MrTomT

Does anyone know how the standard 928 venturi intake tubes compare to after market tubes that have the same diameter the whole way up to the air-box?

Did the GTS do away with the venturi setup?

& has anyone found a better replacement for the factory tubes?

Thanks in advance,

Tom

Mike Frye

Hey Tom,

There have been a few discussions about the 'venturi effect' of the intake tubes and I think the common opinion is that there really is none. The shape of the intake tubes is completely negated by the turn at the end, and by the air going through the filter at the back, then going through all those other bends.

The Venturi effect (according to wikipedia) is " the reduction in fluid pressure that results when a fluid flows through a constricted section of pipe. The fluid velocity must increase through the constriction to satisfy the equation of continuity, while its pressure must decrease due to conservation of energy: the gain in kinetic energy is balanced by a drop in pressure or a pressure gradient force. An equation for the drop in pressure due to venturi effect may be derived from a combination of Bernoulli's principle and the equation of continuity."

I think it's just too far removed from the intake to be a factor.

The shape of the intake tubes, according to Projekt 928, is more to reduce the noise of the intake than for any other reason. People have used straight tubes with very little effect on performance and noise from what I've heard. I also think there are some Mercedes Benz intake tubes that work. I'm sure you can find something if you look around for intake tubes and MB

Mrmerlin

another side benefit of the intake tubes is that if your having belt problems simply look at the air cleaner, if the tubes are connected to the timing covers then they will suck the belt pieces into the filter , this will let you know you may have a potential problem with the belt run

soontobered84

928Rus has replacement tubes that are better looking than the originals IMHO

heinrich

The intake on our cars is extremely volatile and easily affected by airflow pattern changes IF you are normally aspirated and IF you care about that last 20hp. The tubes won't give you 20 but they, along with good airbox etc will.

Jim bailey - 928 International

The neck down was to reduce intake NOISE. The Very expensive GTS tubes are covered with foam to absorb the noise. There is little or no increase/ decrease in power .....this is one very dead horse

928SS

I heard that wrapping them in bubble gum wrapper helps give 15-30rwhp. it has something to do with insulation and providing less internal resanonce with the ventruical air flow calibration interaction with the MAF.

mark kibort

PLUS,the tubes leak and much of the air flow comes from around the filter and where the intake tubes attach to the air box.

I use 3" brake tubing, but wish I had something smoother and better looking.

mk

Mike Frye

You forgot about the effect of the laminar flow caused by the MAF screen!

mark kibort

You mean, "lack of " laminar flow from screen use

mk

hacker-pschorr

Don't forget about keeping a pair of hedgehogs (see avatar) above the air cleaner to run around and help the tumble and swirl of the air going into the MAF.

928SS

oops...

imho, the gts tubes are the nicest looking. but they are expensive and will eventually crack. after mine cracked, I wrapped them in black duct tape, then coated that with "dip it", a rubberized coating to make them look smooth. seem ok for now.

if that starts to fail, I might just keep the nice fitted ends and slap some kind of tube in between....

Mike Frye

Oh, crap, I thought you got the nice laminar flow from using two screens, but you get burbling from using just one. Kind of like a modulator/demodulator.

Now I have to check and make sure I've got the top and bottom screens in the right place and facing in the right direction!

mark kibort

make sure the MAF is facing the right direction!

there is no real good from the screens. I would like to see a true dyno run from the screens but it has the same effect as a poor efficiency venturi! nothing but bad can come from it, except tubulence that the maf sensor wire gets, which can be good or bad. What i found as well as mark anderson , was that making the larger MAF body, of course without a screen, actually cleaned up fuel air curves.

anyway, think about screens. they are so effective at making air flow INefficient, that when the Harrier jet was first designed with them on the inlets, it couldnt even get off the ground! mass flow reduction was pretty dramatic caused by the turbulent flow and pressure drop across the screens.

I guess the main reason for them, is to keep a hot wire piece from entering the engine, or something that gets passed the air filter, to get into the MAF. bmw uses some pretty substantial plastic formed screens which seem to give good gains when removed.

mk

MrTomT

Thanks for all the info.
I've found a few different options on ebay for 90mm tubing. What's the temperature in the engine bay that the tubing needs to withstand?
Some stuff only handles 150 deg C, while other tubing is ok at 300 deg C.
Is vacuum bar an issue?
p.s. 928rus seems offline.
Thanks again,
Tom