Ed Ruiz

I think (don't know for sure) that the GTS intake tubes do NOT have the constriction. I think they are straight tubes that are insulated.

Please, someone correct me if I'm wrong. If I'm not, then Porsche apparently felt the constriction was irrelevant.

My straight intake tubes don't seem to have any ill effects on my GT's performance. YMMV.

Jay Wellwood

Ed's right about the GTS tubes not having a neck down feature.

I examined one closely at the SITM in May. The GTS tubes are basically a spriral spring with a plastic boot on each end. The spring is covered with insulation (black foam, obviously heat resistant).

I had intially thought that the neck potion on the all plastic tubes was intended to creat a venturi effect, thus providing for 1) an increase in velocity, and 2) a possible corresponding drop in temperature. This is based on thermodynamics (probablly in error - but hey, I work in Nuclear Power - so...)

Since the GTS tubes don't have the venturi, I would tend to think that the Porsche Engineers threw that theory out, and decided insulating was the best approach for optimum air flow and desity.

In retrospect, I have thought that an intercooler approach might be the best at an honest attempt to lower intake air temperature and increase density of the air mass. Now how to devise such anitem, and make it workable, efficeint, and above all reliable is another story.

DoubleNutz

If I may "chime in here for a minute. I would like to keep this simple for ya. In addition to Wally's answer above as it relates to heat, the air tubes serve an additional purpose. If you are fortunate enough to have an earlier brochure it depicts in there some rather brilliant engineering on Porsche's behalf.

The absolute lowest point of air pressure on the (when it is moving in a forward direction, of course) 928 is just before the intake of the airtube inlet at the front of the car. This creates a rather efficient ram air inlet. Additionally, there is a functional and purposely built features built in the air tube like its ridges and a venturi (uh- oh, I said the "V" word) at the front of the airtube to step up the velocity of the air going into the airbox. Not, to mention the cooling port for the timing belt. When it came to this engine, Porsche engineered everything into for a reason. that is part of the reason why cheap horsepower gains are very difficult to find on the 928.

Ed, I beg to differ in the value of thermally insulated air tubes. Louis Ott has done some extensive testing on thermally insulating the air tubes and found no significant benefit in horsepower or performance when the car is moving forward. The air is never in the tube long enough for the insulation to make a real difference. However, if the tube is completly removed, drawing air from around the higher temperature engine block will have a severely negative impact on the engines performance.

The Devek white car does remove the tubes from the car, however, fresh air is also drafted, from behind the windshield (another low pressure point on the 928). The draw back to that...RAIN! Nothing like a water flooded airbox to "jack up your day day!"

CSBEAUMONT

Hey Pat,
I tend to agree with you. I think Porsche did a bang up job with designing it. I try not to hang on brand names but the "looks" of the system seem to imply some funky design. I think they did design the Condiv nozzle to actually ram the air at a certain design point. I can't tell you what RPM that would be but under that, it has no effect whatsoever. If it is a venturi the velocity of the air would be the same on either side if the diameter of the tube was the same. But if the air reaches Mach 1 at the middle of the constriction, the air velocity and pressure as well would be higher than before the constriction and quite possibly a higher pressure than ambient air.

My question is whether the GTS has a dual stage plenum system similar to the newer lexuses and high performance engines. This may be why they abandoned the "venturi".

Regards and always a little confused,
Curtis

Ed Ruiz

I actually met and talked with Lou Ott at the 2000 928OC-IC in Wichita, KS. He told me that the insulated tubes DID make a difference.

Apparently, Porsche came to that conclusion when they changed from constricted tubes to insulated tubes for the GTS. YMMV.

~ Merry motoring ~

928 Maniac

Now this is probably gonna sound ridiculous, so please excuse my ignorance. Has anyone ever thought of contacting the Porsche factory and ask them, or is that not possible?

Come to think of it, is there any way a group of enthusiasts like Rennlist could communicate with Porsche regarding technical aspects, or is it something that's just not done at all?

Ed Ruiz

Go for it! Call them at 1-800-PORSCHE, and let us know what they tell you.

~ merry motoring ~

Dave H.

so will the GTs tubes retrofit into an S4 like you can with the GTS oil breather system? whould there be any point?

Ed Ruiz

I'm not sure I follow you. If you are talking about the holes that fit over the cam-belt cover breathers, YES.

Dave H.

what i waas getting at was that on a GTS the oil breather system between the cam covers and oil filler is different from the S4/GT's (there's a diagram in the maint. manuals). supposedly improved breathing, it can be retrofitted to the S4/GT's.

i was wondering if the S4/GT would benefit from having GTS air intake tubes installed as well. if Porsche redesigned the tubes for a reason, maybe we can take advantage of it also...

Ed Ruiz

Interesting. I had not noticed the difference. I do know that early GTS models have a slightly higher oil consumption rate than S4s or GTs. I wonder if the "improved" breather is contributing to the additional oil consuption?!

~ Merry motoring ~

DoubleNutz

Professor Ott Speaks and has this to say about the insulated vs non-insulated air tubes:

P AKA sharkdog

The 928 is a high performance luxury car, and part of the function served by the air intake tubes is to "attenuate" the sounds of the motor, believe it or not. In the recording studio business, people know that if you have an air duct that is making a bit of noise and youre picking it up on 32 tracks, you simply extend the duct run, and tie it in a knot like a pretzle, and the sound will be attenuated due to the route the air has to take, flow being the same. The intakes of the 928 have enough variance to (help)attenuate "undesirable" noise, but most of us would probably like the sound anyway! I also agree with the laminar air flow, although I don't view the intake tubes as too much of an engineering masterpiece. They provide basic clean air, attenuate a tidbit of sound, and look cool.

The cold air is a relevant point, but I don't know how much temp difference ther would actually be by running a thin wall piece of plastic duct over top of the headers and valve covers. Seems like the air temp would almost be the same.

Porsche has traditionally engineered at the limit, and they've made some major changes from time to time, that will provide just a smidgen of performance. I don't think it's particularly sacred about the intakes, but everyone knows it's better to get clean air to the intake, but many many many cars do very well with a carb sitting on top of a hot engine compartment.

Doing an insulated air intake tube might help, but I think it's more of a state of mind, rather than anything you'll be able to measure in the way of increased performance. The Devek ram air system, off the bottom of the windshield, has been proven to work with a lot of differnet cars, including the "cowl induction" big block muscle cars of the past. Running one in the rain would be risky, without doing some kind of a dorade box (you sailors will know what I'm talking about) to pull the water out of the incoming air stream.

P "sharkdog" Nashvegas, TN

Ed Ruiz

Those of you who have seen either Lou Ott's insulated intake tubes or mine know that they are more than just a thin cover over the tubes. As Lou did, I also isulated the area above the radiator and above the air filter-box. As suggested by Lou, I also raised the filter-box to get it a bit farther from the heat source.

The results have been astounding! The GT can now do 0 to 60 MPH in 5.7 seconds. It usedd to take 5.71 seconds. If I complelety insulate the engine, I can probably get it below 5.7 seconds.

P.S. It now runs 0.1 degrees F cooler than before, and the AC is now causing icicles to
form on the center vent.

[ 07-11-2001: Message edited by: Ed Ruiz ]

Jim Nowak

I think sharkdog's summary is correct. From a reliable source: "Porsche changed to insulated tubes because of noise ordinances passed in Germany and the 928 GTS needed sound deadening insulation around the intake tubes to meet the new noise requirements."

On a seperate but somewhat related event, when I took my car from 4.5L to 5.4L the engine and exhaust became significantly louder. In fact, much, much louder than before. I've been asked by many "car nuts" if I had a big block chevy under the hood.

Jim Nowak