Project Silvia’s Grlfriend: Part 3 – Making it Pretty

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Q45 Airbox

 

Okay, it may seem strange to count an air filter box as part of making the car pretty, but here’s the deal: this air box is all about appearances. If you’re trying to impress the kids at the car show, you’ll do incredibly pointless, silly things underhood. Like color coordinating the coolant in your overflow bottle with your radiator hoses, or polishing the outside of intercooler pipes. Hell, even Project Silvia had intercooler hoses that matched the valve cover. Sarah’s engine bay goes exactly the opposite way. This car is meant to be invisible to anyone who isn’t already an S13 nerd, and as part of that goal, we went out of our way to make the engine bay look like a factory engine bay from some anonymous brown ‘90s Japaense car. Hence the quest for a stock air filter box that is up to the flow requirements of a mildly boosted SR20.

Nissan, in the ‘90s, used two different air filters to cover everything they sold. The smaller one was used in 240SXs, Silvias, Sentras, Stanzas, hell, even Subaru Legacys (Nissan and Subaru shared quite a few suppliers in the ‘80s and ‘90s). The big filter only appeared in the really high output stuff like the 300ZX, Skyline and Q45. As long as the air box itself is designed well, this big filter has enough flow capacity to support 300 hp with no issues. I know this because I’ve dyno tested stock R32 GT-Rs with and without the air filter and bottom half of the air box, and there was absolutely no difference in power.

So, step one in the stockification of the engine bay was getting this Q45 air box from the junkyard. The big waffle-stamped tumors on the box are helmholtz resonators meant to prevent any cool V8 intake note form escaping. They are easily removed.

 

 

Though the Q45 air box looks like it can easily support our flow needs, the big Q’s airflow meter is too damn big for the SR20. Though Jim Wolf Technology was tuning our stock ECU, and could use any airflow meter profile we wanted, the Q45’s is simply sized wrong for the application. Because it was designed for a big, naturally aspirated engine, it doesn’t have the necessary resolution at low flow to accurately measure idle airflow on a little 2.0-liter engine. Strangely, Ford Cobra airflow meters do have the necessary low-flow resolution, and are a viable option for JWT-tuned SR20s… Anyways, we had a Z32 300ZX Twin Turbo airflow meter from the same car that donated the brakes and rear uprights, and those also work just fine, so that’s what we did. The first trick, then, was somehow making the smaller Z32 meter fit the big Q45 bolt pattern.

Luckily, the air horn on the inside of the box is also the threaded mounting flange for the airflow meter, and two of the four holes already matched.

 

 

All the adapter has to do is block off the excessively large Q45 hole and seal the box when it gets sandwiched between the air horn and airflow meter. Because I am me, the adapter is made from the bottom of a paint roller pan. It bolts to the air box using the lower two holes, which didn’t match the Z32 meter, and seals against the box with RTV. When the Z32 meter is bolted on, the adapter is sandwiched and everything is air tight.

 

 

The bottom of the box doesn’t come anywhere close to fitting, but it doesn’t really have to. I cut the whole bottom away, leaving just a ring to hold the air filter and give the upper box something to clamp to. This makes the stock-looking box an open-element intake, so there’s no chance of any inlet restriction upstream of the air filter, but we still have the filtering effectiveness of the stock filter.

 

 

The air box looks like it belongs. The box did have some fading and junkyard paint marks on it, so we “restored” the plastic finish by giving it a light coat of spray can truck bed liner. This gives a nice, even, textured finish that looks like some kind of factory plastic.

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