your average modern street rod cams open the intake valve about 24-36 degrees before the exhaust stroke reaches TDC. On a 7,000 RPM cam, that is increased to about 40-60 degrees of overlap. On my 8,500 RPM big block, I was running 110 degrees of overlap.

Understand what the overlap does. Literally, as the exhaust valve is closing and the piston is chasing it back into the head, your intake valve begins to open up. You might incorrectly think that the piston coming up would push the fuel back into the intake, but the opposite actually happens at high rpm. The exhaust rushing out the exhaust port creates a vacume that pulls the air fuel mixture into the cylinder as the piston comes to top dead center. The amount of air and fuel we actually squeeze into this overlap area determines how much extra cylinder pressure we build at a given RPM.

Lets look at the problem with too much overlap. As the intake and exhaust valve remain open longer and longer, unburned fuel moves into the exhaust port and exhaust pulse energy remains in the combustion chamber. If the exhaust flow is hampered by any event like a couple of cylinders misfiring, the exhaust flow collapses and now the easiest way for it to flow is through the open intake valve, into the intake manifold and BOOM. This is why we see blown fuel engines launch their intake and blowers up in the air. In the 60's they could fly as high as 15 or 20 ft in the air and sometimes killed the driver or spectators. Now we use engine straps to contain the parts.

So knowing we need a proper amount of overlap, we can just measure what we need and get the cam made to our specifications. Right? Well... YES AND NO. You also have to understand intake velocity.

The third point is that intake velocity is just as important as overlap to increase the cylinder pressure. As intake ports are widened and as the valve diameters increase, your intake velocity decreases. The vacume of the exhaust flow during overlap pulls fuel in, but this is really a small fraction of time during the crank rotation. As the intake valve opens larger and larger and the piston now begins moving down, the intake charge begins to slow down.

In flat head days we found that by tuning the intake ports to specific volume, we could get more fuel into the cylinder at given rpm. Your typical intake is a single plane or dual plane and it does not individually tune the cylinder very well. But look at your high rpm, low ET race cars and you will see individual intake runners going to the cylinders. Then we can use a tunnel ram set up to make huge volumes of dense air fuel mixture that as they move from the top of the intake, compress and force the air fuel below into the cylinders that are a foot or more below it. This is the power of inertia. Once we get that heavy air and fuel moving, it wants to STAY MOVING.

So in whatever bracket you race, don't overport, don't over cam, don't over carburate, don't over intake. You need to run the package that moves the most air and fuel the very fastest speed possible in your target RPM range.

Fourth point, air fuel density.....

Ever wonder why the race car that beat you was exactly like yours in every way. Your car has the proper 600 cfm rating for the 306 engine you built. His has a 750 which should be way too big......

Smokey Yunich worked with Chevy for many years to develop maximum rpm on their race engines. As he built more and more record producting power plants, he found one thing to be true. You need about 2 cfm of carb for every horsepower you plan to build.

This flys in the face of carb selection formulas that say a 289 should run best with 500 cfm of flow. So why was Shelby running a 715 on the Cobra R? Because he was producing 350 hp from a good portion of these engines and you need two cfm to build one horsepower.

I am going to leave this sketched out like this now, because carb technology should be the subject of its own speed secret, and many of you are using tuned port, sequential fuel injection systems that eliminate the carb sizing all together. However for you fuel injection guys, on our 214 mph Pantera engine, we sqirted the closed intake valve during the opening of the exhaust valve, and then again when the intake openned. This got us the fuel density we needed to make over 900 horsepower.

Fifth point, ignition timing.

So now our stroked 351 Cleveland with excellent quench of .039 at the piston edges, and .085 at the wedge and .18 at the spark kernal is built with a cam that has proper overlap, really fast intake velocity from a custom tunnel ram intake with tuned runners, great fuel density charge from proper carburetor size, venturis, and ciruit tuning, and an awesome exhaust that literally sucks the intake charge into the cylinder.

Now in order to use all that great cylinder pressure created by the intake stroke we need to properly fire it. This is where a lot of dyno time and track testing are required to find the right ignition curve to properly ignite the mixture from 5,500 rpm to 8,500 rpm.

The difficulty is that a great many things change during the acceleration process. These include changes in the exhaust flow and back pressure, the reveberant energy due to overlap, misfiring in cylinders and a lot more. It may be necessary to run 36 degrees at 6,500 rpm, but 26 degrees at 7,000 rpm and 34 degrees at 7,500 rpm, and 32 degrees at 8,000 rpm.

See what I mean. We are after the best burn cycle and the best breathing possible and ignition timing effects this in really significant ways. Your air fuel mixture does not remain constant at any rpm. It is constantly variable. So you need to find out what works for your engine, then remember that it is effected by bariometric pressure (weather), air temperature, air density, altitude, track temperature, engine temperature, and even more than I want to think about.....

This too needs to be properly discussed later, so I will not go into all my ways to vary ignition timing.

I hope the whole point I am making does not get lost on you as you read my NOVELS.

Compression ratio is more than static compression. In fact in my mind, static compression is a worthless figure to me except as the rule book definition of what parts I put into the engine block.

In every race engine I build, I am looking for the dynamic compression of the engine at the rpms that are meaningful to win. PERIOD. End of story.

Now I want to hear your comments GOOD and BAD. I have drastically oversimplified alot of this just to keep this short and sweet and to test those of you out there that really know your stuff about engines and making power.

So please comment on this third speed secret, and help fill in the blanks so we can keep FORD "FIRST ON RACE DAY!"