Tag Archives: Aircooled

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.

There are two ways to port cylinder heads: The right way and the wrong way.

The right way is to refine the flow characteristics of the head and intake manifold so as much air as possible enters the cylinders at the engine’s peak power curve. Every engine is different so there’s no ‘standard’ port configuration that is guaranteed to deliver maximum air flow on every application. The port profile that works best will be limited by the physical dimensions of the cylinder head.

Limiting factors include the size, position and angle of the stock ports, the size configuration and angle of the valves, the thickness of the casting around the ports.

But other factors must be taken into account, too, such as engine displacement, the engine’s bore and stroke, the shape of the combustion chambers, compression ratio, the depth and angles on the valve seats, total valve lift, camshaft profile (duration, overlap,), and type of intake manifold and induction system.

One of the basic goals of head porting is to minimize obstructions so air can flow relatively unimpeded from the throttle plate to the valves.Two things that get in the way are the valve guides and valve guide bosses. Using valves that are necked down just above the valve head improve the air flow.

Transition areas in the port also need to be reworked so air will flow more easily around corners with a sharp radius and into the seat throat just above the valves. Any sudden changes in the cross-section of the port can disrupt this effect and restrict air flow.

The point where the intake manifold and cylinder head intake port meet also is a critical area. If the runners in the rubber intake manifolds are not perfectly aligned with the ports in the head, sharp edges can interrupt normal air flow and impair performance. The same goes for exhaust ports. The head ports must be aligned with the header openings so the exhaust gases can pass freely out of the engine without encountering any sharp edges or obstacles.

The right way to improve air flow is to locate the best places to remove metal. This takes experience, knowing what changes work and what ones don’t and using the right tools for reworking the various portions of the ports, valve pockets and intake manifold

The wrong way to go at it is to grab a die grinder and start hogging out the intake and exhaust ports with no idea of where you’re going or what you’re trying to accomplish other than to open up the ports.

Bigger is not always better. Grind away too much metal and you may end up ruining the casting. But even if you don’t grind all the way through, removing metal in the wrong places can actually end up hurting air flow more than it helps.

“THE SECRET TO MAXIMIZING AIR FLOW AND ENGINE PERFORMANCE IS TO MAXIMIZE VOLUMETRIC EFFICIENCY AND AIR FLOW VELOCITY (SSR)”.

Big ports with lots of volume will obviously flow more air than a smaller port with less volume, but only at higher rpm. A lot of people don’t know that. At lower rpm and mid-range, a smaller port actually flows more efficiently and delivers better torque and performance because the air moves through the port at higher speed. This helps push more air and fuel into the cylinder every time the valve opens. At higher rpm, the momentum of the air helps ram in more air, so a larger port can flow more air when the engine needs it.

The bottom line is this, to realize the most power and performance out of an engine, air flow has to match the breathing requirements of the engine within the engine’s rpm range where it is designed to make the most power.

As a rule, the roof of an intake or exhaust port has much more influence on air flow than the floor or sides of the port. The greatest gains in air flow can often be realized by removing metal from the top of the port only and leaving the sides and floor relatively untouched. The shape of the port is far more critical than the overall size of the port. The largest gains in horsepower are found on the intake side by raising the roof of the port. On exhaust ports, if you tried to match the port to a header gasket you’d probably destroy the port. The secret of exhaust porting today is not how big the port is, but the shape of the port and the velocity of the exhaust flowing through it. Any time you start making the ports bigger on the exhaust side, you usually end up killing air flow in the head.

As for polishing, a smooth finish is great for exhaust ports, but a rougher finish flows better on the intake side. A slightly rough surface texture in the intake ports creates a boundary layer of air that keeps the rest of the air column flowing smoothly and quickly through the port.

You can optimize the short-side turn of a cylinder’s intake port by expanding the sides of the port. This is necessary in order to address both of the aspects in order to make the turn more effectively and to compensate for the valve guide boss and valve stem which uses some of the available cross sectional area. A well streamlined valve guide boss can enhance results especially swirl rather than hinder it. Expanding the cylinder head walls helps to accomplish the filling of a cylinder when the port and valve is feeding a pair of intake valves in a multi-valve head.

When cylinder head modifications are limited to removing metal dealing with the short-side turn means making the most of whatever is already there. Most production heads have a more abrupt turn than is necessary due to the result of machining the valve throat below the seat. Rounding this off is the best possible solution to what can be done to improve the form of the short-side turn, once the smoothing out of the contours in the valve throat have been completed.

The best way to get the air to move to the back of the valve is to slow it down so that it can make that turn, expanding the intake port’s wall area creates a significant change. When the port is progressively widened and the intake port’s roof is raised in the turn area the slowing of the air just before it reaches the valve can create some substantial HP and Torque gains.

The majority of the air wants to flow in the top half of the intake port, so that area should be favored when removing metal. The increase in cross-sectional area in the valve’s throat area will also create an improvement by converting some of the high velocity into pressure energy, thus intensifying the air / fuel mixture charge into the cylinder.

Even when the exhaust valve is at relatively low lift, the exhaust gasses can exit the exhaust valve’s seat area at super sonic speeds, during this phase the exhaust gasses responds more to the opening area of the exhaust valve rather than the shape of the exhaust port.

As the exhaust valve’s lift increases, the exhaust gas velocity drops to subsonic, and now the shape of the exhaust port becomes the overall factor towards creating high flow. It is also worth noting that for a given size, an exhaust port flows better than an intake port. This is due to the fact that as the exhaust passes from the cylinder into the exhaust port, the flow becomes more organized, which is just the opposite of what takes place at the intake valve.

Another condition that helps the exhaust gasses reach a higher flow efficiency is due to the exhaust valve is typically lifted higher in proportion to its diameter than the intake valve, thus creating the situation that the valve head spends more time out of the influence of the exhaust valve seat.

If an exhaust port has a steep up draft angle and a large short side turn, then the exhaust port begins to resemble a venture like nozzle and a pressure recovery will occur after the gases have passed through the port’s minor diameter. If an exhaust port has a reasonable up draft angle and a short side turn, it will work well for evacuating the cylinder’s exhaust gasses.

Due to the way the exhaust works, a good exhaust port must have an efficient approach to and from the actual exhaust valve seat, otherwise there will be no effectiveness no matter how good the rest of the exhaust port is.

For a general rule, the minor diameter right under the valve seat needs to be 85% to 88% of the exhaust valve’s diameter. Additionally the outward taper from the minor diameter needs to be about 4 to 6 degrees. When the port area gets to be equal to the exhaust valve, for most purposes, it is as large as it needs to be. Also you must make sure there are no low flow spots in the port, as these areas will amplify the low-speed losses seen with Hi-performance camshafts.

Cylinder Heads (3 or 5 Angle Valve Seats)

The greatest flow restriction in any engine is the cylinder head. Having the air / fuel mixture to efficiently pass through this restriction will increase an engines HP and Torque.

The Intake and Exhaust Valves are part of the cylinder ports, and when they are closed their ability to flow is zero. This means until they have opened to a very large opening, the valves are the main restriction to the engine’s cylinder head’s airflow.

Even when the valve is at a large lift, it still presents a difficult path for the air to travel on its way into or out of the cylinder. The priority is to make the valve capable of passing as much air as possible, whatever the lift is. To do this both the valve size and the valve’s seat must be considered.

Although it may be the last operation during a porting job, the valve’s seat design is the most important priority toward effectively filling the cylinder. It would seem that the hole under the valve head needs to be as large as possible so as to flow the most air. Before flow benches were developed, it was a common practice to make a valve seat as thin as possible in order to achieve the maximum throat diameter. Objective flow bench testing found this to be untrue. In the real world the maximum flow is always a combination of size and form around the valve before and after the seat.

Air has mass and does not like to hug a port wall around a short-side turn. With low-angle ports, the air at mid and high valve lifts do not make the transition around the short-side turn very well. As a result, most of the air goes out of the long-side turn.

This situation is even greater as the higher the valve lift becomes. As a result, the streamlining of the port on the long side needs to be addressed for low, medium and high lifts, while the valve seat approach on the short side needs only to deal with the requirements of low-lift flow.

It does not matter if it is the intake or exhaust port, the worst part of the port for air to travel is the short-side turn. If the air fails to make it around the short-side turn, there obviously won’t be much air exiting the valve in that area.

Changes in the valve’s seat angles can make the valve appear bigger than it really is and flow more air during the beginning of its opening phase.

Cylinder Block

Well looking back at this thread my expectation was to encourage additional information from other OSS members to be added to this thread, although there was some I guess my expectations were greater. That being the case this will be my last input of information in this thread for the remainder of this year.

The following information can be used for race or street engines, several Worldclass engine builders have known and have been using the technique for decades.

If you REALLY want to have cylindrical cylinder bores when it comes time to hone your cylinder block, the block needs to be at operating temperatures. Getting the block up over 120f degrees is a good temperture to hone it, but if you can get it to 150f degrees, it’s even better. In addtion any honing oil that you use must be of equal temperture of the cylinder block, when the cylinder block is heated and you douse cold honing oil on it and then hone it, the localized cooling is going to distort the bore.

Many times people ask me how I am able to consistantly build long term reliable high HP race engines, well M8’s this is one of my secrets.

Copper Head Gaskets.

Copper head gaskets are great for extremely high compression ratio (over 13:1), turbocharged or supercharged engines that are running lots of boost pressure (over 15 psi), or engines with nitrous oxide that add an extra 150 to 200 or more horsepower.

Due to the fact that Copper conducts heat much better than most other metals, Copper will help to stabilize a cylinder head and cylinder block temperatures. This will prevent any hot spots that can cause detonation or head warpage, and a Copper head gasket reduces the risk of the head gasket blowing out or burning through.

Copper has a 25 percent coefficient of elasticity which allows it to stretch before it will fail. if an engine starts to detonate because the air / fuel mixture leans out, or there is excessive ignition advance or too much compression and low fuel octane, a Copper head gasket will provide a margin of safety. Copper is strong and the alloys used for copper head gaskets may have a tensile strength of up to 32,000 psi, which is many times stronger than that of the materials used in conventional performance head gaskets.

Copper head gaskets are reusable for a limited number of times (3). This is a plus in situations where the heads are on and off the block between races, or frequent tear downs are required. One of the downsides of Copper head gaskets, though, is that they do not seal oil very well. A Copper head gasket must be coated with some type of sealer, and both mating surfaces must be absolutely flat and clean.

The way to anneal a Copper head gasket is that the gasket should only be heated until it is a dark red color and no more. After the Copper head gasket gasket has air cooled, the surface needs to be cleaned with a brush or abrasive pad to remove oxide from the surface. The Copper head gasket should then be cleaned with brake cleaner or a similar product and allowed to dry before it is coated with a sealer.

The sealer must be allowed to dry before the gasket is installed. Some aerosol sealers may require multiple coats for the best results. RTV silicone also works, and may be applied around oil galley openings in the cylinder head gasket, the cylinder block or cylinder head, only a thin coating should be used, and it must be allowed to set before the gasket is installed.

Copper is a soft metal and it does not provide much conformability. This is a good aspect because the gasket doesn’t crush when the head bolts are torqued down, thus the thickness of the gasket remains the same and does not change. Unfortunately a Copper head gasket does not conform very well to small indentations and surface irregularities in the cylinder head or cylinder block

If a copper head gasket is accidentally bent during removal, it can be straightened and annealed. But if the gasket has kinked, it should be replaced because a kink concentrates stress and hardens the metal. This will increase the risk of a Copper head gasket cracking. Copper head gaskets should not be cleaned by bead blasting because it will harden the metal. The same is true for hammering the metal.

On all applications using Copper head gaskets there should be annealed / softened Stainless Steel or Copper wire O-rings installed in grooves that machined into the block or cylinder head. The wire rings help concentrate loading around the cylinders to prevent combustion pressure from blowing past the gasket. These wire rings are typically .041″ in diameter, and are placed in a .039″ wide x .030″ deep groove. The wires should protrude only about .010″ above the surface of the deck, and the thickness of the gasket should be about four times the protrusion of the wires in their grooves, or about .040″. Engines that produce over three horsepower per cubic inch should also have a corresponding receiver groove machined into the head opposite the O-rings in the block for optimum sealing. The depth of the receiver grooves should be 75 percent of the O-ring protrusion and the width of the grooves should be 1.5 times that of the wire.

COMPRESSION TEST GAUGE READINGS.

Your Motorcycle will always have a higher reading on your compression tester gauge with a stock or low duration cam, due to the fact that you will be closing the intake valve earlier on the compression stroke. This longer effective compression stroke always delivers a higher gauge reading.

Installing a longer duration cam, your intake valve will close later, thus giving a lower gauge reading because of the shorter effective compression stroke. Some individuals feel that this is impossible, claiming that if it were true, why will your Motorcycle go faster with the bigger cam? The reason is that a bigger cam will have higher compression effect in the cylinder at higher engine speeds where all that additional valve timing can do you some good, but at lower speeds and especially at starter-cranking speeds, the effect will be lower.