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In Dragster No.483 we published "Matters Of flow", an article that was written by Michael Marriott. This article dealt with some of the priorities, and complexities, of making an engine breathe. In his article Marriott touched upon flow benches and Warren Eames from Flowbench Manufacturing Australia sent us a letter which relates to Marriott's article, as well as sharing some of his own experiences. Please read and enjoy:

I read with agreement the article by Michael Marriott. I certainly do agree with Marriott's opinion on paper HP using the formula given, or by any other formula. I must admit that I am also weary of calculations that tell you how much HP it takes to push a certain weight of car to a certain ET. There are too many variables to be able to say that an engine is going to produce "X" amount of HP from CFM flow reading.

With our FMA ( Flowbench Manufacturing Australia ) MK9-26 Flowbench we include a computer program to work out a customer's HP. This was done at the request of customers but against my "better?" judgment and our customers are advised of this by me. It can be used as a rough guide only due to the many variables. The "Before and After", plus the "Exhaust to Inlet Flow Ratio Charts" program (also included) is a more worthwhile and useful tool. Your flow can be plotted all the way through the lift range, both on inlet and exhaust, not just at peak flow, which should help make cam choice easier. Because you are plotting your exhaust as well, this can help with your final choice of exhaust system.

The greater the negative pressure (vacuum) that you can create by the exhaust the greater the pull will be for the intake charge when the intake valve opens. This sounds quite easy but l am sure a cam designer will not agree as the timing has to be just right (it will only be spot-on in a narrow RPM band), so the inlet charge does not go out the exhaust (and therefore wasted to a large degree, as it can be used to cool the engine internals as well) as well as making the best use of the inertia mentioned by Marriott.

This may lead you to think that you need great flow readings on the exhaust. I don't believe so. You must remember the exhaust is going out under pressure. The only clue I can give here is that it seems that the exhaust flow should be between 60 and 80 percent of the inlet. This was proved many years ago in the good old VW by placing a washer with a 5/8 in. hole in both exhaust pipes right back near the outlet on a standard engine. A noticeable Increase in torque was gained. This was done by a customer of mine after I told him of my findings when working on Formula Vee engines which, sometimes showed in excess of 100 percent flow of the inlet flow, on the exhaust.

I am a firm believer in flowing everything on the inlet side together (ie: head, manifold, carburetor and air cleaner) and the same on the exhaust, as they all influence one another. Of course it's a matter of trial and error to get the best combination within the rules of your class.

With reference to pressure, the ideal seems to be to work towards a funnel or "V" effect (which Marriott hinted at), with the largest opening at the air entrance and the smallest at the valve seat. This is the ideal but virtually impossible to achieve, however if you think along those lines you can't go too far wrong.

The "in-thing" at the moment seems to be air speed which is the one thing that finally prompted me to write this article. People seem to want to spend more money to measure their air speed. If you have one of our FMA MK9 flowbenches, or any other flowbench, you can see if you are getting a greater air speed without any modifications at all. All you have to do is remember, to get more air through a given hole, or orifice, you increase the pressure, the air speed or both, It's as simple as that. If you have an orifice of 50 mm and a pressure of 10 in. of water which do not change (as when flow testing), the only way to get more air through the hole is to increase the air speed which will in turn give you more air flow (cfm). Relate this to your engine by having the valve seat slightly smaller than the port entry into the head (remember the funnel or "V"). If you want to keep your air speed up, work between these two openings. This is, of course, when you get somewhere near your target airflow. Remember, you can add material in the port if the rules allow. If you do this without touching the seat area or the air entrance area and your cfm increases (without increasing pressure) you have increased your air speed.

Some ways to achieve this are:

  1. To have a pocket under the seat
  2. Use the funnel or "V" effect
  3. Eliminate changes in port area ( as much as possible)
  4. Add material into the port area

Remember: If you use a hot wire probe in your port to measure your speed, it does not capture all the air and therefore will not give you an accurate reading. If you use a vane anemometer it needs to be the same size as the port to capture all the air. This creates a few problems:

  1. Imagine all the different vane sizes you would need
  2. Whatever you put in the port will disrupt the airflow characteristics
  3. Whether it's a hot wire or vane, it has area and volume so the air speed will be different when you take it away anyway.

If you increase your air exit area into the combustion chamber, your air speed will go down. One very important fact to remember is that everyone who knows anything at all about flowing engine parts has and or uses a flowbench. Do yourself a favour, at least consider our Australian manufactured FMA MK9 flowbench. I hope this helps you in the science and art of flow bench testing. Even though I design and manufacture our FMA flow benches, I am by no means an expert on flowing.


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