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Old 09-04-2002, 12:43 PM
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Post Porting heads

would like opinions on port matching heads to intake and exhaust headers. Would you match the ports to the gasket size or match the ports to match each port. This is on a 350 with vortec heads and edlebrock performer rpm intake. I'm not wanting to port & polish the heads just want better flow.

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Old 09-04-2002, 03:48 PM
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Match to the gasket
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Old 09-04-2002, 06:15 PM
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Match to the gasket, but keep in that the size of gasket opening may not be the ideal size. Make sure and match it to the recomended gasket for the head and intake. For the longest time my intake runner was smaller than my cylinder head runner, gasket matching them netted me 3tenths in the quarter.
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Old 09-04-2002, 06:54 PM
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SameO SameO.. Match to the gasket.......
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Old 09-05-2002, 05:16 AM
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What would you do if the gasket is smaller in some places than the ports. Would you cut the gasket to match the port? But cutting the gasket would reduce the sealing area, this is on the exhaust gasket. Would it be better to use the exhaust gasket from the engine gasket set or use the header gaskets that came with the headers?
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Old 09-05-2002, 08:22 AM
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Use the gaskets that came with the headers, or a good set of aftermarket header gaskets. Only problem with the gaskets that come with the headers is that they are usually supplied by the lowest bidder. If the header gasket is slighty smaller than the port it is OK (and beneficial) to trim it slightly.
Performance in a street/strip engine is enhanced by matching the ports, cleaning up any flashing in the ports and combustion chambers, slightly radiusing the "short side" of the intake port and cleaning up the valve bowls (including the guide bosses). Don't get carried away and try to remove a lot of material, as it is not productive and might even be determental to performance.
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Old 09-05-2002, 12:55 PM
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on a street mobile i'm not entirely sold on the benies of port matching the exhaust if the head port is smaller than the exhaust port, i.e. the gas is going into a bigger opening than it came out of. i am totally sold on matching the intakes tho, so if you would clue me in to how the exhaust port have to match, please do. just like polishing is a big mistake for the street as it kills low end torque. si? a certain amount of turbulence is required (at least with a carb). inch and 5/8's headers are better than inch and 3/4's for street so, SET ME STRAIGHT!! I GOTS T'KNOW!! I GOTS T'KNOW!!
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Old 09-05-2002, 01:10 PM
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I think matching the exhaust port of the head to the header would be benificial in added exhaust flow, as long as the head matched the header diameter and not any bigger.

Is this what you are reffering to?
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Old 09-05-2002, 01:17 PM
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Just for your information, I was really suprised when I pulled the vortec heads out of the box. There was not any flashing left in the intake or exhaust ports, really clean ports..
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Old 09-05-2002, 01:47 PM
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The function of a properly designed exhaust header pipe is not primarily pressure reduction. In fact, too much pressure reduction (too large diameter in relation to gas velocity) will reduce the effectiveness of the pipe. Pipes are designed using organ pipe theory or Hemholtz resenator theory which are essentially the same thing. When the exhaust valve opens, a positive pressure wave traveling at the speed of sound, much faster than the exhaust gas, is sent down the pipe. When the positive pressure wave reaches a change in diameter as in the case of a header collector, a negative pressure wave is reflected back up the pipe. Since these waves travel at the speed of sound, the pipe length can be designed to have the reflected negative pressure wave reach the exhaust valve just as it is closing which provides a free scavenging of the cylinder, thus a power boost. This effect must be designed to a specific speed of the engine so a stock, lower speed engine benefits from longer pipes and a racing, high speed engine from shorter pipes. The principle is just as useful in a street engine as in a racing engine.

Port matching becomes important because everywhere the pressure waves reach a discontinuity in diameter, a wave reflection occurs. The ideal situation is to have a very smooth, constant diameter from the exhaust valve to the header collector. That is also why grinding out too much of a relief in valve ports hurts rather than helps performance - it screws up the pressure pulses that the designer is trying to control. This priciple gives a very measurable boost in performance at a specific RPM but can hurt power at other speeds. Engieers of stock engines in an attempt to smooth out the torque curve and have a smoother engine, design exhaust headers with no length thus eliminating any pressure pulse effect.

Surprisingly, bends and turns in the pipe has no effect on performance with reference to the organ pipe effect.

For a street/strip engine there is some serendipity in that there is a sympethetic harmonic pressure pulse that occurs at multiples of the design RPM so a pipe designed for 1800 RPM will show a boost at 3600 RPM and 7200 RPM.

Same principle applies for intake manifolds too. Moral of story - match ports.
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Old 09-05-2002, 07:10 PM
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WOW!!!!!
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Old 09-05-2002, 07:55 PM
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Only one thing to add to that great description of pressure pulse theory, there is no reflected wave on expansions of diameter...it only changes the shape of the pulse. Divergent and convergent cones can be used to shape the return pulse to give it a longer profile which can extend the duration of the scavenging effect over a greater rpm range. This is best seen on two stroke exhaust tuned pipes by comparing enduro bikes and race bikes with the same engine. The race bike will have short highly divergent and convergent cones to narrow the rpm range where the scavenging effect will work. This gives a peaky power profile but also the greatest peak. Long divergent and convergent cones give the pulse a long profile which extends the rpm where the effect will occur.

This explains why crossover pipes can add power, the harmonics of the pulse from the left and right side can be made to work together and strengthen each other and conversely cancel each other out. Most high perf auto setups will have three major pulses acting on the system, the pulse returned at the collector, the pulse returned at the muffler and the pulse returned when the exhaust finally reaches atmospheric pressure. Of course the atmospheric return pulse is destroyed more or less by the muffler but on race engines (without muffler) this pulse is significant. On rotary engines this atmospheric pulse is very important and a divergent cone used as an exhaust tip can gain 5 or more HP over one without the cone. Looks cool too. :p

All this talk of cones has me thinking of Madonna and ice-cream, tutti frutti anyone? Come on Willy's let's get some icecream, I'm buying if you let me ride shotgun in that altered fuel gasser you call a street car. Let me get my nomex jacket first though! Will I need my goggles and helmet? Maybe a change of underwear?
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