Midget Airbox & Cold Air Intake

[Loren]

When I upgraded the carb from a CD150 to a CD175, the stock airbox no longer fit.

My first thought was to just fit a "pancake filter" to it, which is common and looks cool. But, the only ones available for a CD175 are for TR6's, and go for like $150. I settled for a cheap one that fit a CD150 from Amazon for about $16. Of course, I had to modify it to fit the bigger carb. So, $16 plus a couple hours labor. But, that got it on the car. And I could immediately tell that it wasn't flowing enough air! Because the filter was 5" diameter and only 1" thick.

Math break:
5" diameter and 1" thick gives a surface area of 15.7 square inches at best. Calculators I've found online say that my engine should need somewhere around 36-38 square inches!

I measured that diameter, and found a 2" filter to replace that 1" filter. Still not enough flow for high RPM, but should be adequate for the driving that I do. Of course, I had to modify the filter cover attachment hardware to get it to fit. But, that got it on the car, and it worked well enough for a while.

But, I started noticing that the car behaved a LOT better and felt peppier on cool evenings. Why? Well, cars do that anyway, but... the exhaust manifold is directly under the carb, so this pancake filter is picking up hot air directly from the exhaust, especially on a hot day in stop & go traffic.

So, I want to fit a CAI of some sort. Settled on making an adapter to fit a 3" hose to the carb, neck that down to 2.5" and run that to a 2.5" cone filter in front of the radiator. That was the rough plan, leaving out many mundane and difficult steps. I ordered about $60 worth of silicone hose bits and a cone filter to get started on that. Stopped short of buying the long piece of 2.5" hose, figured I'd do a "shorty" intake as proof-of-concept first.

The more I looked at it, the more of a hassle it appeared to be. Not impossible, just a major pain due to the tight space available.

Brainstorming one night, I was thinking... it would make more sense to build an airbox right off of the carb and use a panel filter, then run 2-2.5" hose from there to someplace cool. An airbox. With a panel filter.

JUST LIKE THE STOCK AIR BOX!!!

So, I've come full circle. Back to the stock airbox with nearly unobtanium panel filter.

Stumbled upon an MG forum thread that indicated that the 2014 Mitsu Mirage (heh!) filter actually fits that airbox. Not so unobtanium. Cool.

And the stock airbox exits to a 2" diameter pipe near the back of the engine. I can either run a short hose into the passenger side fender well, or if that's not good enough, run forward somewhere.

Okay, I have a plan! Modify the stock airbox to fit the CD175 carb!

[Loren]

Here's what I'm working with, gotta make this airbox fit that gasket:





Here's a shot of the inside. The mounting flange is doubled for strength, but it still has a bunch of stress cracks. Interesting.



I've transferred the gasket shape to a piece of scrap 1/8" sheet steel. This is roughly where it will go. You can see where the top part interferes with the air box body slanting back towards the carb.



You can see that interference better in this photo. My initial plan was to clearance the box and leave the mounting flange where it was. I later learned that doing that would have my flange interfering with the air filter!



This hand-dandy popsicle stick demonstrates how much trying to straighten that angle above the flange would get into the air filter space. Bah!



But, I pressed on and cut out my flange to fit the new carb. I didn't have a means to properly cut that big hole (or much of anything else). Made liberal use of a jigsaw, some small sanding drums on my drill, and a few other inappropriate tools. Very happy with the result, and it didn't take as long as I expected it to!

[Loren]

This is roughly where the new flange will go, but I need to test-fit to the car and figure out what needs to happen.



Drilled the top hole and mounted the new flange to the box with an extra nut between the flange and the box. This solved the clearance problem with the flange against the box, and might also be enough to give clearance to the top of the carb. (the CD175 has a MUCH larger top than the CD150)



And, yes, it did all of that! You can see the nut shining in there. The flange is flush. The box has a slight angle that it didn't have before, but it's not creating any additional clearance problems with anything else under the hood. Good!



Time to get serious! I'll need to do a lot of filler metal around the new flange to mesh with the back of the box. This part that I cut out is no longer needed and would surely create a bunch of turbulence and restriction of airflow if I left it there. I cut out all but a tab that's holding my bolt so that I still have good location reference to tack everything together.



Tacked together lightly (thankfully), and gave it another test fit. Did you notice that the little cut-out in the flange is on the wrong side? I almost didn't! But, otherwise, things are looking good.



Flipped the flange around, tacked it back in and also tacked in the right side (front as it's installed in the car) filler piece. There was a lot of cutting, trimming and bending to get those filler pieces to fit just right. Even then... there were still some big gaps that I had to weld together here and there... made for some ugly welds!



Here, you can see that there's JUST enough clearance between that filler piece and the top of the carb. That'll work just fine. Match the other filler piece to that one, and we're in business!



And that's as far as I got on day one. Well, I did get the other side piece cut out and tacked in... actually tried to start welding it, but it was getting dark and my bad welds got a LOT worse, so no photo of that mess!

[Loren]

Day 2, I cut the last filler piece to go across the top, then did all of the welding. It's not pretty, but it'll work.

I'm not an expert welder, and welding 1/8" steel to thin sheet metal is not easy! I was amazed that I didn't make Swiss cheese out of it blowing holes through the sheet metal. (I did do 2-3 little holes that I had to fill in!) I had to turn the voltage and wire speed on WAY down to weld this, and I already use thin wire (.030").

I kept it clamped to the work table to keep the warpage to a minimum.

Anyway, here she is in all her Frankenstein glory:



After cooling off, I took a grinder and wire wheel to it (and my left thumb) and gave it a shot of primer. Not bad! Once it's painted black, it will pretty much look stock!



Final test fit. Good clearance against the carb.



And here's a shot of the inside. I'm not going to try to cut out any more of that excess metal in there. It's good enough.



The only thing I've got to do yet is fix one of the top latches (one was missing, I'm replacing it with one from the bottom), and I'm going to weld some kind of "rail" to capture the bottom of the air box cover rather than putting latches under there that can't be reached. That and some paint... and a new filter... and maybe some 2" hose.

[Loren]

Had some time this afternoon. Decided to fix up the latches, clean up the inside and paint it.



Doing away with the lower latches ended up being very easy, didn't even have to weld anything! On the cover part, there's a reverse lip that you can't see in the photo. As pictured, the outside edge rolls over and down about 1/8". You can see where I made small cuts through that lip. Then, I folded that part of the lip down and hammered it flat to make a pair of "tabs" where the old lower latches used to be.

On the carb side, I assembled the cover to it and simply folded that lip over the other lip. Absurdly simple, nice and tight, easy to assemble. Best of all, I'll never have to fight with latches under the air cleaner!

Next Up: Once the paint is good and dry, I'll do some cleaning of the outside and latches and paint the outside black.

Fun fact... I was thinking about this today...

This air cleaner, designed probably somewhere around 1972 and constructed without the aid of computers or robots, is surprisingly complex! It's made of 8 stamped steel pieces all spot-welded together. Plus each of the four latches is made of 3 pieces, plus 2 rivet/pins. 28 individual parts just to make this air cleaner! And this isn't even all of it. There's another part that attaches to the inlet that I don't even have. It's a thermostatically controlled diverter to pull air either from near the manifold, or back near the cowl. That thing probably has another 5-6 sheet metal parts, two hoses, 4 clamps, etc. They took this whole air cleaner system pretty seriously!

It was all for emissions. They added complexity to the carb to help it idle cleaner and reduce lift-throttle emissions. They had an air pump that sent fresh air into the exhaust ports to... Idunno, I guess it helped the cat work better? Plus the cat itself. All of that stuff choked the old 1300 engine so much that they had to stroke it to 1500 to make it acceptable to drive!

The previous 1300 engine (in the Spitfire) was pushing 75 hp and 75 ft/lbs in euro spec with dual 1.25" carbs. The 1500 in euro spec with dual 1.5" carbs was the most powerful factory version of the engine at 71 hp and 82 ft/lbs. But, the US spec single 1.5" carb, emissions choked 1500... 51 hp and 73 ft/lbs. (mine's a little better being a '76, the only year with higher compression... HP is probably 60-65ish with a little more torque)

So, obviously, opening the exhaust, fitting a bigger carb and putting some cooler air into it is well worth the effort on the Midget 1500! I should be able to get close to the euro spec 75 HP, and surely have torque in the 80's. Torque is this engine's strong suit due to its long stroke. It's never gonna be a (safe) high-revving engine.

[Loren]

Finished all the fab and paint work. Time for an air filter!

The stock air filter is still available. Sort of. They're not "readily" available, and they go for like $40!

Me being the frugal sort, I searched. And I found a thread on the MG forum that pointed to the 2014 Mitsubishi Mirage filter as being a replacement. Cool! That's readily available and cheap!

But, while some people said "yes it fits", others said it was too tall. So, I dug into it some more.

The Mirage filter is just about the ONLY filter that's ~5x10". Why do some people say it's too tall? Because the thickness on them varies between about 34 and 37.5 mm depending on the brand!

I measured the available space at almost exactly 1.25", which is just under 32mm. All of these filters have about a 1/4" thick gasket on them (apologies for flip-flopping between metric and standard measurements), so any 34-35mm thick filter should fit without issue. I can see the thicker ones maybe being a problem.

So, I ordered and ECOGARD XA10388 which claims a 1.34" height. (careful when researching filter specs, many sellers list the size of the BOX) 1.34" = 34mm. That should work. Allows for .09" of gasket compression.

Received the filter and tried to fit it. It fits, but... photos tell the story...

Here's the stock filter next to the Mirage filter. Notice that the stock filter is in a box. It has a frame around it, and that presses the gasket against the mounting surface on the carb side. The Mirage filter doesn't have that, so there's nothing to force the gasket to seal.



It does fit quite nicely!





But, here's the problem. In theory, maybe the suction from the engine pulling air through the carb would keep the filter gasket sealed. But, if it happened to take a set in the wrong position, unfiltered air could leak around the gasket. That's no fun. (I've been running without a filter for a few weeks... trying to get AWAY from that!)



The paper filter element can slide into the opening, like so:



My solution was this... rather than further modifying the air box that I'd just painted (could have maybe done a similar solution with steel and welding), I opted to make a little wood box to surround the filter and provide a solid seal on the gasket. Like the stock filter's cardboard box, but more rigid. It's cut to fit and epoxied together. It's not glued to the air box, just a snug fit. I'm no woodworker, but I like it. I should paint it. I think I'll paint it.

The box is 1" tall (maybe 15/16"), side rails are 3/8", end rails are more like 3/16". I didn't want to try to nail such thin wood, so I used quick set epoxy.



Filter fits in there just right!



Gasket has just a little bit of compression and stays tight!



And, I guess it's good that I'm still "backwards compatible" with the stock filter. Just slide my little box out.

With a little bit of scrap wood and some time, I can now use a Mitsubishi Mirage filter! $12 air filters all day long.