I have been researching on home porting heads, whatever kind they may be, street bike, ATV's and ofcourse LT1's.

Here's the article that started this: http://mototuneusa.com/think_fast_intake_porting.htm


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------This is what it read:

It's just like a roadrace; you can lead for 3 laps, then run out of steam and end up in 7th place.

In racing, only the last lap counts, because the one who leads at the checkered flag wins.


"In the intake cycle "race" the last lap is the charging phase,
and the checkered flag is the intake valves closing.


So in the final result, the first 2 phases don't matter if the 3rd phase is unsuccessful !! The total intake volume that will be burned is determined by the amount that remains in the cylinders after the intake valves close. That means that an early gain during the suction phase can be easily lost during the charging phase. And, if the intake charge returns back into the port during the piston's upstroke, the result is going to be a net loss !!

It gets worse !
On a carbureted bike the reversing charge will create an effect called double carburetion, which means the mixture will become even richer with fuel as it passes back over the main jet nozzle for a second time.

Have we lost enough power yet ??

Wait... it gets even worse !!
Here's a real "out of the box" idea: high flow ports also flow really well backwards !!! All that work on the flowbench comes back to haunt you when the piston now has even less resistance to pushing the intake charge back into the " high flowing port " !!!!!!! When you begin to consider the consequences of all this, the whole idea of "more flow is better" comes crashing down like a lead balloon.

What's The Secret ??
Using the race analogy, if you increase the Port Velocity, by making the port smaller, the intake cycle "race" starts out slower, gains momentum and makes a tremendous charge on the "last lap" to overtake the high flow port and win the "race". The interesting thing is, this type of port will always lose in a flowbench contest !!!"

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I found an article from chevy high performance, and while they don't exacitally mention alot about the Velocity idea, they definitally mention not to go grinding it out bigger, just make the travel smother/faster.

Now I know this guy uses this technology with street bike motors that Spin 13-16,000 rpm, but does that mean it shouldn't be looked at?

I wanted to home port my 400ex ATV head and this is what started it. But this was Very new to me.

??? what do you think? is he crazy, or a genius?

 

I've been reading everything I could get my hands on regarding head flow and velocity, for several years now.

I'm still not a porter, I'm not one of these guys like Lloyd Elliot, but I have learned enough to know that velocity is very important. I have also watched some very high end discussions on another board, and what this guy is saying is very similar to the things I have heard there.


But this is hard stuff to get your head around, trying to visualize all 4 cycles in your head and play with different valve timing ideas.

Probably the best advice I have heard is that when you do a home port, visualize water flowing through the port and have a contest with yourself to see how little material you can remove and still smooth things out so water would flow well. Also, keep in mind that on the backside of the valve, along the axis of the stem, you want that to be as straight as you can manage.
In other words, the last part of the journey, when the water is lining up to hit the back side of the valve and spread out into the chamber.

 

I want to see where this post goes.

 

The intake port on head is larger than the intake valve side in the combustion chamber for one reason. Velocity. The basic idea is to flow the air through a small hole (intake valve) at the same mass flow rate as it's coming in at the huge intake port. Polish and smooth up the last half of the intake runner and you'll get a lot better flow, with the same velocity or slightly better. Gasket match the intake port and gradually narrow it and polish it back to the same size as where the last half of the runner starts and say hello to some good numbers.

 

There might be a few folks who disagree with this advice.

 

Anyone that's researched or been in the field know that polishing intake ports is for show. People always think shiny will flow smooth and faster, when in reality a slightly rough surface will flow more. Why? Because the slightly rough surface creates a boundary layer of air, which has less friction for the rest of the air to flow over.

If you want an interesting example of this do a google search on why golf ***** have dimples. Here's a quick read:
http://entertainment.howstuffworks.com/question37.htm

Example in automotive use: Look at the undercarriage of the new Lexus LS, they underside is completely covered with a "dimple shield" to lower the cd.

So next time you see a shiney intake port just think one thing "BLING" because that's all it is, Jewelry doesn't win races.

Also anyone that's done cross sectional measurements on an LT1 head quickly realizes that gasket matching is worthless, that's not the minimum Cross Sectional Area, so it's not going to help increase the potential flow of the port. The smallest cross sectional areas are the hardest to get to and take the most time to shape.

Cylinder heading porting is best left to professionals that have the time, equipment, and knowledge to do the job right. Next time you want a set of ported cylinder heads ask the porter what schools they went to (SAM, Mondello, etc) or how many years they've done cylinder head work. You'll be glad you did.

-Christian

 

I thought the dimples on golf ***** were due to the ball rotating and causing turulance ,so as in the artical the dimples would help , but as in the artical sometimes a smooth surface would be benificial ,This is from the artical...

" Therefore, if the shape of an object is such that separation occurs easily, it is better to turbulate the boundary layer (at the slight cost of increased drag) in order to increase adhesion and reduce eddies (which means a significant reduction in drag). Dimples on golf ***** turbulate the boundary layer. "

Personally it seems that an intake runner would benifit from being smooth since none of the surfaces change in relation to the surroundings like the bottom of a car or a golf ball.
just my opinion... and thats why i'm still reading up on articals like this!
one more thing... do you work for a head porting business?

 

Velocity versus volume has been an ongoing debate for years. You will notice how much intake ports for SBC have increased in the past 20 years, but it is mainly due to the availabilty of larger displacements ( a 454 SBC needs a little more than 170 cfm).

Polishing an intake port used to be a big no-no with carb motors, but now since the FI engines are dry flow, people think that it will do better. But air also doesn't like to turn around corners, so some combination of finish in the port to help the air "adjust" as it turns can be benificial. I still wouldn't fully polish an intake port, for the above mentioned reasons of dimpling golf *****.

But carving an intake port that tapers to the valve will cause a stall point behind the valve, just like a funnel.

What really needs to be done is research the port versus cam timing, and size your port to match the timing of your intake events. And the valve seat and valve head/neck play a large part in the flow also..

 

It might be easier to match the valve timing to the port configuration, length and flow capacity rather than the other way around. It might even be more effective.