bbarbulo

iTrader: (2)

While port matching sounds like a fine idea, in practice it is in fact a bad idea. Port matching the intake and exhaust manifolds to the cylinderhead aids in the production of reversion, which is robs power, and contaminates the intake charge. Reversion is a term that refers to the backward flow of the air/exhaust in the cylinderhead. What alot of people don't realize is the incoming air, and exhaust, flows both ways- ie in and out. I won't explain fully how and why it occurs, as its full explanation would fill pages, but I will give a short explanation: Ports do not flow one in just one direction. Both the Intake and exhaust ports pulsate and flow in and out. This happens during valve overlap, when one cylinder still has positive exhaust trying to exit the exhaust port. This is also when exhaust back pressure from the exhaust manifold is pushing backwards. So when the intake valve opens, the exhaust gasses seeks the path of least pressure/resistance, which is back out the intake port. Reversion. This reversion, in both the intake and exhaust, carries over into the combustion chamber. This where reversion really does damage to power production. The reversion contaminates the intake charge with vapors and residue already burned by the previous cycle. This robs alot of power, especially in high horsepower engines. And port matching only makes this worse. As a general rule, the intake ports of the cylinderhead should be larger than the ports on the intake manifold. And conversely, the exhaust manifold openings should be larger than the exhaust ports on the cylinderhead. This helps in resisting reversion, as the smaller openings on the intake manifold, and the smaller exhaust ports, give the backwards flow of air (both intake and exhaust) a measure of resistance, while at the same time offering no resistance to the forward flow of air and exhaust. Reversion is something professional engine builders despise, and they use a variety of techniques to keep reversion to a minimum. These include the afore mentioned port mis-matching, to more sophisticated techniques, like 'unique' valve seats and trench cut valves, both made popular in NHRA Pro Stock cars in the 70's. And since stuff they did in the 70's in only now becoming available, one can only wonder at the current techniques and technologies employed by modern race teams.

That's not say all port matching is bad. One example of this is port matching the throttle body to the intake manifold. Alot of enthusiasts spend the money on bored and polished throttle bodies, yet don't take the time to fit the throttle body to the intake manifold. The stock intake manifold is designed for a stock bore throttle body, and thus the cross section of the throttle body mounting plate is considerably smaller than the bored throttle body. This represents a restriction to the path of air. Spending some time port matching components in this situation is definitely a worthwhile endeavor.

Steve... any thoughts on this??

imported_gatherer

well put Bruno... been read at the theoldone.com have we?

personally I agree.. on the D series board there was a dude who designed a exshaust manifold that would have the same port mismatching but continue it all the way down the pipes .... this allowed for a sort of multi stage mis match to take place and further restrict reversion without hindering forward flow....

spracingsports

you guys are way too smart for your own damn good... i wish i knew half you guys know... although i have learned alot readin your posts.. keep these tecnical threads coming, they are very interesting to read

imported_gatherer

here become smart too

start reading it all

www.theoldone.com

have fun

B18C|

for real...you guys know your stuff INDEED!!!

bbarbulo

iTrader: (2)

Steve, congratulations on the Endyn and Crower parts. Those are some yummy bits!! I really wish more ppl had more disposable income. It almost hurts me to see ppl passing up such great parts for some CRAP... like tail lights and whatnot.

yaseen2477

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