Greggg

So if we add a small amount of force to the incoming charge of air, it would stablize some of this "bouncing" effect of the returning air...... <img src="graemlins/crying.gif" border="0" alt="[crying]" /> <img src="graemlins/c.gif" border="0" alt="[ouch]" /> <img src="graemlins/crying.gif" border="0" alt="[crying]" />

jpitman

This is fazackerly the same as a tuned length exhaust on a single cylinder 2 stroke engine, except its length is dynamically variable. If you look at the Vortec(sp?) 350 sbc engine intake system (Yukons etc), you will see a black plastic thing which acts as a plenum chamber to improve inlet charging.
But, yes a venturi does speed up the air as it passes, and then it slows down again.
jp

Donald

Better try a large amount of force, i.e. greater than the pressure wave.
jpitman- bon point, ditto Honda Vtec w/ dual plenum intake
Donald

Mike Schmidt

Dyno testing with and without the tubes really isn't a good way to do it. Without the tubes, the air will be getting sucked in through the square cut off ends of the airbox openings. A square cut off tube opening like that will not flow as much air as an opening with a bellmouth on it, like those at front of the car above the radiator. You'd also be picking up air from a different location in the engine compartment, so it's temperature might be different. The way to do the dyno test would be with the stock tubes and then with straight tubes that do not have a venturi.

drnick

changing from the standard tubes to later model insulated and 'unrestricted' ones does improve the engine noise!

Randy V

Wouldn't the car have to be in a wind tunnel to replicate forward movement if you wanted a 'real world' dyno comparison?

Mike Schmidt

Having the car in a wind tunnel when doing the dyno test would give the real world power figures, but shouldn't really matter as far as seeing if there are any differences in the two styles of tubes. In the wind tunnel there would be a higher pressure area at the openings over the radiator, so you could have a little more power than if stationary. The pressure where the air was being drawn from would be the same with either style of tube though. If the car is stationary, the pressure there would be lower than if the car was in a wind tunnel., It would again be the same for either type of tube though, because you're using the same intake location and opening for each. As long as you keep everything the same excecpt for the venturi or no venturi tubes, it should show if there's any difference between the two. Do three dyno pulls right in a row with one style of tube, switch tubes, do another three pulls right in a row with the other tubes, and then compare the two different number three runs with each style of tubes.

chris928

As far as slapping the guy who opened this discussion, my thread is a response to this being mentioned in another thread.

As for my statement about the air speeding up. I am correct. It speeds up only in the venturi. It does not supercharge the induction system. By the time the air reaches the combustion chamber it has less energy than if there were no venturi. As Dr Bob mentioned, a well designed venturi is very efficient, maybe 97%. So in reality, you loose 3% of your flow energy. If that means 3% of your horsepower, that's not cool..

As to my mention of heating, the air will heat through a venturi. Where do you think this 3% loss comes from??? Yes it cools as it expands when exiting the venturi, but that is after it has heated due to compression.

The process of air moving through a venturi is not reversable and therefore not adiabatic (isentropic constant . The entropy increases and you have losses. Minor losses, but losses nonetheless.

So the venturi does choke the flow (just a tiny amount) and it does not supercharge the intake. It seems that the most reasonable answer is the one regarding sound due to the change on the later models. I don't know, but i'm sure it isn't a magical device to speed up the air. You would need a bilge pump for that...

John Struthers

Depends on where the venturi is located in/on the intake in relation to the plenum. Yes it would be charging individual combustion chambers but in principle it is actually charging the plenum then on to each cylinder via - in my case the individual tuned intake runners -spider-.
My point was and is there could be an advantage in making a system that is larger than the stock intake system but smaller than the straight tubes you are planning on. With a properly located venturi, I suppose there is a rule of thumb,or, formula out there somewhere that factors in overall length, I.D. of the tubes, the various turns and bends, combustion chamber CC's, intake valve dia,... .
Unfortunately, I don't have it.
You are welcome to try whatever you want with no arguement from me. Its your ride.
But, hey! Could you explain the 3% loss on say...a simple blacksmith's bellows - small end venturi,large end variable length intake.
Mike,
It all seems so simple, now. Then five minutes later I have to reread your explanation.
Nothing wrong with your explanation - I'm sloooww.
thanks <img src="graemlins/bigok.gif" border="0" alt="[thumbsup]" />
John S.

Steve J.

No need to guess or speculate. Let's see what Porsche says. This is from page 5 of the '81 Service Information Manual (WKD 450.221)

Greggg

Its nice to see how these posts provide multiple sources of information and not just one persons' guesstimation. <img src="graemlins/crying.gif" border="0" alt="[crying]" />

John Struthers

Or two peoples' guesstimations

Mongo

ya know...i never thought there was that much technology in those lil plastic thingies I thought 1 intake tube is just as good as the other <img src="graemlins/oops.gif" border="0" alt="[oops]" />

Mongo

ya know...i never thought there was that much technology in those lil plastic thingies I thought 1 intake tube is just as good as the other <img src="graemlins/oops.gif" border="0" alt="[oops]" />

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

SMTCapeCod

I wasn't sure that this was settled, so just thought I would post it back up.

Happy Halloween!