Weber carbs, What's the difference? Options

[914 Monster]

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.

[Cap'n Krusty]

Because the builders don't want to overcarb their engines.

The Cap'n

[GeorgeRud]

Don't go too large on the venturi diameter, or you will not have enough of a "pulse" at idle and at low rpms! There is a formula that has been used to try to match the size to the size of the engine. The factory 914-6 engines came with 40mm diameter throttle bodies, but all the way down to 27mm venturis (to get a good enough signal for emission control at idle).

The 44s and 48s are usually best for race engines that will be run at high revs, not for street cars INHO.

[Rand]

The answer is in your post. 2.0. There are a ton of variables with a custom build, so we can't say with those details. But typically a 2.0 doesn't need 48s. More fuel != more power. Tune to build.

[914 Monster]

How exactly does one overcarb an engine?

[914rat]

Too much gas+too much air=overcarbed engine.More gas and air do not equal more power.

[Rand]

How exactly does one overcarb an engine?

Are you serious?
:choke: :choke: :drown: :drown: :lose:
A: Dumping way more gas down it's throat than it can efficiently use.

[tat2dphreak]

How exactly does one overcarb an engine?

the question should be "how does one NOT overcarb an engine" (IMG:style_emoticons/default/smile.gif) over carbing is basically having the port so open that you will run way rich, and just sloppy.

a 2.0 doesn't need more than a properly jetted 40. properly jetted you will have smooth acceleration through the power-band.

I suggest reading the tech articles on aircooled.net. they will really open your eyes (IMG:style_emoticons/default/smile.gif)

Too much gas+too much air=overcarbed engine.More gas and air do not equal more power.

agree... in fact too much will BOG down the engine

[ArtechnikA]

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...

[zymurgist]

+1 (IMG:style_emoticons/default/agree.gif)

[ME733]

.........well done Artechnika....I agree.....and to 914MONSTER, I would suggest that you provide the forum with the camshaft #, manufacturer,and camshaft specifications,(they provided or you measure), along with the valve sizes, heads (conferm 2.0 heads)...and the compression ratio would help...there are many people on this forum who can help you "dial in " the ideal venturis and jet sizes to optmize your engines H.P. output,and driveability...........................................to consider. murray.

[IronHillRestorations]

IMHO you want your 40's with 38 venturis. If you are raising the displacement and going with a more radical cam, you would want to go with larger venturis.

The Bigger=Better school of thought does not apply to correctly matching a carburator to your 914 engine.

[Jake Raby]

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.

Because the 44s make more power and yield better efficiency.

I START using a 48mm carb at 2.7 liters of engine displacement, below that the 44 will kick the pants off the 48.
I've ran them back to back on dozens of engines.

For a carb to work well it must "Carburate".. Big carbs don't do that very well.

I can count on one hand how many times "bigger has been better" out of hundreds of experiences, even on large engines.

[pbanders]

When airflow is low (e.g. idle) a large venturi results in a low airspeed, which leads to poor fuel metering and poor atomization. The poor atomization means you have to increase idle richness to get it to run at all, and it still will idle poorly. Carb venturi size is a compromise between good running characteristics across a wide range of engine speed and load levels, and 40's and 44's have proven to be the best compromise for T4's of 2.0L or somewhat greater displacement.