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Motor Types Question


Well-known member
I have noticed that some AC type engines and/or GPU's have the carb on either the top or bottom of the motor. What are some of the model names for those different types and are there any advantages to one set up vs the other.

Is that what is reffered to as an "updraft" design (bottom carb design)?

Lastly, I have seen some AC type motors with what looks like custom intakes that allow the mounting of carbs such as Holly brand carbs... does this fuel system have distinct advantages from say my standard 0470 GPU's Zenith carb set up?

Thanks, just trying to learn more...
Carbs on bottom of engine are called updraft.If you are running a Zenith try to convert your intake to a Holley.Either 500 cfm 2 barrel or 650 4 barrel.But get rid of the Zenith.You will diffenetly like the change.I run a 930cfm Predator on my Cont 520.Most of your AC motors are setup updraft standard(Lycoming,Contentinal).
A/C engines usually have their carburetors mounted below the engine because of the nature of the application they were designed for.
An aircraft engine is a slow turning / big torque motor, and long intake runners usually contribute to torque. They're also usually in a cowling when mounted on an airplane, so having the carb below the engine makes for a more streamlined package.
And third, having the carb located down in the vicinity of the exhaust stacks and mufflers keeps the carb warmer to prevent carburetor icing at altitude. Most, in fact, have a specially designed airbox that uses exhaust heat to actively warm the intake air for that reason.

I think there's a lot of horsepower and fuel economy waiting to be found for most A/C set-ups that are still running the old updraft carbs. That system is begging to be redesigned for an airboat.

Also, if you look in the oil pan of an updraft, you'll see the air flow piping has fins on it. The air cools the oil, the oil pre-heats the air. Pretty snazzy for a cold temp environment like up north in the winter or 20,000 feet. Not sure how effective it is here at sea level on a 95 degree summer day.

Updrafts are easier to make a throttle linkage for too because the carb is right down there on the same level as your feet.

I like the look of that clean, low profile motor too. The crankcase seam is the highest part of the motor! If you are measuring ounces to get a lower center of gravity, that's gotta help.