Mike Trepicchio

Hey i see in some pics of engines of 928's that some of the owners cut off the intake tubes from the airbox. I understand that some people make the tubes larger ect. But i was wondering, is it safe to just hack most of the tubes off? Bare with me here im only 17. I went to the 928's specialst page (www.928gt.com) and if you check out an engine on the page from a red 928 you'll see what im talking aobut? Does this improve performance? If so noticalbe? HELP!!!

WallyP

Sorry - misleading photo. The inlet tubes were removed for the photo to show the engine compartment better. David runs with the tubes on.

Removing the tubes would lower performance by feeding the engine hot air.

Mike Trepicchio

Is there anything i can do to, futher help my intake, besides the K&N that i have? Does anyone have any directions on enlarging the tubing? Is it hard?

928 Maniac

I agree with you Wally. Like your's, I've seen many pics of 928 engine bays with the tubes removed and I've always assumed the reason being that it would show-off more of the detailing...

But that's that. Common sense tells me that Porsche designed the tubes for a reason. The narrowing of the tubes around the area where they pass underneath the X brace creates a `venturi effect', or am I wrong?

However, what I'm getting at and would like your opinion on, is whether there are any worthwile mods we could do enhance performance through the tubes...

My novice questions are:

Firstly, the tubes are supposed to feed in fresh cool air, right? Well, why are the intakes situated on top of the engine and above the radiator, where the air is already hot and somewhat turbulent?

I realise that the system is not designed for, or supposed to be a `ram air' system, but when I look at it the design makes me wonder that Porsche engineers might've overlooked a very important performance cue at the time...

I've seen two pics on tube converions: One was a Koenig design with vents cut into the hood that lined up with the tube intakes, (makes sense to me) and the other was by some British company (can't remember exactly where I saw it) where they changed the entire airbox and replaced the tubes whith two cone-like K&N filters either side of the box (hot air - don't make sense) however, they claimed + 10 - 15hp. Hard to believe.

Anyway, every time I open the hood of my 928 I wonder about the horses lurking in these tubes...

Have you ever thought of it?

Richard S

This subject has been discussed extensively in the 928 e-mail list. Basic results seem to indicate that keeping things as they are is the best course. Those who have tested the temp reading with the tubes find that the air doesn't heat up much while the car is moving. As far as ram-air effect, that doesn't really help until you are talking VERY high speeds. Mainly what you want is fresh air. Devek modified their white car to pull in air from the base of the windshield, that being a high pressure area. But they stress that any modifications that might allow water (ie when its raining) to enter the intake may result in destroying the engine. They will not sell this system to customers for this reason. Later 928's used insulated tubes, some owners have modified their old tubes with insulation. I've seen the ducted hoods, but don't know what effect they have. There doesn't seem to be a solution that would offer HP gains to justify the modifications.
Richard
79 A/T
80 5 spd

DaveG

Mike,
There's a site called "My 1987 Porsche 92824" (not sure if its one word) this guy modified his intake tubes, he walks you through the process, along with other mods
Dave

Jay Wellwood

That guyt would be Tony H - his site is here on the rennlist-

scroll down for 'v1uhoh'. look around...you'll be glad you did - I promise!

Darren '85S

Can water in the intake really do that much damage?

Richard S

Liquids can't compress, too much water in cylinders will cause connecting rod failure, possibly other major problems. I don't know how much is required to cause this, but Devek seems to imply that it really isn't that much. Probably a bit less than the volume of a cylinder at TDC, but that's just a guess.
Rich.

CSBEAUMONT

Just a note,
I would strongly not recommend modifying the intake tubes. From an engineering standpoint and just looking at the tubes on my 83' S I think porsche did a fantastic job creating these. A couple things about compressible flow dynamics(bare with me and please correct anything I slip on John D), laminar flow is not a good thing when it comes to volumetric flow rate(which is inherent to engine power). When a laminar flow of any viscous fluid(air) travels through a pipe a good assumption is that there is no slip at the wall(meaning the air at the wall acts as if it is standing still, friction from the wall keeps it from moving), as you move out to the center of the tube the air will move faster until you reach the center. It is relatively parabolic. (simply incompressed air at this stage). When you add ridges and things in the tube you create turbulence(good thing). This turbulence increases the Reynold's number of the air and after a given point it becomes turbulent. When this happens, the boundary layer of stagnant air at the wall surface gets thinner until the velocity profile looks rather square rather than parabolic, all of the air except a small thin film near the edge is moving at the same relative speed.(higher volumetric flow=CFM). Keep the ridges, anyone who has ported an engine knows that the ports need to be properly "roughed".

Also, my car has the "venturi tubes" in the intake. Under vacuum, you can only "suck" so much air through a tube(velocity wise). When you design a venturi like they have done, you look at the operating pressure differences and then there are area differentials you use to determine when the air flow will reach supersonic. Unless the flow goes sonic at the narrow point in the venturi, you will "choke the air flow", this will not cut off the air flow but you will plateau at a certain velocity. This venturi is modt likely the German Engineers finest achievment in terms of intake modifications. Unless you are going to precharge the air, I would leave the venturi and the tubes in place. I don't want to discourage anyone from doing modifications, but I marvel at the forthought of this intake system.

Ed Ruiz

Let's not over analyse the wonders and benefits of the OEM intake tubes. Even Porsche abandoned them for insulated tubes for the GTS. I too have installed insulated tubes, and the GT still runs like a scalded cat.

All that air gets constricted at the MFS and then the throttle body. IMO, improvements to air flow before those constrictions would be minimal at best. Reducing the air temp would be more beneficial, IMO. Apparently, Porsche seems to have thought so as well, at least for GTS models. YMMV.

[ 07-05-2001: Message edited by: Ed Ruiz ]

928 Maniac

OOOmmphh. Were might be missing the point here. You've heard the terms `Forced Air Induction' and `Ram-Air'... Okay, the goal is to get fresh cool air to the airbox as quickly and undulated as possible, right? After all, we're mixing fuel 'n oxigen to create combustion... It's the principle behind carbs, injection, blowers and turbo's - hell, even jet engines rely on this concept!

So let's travel along with the air flow and let common sense prevail...

Open the hood and look at your 928 engine bay. Look at where the intakes are and how far away the airbox is. Now think about the air going into the tubes - already we're picking up dirty air at the intakes (hot and turbulent), but still, the air has quite a distance to travel...

So what do them clever Porsche designers do? - They create a venturi effect to excellerate the air midway between the intake to the airbox. That's like using the moon to slingshot you to Mars. The problem is that by the time you reach Mars your spacecraft have lost momentum (airflow), but now, those same clever Porsche desighers make the air do a U-turn, put it through a filter and still make the engine perform...

Naaahhh... Unless you're a complete desciple of brandname myth, this Porsche airflow design is not making sense... I don't have the answer but the design is questionable to me. Some Gemballa conversions have ducts in the hood that lines up with the intakes, and some Strosek desighs have intakes closer to the airbox...

What could we as common sense enthusiasts come up with?

Ed Ruiz

I'm not sure that it really matters that much. I've been told by several engine tuners that the 928 engine would benefit greatly by having SMALLER (but ported) inlets to each cumbustion chamber. The smaller inlet would actually help charge the cylinder as the piston reached BDC and the intake valve closed. If this is basically true, any increased amount of air-flow before the throttle body would almost be irrelevant.

Since I am not a mechanical engineer, I have no experience or knowledge to either agree or disagree with these engine tuners. I do know that Lou Ott has done temp measurements before and after installing insulated air tubes. After installing the insulated tubes the temps were about 14 degrees F cooler at idle and about 5 degrees cooler F at WOT. YMMV. Based on that info, I installed insulated tubes on my GT. If there is no benefit, it only cost me $10 in hardware from Home Depot, and I can always put the OEM tubes back.

[ 07-05-2001: Message edited by: Ed Ruiz ]

CSBEAUMONT

The one thing that these tubes are good for is the fact that they are insulated. That is good, the fact that you are drawing in cooler air means that in fact it should be denser and therefor you should have a higher volumetric flowrate. but 14 degrees F at 120 degrees F is really only a theoretical 2.5% increase in the density of the air.(remember temperature differences are measured off an absolute scale, 120 degrees F is 580 degerees Rankine). What I would do is wrap the pre-existing tubes in insulation like the mod-designers, but keep the venturi. Using the venturi can actually cause a sonic acceleration which may ram air into the throttle body. When you deal with compressible flow issues, you can't look at the venturi horn as being like a moon sling shot. If the flow reaches mach at the narrowing of the horn, shock waves build up on the backside and cause a pressure build up. This is how Turbine engines propel supersonic aircraft(converging/diverging nozzles). I would in fact insulate the tubes but I would leave them intact, I think they may have done some serious research into them.
Regards BSME Thermodynamics and Fluid Flow

Kaz

Saw a 928 outside of a bodyshop with the entire engine exposed due to extensive body damage. First thing I noticed besides that were the insulation wrapped intake tubes.