Alright today I cut the bottom of my air box off like the Darintake mod, however I ran out of time to do all I wanted so this is a preliminary post and when I do a full write up maybe it will get moved to the DIY area.

My goal is to find the most efficient combination of DIY intake parts, this involves, the configuration of the intake/air box, the type and material of filter used and the extent of insulation that will provide a colder intake charge.

I am no stranger to DIY or homemade intakes, I hand laid a one off custom fiberglass air box for my Subaru that flowed better than any aftermarket intake.

I don't know if a custom air box is appropriate for the S4 but we'll see where we end up. The advantage to the Subaru was space, there was room in the engine compartment to route an appropriate snorkel to a fender well to obtain a source of positive pressure cold air, the S4 seems to lack this luxury. The S4 is 10 pounds of potatoes in a 5 pound sack.

I won't get into any technical discussion right now, just comment on today’s work.

My car is a 2001 stock except for an APR Bi-Pipe and a K&N panel filter.

I cut off more than the Darintake on the premise that a larger opening will flow more air, unfortunately the gains were not spectacular, I logged 3 runs stock, made the cut and logged 3 more runs after the mod (one was in English units so I had to discard it)

I made all the runs within an hour of each other, up the same grade in the same highway at the same speed with shift points at the same landmarks. Runs went from stopped to about 90-95mph, as consistent as I could get other than finding an all wheel drive dyno. The ambient air temp was within a degree for all the tests, and the altitude is about 7000 feet above sea level so keep that in mind (I am dealing with thin air)

I have posted 3 graphs, The best stock run, the best run after the mod and then a graph with all runs (the mod runs are red, the stock are blue, you can see the mod runs are consistently higher indicating a slight gain)

The data shows an increase of 2g/sec peak, not huge but a start in the correct direction, I have a lot of ideas left (even a hand laid fiberglass or welded aluminum air box are not out of the question, we'll see)

Butt dyno results: I am not sure if the car seems smoother, it might, boost seems to build slightly faster and seems that throttle response is a bit more reactive than stock, I’ll know more after my 80 mile mountain commute tomorrow.

The biggest surprise is the sound, the spooling is slightly louder but what really surprised me was the sound of the diverter valves. Yes you can actually hear the diverter valves now, pretty well in first and second before wind noise overcomes the sound. With the windows up the car is as silent as stock, some kids heard it and yelled "Nice car!" I would like to hear it from outside as the car goes by, I don't think trying to record it would work, I don't think it's loud enough.

Anyway more to come as I get the time, I would really like to take a weekend and try all the ideas I have, I want to log runs for every variation and see what combination works the best, I think it has a lot of potential.

Mason


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Nice post and nice data. It's good to see someone getting technical on here. I'm interested to hear if a fiberglass fab would work any magic.

 

THE FOLLOWING POST CONTAINS CALCULATIONS THAT YOU MAY OR MAY NOT UNDERSTAND OR AGREE WITH, BE FORE WARNED.

Will do however I went Aluminum sheet metal air box route just because It's cheaper and easier for me to do at the moment. This way I can make a model out of cardboard and fit it before I fab the final one up. Then all I have to do is make untape the cardboard and trace the pieces on the aluminum and then cut and weld, should be easier than the fiberglass to make the prototype.

Also I was thinking about some math and this is what I came up with, please add you input if you think my thinking is off or have any suggestions...

Before the air box mood my car was stock so lets assume it made the 250hp and 258lbs/tq (I understand that this is a factory estimate but for this calculation we'll assume it to be true, I know that the number won't all be exact but without a true dyno they never will be but this will at least give us a comparison ratio that if always calculated in this manner will tell us if we are making more power or less power and roughly how much from one mod to the next, remember don't focus on actual numbers just the ratio between them because everything is relative.)

So stock the average peak air flow through the MAF was 185 grams per second. Lets assume that the car makes peak power when there is the most air flowing into the engine (more air equals more gas equals more power) So lets say that the car was producing the 250 hp and 258lbs/tq when it was consuming 185 grams per second of air. If we divide both the Hp and Tq by the grams per second of air that should theoretically give us grams per second of air per Hp and grams per second of air per Tq. (yes I know that colder air is more dense so for the sake of this argument we will assume that the intake air temp is constant and therefore will cancel it self out of all equations, I know it isn’t realistic but we don't have a true dyno so nothing is realistic, just keep in mind this is a relative compro.)(still with me?)

250hp / 185g/sec =(approx) 1.35 (constant that means that it will take 1.35 grams per second of air to create one additional horsepower)

and

258Tq / 185g/sec = (approx) 1.39(constant that means it will take 1.39 grams per second of air to create one additional foot pound of torque)

alright so this is a base calculation for grams per second of Hp and Tq respectively. (still with me?)

so if we gained 2.75 grams per second of air from the hole I cut in the air box we can calculate the theoretical gain in Hp and Tq. All we have to do is multiply the constants by the increase in air flow:

1.35 (our constant for Hp) * 2.75 (grams per second of air increase that we can attribute to the hole I cut in the air box) = 3.71 Hp

now torque:

1.39 (our constant for Tq) * 2.75 (grams per second of air increase that we can attribute to the hole I cut in the air box) = 3.82 foot pounds of Tq

So the hole in the air box did create a definite gain although at this point we can only speculate as to how much, should be interesting to compare once I try a cone filter and try and increase air flow to the air box as well as eventually (if it is feasible) fabricating an aluminum air box that should be superior to the stock one.

I would appreciate comments, questions, flames or any other input you may have, also has anyone measured or know of anyone who has measured stock air flow through the MAF? Are my numbers close to others? How much more is the air flow if you are chipped? Other mods that have effected the air flow (keep in mind that just like a Dyno results will vary but it is interesting to compare anyway.)

Mason

 

I follow you, and it seems theoretically sound, but not sure about practical application. It's early, let me think on this some more.

 

Just a little FYI on making graphs.

Use the X/Y chart and put RPM or time on the bottom. It makes it much easier to read & understand.



Back to the intake....
Where did you cut the holes? How much did you cut?
The bad part is if you take to much, it will disrupt airflow, and can cut flow.
If the holes are in the wrong place, they can suck in hot air, also decreasing flow.

The Darintake has proven over & over to flow about 20g more then the stock airbox.
Of course when you upgrade the car the flow will change.

The on design that seams to work best on modded S4's in the Racer X-1 intake.

 

Alright if every 1.35 grams per second is approx 1 hp and every 1.39 grams per second is approx one foot pound of torque, why would I divide the increase in airflow? I want to know how many hp and Tq respectively each "new" gram per second of air gained. So I would multiply each new gram per second of air by how much each gram per second of air contributes to Hp and Tq respectively....(does everyone agree that that is the correct method of calculating the ratio of grams per second of air to how much Hp and Tq is produced(not that it is accurate but that the math is correct in guestimating a ratio between the two))

and Yes i have looked at aftermarket intakes, and yes I do feel that they are over priced and I do believe that the Darintake did net something like 20 g/sec but I believe his car was chipped, my car is stock, and like i said this is the first out of many ideas.

here is my thoughts on the stock intake.... more math so hold on....

I am not sure if altitude is a factor, I will try and call Mike at Pure and see what the standard code is to activate a program.

as for the intake, I am not sure how much it can be improved if the car is stock. The biggest thing I have a problem with is looking at the relatively tiny snorkel that runs from the grill to the air box. Both my S2000 and Subaru had some form of after market intake, the S2000 was 3" in diameter and the Subaru's was 4" in diameter. if you calculate the cross sectional area (Pi *r*r) or Pi times radius squared the 3" would have an area of 1.5* 3.14 = 7.06 now this is not a flow rate but is the maximum area that ALL of the intake air must flow through in order to enter the combustion chamber of the engine. The S2000 also redlined at 9000 rpms so you can imagine the velocity at which the air must be flowing through the intake in order to fill the combustion chamber 4500 times in a minute(a four stroke engine has a combustion cycle which is 3600 degrees and an exhaust cycle which is also 360 degrees, which means that it fires only every two revolutions.) The Subaru on the other hand had a 4" intake so we calculate 2*2*3.14 = 12.56 that's 5.5 more square inches of cross sectional area that intake air can occupy. That is a 78% increase in cross sectional area over a 3" diameter intake. Now again this is not flow rate, but that means that the given air that will be sucked into the intake by the engine has 78% more area to occupy. We can compare this to a hu8man trying to suck a given volume of liquid say a can of soda through a tiny straw or through a garden hose. It is going to be much much easier to such the soda through the garden hose and much more efficient as well, which means it will take less time to suck the can of soda using the garden hose. I look at the stock snorkel on the S4 and it seems smaller than a 3" intake. I am not sure of the exact measurements but it seems like it is the choking point of the stock intake system. One final point I would like to emphasize is that the S2000 and Subaru were both N/A which means that they had to basically suck all their air into the intake from the negative pressure created by the down stroke of each piston. The S4 id Bi Turbo which means the turbos compress air in the intake(after the air passes through the compressor housing on the turbo of course) and the air is actually being forced or pushed into the combustion chamber. This means that there is more air being consumed by the S4's engine than a natural aspirated car and that all of that air must flow through the relatively small snorkel that goes to the stock air box. This in my mind means that eliminating restrictions such as the snorkel are even more important on this car because of the increased air flow that the turbos present.

Again please let me know if my logic is flawed or if you have any other comments or questions...

Mason

 

2k S4....

this is the only problem I have with that intake, it doesn't address my biggest concern, this seems like the absolute choke point for the stock system and this aftermarket system: notice that this intake also relys on the the reletively small opening in the grill to flow ALL of the air that will enter the combustion chamber..... (notice the red circle)

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First off, you can't count airflow & correlate it to direct HP. It takes more then air to get HP.
You have to take into account fuel, ignition timing, valve timing, boost, etc...

The stock small snorkel flows a lot more then you think, it's placed in a low pressure area.
There are a lot of high HP K04 cars running with the stock airbox, or the RS4 one.

The X1 intake is also open behind the headlight. In test has proved to have some of the highest
flow. But most people using it are heavily modded, and have a much bigger MAF.

Most tuners seam to like the stock or RS4 airbox. The consensus is unless you have K04's
there really isn't much to be gained on the intake. The RS4 box has a flapper in the side
that opens under WOT, just think darintake, with a rubber flapper.

Forgot to add...
When doing CFM with car intakes, you have also account for the air pressure around the part.
That can add or reduce flow. There are other factors as well. Really while doing the math is nice
it only tells part of the story, you really have to test each application on the car & do logging to
see what the best compromise is.

One last thing, don't forget the MAF likes nice smooth airflow. Some times putting to much high
turbulent pressure at the MAF will make the car run worse performance wise.

 

wow - kudos to you socal - and great input here as well 2k. I'm no auto techi myself (pure driver here!)..so admittedly I just skip the numbers part and try to pick up on the jist of what your getting at - and I think this is an area where we might get some gains with our cars. (having no stock airbox when I purchased the car used) I went with an open element EVO and have been fairly pleased with the performance (and engine bay sounds!) - but the very hot engine and limited air entry to the filter/MAF do concern me and leave me wondering if I could get noticable performance gains from just getting more/cooler air. I really like that cf heatsheild with the X-1 intake - is that fitable to my EVO intake ya'think? I've also wondered how one might get more cool air into their in general and am glad to see the likes of socal expireamenting in this area. Any chance of some pics?

 

I know I know I know

I appreciate your input and understand how flawed my data and calculations may be, but frankly it's the best I can do with the resources available to me.

Not sure exactly how much you read, I said many times that this is not sufficient to calculate Hp and Tq only that if I always use the same formulas I can compare the data from each different modification and see if there is a net gain or loss. Audi has spent tons and tons of money on the engineering of their products, I will never be able to compete with them or any after market intake company, I do this because I ENJOY it, not to make money or re invent the wheel so to speak. I am looking at 40k in student loans so if I can gain a few Hp for free or relatively cheap then I will take them.

I understand the concept of turbulence I attended Cal Poly Pomona and attended the State Science Fair three years, one year with a full size working wind tunnel. No matter if the intake is stock, RS4, Aftermarket or home made the concept is the same:

The more air the more fuel the more power. How you deliver the air to the combustion chamber is also important, if the air flow is smooth and less turbulent then it is more efficient which means delivering more air in less time. There is an absolute amount of air the engine will draw even if the intake is completely unrestricted, this means that it can only suck so much, the key is to make whatever intake you have installed on the car get as close to this point as possible while still sufficiently filtering the air as to not increase wear on the engine from dirty air. Ideally we would want the coldest, most dense, least turbulent air flow possible. However, this is impossible due to the fact the 1. the engine will always create heat and that heat will always heat the incoming air. 2. as air increases in temperature it becomes more excited and less dense (intercoolers help cool the air after compression by the turbos) and 3. the air box is inside the engine compartment creates poor air flow characteristics due to the fact the the air has to basically navigate a maze from the atmosphere to the combustion chamber, there will always be turbulence, and unfortunately there is not a whole lot of room in the engine compartment to minimize the path of the intake air (for example if the headlight was removed from the the passenger side I am sure a HUGE gain would be accomplished but other than a drag strip or track day that is just not plausible)

keep the comments and thoughts coming, two brains are better than one and all of us are better than just me...


Mason