Baldy

So I often read about how larger ports/intake are good for high-end power, but low-end suffers. Why? Why does a smaller port or smaller intake diameter cause more power in the low range?

phinsup

I don't know for sure, but I will take a stab.



I am going to with bernoulli's principle on this one. Possbily at low RPM, the smaller diameter port provides for greater pressure and better fuel atomozition (eddy's). At higher RPM the same concept inhibits the amount of air and fuel that can travel through, small cone, big pressure, lower volume, big cone lower pressure higher volume.



Seeing as my background is primary aircraft.....

Just a theory...



here's a cool site where you can play with the venturi



http://home.earthlink.net/~mmc1919/venturi.html

Feds

When the port opens, and there is a pressure difference between the atmosphere and the camber, the air in the runners is compelled to more toward the low pressure. The smaller the runner, the less intertia the charge has, and the quicker air moves into the chamber. Since there are fewer intake events per second at low RPM, the flow through the runner is small, and the runner is not a restriction, so low intertia wins.



At high RPM, a small runner can't flow enough to feed the motor, and thus HP is limited. A large diameter runner has the size to accomodate the flow, and therefore can make more power.

Baldy

So then, it's less about the amount of air at low rpm's, and more about how that air treats the fuel? Is that right?



Like, sure smaller intake would inhibit flow, but at low rpm's it's not an issue, so you want more velocity to suck and distribute fuel. Is that it?





...now that I've read the post above mine, I think I get it. So it's more about how much energy it takes (away from the moving rotor) to get the air moving into the port...right? Bigger port, more air to move, takes more energy to get it moving.

z8cw

One other point to consider is that air flow with higher velocity in a smaller port will be harder to interrupt or stop or reverse. So at low rpms from intake closing to new intake opening the higher velocity will help to keep the intake flow moving. Where at higher rpms that becomes less of an issue.

heretic

Intake velocity is ALWAYS an issue.



Remember even on the smallest ports, the intake port closes well past BDC. If you have decent intake velocity, the slug of air in the port will have enough inertia to counteract, or preferably overcome, the force of the rotor trying to squeeze the charge back through the intake port.



Huge ports means poor velocity at low RPM, and you get reversion in the intake tract. Not only does this hurt VE (and therefore power) but just as it takes energy to stop the airflow and push it backwards, it takes more energy to stop it again and get it moving forwards again. So you lose at both the beginning and the end of the intake cycle.



On the flip side, if your ports are too small, it takes excessive energy to try to move the air through. Roughly .5 Mach is where the cost/benefit ratio turns against you.



Fuel atomization/etc doesn't particularly enter into it unless you are adding your fuel way up at the top of the intake runner. Note that Mazda puts the primary injectors as close to the chamber as physically possible, but they put the secondaries way up the runners. Works good at low speed, and high.

Baldy

awesome, thanks for the clarification.

rotary_pwr

nice post heretic really informative

mike_rudy

is that the same reason then why longer intake runners create better low and and less top end power?

1revnrex

[quote name='mike_rudy' date='Mar 15 2005, 11:35 PM']is that the same reason then why longer intake runners create better low and and less top end power?


[/quote]





Search for info on variable length intake stacks that the 26b used in the 787 World Car. It will explain the reason behind the length of runners.