Thread: Question on Carb. size

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Little advice on what size carb you would ues. I had a 383 stroker built 460 hp it has a 800 edelbrock on it just seems like an over kill what size would you suggest, just looking for some input?

There are a few sites to calculate. Here's one.

Carburetor Size Calculator

I run a 750 Edelbrock on my 383 (450 hp). It works fine but it was too rich out of the box and we had to change out the jets and metering rods to lean it out when it was run on the dyno. You 800 may be a little big but should be all right if you tune it for proper A/F ratio.

We seem to have similar discussions, re; “carburetor size” on a fairly regular basis on this site. I certainly can associate with a lot of the questions as this issue is only next to “cam selection” in complexity. There is a tendency to “over carburetor” because most people think bigger is better. Well, size does matter, but in the case of carburetors, if they are sized too big, you’re pouring gas “through” your engine rather than optimizing the combustion characteristics of atomized fuel at a point where the most energy may be realized. To get real “serious” about carburetor sizing, it is important to have at least a basic understanding of “volumetric efficiency”.

If you want a real detailed explanation (college level) look at: Volumetric Efficiency:* Calculating your cars volumetric efficiency

If not, here’s my simplistic (layman’s) explanation that may be beneficial: Volumetric efficiency (VE) is a measurement of how well an engine can move atomized fuel into and spent gases out of the cylinders. VE is a ratio (measured as a percentage) of what quantity of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. If an engine creates higher induction manifold pressures (above ambient air conditions) it is possible efficiencies greater than 100% - however it is unlikely in normally aspirated engines. Engines with higher VE will generally be able to run at higher RPMs and produce greater output.

Stock (carbureted) engines typically have VE of .75 - .85 (our German and Japanese friends have several engines in the .9 and above - but they are turbo’ed, i.e., forced induction)

A good build may get a normally aspirated engine in the .85 - .90

Full tilt boogie build (like Pat and Jerry build) will be in the .90 plus range

It sounds like you’ve got a pretty healthy build on your 383. Without knowing the specifics I’d not try and second guess your builder so I’ll make some assumptions and offer my opinion.

A 383 of your build is probably in the .85 -.9 range. At .85 with a maximum RPM of 7,000 – you’d be well served with 650CFM. At .9 we would push 700CFM at 7,000.

All that to say – I think your 800CFM (1412/1413) Edelbrock is a bit much and you’re probably not reaping any benefits over a 650CFM.

I like Edelbrocks but have found they tend to run rich out of the box and more often than not, I’ve re-jetted and installed new metering rods to prevent them from single handedly supporting OPEC. Your builder may have addressed this condition and if you’re happy with the performance, well just driver ‘er hard and enjoy!

Regards,
Glenn

Thanks Glenn, I had no idea all that came to play

I actually think carbs are built backwarks to what is truly best for our needs(street driven)--I like the idea of the q-jets and edel but think they are designed backwards--I would like a bigger primary side to stay reasonable efficent at low-moderate cruise speeds and add the smaller richer calibrated secondaries for WOT ops---I went round and round with some of the guys at Edelbrock back in the early seventies about the cfm size we wanted for our trucks towing the dragster/funny car. I wanted 800 to 850 so we could run in the more effiecent /leaner primary side and only use seconary when passing/hill, etc.


However------they all continued with the BIG seconary deal so now we're stuck with overly rich calibration at upper crz rpm as the seconaries start to open and rediculously rich at WOT--which in my opinion is about the same as trying to crz on a 200cfm carb

I concur with Jerry - but I do so like the sound of the Q-Jet secondaries opening up when you punch it!

you'd also be hearing my exhaust back there with that q jet

Don't you mean quadra BOG?

Originally Posted by 34_40

Don't you mean quadra BOG?

That's funny - however; most of the time Q-jets get a bad rap from those who "just don't understand them". Set up correctly, they are one fine carburetor.

Originally Posted by glennsexton

That's funny - however; most of the time Q-jets get a bad rap from those who "just don't understand them". Set up correctly, they are one fine carburetor.

I would agree with you.. That's just the name we gave them up here! Some like the sound of those big secondaries...