Stock Air Cleaner Mod

Cratecruncher

Cratecruncher

True Classic
I usually don't go for big mods on my X but there is one thing that began to bother me. If you look at the stock FI air cleaner (4453445) it has an intake port about the size of a urethra. Even at idle speeds the air velocity through the inlet nozzle is brisk. I calculated the airspeed at 6000 rpm and decided it must be improved.

First I laid out 40 holes equally spaced around the perimeter. Then drilled them out to 7.7 mm diameter to effectively double the intake area of the air cleaner. When installed it looks totally stock.

The first time I took it for a test drive I noticed a subtle growl of induction noise at lower rpm and what feels to me like more torque. I won't know until I dyno the difference between stock and modded. I also have a spare unmodified air cleaner for just such an occasion.
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A common way to make the air cleaner intake less restrictive back when these cars were new was to remove the stock air cleaner cover and replace it with an appropriately sized aluminum pie pan with three holes drilled in it for the mounting studs. That way, the entire periphery of the air cleaner is open to the air.

I removed the snorkel on my stock air cleaner and replaced it with a fitting that allowed it to connect to 4" dryer hose. I did the same thing to the driver's side engine compartment intake scoop and connected it to the air cleaner with the 4" hose. I can't say that I could tell a difference using the outside air or not, but it seemed like opening up the air cleaner did make a difference. For sure, it sounded better.
 
It's all about the sound. I have no direct involvement but apparently folks here have actually had dyno runs with little or no change with freeing up the air filter. The sound alone is worth the mod, everyone likes intake growl and we all know that affects butt dynos!
 
This is a common mod across most platforms. Commonly called "Swiss Cheese" air box. As with real Swiss cheese, the holes vary from mod to mod. Some choose a series of small holes, some choose a couple strategic large holes, while others choose several medium nickle-quarter sized holes.

Results vary. Ive never choose to dispose of income on dynos, but I have felt some improvements. I doubt that there is huge gains in power or torque. From my experience, the difference that you may be feeling is that the car isn't having to work as hard, and better throttle response. Enjoy the renewed love with your car, and enjoy the lovely voice that has been locked up, for too long.
 
My 85 doesn't have the stock air cleaner on it anymore. It came with a cylinder style filter clamped to the AFM. The old one was pretty grungy, so I replaced it with a K&N cone shaped filter. I haven't paid much attention to how wet it gets, but then again, I try not to take it out in the rain. :)
FI air filter 1.JPG FI air filter 3.JPG FI air filter 2.JPG
 
I would like to think the engineers would try not to kill horsepower on their cars with a crappy restrictive air filter housings and would work hard to make sure they get it done right. Sure there are noise constraints they have to meet and it all has to fit in the car. I'm amazed at some modern intake paths with all manner of Helmholtz growths and convoluted tubing. What surprises me is that "sporty" cars don't come with a purposely designed housing that would provide intake growl...surely any Fiat or Miata owner would enjoy that and the manufacturers could then keep the exhaust note boringly quiet.
 
I tend to agree with Carl's comment that the original design probably won't be that restrictive, despite looking like it would be. At least on a stock engine. There was a great study about intake air flow on BMW's several years ago. A very well designed study by a certified lab with all the right resources and knowledge. The principle goal was to compare the stock paper filter with popular aftermarket items (like the K&N types). But they also evaluated things like the air filter box/housing vs a custom one, cold air ducting vs stock, etc, etc. The bottom line was the engine only flows a given maximum volume/velocity of air and the original factory set up met that need. Anything beyond that yielded no improvements on the dyno. All the added flow capacity in the world won't help if the engine does not draw any more than the baseline intake did.

That's not to say that Fiat failed to meet the engine's basic requirements, I don't know. The stock original design may indeed be reducing performance if that's the case. And certainly a modified engine should be capable of flowing more air (assuming the mods were properly done), so the intake requirements would go up. Also if the owner does not regularly service/maintain his vehicle (i.e. has a dirty air filter) then it may not be allowing enough flow to match the engine's needs. I'm sure other variables may be in effect as well; climate conditions, driving habits, etc. But if anything those would decrease the air flow requirements (like a dirty filter would), not increase it.
 
"I would like to think the engineers would try not to kill horsepower on their cars with a crappy restrictive air filter housings and would work hard to make sure they get it done right."

I hear you Carl but we all know engineers nerf engines constantly to comply with local emissions requirements and other regulations. Also, what if this cleaner was just an off the shelf part designed for a smaller capacity engine? One of the brackets is spot welded right over part of the Fiat logo leading me to think this part started with a different model.

There may be an additional benefit to this mod. Before the change there was a dirty patch on the filter the shape and size of the inlet just opposite the inlet. In fact there were two or three indicating I'd not replaced the filter the same way each time I'd removed it. I'm confident the added air holes will distribute the flow more evenly around the filter reducing progressive restriction due to dirt.

Just as a fun exercise I suggest the naysayers calculate the volume of air flowing through a stock 1.5 liter engine at 6000 rpm (100 rev/s). Then divide it by that tiny opening (1872mm^2).
 
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I get suspicious when the cross sectional area of the intake snorkel is much smaller than that of the carb throats.
 
It's a nice day here so I decided to try the air cleaners back to back on the same stretch of freeway and backroads. I've been driving with the modded one for a couple of weeks so started with the stock part. I noticed right away the engine was much quieter but there was still a lot of usable torque. Then I switched back to the modded unit and there was a loud induction drone at high rpms but I must be honest, it didn't seem much quicker. I think if there are any hp gained it will be at the very top end which is really hard to test in traffic. Can any butt really feel the difference in 1 or 2 hp?
 
Fluid dynamics was my worst subject in engineering school so I can't speak to theory...but I do know fluid flow is pretty sneaky. Ever look at the size of the openings in a NASCAR restrictor plate, they are tiny. It's not just size of the opening but also flow through it...just like the design of a carb body.

Butt dyno readings are just based on sound!
 
carl said:
Fluid dynamics was my worst subject in engineering school so I can't speak to theory...but I do know fluid flow is pretty sneaky. Ever look at the size of the openings in a NASCAR restrictor plate, they are tiny. It's not just size of the opening but also flow through it...just like the design of a carb body.

Butt dyno readings are just based on sound!
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Whew, when I started reading that last sentence, I was afraid you were going to rate butt dynos by size of the opening as well. :eek:
 
You guys are even worse that Miata.com. My post there about what lube to use in my rear end (Miata) set off a similar storm of anal jokes. Apparently, X or Miata, we are all in the same fourth grade class...actually you guys are, I am the adult in the room and that's really scary.
 
Hear that guys, we've been promoted to the fourth grade!!! And it only took us 20 years to get there.
 
Karl, we've evolved while you were away. The nature of our humor has become much more refined. As Carl confirmed, we are now up to the 4th grade level. :)
 
carl said:
I gave you guys too much credit, apparently kindergarten is more appropriate.
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Hey, at least we didn't appoint ourselves as the adult in the room. That is the kind of idea you should run up the lightpole first to see if anyone salutes. Er... I mean flagpole. Ok, I see what you mean. :cool::p:D
 
I have to debunk this whole thing. While at the dyno I disconnected the intake and sucked in just straight air for a run to test my theory that it was restrictive from stock to wide open no filter or air box I gained .5 hp gain. The biggiest gains I had was opening my dump valve and bypassing the muffler. That was only 5 hp. I did total of three runs without a cat everything else stock I was running 78.9 hp at the rear wheel with no cat. Dump valve open 83 hp. As you can see from my avitar I am a stock 86 FI running with no cat.
 
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