Airflow and Performance Enhancement

A Mix of Science and Politics

So you've gotten bored with the performance of your V-twin  and you want to do something about it. We feel your pain. For some of us who were born with a torque wrench in one hand and an engine in the other (of which I am NOT one) all of this old news. For the rest of us the path is not so clear and we must wade through the mire of the multitude of choices. In the rather expensive area of engine performance a mistake can be extremely costly.

In the manufaturing process the bottom line rules. Manufactures have to be concerned with two things; Compliance with the law (both in the manufacture of, and the finished product itself) and the bottom line.  In new vehicle manufacture the product has to pass all the Federal AND State standards of our EPA concerning emmission control (both fumes and noise) and effency as well as the National Highway Traffic Safety Administration (NHTSA) issues with safety.  At the same time this wonder machine has to be appealing enough to purchise. And this is complicated even further if this vehicle is made for export with all the additional rules and regulations. Some anal-retentive States require that the vehicle uphold these standards throughout the vehicles lifetime. If you live in such a State, sucks to be you. The "Aftermarket" parts manufacture are not held to such stringent regulations or "qualify" their parts with "not for street use" or "for race use only" this resource along with living in a more "liberal" state allow some of us to experiance the wonder of pumping up our bikes without  being dogged about it.  That being said the single most cost effective performance up grade you can give your V-twin is replace the stock exhaust and air cleaner for a more efficient aftermarket set (if your State allows). This mod allone can give 5-10 hp gain with the cost of just a few hundred bucks (bolt-on operation with no special tools or area to work with).

The Manufacture's bottom line is the other area to be addressed in performance mods. The manufacture has to balance looks, reliability, and  performance with cost of production. In this effort things like ideal fitment and practical fitment are weighed. Ideally you want the fitment to have the greatest efficiency and reliability producing the least amount of mechanical noise. In practice when hundreds or thousands have to be produced in a short amount of time a middle ground is reached where an "acceptable" amount of noise and "reasonable" efficiency and reliability are reached for the "average" user. This is the domain that can get extremely expensive as we identify certain weaknesses in our motorcycles and correct them with either replacement of, or machine work tostock parts. In this area the cheapest "fix" is in the bolt-on replacement of the stock ignition (coil, plugs, wires, and ECM).

OK, so you got the bolt-ons done and that just wetted your appetite, you have a fair amount of mechanical skill, a place you can work on your monster, and best of all you have some cash to invest. Now what do you do? You try to make sense of all these numbers; dyno results, cam lifts and duration, gear ratios, etc., as you are bombarded by hundreds of aftermarket manufacturers with their latest greatest "silver bullet". As far as who's silver bullet to buy well that is more a personal choice. With the information and opinions so readily available it's gonna take so time in research to make an informed decision, you just have to be able to cut out the B.S. propaganda from the real information.  I good source are Forums on the internet (careful of the ones hosted by the manufacture). Informed independent opinions are extremely valuable.  As far as performance shops, most can and do provide some good ideas and tips freely but again most of the speed shops offer a few product lines (they won't sell Coke and Pepsi).  Any good speed shop will produce good results but may not use the same make parts or even the same sized. They have put together an assembly of individual parts to produce a extraordinary result for a particular need. Because the relationship of these parts are interdependent (a change in one part requires a change in one or more other parts to effect a positive result), and the customers needs vary, these assemblies can be just as varied form one shop to the next to produce simular results.  So what we can do here is get you down the path suited for you.

Question: What will make your engine run best for your type of riding? Should you go in and hog out the inside of the ports to the max, or just match up the edges of where the parts fit together to give a smooth airflow.

Nurd answer: The effects of airflow determine how well the cylinders are filling with air/fuel mixture (volumetric efficiency) at the different rpm ranges that the engine runs. The greater the volume of air/fuel mix, the more air/fuel mix will flow into the cylinder which will pack a cylinder more than the stock setup. For best performance you want airflow volume so fast it cram-packs (compresses) the air/fuel mix in the cylinder to its maximum capacity (greatest volumetric efficiency). Get the idea?

If you want the long explanation; In a four-stroke naturally aspirated engine, the theoretical maximum volume of air that each cylinder can ingest during the intake cycle is equal to the swept volume of that cylinder (0.7854 x bore x bore x stroke). Since each cylinder has one intake stroke every two revolutions of the crankshaft, then the theoretical maximum volume of air it can ingest during each rotation of the crankshaft is equal to one-half its displacement. The actual amount of air the engine ingests compared to the theoretical maximum is called volumetric efficiency (VE). An engine operating at 100% VE is ingesting its total displacement every two crankshaft revolutions.

Doing this at different rpm ranges like the one you ride down the road in, and the range you race down the track at isn’t possible. You have to pick a happy medium with parts that will set up your engine’s airflow volume for a broader range of riding speeds, street, strip, or something in between, it’s your call. To operate at street speeds or high-end rpm racing greatly impacts how long your engine has before you have to rebuild or perform more frequent preventive maintenance. I want to emphasize  this and tell you how it is; if you’re going to build performance engines,  parts are going to be subject to higher stresses and closer tolerances so they will breakdown faster. Be realistic; work within your budget & capabilities. “You learn to crawl before you walk and then run”.  Getting the performance you want out of the street/strip engine is knowing how to go about it. From start to finish, be prepared, and plan carefully, a mid-stream change can be costly.

Practical answer: Determining how to achieve correct airflow speeds: Creating good airflow speed is made up of a combination of carb, manifold, head porting, valve shapes, valve spring pressure, cam sizes, and exhaust pipes that all work in unison with the cubic inch and rpm range of the engine. This article does not give all the technical information but will skim over the top to help give a better understanding of how to get the best power band for your street or strip engine. Stock carb and manifold sizes are made for street riding power bands. Size determines how much airflow is picked up. Minimum modifications of pipes, heads, cams, carb & ignition tuning can produce a better power band for street engines at a lower cost, compared to expensive all-out maximum engine modifications. These modifications allow the Engine to run smoother, as parts work together more efficiently increasing engine life. This puts the torque and horsepower output into the power band that produces the best performance for the type of street riding or racing you want to do. The trick is building that power band into your engine.

Starting Point: Airflow starts in the carb and manifold setup. Build the engine with consideration towards cubic inch displacement in the rpm range that the engine will mostly be running. Your Displacement and "normal riding" RPM range determine how to plan and choose the right parts to achieve the performance level you want to operate in.  Bigger is not necessarily better when it comes to the carburetor intake manifold.

For a better understanding of how to change airflow volume so that a broader power band can be reached and to give better engine performance let's start with the heads. This is where a minimum amount of work will achieve maximum results. Pictures can show how port shapes and valve angles allow a faster airflow through. The air ramps up at the start of the intake port to the top of the roof, and down on a better straight-in flow path over the valves, into the cylinder chamber. It is said that higher roof in the head gives a better straight downflow. Get the basics right with the heads and you can change out most other parts (carb, manifold, cams, pistons, ignition, air cleaner, pipes) to make the engine perform as you like.Taking the heads up to the next step would involve big bucks; finish porting and polishing the heads by hand.  Proper head porting is as much an art as a science because a proper balance must be achieved  between the polished surface to reduce drag to permit smooth airflow and a rough surface to get proper atomization from the air fuel mix.

"The porter clamps your cylinder head to his porting bench, and proceeds to sand, shave, grind and polish in all the right places, to remove the inconsistencies of mass production, as well as known deficiencies in your engine’s design. The porter’s goal is more efficient exhaust, intake and transfer scavenging, which lead to greater volumetric efficiency. He works to optimize the radius around the valve seat, the width of the valve seating area, transition into the combustion chamber, and radius on the back side of the valve. The goal is smooth flow, not sheer volume. A common misconception is that bigger ports are better—on street engines the opposite is more often true! Harley engines are very sensitive about their exhaust-to-intake ratio. They run best when that ratio is around 90 percent, but factory levels are more like 78 percent. A good porting job can give you the optimal exhaust-to-intake ratio for your Hog, but a bad one can make it worse."- Perry Klime, Flo Headworks. For more detailed information and access Perry's services I highly suggest you visit the  Flo Headworks site .  It takes machine precision, an artistic talent, and a good flow test bench to get the job done right. This level of work could be done in different stages all the way up to super high-roof big valve head castings.

Another thing to consider is valve shape which can add to airflow velocity also by increasing valve size to a larger diameter and making the valve tulip shaped as pictured. The tulip shape takes up more room in the larger ports, so even with these large valves installed, airflow is the same as with standard valves. By streamlining, velocity is increased by reducing drag to help give the wider power band we talked about. Bottom line, with high performance valves installed in a stock engine, it will run well with heads just ported (not polished) by grinding the carb/manifold edges & valve seat/casting edges together for a smooth fit. The rough sand casting inside seems to make for a better turbulent motion for mixing the air/fuel together. This helps cheap pump gas atomize better, giving more complete combustion, a plus for the street rider. If you truly have a passion for power, and speed, the higher performance level you build an engine seems to be measured by “cubic bucks” and time well spent.

Let me say, all parts are NOT created equal. Test and tuning of parts show some parts give better performance with less work done to modify them. The labor of modifying stock parts can cost more and be of lesser quality, than if you bought a custom built part to give you the performance you wanted in the first place. I hope this gives you some insight on how to best upgrade your engine and where to start.

Feedback is always welcome; praise, criticism, tips, tricks. The more info we get the better

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