How to "make" a custom intake for mpg?

[Honda100]

Hi there,

Now I know that I am not educated in terms of optimal flow rates, but am just toying around with an idea since my Toyota bit the dust.

EDIT: I also forgot to add that I wanted to replace the carb with something smaller. I have 6 or 7 carb laying around from anything for a Honda Super Cub 90 to a Honda Rebel 250. Maybe I could make an intake that could accept one of these carbs?

I have an SR5 Corolla with a ton of miles that isn't worth the $300 for a new carburetor, so I thought it would be a fun idea to "engineer" it for fuel economy. Please correct me where needed, I know that I am just an amateur when it comes to engine design and operating principles.

So any intake that is oriented towards performance is usually a bigger diameter right? So in principle, a smaller diameter intake would restrict the total mixture allowed in right?

So does that mean I can just fabricate myself a smaller intake manifold port and attach a smaller carburetor on it for better mpg?

I know this probably isn't right as I suspect from reading around that air speed/charge speed has something to do with why, but that's why I am here! TO learn. Thanks!

[niky]

May be too much work. You can simply fabricate a mount adapter between the carb and the manifold.

A 250 bike carb on a big engine won't run well. It'll probably run, but you'll experience bad choking the moment you ask for more power. I'm running a Toyota 4K carb on a Nissan 1.6 and it only runs to 4k rpm before choking.

Neat trick, that. Wonder if the Toyota 4k was named for that.

[AndrzejM]

Hi, carburators are not so simple to change between engines and cars. It can be done and I've done it before, but you shouldn't change the diameter significantly. Carb is all about airflow and if you put diameter too big fuel won't be atomized properly and your mpg will drop, if you put carb with diameter too small for your engine you won';t get enough air and engine will suck the fuel because of huge vacum in the intake mainfold.

So stay with the similar diameter to the original one. What you can do to improve FE, first of all you can put spacer between carb and the mainfold (required anyway if you want to put different carb anyway, because of mounting points. You'll have the advantage of better fuel atomization. If you want to have better low rpm response put straight pipe at the inlet of the carb (it can be bended to fit under the hood of course but try to stick with the diameter as close as possible to the diameter of carb's inlet. The longer the better. You'll have slightly increased pumping looses but much better low end response.

Or if you want you can switch to EFI with Megasquirt, it's a lot of fun and it's always an interesting project. But not $ efficient though

[ae86driftmaster]

what year is your SR-5? i have a 87
a cool cheap trick i learned on the 4ac....is to place aluminum foil (reynolds wrap) between the carb and exhaust manifold....

[freebeard]

Well if you really want to learn something:

Metal Casting Part-1

Metal Casting Part-2

Or do what NASCAR does—use a restrictor plate, with the same manifold and carburetor. For a highly engineered product: the Gene Berg Jet-A-Vator.

Else just grab an aluminum or phenolic plate and a drill and have at it. NASCAR uses one hole per port. I can't find a good picture of the Berg product, but it has ~1/8" holes in 60-80% of the port area. This gives some additional benefit, anti-reversion and better atomization.

[Honda100]

Sorry, I am sick in bed so I might be a little out of it.

Anyway, I have an 87 also, love it to bits. I completely forgot to add that I was interested in using the 250 carbs in a sort of ITB setup, since each cylinder displaces around 400 cc on the Corolla, I figured it would work close enough.

I did some further research into ITBs though and gave up on the idea because of carb synchronization issues.

Either or, I ended up rebuilding the carburetor ultimately for cheap, so that wasn't too bad. Car runs again and gets okay mileage.

[freebeard]

You could still do a restrictor plate. You can test whether you'd like it by putting a block under the gas pedal.

A screen sandwiched between two gaskets is *supposed* to improve atomization. Something like the Jet-A-Vator will prevent reversion (backfiring through the carb). A phenolic plastic 1/4" thick will block heat and might prevent vapor locking.

Hope you're feeling better.