Stock airbox vs aftermarket filters. Does anyone have dyno results?
 

So, i have been told by several members, and read in other threads, that the stock air intake is restricting the breathing potential of the engine. Being a skeptic to any information i get served like this without any backing information, i would love to see some dyno results from a back to back test of a stock airbox, vs open air filters or other solutions.
Looking at the intake system, i cant for the life of me understand how any airfilter box can cause more restriction than the intercooler that has to be on the intake no matter what mod i do, even if they are on different sides of the turbo.

Not dyno results from people who sell them, i dont trust that at all.

Has anyone done any trials like this, in a more scientific way than by a butt dyno?
I have tried youtube, but the only thing i can find is the more general mythbusting from "Mighty car mods"

There are a test done probably around 10 years ago. However, even the people reporting the results didn't physically do the testing so they were a little skeptical if the results were doctored to favor certain aftermarket companies.

In short, there is no real performance reason to change out the stock airbox on the FD. There are vanity and space reasons. On space, some aftermarket intercoolers will not fit with the stock airbox (whether it's a larger SMIC or the piping for a FMIC). On vanity, some people just like the look of open air intakes. Nothing wrong with that, but if you are looking for a performance gain spend your money elsewhere on the car.

except for changing the intake location, either to the 99+ or the cheap bastard spot, so that the intake doesn't draw air backwards through the intercooler, the stock airbox flows enough air to do ~370rwhp, and not be a huge restriction

Also, remember, don't over think it too much. There's no magic filter. If it flows better and has the same surface area (like a K&N drop in), it filters less, and vice versa.

Its an in take on a turbo car... Meaning what little gains or loses between different types of intakes is nullified by the turbo setup.

The main reason to change out the stock intake is for room. Given that, I only have two issues with the stock intake:
1) it doesn't get direct air into the filter (easy fix with Adam C's cheap bastard mod)
2) the size

However the thermal properties of the stock box are by far better than any intake out there. See for yourself:


Intakes

Make sure you read the comparison....

"Sonny" did a back to back dyno test. He used a stock intake vs a stock intake with my "Cheap Bastard" airbox mod. There was an additional 12 rwhp with the modified airbox. The dyno is no longer visible on the thread, but here it is:

https://www.rx7club.com/3rd-generati...-3-ecu-362737/

I've heard this can happen at WOT. But even if it's not drawing air backwards, being able to draw ambient air without drawing it from the intercooler duct means that air pressure is increased at the front of that intercooler...all else being equal. That means better IATs...all else being equal.

I think he was comparing hp at 7500 vs 5500 rpm and only lost 12 hp with the cheap bastard. It breathed better than the stock one at high rpm.

No. He lost 12 rwhp when re-installing the stock intake. The modified intake provided 12 additional rwhp. The additional hp gains were in the higher rpm range. This was to be expected, as the unmodified stock airbox can only flow so much air.

"The third-generation RX-7 had come off Mazda's rigorous development test programs on the bench and on the demanding Global Road Circuit section of the Miyoshi Proving Ground with flying colors. Yet, there was one arduous test left undone. When the car was taken to a race track near Tokyo known
for its tight turns requiring short bursts of speed followed by fierce deceleration, the pride of Mazda's rotary rocket team quickly cooked its powerplant when pushed to the limit. Subsequent investigation revealed that air temperature at the entry area had risen as high as 50 C (122 F). Fresh air for the engine's consumption was taken from the single intake that also fed to the air-to-air intercooler. On wide-open driving, air flow reversed its course from the intercooler and went straight into the engine's intake.
The intercooler was acting as an inter-heater! In the updated RX-7 with a designed fascia, fresh air is taken through a separate, dedicated duct guided by a newly installed air-guide. Air temperature at the engine's intake entry area has been halved to about 25 C ( 77 F), which adds about 7 kW (10bhp) to the output."

from the 3rd article down, the SAE paper, '99 FD3S

Stock FD intake robs power through restriction to flow and high intake temps.

I was running 10psi no creep with the dual MBC mod, downpipe, highflow cat, catback, SMIC, and K&N drop in filter on my FD.

Added Blitz dual element intake with hard pipes and minimum boost was 14psi no matter what I did.

The aftermarket intake could flow a lot more air.

In some situations it was hotter heat soaked air straight from the engine bay.

Sometimes it was cooler air (from not being drawn from the engine bay through the hot IC).

I would have been even better if it was high flowing and had a path to cooler air (I later bought an M2 box)

All you have to do is look at the limited production homologation special the RX-7 SP that was built to win at Bathurst production car races.

If the stock air box arrangement worked well, they would have kept it.

Few systems were changed on that car to make a big difference. Exhaust, intake box/pipes/IC & duct, a vented hood, front bumper, functional rear wing, large fuel tank, suspension, wheels & tires.

http://img.photobucket.com/albums/v2...ebaycarbon.jpg

So...I'm not sure what your point was in quoting me. I agreed that at WOT air flow reverses itself through the IC and into the duct. My point was that even if this isn't happening, higher pressure at the face of the IC is available with an intake design that's not scavenging air from the duct. And with higher pressure at the face of the IC, IATs should naturally improve.

we think that the IC works backwards because it is documented in the factory literature, its not guess work.

The stock airbox is known to steel air from the intercooler. The "cheap bastard" intake mod will produce similar results to a proper aftermarket intake. The main benefit of eliminating the stock airbox is to make room in the engine bay and for looks.

They also did filter comparisons during the development, finding stock the best compromise of flow and filtration in the SP box. When running that pressurized system with a K&N, we did find a bit of compressor erosion, despite being properly oiled.

j9fd3s point makes me scratch my head with the number of these cars running cone filters on the discharge of big arse ICs, expedient but dumb!

Obviously the temperature of the air supplied to the turbo is important, and some unducted systems actually heat up the air. That isn't a good thing. However, on a turbo car intakes are more complex than people realize. There are three different things at work here that you can't see from doing a test like what's been done in the OP's video he posted. You really need an engine dyno to separate these effects:

1) the pressure drop at the compressor inlet due to restriction. This is what changing the air filter and intake piping affects. Any intake system has an exponential mass air flow vs restriction curve. So the more air you are trying to flow, the more the design of the intake piping and filter matters.

https://www.rx7club.com/attachment.p...1&d=1415043012

See that really crappy intake, the one with the most restrictive curve? It works the crap out of your turbo. The harder the turbo works, the more your compressor speed increases and the more you are shifted on the compressor map.

2) the temperature at the compressor inlet, which affects the temperature at the compressor outlet after the air has been compressed. We've discussed this to death - the colder you can get, the better, but with a turbo car it's more complicated because you have the turbo heating up the air.

3) the temperature at the compressor outlet, which in a peak power kind of dyno run is greatly affected by the pressure ratio of the turbo. The pressure ratio is the outlet pressure (boost pressure) divided by the inlet pressure. I am not referring to the manifold pressure. The y axis of a compressor map is only indirectly related to the manifold pressure. The turbo creates a pressure ratio. The pressure ratio's biggest components are the turbine/compressor wheel speed, controlled by the mass flow to the turbine, and the localized compressor inlet pressure.

The more restrictive your intake system is, the harder the turbo works, and the hotter the air will be.
The lower the ambient pressure due to altitude or weather, the harder your turbo works, and the hotter the compressed air will be.

15psi manifold pressure on a turbo with a stock intake and air filter works the turbo harder than 15psi boost with a free flowing intake system. That's because the pressure ratio is lower.

https://www.rx7club.com/attachment.p...1&d=1415043012

I'll explain it again now with a compressor map. Those of you who have looked at some compressor maps before probably take the Y axis as being relative to atmospheric pressure. So 2.0 must mean about 15psi boost, because 1.0 = an atmosphere and the first atmosphere is ambient. That's not actually how it works. The more restriction you have, the higher you move on the Y axis, and in many cases the lower your efficiency goes. When the efficiency goes down, the compressor outlet temperature goes up. With a stock turbo compressor speed becomes an issue, because you have to spin the turbo more to make boost in a restrictive intake than a mostly open one. The more you overspeed the compressor with a high pressure ratio the more you reduce the life of your turbo.

3) the temperature inside the actual intake manifold near the intake ports, after the compressed air has passed through the intercooler and all the underhood piping. A lot of that has to do with how well the intercooler is cooling down the air, and any heatsoak affecting the air as it moves from the intercooler outlet to the engine.

Cool intake temps mean more dense air entering the engine and less chance of knocking. If you have a restrictive intake system making your turbo work harder, your intercooler has to work harder to reduce the air temperature in the actual intake manifold.

So when we are talking about increasing peak power, it gets complicated. You may have an intake system that is less restrictive, making the turbo work less and reducing compressor outlet temperatures which then helps intercooler efficiency and reduces manifold temperatures. But it could also draw in more hot air to the turbo. So you have two effects working against each other.

The colder the ambient temperature, the less the "cold air" part matters and the more important the restriction level is. Your primary contributor to manifold air temperature is the work of the turbocharger, and that's related to pressure ratio (which is affected by how restrictive the intake is). You can actually take a stock intake system and run each part of it through a flow bench to see what is the most restrictive. Is it the filter? The airbox itself? The scoop pulling in ambient air? The ducting to the turbo?

I solved the issue years ago, and nobody was interested at the time, as they were advocating that "pod/cone filters" in the engine bay were the way of the future- so I didn't elaborate on what I did, because everything else was seen as inferior to the pod/cone filters dangling in the engine bay, sucking in hot air.

Retaining all factory airbox and plastics and having two separate cold air feeds for the intercooler & the airbox is easy.

You must:

Block the big flat plastic crossover piece that feeds off the intercooler duct. Block it where it sits into the top of the intercooler duct.

Leave the intercooler duct alone now. It is now soley for feeding cold air to the intercooler.

Then, look again at the big flat hollow plastic cross over piece. Turn it over.

Cut a large sized hole underneath it, equal in size to the port you just blocked off on the end of it.
Cut the hole closer to the airbox end of the big hollow flat plastic thing.

This hole now feeds the airbox cold fresh air from the bonnet gap at the front of the car, and the nose cone.

Done. Problem fixed. Airbox gets cold air, and the intercooler gets its own cold air, both are separate.

Cost? Some black race tape or similar to block one hole in the intercooler duct.
Cut another hole- free.

Benefit- nobody will ever know, and it looks 100% stock standard. Takes half an hour.

I devised this because I also have a 2000 Series 8 FD here, and this is what the factory did to solve the problems you mentioned above.

Did this to my 1992 car, and it has been running with cool intake temps for years with no issues guys :)

Stealth dedicated cold air intake for stock cars
 

Guys, this is such an easy modification for a stock standard car, and you dont even need to take the airbox out to do it. You only need to remove the plastic hollow panel (2x M10 bolts).

Photos are really self explanatory (sorry for the awful camera phone pics, and dirty engine bay)
I've run this stealth cold air setup for a few years of daily driving now and it works fine with no issues. The intercooler gets dedicated cold air, and the airbox gets its own cold air feed from near the front of the hood and nose cone.

I have access to a JDM 2000 car and this is essentially similar to how the factory solved the issues of the early cars (your USDM cars) scavenging hot air from through the intercooler core and into the airbox.

Attachment 635783

Attachment 635784

Attachment 635785

Takes very little time to cut intake hole 'A', and block off the problem child hole 'Z'. Then you're good to go.

Nobody looking under the hood will ever see it either.

SA3R,

I like the basic idea of what you have done, but it doesnt solve the issue of airflow restriction of the stock airbox. The big flow restriction of the stock box is what you have labeled as intake hole "B". That hole is simply too small to flow enough air for anything but "stockish" applications.

My "Cheap Bastard" mod resolves the intercooler issues, flows plenty of air for just about any application, and retains the oem look (like yours does).

Why not cut out a portion of where the red area is and you'll get a better source of air ...
It might not look nice ... but should be more functional.

I've had great results with a version of the cheap bastard intake mod. I simply cut some additional holes in the bottom of the box and ran pvc pipe to the front of the car. Looks stock but eliminates the I/C reversion issue.