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> Weber carbs, What's the difference?
914 Monster
post Apr 13 2010, 09:13 PM
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Since I'm in the process of rebuiling my 2.0 engine, I was wondering why 914 high performance engines use 44 IDF carbs instead if 48 IDF's or IDA's. I'm not trying to build a race car or anything to much but want to upgrade my 40 IDF's.
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914 Monster
post Apr 13 2010, 09:42 PM
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How exactly does one overcarb an engine?
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ArtechnikA
post Apr 14 2010, 04:59 AM
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QUOTE(914 Monster @ Apr 13 2010, 11:42 PM) *

How exactly does one overcarb an engine?

Here's a more detailed response...

Q: What does a carburetor do?
A: It measures airflow and delivers a proportionally correct amount of fuel.

So there's a few factors at work here. Let's look at measurement first. (Be aware, some oversimplifications follow. We're after The Bigger Picture here, not details...)

Picture a tachometer like the one in your car. Except that instead of reading to 8000 rpm, it reads to a million. You want to know where to shift, but since your engine only goes to 5000 rpm, the needle barely moves. You just can't see what you're trying to measure. You could measure to a million if you could turn the engine that fast, but you can't.

So - Big Hole flows more air than Little Hole. But how much air can you _actually_ flow? In round numbers, at full throttle, you are breathing in 1 liter / rev. A hole bigger than one that can flow 1 liter/rev won't buy you anything. In fact, it's like that part of the tachometer between 5000 rpm and a million - it gets in the way of measuring what you really care about.

So that's Part One of why too-big is bad - you cannot obtain an accurate metering signal with too little actual airflow.

But that's not even the big problem. The big problem is _how_ carburetors work. They use the suction in the airstream as is passes through the throttle body and venturi to deliver fuel. Suction is the key. In an engine it's called manifold vacuum, and it's the difference between atmospheric pressure and the piston trying to suck in air through the opening.

But recall that it is air _FLOWING THROUGH_ the carburetor that delivers the fuel.

When you open a BIG throttle butterfly on a BIG bore carburetor, there is little vacuum. The cylinder is happy because it got its big gulp of air. But since there was no restriction, there was no vacuum, and you got NO FUEL.

You stand on the gas and the fire goes out because it's too lean (too little fuel) for the engine to run. Amazingly, the big problem with too-big carburetors is not enough gas... When the hole's too big you have no vacuum for the carburetor to measure to dispense the right amount of fuel.

Race engines run 'bigger' carburetors because they understand the tradeoffs and accept truly crappy running at low (say, under 5000) rpm in exchange for a little more peak performance over a very narrow (but very high) rpm range. And they accept the really crappy gas consumption rate that goes with it.

I could reference a bunch of really excellent books on design and tuning of automotive intake and exhaust systems, but I think I probably already have somewhere before. Start with the basics, and never forget: port velocity is your friend...
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