Throttle Body!! More air!! Torque?????????

[Farqineh]

I'm Curious as hell here.

 

I have replaced oodles of engins both gas and diesel. I get the point behind volumetric efficiency 'n' shit. ( covered all that crap in school )

( heavy equipment mech)

But.... Since I have been here and over the course of the last couple of days with this reading, and reading, and reading...... One thing really perplexes me.

How does bottom end, vs. top end compare in a V6 (torque), and how in the hell is it that the amount of restriction to air flow could be detremental to one or the other. Logic dictates (my logic) that on a N/A engine, the piston goes down and the volume created by the voided space must fill with air. Negative pressure is created and the air moves in at the respective atmospheric pressure.

Now, I realize that different atmospheric pressures (altitudes) can affect the volume of air achieved (density also as a factor), as well as the restriction to flow. However, I fail to understand how this phenomenon (as I see it) affects high or low end torque, as opposed to overall engine performance over the entire curve.

 

Could someone explain this one to me!!!!!

 

Thanx a pile

[BIGBULS]

High rpm engines want a LOT of air, and they want it to get to them as simply as possible (no bends etc). So, short and fat intake runners (that are as straight as possible) and a cam with a lot of lift and overlap (to help get the air in and out). Also larger primaries on the exhaust manifold. You also want to take advantage of these attributes by designing an engine that revs higher..............if you can keep producing torque at high rpm, you can then gear the car down so much that it gets an artificial boost at low engine speeds as well (by the torque multiplication that the gearing provides).

 

Low rpm engines want longer intake runners to help promote swirl for better combustion and higher velocity incoming air...........they also want long, thin exhaust manifold runners to help promote exhaust gas velocity and thus, the mild scavenging effect you get from the pressure pulses of the outgoing gasses. Match these with a cam designed around lesser quntities of air (lower lift and less overlap), and low rpm muscle is yours. The single biggest thing affecting low rpm power though, is displacement............I don't care HOW you try to tune a 1.6L I-4...........it just won't make much torque without forced induction.

 

 

 

This is why a combination of a variable valve lift setup (and timing too) such as VTEC (or i-VTEC) is such a help. You have essentially both a low rpm cam lobe, AND one for up top. Now match a VTEC style setup with variable intake runners (such as the Ford Taurus SHO has), and you'd have the best of both worlds (and a DAMN fast car).

 

 

When it's all said and done, a properly geared, high reving powerplant will be faster............but you will have to wrap the shit out of it to get it to go, so day to day, it probably won't be as fun, nor will it FEEL as fast.

 

This is why so many small displacement Imports FEEL kind of slow, but when matched up against a larger domestic V6, are actually able to accelerate faster (weight is obviously an issue here too).

[loominaz34]

My highly uneducated guess is that at lower engine speeds there is more time for the negative pressure to form than at higher RPM.

The time it takes for the air to fill the now empty space is limited by how fast it can move (Theres where air restriction comes in)

 

In short

 

1 BOOM! gas explodes, piston is forced down while exhaust is forced out.

2 Negative pressure is formed by downward movement of piston. During this stage, without forced induction, air may be pulled below atmospheric pressure because it can't move fast enough to enter before the valve closes again.

3 The air/ fuel mix takes a certain amount of time to get into the combustion chamber. Less restriction means more air is able to move faster and more fully enter the combustion chamber.

 

Again, my highly uneducated, 15 year-old mind at work.

How right am I?

 

 

 

Again totally uneducated and pretty unclear.

[Vegeta]

Its all about velocity. For NA, having a short fat runner means very little velocity. It takes higher RPM to get the velocity up to snuff for the cylinder to fill up with air at a rate equal to the valve open event.

 

For the TB, you have to look at the runner volume and the plenum volume to determine at what point throttle resolution will be off (having enough opening to = WOT when the TB plate isnt at WOT yet). Smaller plenum needs a larger TB plate to keep the air supply readily available to all the runners. Too small a TB with a small plenum area means awesome low end torque because the velocity will be extremely high. It also means the mid and top end will hurt and the benefit of that low end will be negated by the mid and top end loss.

[Farqineh]

 

Okey Dokey then! So in a nutshell, (large picture speaking) high revving motors require as little restriction as possible to aquire their air. Thus the shorter runners and larger bore TB's. This seems directly perportional to the overall engineering of the engine and is then, of course relative to the gearing of the machine. Naturally a lower revving engine has more time to fill the 'hole', so to speak. So manipulating combustion charachteristics to aquire more bang via swirl, and so forth makes sense.

 

Nothing substitutes cubic inches for torque, of course!!!

 

However, my query still remains somewhat unsatisfied. How is it that an engine could suffer some sort of torque loss with the addition of less air restriction at lower rpm. For instance, lets say your 3.1 is the proud recipiant of some major trimmings. A fat assed intake and a thicker throttlebody. Camshaft with a little higher lift and a touch more duration. No bottom end or trans. upgrades. My logic dictates that you would have a power gain throughout the torque curve no???

Perhaps I am getting mixed signals, or I have a perception issue, maybe I'm just a moron, I dunno. For whatever reason, I am getting the impression from some sources that the torque would drop at lower rpm, and increase at higher rpm only. Hell, maybe they're right, but for the life of me I cant see why!!!

 

Enlighten my dumb ass

[Farqineh]

Its all about velocity. For NA, having a short fat runner means very little velocity. It takes higher RPM to get the velocity up to snuff for the cylinder to fill up with air at a rate equal to the valve open event.

 

Velocity???? Isn't velocity directly perportional to volume and resistance??????

 

For example, suck and arbitrary volume of air through straw, then suck the same volume of air through a chunk of PVC pipe. The larger pipe will experience less air turbulance and velocity, aquire the same volume of air in less time and with less work. Which = less chance of you passing out

[BIGBULS]

That's why you want the smaller diameter TB and intake runners (for low rpm), plus the less radical cam at lower rpm.............to keep velocity of the air charge high. This lets the cylinder fill more efficiently as well, and when combined with the long, narrow exhaust primaries equals great combustion at lower engine speeds.

 

Make more sense?

 

Higher rpm optimised engines (with the short fat runners, more radical cam and just bigger overall "holes") lack the velocity when the engine is turning slow.........but up high where the air really gets moving.........

[Farqineh]

:shock: :shock: :shock: :shock: :shock: :shock:

 

OK Alrighty

I'm a dumass. I'll admit it. Ha!! I'M THERE DUDE!!

 

The turbulance is the key. Ya Ya ok I was right on top of it and someone had to kick me in the giggle berries to get me to look down.

 

Basicly the lower rpm engine is manipulating the air as opposed to trying to get it for combustion. If you rob its ability to do that ie. throttle body, blah blah blah, you loose some low end torque because you drop the velocity down.

 

See....Good thing I thought of that eh??????

 

Jokes............Thanks alot guys. I see the light!!!!!