I wonder about the stock airbox. Is it very restrictive since everybody seems to go with other solutions? I like the stock look but is there a limit as to when it is time to go to other alternatives?

I know vitesse has a maf version hidden and attached to the stock airbox which is nice but is or would there be a big difference to use an open filter like everybody seems to be doing?

Would be nice to know if anybody has some real data showing if or when stock gets restrictive.

Regards
Peder

 

I don't have airflow data, but the actual barn door is physically bigger than most MAFs. The restriction appears to be in the snorkel, in particular where it fans out and attaches to the filter box.

The more immediate problem is the voltage curve of the stock air flow meter. The stock AFM maxes out at 5v at a relatively low flow. Once the barn door is open all the way (at somewhere around 300hp depending on temp, alt, etc.) then the DME will have no way of knowing how much more air might be coming in. The DME will see the same 5 volts whether the motor is consuming 300 or 600hp worth of air. Some folks have tried to stiffen the spring to change the curve, but then you have to remap the whole curve, so a pretty big undertaking. Others have added a MAP sensor and blended the AFM and MAP signal to create their own curve, but that also requires a full remap...

You might send John a note at Vitesse and ask him if the stealth set up compromises flow at all. My guess is that it will depend on whether you keep the factory snorkel in place (without drilling holes in strategic unseen places for better breathing). The corvette guys used to drill holes in the top of their (similar) filter box and pick up 10+hp. Venders would actually sell filter box tops with symmetrical/artsy holes...

 

Thanks for your reply. It is the stock airbox/snorkel I am wondering about. I just think it looks wrong without the airbox leaving a big hole. So if as you say the snorkel is the restriction maybe a setup like the 968 TS with a naca duct and a cut down snorkel would be great? That way the airbox can be retained. Also I have thought of why Porsche with the stock j boot attached the hose coming from the AOS right at the middle of the inside bend of the J boot were the air is at it's slowest? They also used a valve in that AOS hose. With aftermarket setups that AOS hose attaches at a point were the air is moving alot faster and that valve is not longer used. Would that not be a good thing to replicate even on modified cars? I mean would a 968 TS setup and J boot/AOS hose and valve location not be the best way to go and still have a very stock looking setup?



Regards
Peder

 

https://rennlist.com/forums/944-turb...k-air-box.html


This snorkel is nice, I wonder if it is still being made..

 

For what it's worth, the flat elongated opening at the box just doesn't have much area. The snorkel itself could be a foot in diameter, but if it has to flow into the box via its stock opening, that will be the restriction. Some data here:

https://rennlist.com/forums/944-turb...intersted.html

Drilling holes in the top of the air filter lid would solve that issue though, if you don't mind the look, just like the corvette guys do.

 

I like the idea of the bigger snorkel voight posted about (thanks!) but if it is as you say that the opening to the airbox is the issue then the 968 TS solution must be an effective way with the naca duct straight into the airbox? I can see myself doing that to retain a clean look that will still look factory. Ofcourse I wont cut up up my Turbo Cup hood but use an aftermarket one.
It would also be nice to see if that big german made snorkel isnt good enough. The opening into the airbox seems bigger than stock.

 

I've never held one in my hands, and Raj can probably confirm, but from the pictures it looks like the 968 Turbo S used the 951 air box with the same small-mouth opening. They did away with the snorkel part, perhaps just because the fenders are all different. I don't think those motors made enough power to be limited much (if at all) by that air box.

 

Cut out/drill the underside of the really thin part of the snorkel. That way it looks stock but can suck in more air.

I'm sure the intercooler affects things but my logging shows intake air temp is ambient once the car is in motion, with an open filter element (no air box).

 

FWIW When I contacted Vitesse I was told they no longer make the Stealth Kit. I have a Vitesse unit and plan on making my own stealth install. I wondered about making a custom lid that allowed more air in as well. I will be watching this thread with interest.

 

I remember from playing with a piece of string and loosely evaluating how the section of the snorkel varies that the junction where it meets the airbox is actually where its section is the largest. The only advantage of the snorkel is that it will draw air from outside of the warm engine bay, though the upgraded version of ETG has been proven to yield gains on engines that are not "overly" upgraded.

I can't remember if the inlet of the airbox is larger than the rectangular area where it connects to the AFM. Could be calculated easily...

 

So I guess on an engine that is not modified beyond 968 turbo S/RS levels 350-450hp the stock configuration will be ok?!

Also apart that you may loose some power compared to a cone filter setup is there any other disadvatages?

 

I watched Jon Milledge run a stock airbox/snorkle at 400 hp on his engine dyno.

 

951 AFM inlet hole is ~60mm x 50mm rectangle.

 

Here's what I posted back in the other thread (which supports your recollection):


The actual opening looks to be roughly the shape of a right triangle of about 1.5" x 5.75" x 6". That means the tape would measure about 13.25 inches around, or close to the 14" you measured when you add in the wall thickness. The area of a right triangle is equal to the base (1.5) times height (5.75) divided by 2. All of that means the cross-sectional area of the opening is roughly (1.5' x 5.75")/2 or about 4.3 square inches. A round tube with 4.3" inches of cross-sectional area would measure just about 2.3" in diameter, which is pretty close to the 2.2 inches you measured for the round part of the snorkel. Crazy german engineers...

I defer to someone smarter as to whether triangle-shaped and circle-shaped openings flow the same amount of air if they have the same cross-sectional area. They didn't teach us that in geometry class...

 

...and the AFM inlet at 60mmx50mm works out to about 4.7 sqin area...

re:shape, there is a concept called hydraulic radius which is for approximating flow area through a square pipe vs a round one...basically the largest circular section that fits inside the square, so a 60x50mm rectangle would fit a 50mm diameter circle inside at most, d =50 means r=25 or 1 inch, means A=3.14sqin. so the AFM is still the restriction in the system...