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Performance Tuning – part 2 Fluid Flow

Motorcycles are like human beings. No am not talking about girls and bikes and how unpredictable, moody yet lovable they can be. A motorcycle breathes air as fast as it needs, burns calorific fuel and throws out the waste as it goes. So just like you start panting for air and get hungry after strenuous activity, the motorcycle needs to be able to increase its air-intake, fuel-intake, waste elimination, etc. and the strength to not collapse under a cardiac arrest.

Lets take a look at some Fluid Circuits, broken down to necessary detail:

Air Circuit: As the piston sucks in air, air rushes in through the filter, goes in through the carburettor, picks up petrol on its way and goes into the combustion chamber as the “charge”. These days there are other sub-circuits like Catalytic Converters, Bypass tubes and what not, but those are beside the point at the moment.

Now this basic circuit needs to be free and unclogged for normal functioning. A blocked air filter can drastically reduce efficiency. Meaning lesser power, higher fuel wastage and that’s just under normal operation. When you modify ANYTHING, the ideal proportions of air, fuel, restriction at air filter, restriction due to silencers, etc. will all change. And you need to know (or find out the hard way) how much to change where.

Typical elements that can be altered:
1) Air screw
2) Air Filter
3) Carburettor components

Fuel Circuit: Fuel goes from your petrol tank to the carburettor, where it sits in the float and is picked up by the moving air, through various jets and then forms the ‘charge’. You know the rest. The amount of fuel that’s picked up by the air is determined by the jet sizes, among other things.

This circuit too, apart from being unclogged and normal, needs to be altered as per any other alteration.

Typical elements that can be altered:
1) Main jet
2) Pilot Jet
3) Fuel Tap
4) Carburettor Components

Charge Circuit: This is where the drama unfolds. The charge goes in through the Inlet port to the Combustion Chamber to the Exhaust Port to the Exhaust. Time to go deeper.

Inlet Port

The size, shape, length, contour, smoothness of the inlet port decides how much charge goes in, where it goes and how fast it goes in. Each one of these matters a lot. Lets get deeper into this.

How much: It matters, obviously. The more charge, the bigger the bang, the bigger the force, the more the power. See? These equations make sense!

Where: The closer to the plug at ignition point, the lesser the time taken to push the piston, the faster the engine can spin, the lesser fuel that is left unused, lesser pollution, lesser required advance in the ignition. In power point of view, the biggest reason is higher possible rpm.

How fast: This has a very direct and massive effect on the rpm that the engine can reach up to. The faster the circuit flows, the faster we can get done with this stroke and move on to the next.

It should be noted that the ‘how fast’ factor, or the Port Velocity is roughly opposite of ‘how much’. One has to maintain the optimal balance between the Intake Volume and Intake Port velocity. It works just like the watering hose nozzle that is used in the garden. When you block the hose with a finger, reducing the outlet, then the velocity goes up but the amount goes down, the same concept applies here.

To increase both, one has to increase volume of cylinder, apart from other things, but that’s not the point here since we are looking at what we can do to the Intake Port only to optimize power, not how other parts can help the Intake Port.

Specific to Two-Strokes: In a two-stroke, the charge actually goes down to the crankcase and is pushed through the transfer ports. The transfer ports also follow the same laws of size and shape relevance as the intake port; and have to be altered accordingly.

Combustion Chamber

The shape, size blah blah matters a lot blah blah. You can’t do much here so lets get to what you CAN do, without obtaining the use of NASA’s R&D lab. Concentrate on the smoothness on the walls of the cylinder head; keep it free from carbon and other junk. The reason is to increase heat transfer and not to better flow, as some people tend to think. Mirror finish walls and matt finish walls don’t matter much to airflow.

Next, depending on the other modifications, you might need to increase compression. Every motorcycle has a step on the cylinder head that you can reduce so that the final volume reduces. That means the initial volume is same and final volume is lower. Higher compression!

Also, this process called ‘dialing-in the squish band’ helps eliminate inconsistency in your squish band, thus reducing compression losses that occur due to bad quality control. Increases efficiency and gives you some free power.


Not much you can do here except bore out your cylinder, put much wider pistons, basically increase the FORCE in the power equation. A highly skilled operation, it requires huge alterations in all other aspects to be totally effective and worth the effort.

Exhaust Port

Everyone’s favorite. And with good reason, too. It helps a lot. But the effectiveness of the process has given birth to the belief that you can wield a grinder and shave off metal from the exhaust port randomly and then have a fast motorcycle. Like I said, if the metal need not be there, nobody would have put it there.

The size and form of the Exhaust Port determines how fast the burnt fuel can be thrown out and how much can be thrown out. The ‘where’ factor in this case is taken care of by the bend pipe of the silencer. Exhaust Port Tuning is, but a function of the Inlet Port Tuning. If the inlet port is designed to allow better flow, and the exhaust port is not in agreement, it doesn’t make much sense. Everything has to be in symphony.

The exhaust port has a lot of interesting functions and uses in the two-stroke engine, since it plays a much more complex and critical role there. We will look at it in its deserved depth.

Exhaust Ports in Two-Strokes
: In a two-stroke, the piston uncovers the exhaust port and the blast that has just occurred pushes out all the spent gases out of the exhaust. This causes a temporary vacuum that helps pull in the intake charge faster than otherwise. Therefore, a better flowing exhaust port facilitates more rpm, more fuel suction, oh well you get the picture.

To summarize the typical elements to be altered in this circuit:

1) Inlet/exhaust port size, shape, form
2) Compression Ratio
3) Correction of manufacturing errors

One thing to be noted here is that the flow characteristics through an engine is best measured than guessed by feel and intuition. A flow-bench is required to correctly measure the efficiency of any modification.

Specific for four-stroke Valves: This is another specialised field of performance tuning that has its own set of specialists. When modified drastically, the valve system in four-strokes fall behind in performance. Valve float is a common occurrence when the valve does not close quickly enough when engine is revving hard. In this case, one can do various things such as reduce the momentum by reducing weight of valve, installing harder springs, etc.

Improvements in the valve train can include high-lift cams which increase the opening of the valve, very similar to the increase in size of ports in two-strokes. However, these alterations call for other numerous alterations in the valve train that accommodate each other.

Exhaust Pipe

The exhaust pipes have different roles in power play for two-strokers and four-strokers.
Pipes have the following important characteristics:

Two-Strokes: In a two stroke, when the power stroke is happening, there is a duration called the exhaust duration, when the combustion chamber is open to the silencers. This opens up a host of possibilities as some engineers discovered back in the early 1900s.

I have no clue how, but someone noticed that when a pulse (that’s given out by the fuel explosion) travels down an open pipe, it comes back in a negative form. Meaning the sound pulse traveling with the spent gases travelling outward comes back into the exhaust port as a suction wave and helps suck out more spent gases, and thus help in sucking more fuel. Yes, spooky. But it is very effective. It’s like a supercharger without the hassles. Once you get a header that allows the ‘suction wave’ to come to the exhaust at the correct time, it is amazing how much more power you can make.

Therefore, one has to design the expansion chamber for the particular chamber, depending on the various specifications of that engine. Expansion chamber design is a very elaborate science that involves calculation of header lengths, expansion and reduction in volume, etc.

Four-Strokes: Four strokes have their silencers almost isolated to the chaos that goes on in the cylinder during a power stroke. This is due to the luxury of having a separate stroke just for exhaust. The exhaust valve closes down, so that all the force of the explosion is used as much as possible, for maximum efficiency. This leaves only the back pressure to play with. This flow-restriction given to the engine by the silencer needs to be reduced as per the increase in flow of the above-mentioned circuits.

Also, the British racers of the café racer era realized that the ‘extractor silencer’ or the ‘collector silencer’ made famous by the BSA Goldstar is useful in using the available compression better. It is a popular modification done to Enfield motorcycles coupled with free-flow filters and better flowing carburettors.

Okay, now you know what flows where and how to use that to send your engine into piston pushing frenzy? Not yet?

Remember the defining moment when the spark plug fires the hell out of all that compressed charge? Well I will write about how to define that defining moment in the next part of Motorcycle Performance Tuning.

The Performance Tuning Part 1
Performance Tuning – part 3, Ignition
Performance Tuning – part 4, Power Transmission


  1. Joel Valente says:

    Hello, thank you for this information. It is very helpful to me right now especially. I am currently building a custom motorcycle using a 76 kz400. Obviously I need to optimize performance as much as possible in this application. I am also working on a project for my senior studio class concerned with motorcycle exhaust specifically. I just had a few questions. What specific advice would you offer for a range of bikes 400cc to say 800cc. ( I know that’s broad but I really just need some idea for my class since I am not an engineer) I am really just looking for general tube size and length of pipe. I used the stock size for my KZ since it came with open 1.375″ pipes anyways and built a custom exhaust with a builders kit.

    Also specifically what do you mean by the “‘extractor silencer’ or the ‘collector silencer’” I have done a few searches and have not come up with anything specific. Sorry I know that’s a basic question. Im just confused. Thanks again for the info.


  2. Hey,
    Apologies for the late reply. No question is basic, The best of them go back to basics, thats why they are the best!

    The extractor silencer is not very popular these days, however I can give you any information that you need. As for the KZ, the exhaust can not contribute much except to NOT restrict airflow too much and to be light as possible. Most modern performance exhausts wimply are lighter and nothing else. Believe it.
    I will need more details than that to help you. Email me on [email protected], will be glad to help you with what I know.


  3. dr shaikh says:

    hi. can u advice me on good after market exhaust for karizma.can i use old cbz exhaust(big hole exhaust). regards

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