chris928

Some strange and mystical thoughts have arisen here at the rennlist regarding the "venturi" section on the intake tubes.

I can tell you one thing, this neck in the tubes is not to speed up the air because it doesn't! It is without a doubt, a restriction in the flow. Where the air does speed up at the venturi, ask yourselves, what happens after it leaves the venturi? IT SLOWS BACK DOWN!!! And it probably is going slower than when it entered the venturi because it has heated due to the compressive effects of the restriction.

The two best explanations I've heard are 1: it is simply there to clear the crossbar and 2: it is used to help tune the intake for a noise problem.

Please, let's not propigate this myth any further that this venturi speeds up the air. If that were true, there would be no less than 10 venturi's along the intake so that we wouldn't need superchargers...

Mike Schmidt

[quote]Originally posted by chris928:
<strong>it is used to help tune the intake for a noise problem.</strong><hr></blockquote>

It's for the noise. Porsche replaced the venturi tubes with straight insulated ones on the GTS

John Struthers

Chris,
Why is the throttle body necked- down.

John S.

Tony

OK..OK..LOL..lets slap the guy that opened the venterui can of worms again <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

dr bob

Wow!


OK, just to keep things straight, let's talk a few basics.


1) The mass of the air flowing at either end of the tube is the same. Pretty much says that all the air that comes out of the tube starts off by going into the tube, whether there is a venturi in the middle or not.


2) The venturi does in fact cause the speed of the air to increase within the venturi. Since the mass flow through the tube and the venturi is the same at allpoints in the system, (see 1 above...) it will flow faster where the cross section is smaller.


3) The flow downstream of the venturi is not hotter because of the "compressive effects of the restriction" but is likely cooler due to the losses when the gas expands downstream of the restriction. This assumes that there is no heat contributed by conduction through the walls, of course.

Remember that the pump (engine) is reducing the pressure in the tube at the air filter end. That difference in pressure is what causes flow through the tube.


4) The venturi restricts the airflow. Keep in mind that a well-designed venturi recovers almost all of its losses downstream. So comparing it with an orifice plate as a restriction is not good. What are typical losses? As low as 3% with a clean design and constant flow. Hmmm... constant flow. OK, maybe a better term is 'at the design point'


5) The change in velocity and the subsequent changes in density of the flowing air in the various sections can change intake noise. Sound waves change speed with the density changes, and reflect inside the tubes. At various frequencies and at various flow rates, you may see sound pressure oscillations (waves?) that cancel or reinforce themselves.


Now, why would you want to have a venturi in the inlet, with a stub tube sticking up into the middle of the throat from the cam belt cover? The lower pressure at that point would draw hot air through the belt covers, air that has been heated at the front of the block... Does anyone agree that a purpose of that venturi is to keep the belt and hidden bits from getting prematurely crispy from captive heat at the front of the engine? Anybody know why the Cosworth in my Lotus has no belt covers or shields at all? Call it a... hunch.


So, not meaning to slam your logic or anything, let's at least propogate truth when we can.

<img src="graemlins/yltype.gif" border="0" alt="[typing]" />

Steve Cattaneo

The tubes were designed as venturi nozzles to dampen the engine’s intake noise. Due to the aerodynamic shape the AIR STREAM IS NOT RESTRICTED... Air is drawn into the belt , for cooling.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Mike Schmidt

I agree with everything dr bob said except for one correction. The stub for the belt cooling is located several inches forwards of the throat of the venturi. The tube still draws air through the stub to cool the belt though. The way that dr bob described it would have been even more efficient at drawing air through the belt area.

Greggg

So if the venturi effect speeds up the air as it travels over the hottest point in the engine compartment then we get less time for the air to collect heat. Sorry to stir the pot... <img src="graemlins/crying.gif" border="0" alt="[crying]" /> <img src="graemlins/crying.gif" border="0" alt="[crying]" /> <img src="graemlins/crying.gif" border="0" alt="[crying]" />

AO

You go Gregg! As the say in the Motor City, "Don't you take no $h!+ from nobody!" <img src="graemlins/c.gif" border="0" alt="[ouch]" />

Keep stirring, keep stirring! If I still used a charcoal grill, I'd give you my lighter fluid if I thought it'd help. But from the looks of things, I think you'll get someone's panties in a bunch pretty soon!

As soon as you think you've got evrything setup and are pulling 0-60 in the 4's call me and I'll be your witness and scientific recorder.

I never liked physics! We don't need to apply them in our little world! <img src="graemlins/soapbox.gif" border="0" alt="[soapbox]" /> Do we...?

Oh well, it's been a strange day, and the wife is calling me to bed! <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

John Struthers

ALL,
REALLY, DIDN'T MEAN TO DO THAT...
FWIW
Typed in ' engine venturi ' on the old browser
Some good info pops up, to include some stuff on modifying intakes.
Someone really up on intakes?
Could use some enlightenment on the new BMW 745i
intake manifold with the two internal helical elements which are rotated by servo to extend intake length from 8.5" to 23.9"...Howzat?
Description is in June Road and Track. Oddly, the article on the Bernoulli principle is described in Automotive Aerodynamics. Of particular interest is a sentence or two about the BMW wind tunnel located in Munich(Munchen). The tunnel has an overall cross section of 1000sq. ft. (+ or - a little). The fans produce a controlled 18mph breeze. However, when the tunnel necks down to a
200 sq.ft. test section the 18mph breeze is transformed to a 90mph flow thru the narrow section.
I only understand the rudiments of a venturi and realize that there have to be variables when trying to apply the principle to automotive intakes. Specifically, our 928's with engines that vary in size and have various and sundry intakes.
Interesting though.
John S. 82',Auto, Weissach, 'Pattycakes'

Mike Schmidt

Those BMW intake runners are continually variable in length. This means that the Helmholtz chamber tuning is being continually adjusted by the computer for the specific RPM that the engine is currently at.

When an intake valve is open and the piston is moving down, air is sucked through the intake runner for that cylinder. When the valve closes, the air that's still moving down the intake runner is still moving towards the valve from the momentum. The air slams into the back of the closed valve, creating a high pressure area. This high pressure air then bounces back, traveling back up the intake runner as a high pressure wave. When it gets to the plenum of the intake, it causes a reversing, and a high pressure wave starts back down the intake runner again. If the intake valve is still closed, the wave will hit the back of it and go back up the intake runner again. It will keep going back and forth like that, getting a little weaker with every trip. It acts pretty much like a toy Slinky, or a wave of water hitting a wall and starting back the opposite way.

The trick is to have the intake valve be open at the instant that the pressure wave is getting there. With the valve open, the high pressure pulse goes into the cylinder, packing in more air than would normally go in there. This is higher than normal pressure air, so it's kind of like turbo or supercharging in a way. If everything is perfect, you can get volumetric efficiencies of over 100% for the cylinders.

With a regular intake manifold, this ideal situation only occurs at one RPM though. The longer the intake runner, the longer it takes for the pressure pulse to go up to the plenum and back down to the valve. As engine speed increases, the ideal length for the intake runner needs to get shorter in order to have the pressure pulse get back down and be at the valve when it's open. If the runner is a fixed length, things get more and more out of sync as the engine speed changes from the ideal one for the particular runner length. The pressure pulse might be going up away from the cylinder when the valve is open, or it may be moving towards it but still be too far away to get in there before the valve closes.

The intake resonance flap (flappy) on the S4 and later 928 intake manifolds changes the resonance frequency of this tuning. It's basicly like having an intake manifold that's tuned for a low RPM, and then having it switch to one that's tuned for a high RPM when you need it. That's why the S4 and later cars have two torque peaks on their dyno charts. If you've got an S4 or later car, and have felt a surge in acceleration at about 3,800 RPM or so, that's the flap changing position.

The BMW intake runners are variable in length, so they're continually adjusted to the current engine RPM. That should make a nice flat torque curve over the whole RPM range.

AO

Mike:

All I can say is WOW! You know waaaaayyyyyy too much about his stuff. But don't get me wrong, I love it! That write up is awsome! I never thought the intake could be so complicated and could result in that much performance gain (for the BMW). Great job! <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />

slate blue

I stop all the arguement and dyno test my car, with and without the tubes. I am going to dyno it to get a baseline to ascertain the benefit of my custom headers.

<img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

Normy

Andrew, I remember describing that effect to my flight instructor back in 1988-

N!

AO

Wow! You too, Normy? I cannot believe the enormous amount of knowledge that surrounds me! I must be the luckiest person in the world.

(The preceeding comment was an electronic attempt to give Normy an, "ataboy!")

<img src="graemlins/roflmao.gif" border="0" alt="[hiha]" /> <img src="graemlins/drink.gif" border="0" alt="[cherrsagai]" /> <img src="graemlins/roflmao.gif" border="0" alt="[hiha]" /> <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />