The Aftermarket TB/Plenum Advantage

2000GT · Mar 17, 2004

We all know that fuel is made up of gasoline and air. More air + more gasoline = more power. However, in naturally aspirated form, our engines are only capable of ingesting a very specific volume of air. How does changing the stock plenum and TB add any more volume of air into engine? It’s not like a blower where air is being compressed into the engine with pressure.

It just doesn’t make any sense to me. Keep in mind that I am not disputing anybody’s claimed hp. gains, I just don’t understand what causes the gains over the stock plenum. How does more air end up inside the engine from an aftermarket plenum?

Mach428 · Mar 17, 2004

I'm not totally sure what it does diffrently either, But I have herd, and I know i'm going to get crap for this, that it's alot of money for about 5HP.

CottonBurnerz · Mar 17, 2004

agreed... tornado air thingy would work just as good and cost $300 less

Makalani_96GT · Mar 17, 2004

I haven't put my new T/B and Upper intake on just yet but I have heard (and that's one of the reasons I bought it) that your throttle response changes some puls in the long run when I put a power adder in I won''t have to buy a bigger T/B

DB96GT · Mar 17, 2004

It's all about where the airflow bottleneck(s) are. There is apparently a bottleneck in the airflow from the stock plenum. Air flow/exhaust mods are about not so much ELIMINTATING air flow bottlenecks, but just moving them from one place to another, all the while increasing the air flow.

NasaGT · Mar 17, 2004

Its basic fluid mechanics. (fluid being air in this case) The stock plenum has a harsh turn inside of it. As the air moves through the TB and into the plenum, it will become very turbulent and non-laminar at the elbow because of the abrupt turn. The turbulent flow will eventually straighten out if you will, but it limits the velocity of the air traveling into the lower intake, which means that it limits the volume of air able to get into the combustion chamber while the intake valve is open. The aftermarket plenums do a better job of turning the air flow than the stocker and therefore get better air flow into the lower intake.

The larger TB helps because it matches the opening of the plenum. A TB smaller than the plenum opening causes a small area of turbulence where the two meet. Turbulent flow = bad.

Disclaimer - this is a very simple description, don't nitpick it to death.

2000GT · Mar 17, 2004

NasaGT said:
Its basic fluid mechanics. (fluid being air in this case) The stock plenum has a harsh turn inside of it. As the air moves through the TB and into the plenum, it will become very turbulent and non-laminar at the elbow because of the abrupt turn. The turbulent flow will eventually straighten out if you will, but it limits the velocity of the air traveling into the lower intake, which means that it limits the volume of air able to get into the combustion chamber while the intake valve is open. The aftermarket plenums do a better job of turning the air flow than the stocker and therefore get better air flow into the lower intake.

The larger TB helps because it matches the opening of the plenum. A TB smaller than the plenum opening causes a small area of turbulence where the two meet. Turbulent flow = bad.

Disclaimer - this is a very simple description, don't nitpick it to death.

That makes some sense. Thanks for taking a stab at it.

So, if I understand correctly, it’s more about the velocity and smoothness of the airflow entering the engine that in turn allow the engine to intake more volume. With that being said, there is obviously some point where the CFM from the plenum/TB combo is maximized to the point where there is no improvement in performance. I think this was proved in the second plenum test where going from the 70mm to 75mm TB on the C&L plenum produced no gain. According to C&L’s web site, their 75mm combo produced 30 more CFM over the 70mm combo.

Accufab claims their 70mm and 75mm TB/plenum combos produce the same CFM.

RysRed96GT · Mar 17, 2004

basically, less harsh bends means less air friction which means more air volume can travel in faster per time interval.

hognutz · Mar 17, 2004

basically your engine is a vacuum. if there are restrictions in the airflow path that cause the engine not to be able to get the air needed you lose hp. the other angle to the arugment is air velocity and air turbulance. if you just give the engine a huge opening to draw in air you will lose velocity witch will affect powerband and torque. now if the path cause the the air to be turbulent the air will not gain as much velocity.

so the reason an aftermart plenum works better than stock, is stock is not the optimum design. ford did not say lets see how much power we can get out of this motor period. they most likely set a target goal and met it. I have never seen a factory enine that was completely optimized.

that being said we are only talking 5-8hp not 20hp. it is not like it is a night and day mod. most engines will benifit from a free flowing intake to a certain point.

sl03gt · Mar 17, 2004

A friend that owns a speed shop told me one time "You must remember that headers, cat basck, gears, air filters, pulleys, etc.. don't make power. They free up power. Superchargers, nitrous, and turbos make horse power. Hence the name Power Adder. That's why in performance part ads they say "gain up to such and such hp"." Which is true. Maybe the plenum only gains 5 hp but what about the average gains in the mid range in tourqe and horsepower?

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