Author Topic: Porting and polishing  (Read 17376 times)

busajack

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Porting and polishing
« on: Nov 04, 2011, 03:40:01 pm »
For all you hot rodders.
Heres another way to look at it.

 Porting and Polishing
 

"If you get careless or go romanticizing scientific information, giving it a flourish here and there, Nature will make a complete fool out of you. It does it often enough anyway even when you don't give it opportunities." Robert Pirsig, Zen and the Art of Motorcycle Maintenance
In a classic example of the over-valuing of independent thought, many religiously believe that through the simple expedient of grinding out port metal, bucketsfull of power wait to be unleashed from the average motorcycle engine. As with the other subjects in this section, this is a misconception that is steeped in motorcycle culture and lore. Yes, there is such a thing as porting, and engine builders do practice it. However, as you're about to discover, the truth is starkly different from what most people believe. Porting's Objective
The point of porting is of course increased cylinder filling. The more air that is taken into the engine, the more atomized fuel will follow, and the more power will be produced. Simple in concept, and at one level at least, amazingly enough it is entirely correct. It is a proven fact that, all else being equal, roughly 1/2 hp gain in power can be expected for every cubic foot per minute (CFM) increase in an engine's existing intake airflow. That's 5 hp for every 10 CFM, which is much less than 10% of an average port's stock flow. So the reality is there, at least on paper, and it would seem to be not such a hard thing to do. However, from there the situation takes a sharp turn toward reality.
First of all, there is only one reliable way an individual doing porting can know that he or she is accomplishing anything other than making aluminum dust. That way is by using a flowbench. The flowbench is an instrument which simulates having an engine attached to the cylinder head. This machine circulates air through the ports and measures how much air the head can pass in a minute. The builder takes a few swipes with the porting tool, attaches the head to the flowbench, and sees whether the result is good or bad. The experience is sobering. Most people approach their first porting project with more zeal than knowledge. Before the project is done, however, the avid engine builder comes of age, so to speak. He or she discovers how agonizingly difficult it is to increase flowbench readings.

More Airflow Does Not Always Equal a Faster Motorcycle
And that's just the beginning. Because even if flow gains are realized, the bike may end up being slower on the road. Two pitfalls await the unwary. First, just because a head is made to flow more air doesn't mean the engine will achieve that quantity of air movement. After all, the head doesn't move the air through itself, the rest of the engine does. If the engine can't support the head's potential, it is just that -- potential, and not practical reality. Take for example the 1340 Evo Harley-Davidson engine. This engine's heads flow more air on a flowbench than many cars' heads, yet stock 1340s make the least power per cylinder displacement of practically any current engine. This engine just doesn't create the forcefulness of combustion required to move great amounts of air. The second problem is that the flowbench indicates only the amount of airflow a head is capable of. It tells nothing about where in the rpm range this flow is going to happen. Using the 1340 example again, not only does this engine not push its piston down hard enough to move a lot of air, it doesn't do it frequently enough either. That is, this engine is not a high rpm engine. In other words, what good is a flowbench-registered 20 CFM improvement in airflow if subsequent calculations show that it occurs at 3,000 rpm above the engine's redline? That 20 CFM will never show up as improved road performance.

The way to avoid useless and even hamrful porting is to focus on port velocity. The surest way to do this is by avoiding increases in port cross-section. It is merely intuitive that the port with more cross-section, that is, diameter, will flow more air. However, that air's speed will decrease. The reduced velocity will affect mixture distribution in the cylinder, making the air and fuel less thoroughly mixed at lower rpm. Reduced midrange power will result. For the porting job to be a success therefore, every gain in CFM must be obtained without losing port velocity. A special tool called the velocity probe helps here. It is attached to the flowbench, and inserted into the port before and after each touch of the porting tool. But even using this tool isn't as reliable as simply not enlarging the ports to begin with. But how then can airflow be increased, you might ask.

The quest for fast flow begins before the port is ever modified. Typically, the engine builder pours RTV (silicone) into the port to obtain a flexible cast of its contour. After removal, this mold permits visualization of the port's shape in three planes. Only then can you see every part of the port, including where it can be improved, and where it must be left alone. Much of it depends on experience, but there are some hard and fast rules. In most cases, for example, it is impossible to make curved intake tracts straight. The tract must somehow empty into a cylinder, and to do so it will have to turn a corner or two. Usually, the tuner hopes only to make the port "think" it is straighter, by performing whatever shaping tricks are necessary, guided by the flow meter and the velocity probe. A port whose ceiling squeezes downward under a valve spring, for another example, is improved by widening the tract at that particular point. This makes this point in the port's flow area consistent with the rest of the port. Or, as is true in many older engines, the port may need a hump filled in and the radius leading into the valve throat increased. In fact, nearly all engine's ports respond to increasing their radii. That is, reshaping its bends so that both inner and outer parts of the curve have as large a radius as possible. And grinding isn't the only way to do this. In fact, in professional porting, filling with welding or epoxy is much more common than metal removal.

The Importance of the Valve Seat
And speaking of hard and fast rules, there is one part of porting that is generally downplayed but is in fact more important than any other part. That is the simple valve job. When making changes to various parts of the port, it will be noticed that the most immediate and gratifying gains will result from careful work on the valve seat. This means narrowing the seat, and raising it toward the edge of the valve. It also means carefully blending the seat's angles. This blending is the purpose behind the so-called "five-angle" valve job. The five-angle valve job is really just the standard three angles with careful blending between them (which adds two angles). You can have most of the benefits of porting simply by doing the standard valve job meticulously, something that takes fewer special tools and much less skill.

Polishing
Finally, we get to the subject of polishing. Polishing is, if possible, a myth within a myth. It is one of those traditional yet ineffective techniques which has somehow survived in the dank swamp of motorcycle mythology. However, the industry is more to blame than the unwitting public. The truth is, port polishing in a practical sense serves merely to "sell" expensive porting jobs. It's "eye candy." The customer has been conditioned to be more easily impressed by the unreality of what he sees than the reality of what he can't see. Sort of like the detail work done on your car during its 10,000 mile inspection. As just explained, most of the magic in port work is almost invisible -- fully 90% of effective porting takes place within an inch of the valve seat. The parts of the port that are usually polished are far from this area. The polish gives the customer something to look at.

Furthermore, polish is not only unnecessary, it's actually harmful, in two ways. The first concerns the airflow itself. The experience of countless engine builders, plus research by Superflow, Boeing, and various members of the Society of Automotive Engineers, indicate that a smooth surface is not necessarily the slipperiest where air is concerned. Ever notice that an airplane's wings are not shiny smooth, but rough? The rough surface reduces air drag, and the wing glides through the air easier. Olympic bobsled teams stick sandpaper-like skateboard grip tape to the sides of the sled and the tops of their helmets for the same reason. Air tends to get lazy on a smooth surface. It's called the "boundary layer effect," and it refers to the fact that an ultra-smooth surface accumulates air pressure next to it. This pressure is really stagnant air, and it effectively obstructs airflow. A slightly roughened surface eliminates this boundary layer and increases airflow. The second way polish is bad concerns combustion. Intake tracts don't just flow air, remember. They flow an air/fuel mixture, whose behaviour is quite different from plain air. The same lazy air boundaries that constrict a polished port's airflow also make fuel "drop out" of suspension in that air. The mixture then enters the cylinder less thoroughly mixed, and poor combustion and reduced power result. For this reason, most builders glass-bead the port walls instead of polishing them. The slight roughness creates tiny eddy currects which keep fuel droplets suspended in the port airstream. Good combustion is thus ensured.

The Bottom Line
Traditional porting is effective. But far from the glamorous thing it's made out to be, it is difficult work requiring excruciating patience. It's not taking out large amounts of what presumably foolish manufacturers left in there. Often it is more filling than grinding. The experienced engine builder knows that velocity is just as important as total flow, and that port shape is the means of gaining both, not just simple cross-section. Increasing cross-section will get you only the flow, with resulting losses in velocity that will actually reduce power at middle rpm. The professional also saves the polish for the valve covers. It's a waste of time and money in the port, and will usually reduce rideability. Finally, if you have porting done on your cylinder head, insist on flowbench charts documenting the gains. This should include references to port velocity as well as total airflow. If those charts are not available, have the work done someplace where they are. No matter how experienced a builder is, porting without a flowbench is akin to luck; the practitioner doesn't know what is really going to happen. In other words, blue sky, "watered" Florida real estate. But now you know. Oh, and please pass the ketchup...  ;) O8O



I have to grow old but I dont have to grow up.

RoadStarRaider

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    Porting and polishing
    « Reply #1 on: Nov 04, 2011, 07:00:45 pm »
    I will never waste my money on porting and polishing again. I paid $1000 to have my Buell heads done.  I had bigger valves installed as well as better springs. That was just for the machine work.  They were beautiful-a work of art.  I probably spent over $2k after labor and dyno time.  The result? 1.5 HP.  I was like you got to be f'in kidding me. What i didn't mention up front is that the shop that did the labor/dyno told me I was wasting my money when I told them I was going to send the heads out. Never again. The mfg's actuall know what they are doing.
    Ken
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    RIP Tail dragger

    Matt St. John

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    Re: Porting and polishing
    « Reply #2 on: Nov 04, 2011, 08:26:23 pm »
    Busa I've read that before somewhere. Have you posted this somewhere else or is this an article online somewhere?

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      Mr. T

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      Re: Porting and polishing
      « Reply #3 on: Nov 04, 2011, 09:53:05 pm »
      Busa I've read that before somewhere. Have you posted this somewhere else or is this an article online somewhere?

      http://www.motorcycleproject.com/motorcycle/text/cows-porting.html

        July 2009

      If we're not supposed to eat animals ... how come they're made out of meat?  ;D

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      Re: Porting and polishing
      « Reply #4 on: Nov 04, 2011, 11:12:57 pm »
      Busa I've read that before somewhere. Have you posted this somewhere else or is this an article online somewhere?

      http://www.motorcycleproject.com/motorcycle/text/cows-porting.html

      ya i knew I had seen this before.

      Floridaliner

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      Re: Porting and polishing
      « Reply #5 on: Nov 07, 2011, 07:40:01 am »
       Thats good info to save someones heads, or headaches. 

        Dont do motor head work / mill work in your garage.  Let a pro shop do it.   Port velocity and cylinder fill are the final answers to good head work and a fine tuned system. 

       My tuner/mech was telling me about a feller who didnt follow the basic program for a Harley 95ci build.  He chose to have a buddy do the heads in his garage. He got less power than it had to start with.  My mech turns out 110/115 all day with his pro head shop and a program of intakes and exhaust. 

       My build runs like a raped ape.  I used pro shops to do my mill work.  I only had to fight exhaust issues to find the power finaly. 

       I was just reading this morning on another site I am on. A feller wants to get 135hp vs the 122hp he has on his 126ci S&S..  The S&S heads he has are pro heads specialy designed already.  S&S goes through great legnths to make those heads, I think he will be in for a headache, poor feller.  Leave well enough alone.  But stock heads can always use upgrades. 
      If you see this big white race striped Rhinoceros comin at ya, ya might want to scooch over

      oops

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      Re: Porting and polishing
      « Reply #6 on: Nov 07, 2011, 10:53:17 am »
      Thats good info to save someones heads, or headaches. 

        Dont do motor head work / mill work in your garage.  Let a pro shop do it.   Port velocity and cylinder fill are the final answers to good head work and a fine tuned system. 

       My tuner/mech was telling me about a feller who didnt follow the basic program for a Harley 95ci build.  He chose to have a buddy do the heads in his garage. He got less power than it had to start with.  My mech turns out 110/115 all day with his pro head shop and a program of intakes and exhaust. 

       My build runs like a raped ape.  I used pro shops to do my mill work.  I only had to fight exhaust issues to find the power finaly. 

       I was just reading this morning on another site I am on. A feller wants to get 135hp vs the 122hp he has on his 126ci S&S..  The S&S heads he has are pro heads specialy designed already.  S&S goes through great legnths to make those heads, I think he will be in for a headache, poor feller.  Leave well enough alone.  But stock heads can always use upgrades.
      I never did see. What did you finally get out of your bike?
      In life you never know what's around the next curve.

      Floridaliner

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      Re: Porting and polishing
      « Reply #7 on: Nov 07, 2011, 02:34:18 pm »
      Yeah...  ;D  my dyno tuner wants it back to his shop too, he wants to see what it pulls.  Havent been back to the tuner yet. Workin to much at the store, cant get to it. 

      I was hoping to sell this PC5 and get the Cobra Auto Tuner first, then go get it hp/trq dynoed. 
       
      If you see this big white race striped Rhinoceros comin at ya, ya might want to scooch over

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      Re: Porting and polishing
      « Reply #8 on: Nov 07, 2011, 02:38:46 pm »
      going with the cobra tuner now?

      oops

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      Re: Porting and polishing
      « Reply #9 on: Nov 07, 2011, 03:35:31 pm »
      What??? Cobra Auto Tuner instead of a PC3 or PC5? With what you have? Who on earth told you to go with that? Surely couldn't have been a good Dyno Jet tuner.

      I remember a few weeks ago you posted that someone was coming to pick up your bike to take it to the dyno when you got it done. What happened then?
      In life you never know what's around the next curve.

      Floridaliner

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      Re: Porting and polishing
      « Reply #10 on: Nov 07, 2011, 06:37:00 pm »
         I thought the Autotune would be cool, maybe not.    :-X   
       
        Heck the bike is hauling arse now with a 13.8 a/f for the first 3000rpm, he wouldnt tune after that till I break it in.  The dyno man gave me a "break in" tune only, a lean mix of 13.8 only up to 3000rpm. After 3000rpm I am on whatever the previous Raider guy had for a map. 
         
         Been reading on it and it seems realy cool, but I guess you guys are right.  Get the PC5 tuned out. Just getting fickled over new toys on the market.  :P 

         My own suggestion OOPS, my dyno man wouldnt want to loose business.  ;D   

        I cant play the same as I used to.  The bike just goes crazy squating and weaving.  Half throttle is fine now. LOL
      If you see this big white race striped Rhinoceros comin at ya, ya might want to scooch over

      oops

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      Re: Porting and polishing
      « Reply #11 on: Nov 07, 2011, 10:28:48 pm »
      13.8 lean on break in??? What??? Only up to 3000 RPM's??? Then the bike reverts to some unknown map from who know's where? What kind of a tuner do you have there? That guy is a joke. You need to find a different tuner and find him in a hurry!
      « Last Edit: Nov 07, 2011, 10:41:23 pm by oops »
      In life you never know what's around the next curve.

      Floridaliner

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      Re: Porting and polishing
      « Reply #12 on: Nov 08, 2011, 07:57:57 am »
       ;D   no no .   he is good   

         you want to wash down your new rings before they seat ?    you dont ride over 3000rpm for break in.

         This guy has built lots of motors.  He I a retired Navy aviation mech, he put thousands of lives in the sky on his motor knowledge.  I stand behind him.   

        I know what I'm dong and he knows what he's doing.    We had a long talk about internet chat.  I just came on here to sell my PC5, should have left it that way.     :-X
      If you see this big white race striped Rhinoceros comin at ya, ya might want to scooch over

      t84a

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      Re: Porting and polishing
      « Reply #13 on: Nov 08, 2011, 09:32:45 am »
      Your tuner is wrong.  When breaking in a new motor you want to vary the RPMs at pretty much all RPM levels.  Also, running a bike lean at any time is a bad thing.  Either you are misunderstanding him or he's giving you bad info.
      Ken
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      Re: Porting and polishing
      « Reply #14 on: Nov 08, 2011, 09:41:37 am »
      You are talking about heat cycling the motor. Ya, you will do that by going from idle to 3k or so and back down varying the throttle input, it sets the rings. Florida knows this. He understands it. With the new engine his tuner did not want him breaking 3k rpm until after 500. The rings will set without going that high on the RPMs. That's the only thing you are worried about on the break in for the work he did. He didn't set anything up to be lean or otherwise above 3k. He just didn't touch the map above it.