Category Archives: Speedy Steve Racer’s Tuning tips

here are two real world effects that determine how much fuel/air charge can get into the cylinder. The first is that air is compressible, the second is the dynamics (acceleration/deceleration) of the air. The compressibility of the air becomes a factor when the air enters the intake port around the intake valve. The intake port/valve forms a constriction, like the throat of a nozzle. Because air is compressible, it can only be pushed through a constriction so fast. Regardless of how much pressure you apply, the maximum velocity possible through the throat of a nozzle is a velocity equal to the speed of sound .

The same effect happens at the intake valve. The ratio of the typical velocity to the intake sonic velocity is called the inlet Mach index. From the science of fluid mechanics the controlling velocity in a compressible flow system is usually the intake valve opening. For a given cylinder and valve design, the inlet Mach index is proportional to the piston speed, and that the fuel/air charge flows in faster when the piston moves down faster. Of course, at some point the constriction of the valve opening starts to limit this. When the inlet Mach index exceeds 0.5 (intake velocity equal to half the speed of sound), the volumetric efficiency falls rapidly with increasing speed. Therefore, enginest are typically designed so that the inlet Mach index does not exceed 0.5 at the highest rated speed.

The effect of this constriction shows up as a pressure drop through the intake valve. Why don’t we just open the intake valve further? Because when the valve is lifted a distance equal to 1/4 of its diameter, the area of a cylinder around the valve (that the fuel/air charge passes through, not the engine cylinder) is equal to the area of the valve face and intake port, ignoring the valve stem. Mathematically, the area of the cylinder is (2 r)(d/4). Since d = 2r, this evaluates to r2, which is the area of the intake port, the amount of additional flow through the intake port increases very slowly as the lift of the valve increases beyond 1/4 of the valve diameter.

Because of the dynamics of the fuel/air charge, the intake valve normally closes at some time after the piston passes bottom dead center. As the piston moves down, it draws the fuel/air charge into the cylinder. This movement builds up momentum in the intake manifold. When the piston reaches bottom dead center, the fuel/air charge is still flowing into the cylinder as a result of this residual momentum. Thus, at the speed desired for maximum torque, the intake valve closing is timed to correspond with the velocity of the fuel/air charge through the intake port dropping to zero. This closing will occur at some time after the piston has started the compression stroke, and will result in the maximum amount of fuel/air charge being drawn into the cylinder. This maximizes the volumetric efficiency, and maximizes the torque delivered to the crankshaft, ignoring friction effects. The angle of the crankshaft at the time the intake valve closes is called the intake valve closing angle.

So what effects does this later valve closing have at other speeds? At low speeds, the momentum built up in the intake manifold will be small, such that part of the fuel/air charge will be pushed back into the intake manifold as the piston starts up prior to the intake valve closing. At speeds above the speed for maximum torque, the constriction of intake valve opening will cause a pressure loss which will reduce the amount of fuel/air charge entering the cylinder. In either case, the amount of fuel/air charge in the cylinder is reduced, and thus the torque is reduced.

The design of the intake manifold also affects the amount of momentum built up in the flow of the fuel/air charge. The momentum of the fuel/air charge is the sum of the effect of standing waves built up from previous intake strokes keep in mind that any tube will have a resonant frequency and effect the transient wave caused by the current intake stroke. While the standing waves contribute to the overall effect, there are no sudden changes in the volumetric efficiency when the RPM of the engine is an even multiple of the natural frequency of the intake manifold.

Long, skinny intake manifold pipes give high volumetric efficiencies at low piston speeds because high momentum lots of velocity is built up in the pipe during the intake stroke. At high piston speeds, the small diameter of the intake pipe causes a constriction and the volumetric efficiency falls. Fat intake pipes show a maximum volumetric efficiency at intermediate piston speeds. However, at high piston speeds, the larger mass of the fuel/air charge in the fat intake pipe is slow to accelerate, and thus the volumetric efficiency falls off.

As the manifold pipes get shorter, the maximum gain in volumetric efficiency over having no intake manifold at all decreases. However, the gain you do get with shorter intake pipe happens over a greater range of piston speeds. Basically, it comes down to the intake manifold pipe should be designed according to the engine requirements. If you need high torque at slow piston speeds, use long skinny intake pipes. For high torque at intermediate piston speeds, use long fat intake pipes. For high torque over a wide range of piston speeds (i.e. a flat torque curve), use shorter intake pipes.

EXTRA SHIFT DETENT SPRING: “There Is Enough Tension In Drag Racing”

I just read a thread about adding an additional dentent spring in your GS / EFE Transmission. This is an acceptable modification for a stock activated shift linkage.

A lot of my hands on mechanical knowledge has been gained thru many decades of working as a R&D Engineer for various Automobile and Motoccyle Maufactuers such as G.M. Toyota, Isuzu, Suzuki, Yamaha, Kawasaki and H-nda. An advantage of working for these Companies is that I would spend a lot of time with a large variety of data acquisition instruments. Fortunately I was able to use and apply many of these instruments to my EFE Drag Bike.

Many of our OSS members are well aware of the fact that Drag Races are won or lossed by a thousand of a second. I have had the opportunity to do A -B -A testing methods using test instruments that are capable of taking measurements in miliseconds, comparing the use of a single or double detent springs with a MRE Pro-Airshifter.

The results of the tests (confirmed repeatability) is that a bike utilizing an MRE Pro-Airshifter will engauge a gear faster with a single spring as compared to using a double spring.

You can consider the above information as another “SSR Race Trick” donated to the OSS site. I still have several more when it comes to Suzuki transmissions that will remain propriety information.

“May The Shift Be With You”

Part of getting a large fuel/air charge into the cylinder (volumetric efficiency) has to do with getting the combustion products of the previous cycle out of the cylinder. At first thought, it would seem that simply making the exhaust valve bigger would help get the combustion products out. As it turns out, the exhaust valve can be as small as 50% the size of the intake valve without affecting the volumetric efficiency over the usual range of inlet Mach speed. Normally the exhaust valves are at least 60% the size of the intake valve. This effect may arise because the combustion products are “pushed” out of the exhaust port by the piston, while the fuel/air charge is “sucked” in the intake port, pushed only by the manifold pressure.

To enhance the removal of the combustion gasses, the intake valve is opened prior to the end of the exhaust stroke. Since both valves are open at this point, this is referred to as valve overlap. If the pressure in the intake manifold is greater than the pressure in the exhaust manifold, the in rushing fuel/air charge will help scavenge the remaining combustion products in the cylinder as the piston reaches top dead center by pushing them out the exhaust port. While some of the fuel/air charge may go out the exhaust port, an engine tuner tries to design the timing such that the exhaust valve closes just as the last of the combustion gasses leave the exhaust port. An additional benefit of valve overlap is that the intake valve is essentially fully open at the start of the intake stroke, thus reducing the pressure loss through the intake port during the intake stroke. The angle that the crankshaft turns between the intake valve opening and the exhaust valve closing is called the valve overlap angle.

Of course, scavenging does not occur at all speeds. At low speeds, the throttle valve reduces the pressure in the intake manifold, such that the intake manifold pressure is less than the exhaust manifold pressure. In this case, a small portion of the combustion products enter the intake manifold, to be pulled back into the cylinder on the intake stroke. Additionally, the combustion gasess in the space above the piston at top dead center are not scavenged. Even so, at low power settings this is not a problem.

CONCLUSION- In general, we have seen that the torque, and thus the horsepower produced by an engine depends on the amount of air that can be pumped through the engine. The more fuel/air charge drawn into the cylinder, the higher the volumetric efficiency. The higher the volumetric efficiency, the higher the torque. The biggest factor affecting the volumetric efficiency is the valve timing, specifically the valve overlap angle and the intake valve closing angle. Volumetric efficiency can also be improved by the intake manifold design. Since the camshaft used determines the valve timing, changing the camshaft will change the shape of the torque curve, and thus the horsepower curve.

Exhaust Reversion

There is a myth that an earlier opening of the intake valve even by 2 or 3 degrees causes the phenomenon known as reversion. This misconception is false from which other incorrect conclusions are made. When you focus on overlap you are on the wrong end of the cam-timing event.

Reversion or the effect of the backing up of the intake fuel air mixture is normally associated with longer duration high-performance camshafts, is actually caused by the intake valve closing later. The answer is in the basic principles of physics. just as with trigonometry and geometry the truth does not change because a person chooses to ignore it.

When the intake valve opens some 40 or more degrees before T.D.C. at the end of the exhaust stroke, very little exhaust gases remain in the cylinder. The piston is close T.D.C and no threat is posed to the incoming intake charge.

A false reversion theory when taken to an extreme would lead to a false conclusion that any overlapping of the intake and exhaust valves is totally undesirable. Engineers of the late 1800’s and early 1900’s used to think this way and they feared of overlap so much so they actually employed negative overlap of – 5 or -10 degrees to be sure none would occur.

The results were that these engines were severely limited to low speeds and marginal output. Engineers in the early 1920’s performed experiments with longer duration cams and proved that camshaft overlap fears are false, as both power, RPM and performance were actually improved. These engineers demonstrated that overlap did not cause engines to lock or backfire.

To further prove that reversion is not caused by earlier intake opening and the resulting extension of valve overlap, look what happens when you advance any camshaft, the intake and the exhaust valves open earlier, this advancement of the cam does not cause more reversion, yet throttle response and torque are improved.

If this myth were correct an engine would run poorly especially at lower RPM. By investigating what is occurring on the other end of the valve timing event will give you the explanation.

When a camshaft is advanced, not only do both valves open earlier but they also close earlier and there is the answer to reducing intake reversion. Closing the intake valve earlier and the reversion of the intake charge as the piston rises on the compression stroke will be reduced. It is not a mystery it is just the truth.

Exhaust Performance Criteria

When the piston approaches top dead center the spark plug fires a spark kernel igniting the fuel mixture into a fireball just as the piston rocks over into the power stroke. The piston transfers the energy of the expanding gases to the crankshaft as the exhaust valve starts to open in the last part of the power stroke.

The gas pressure is still high (70 to 90 p.s.i.) causing a rapid escape of the gases. A pressure wave is now generated as the valve continues to open. Gases can flow at an average speed of over 350 ft/sec, but the pressure wave travels at the speed of sound (Mach 1) and is dependent on the gas temperature. The expanding exhaust gases now rush into the port and down into the primary header pipe and then the gases and waves converge at the collector. In the collector, the gases expand quickly as the waves enter into all of the available orifices including the other primary tubes. The gases and some of the wave energy flow into the collector outlet and out the exhaust pipe.

Due to the above there are two basic phenomenon that are created in the exhaust system, gas particle movement and pressure wave activity. The absolute pressure difference between the cylinder and the atmosphere determines gas particle speed. When the gases travel down the pipe and expand their speed decreases. The pressure waves, base their speed on the speed of sound (Mach 1). The wave speed also decreases as they travel down the pipe due to gas cooling, the speed will increase again as the wave is reflected back up the pipe towards the cylinder. All the time the speed of the wave action is much greater than the speed of the gas particles.

Waves behave much differently than gas particles when a junction is encountered in the pipe. When two or more pipes come together such as in a collector, the waves travel into all of the available pipes backwards as well as forwards. Waves are also reflected back up the original pipe, but with a negative pressure. The strength of the wave reflection is based on the area change compared to the area of the originating pipe.

The reflecting negative pulse energy is the basis of wave action tuning. The concept is to time the negative wave pulse reflection to coincide with the period of overlap this low pressure will pull in a fresh intake charge as the intake valve is opening and helps to remove the residual exhaust gases before the exhaust valve closes. This phenomenon is controlled by the length of the primary header pipe. Due to the critical timing aspect of this tuning technique, there may be areas of the power curve that may be harmed.

The gas speed characteristics is a double edged sword. Too much gas speed indicates that that the system may be too restrictive hurting top end power and too little gas speed tends to make the power curve very peaky hurting low end torque. Larger diameter tubes allow the gases to expand and this will cool the gases by slowing down both the gases and the waves.

Exhaust system design is a balance all of these events and their timing. Even with the best compromise of exhaust pipe diameter and length, the collector outlet sizing can optimize or minimize the best design.

The bottom line on any racing exhaust system is to develop the most useful power curve. the final design is how the engine responds to the exhaust tuning on both the dyno and on the race track.

The following components must be considered, Header primary pipe diameter whether constant size or stepped pipes, the primary pipe overall length, the collector design including the number of pipes per collector and the outlet sizing and the megaphone design.

The header pipe sizing and the primary pipe sizing is related to exhaust valve and port size. A header pipe length is dependent on wave tuning. Usually longer pipes tune for lower r.p.m. power and the shorter pipes favor high r.p.m. power. The collector package is dependent on the number of cylinders, and their configuration firing order and their design objectives and the collector outlet size is determined by primary pipe size and exhaust cam timing.