Had some questions on carbi selection so expanded on and added more detailed elements and approached it slightly differently to my full article. Hope it helps.

Enjoy.

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One of the most common performance upgrades applied is to install a Holley (or equivalent) four barrel carburetor. The early V8 Mustang's generally were supplied with either the two barrel carburetor for the standard models and a four barrel carburetor for the higher performance models. So how do we select the correctly sized carburetor for our engine? As expected it depends on a number of factors such as the air flow required at the maximum HP RPM peak, the intended use of the vehicle, and the design of the carburetor.

The first step in this process is to calculate the air flow required by the engine at peak HP RPM.

Formula for air flow required by your engine.
CFM Carburetor flow (minimum) required.
CID Engine Cubic Inch Displacement
RPM Peak RPM
VE Volumetric Efficiency of the engine
CFM = (CID * RPM * VE) / 3456

The volumetric efficiency of an engine is a representation of the power potential the engine. For example, a stock engine is relatively low powered hence would have a VE of 0.70 to 0.75 while a mild street engine would be 0.80 to 0.85 and a serious street performance engine would be 0.85 to 0.95 and racing engines would be in the order of 1.0 to 1.20. The engine's average piston speed can be used to determine an expected VE range. To keep it simple use 0.85 for a mild street only engine.

So applying the formula to a 351 with a target peak HP at 6000 RPM and expected VE of 0.85;

The engine requires a calculated flow of 520 cfm while this is an excellent starting point it is certainly not correct to select a 500 cfm carburetor. This is a common mistake as the formula has calculated required unrestricted flow.

A carburetor by its nature contributes a level of air flow resistance. In fact the carburetor would not work if there were no pressure drop. The most important aspect of the carburetor is its ability to atomise the fuel properly in the venturi boosters. For example, employing to large a carburetor could result in it being unable to get the air shear required to atomise the fuel. Alternatively, installing to small of a carburetor will restrict air flow affecting the full performance potential of the engine. The choice needs to find the balance of these two requirements.
Traditionally, the CFM rating on Holley carburetors is calculated with a vacuum pressure drop of 1.5in/Hg for a four barrel carburetor and 3.0in/Hg for a two barrel carburetor. The higher the pressure drop the higher the restriction. For example, a 500 cfm two barrel Holley does not flow the same air volume as a 500 cfm four barrel Holley. For example, to calculate the equivalent cfm at 1.5in/Hg use the following formula;

Formula for conversion of flow at different pressure drops
CFM Carburetor flow recalculated.
CFMA Carburetor flow initial
PD1 Pressure drop initial
PD2 Pressure drop target
CFM = (CFMA / SQRT(PD1) * SQRT(PD2)

So applying the formula to the example 500 cfm two barrel Holley for equivalent from at 1.5in/Hg vacuum;

CFM = (500 / SQRT(3)) * SQRT(1.5)
CFM = 354 cfm

The 500 cfm two barrel Holley is flow equivalent to a 354 cfm four barrel Holley carburetor.

Modern carburetors are able to atomise the fuel perfectly well with far lower vacuum pressure drop signals than 1.5in/Hg. This is due to improvements in many design areas especially in the design of the boosters. As a result instead of using an old carburetor that required 1.5in/Hg vacuum pressure drop to atomise the fuel, a replacement carburetor capable of the same atomisation at 1in/Hg can be employed. For example, it would be appropriate to replace an old 60s 650 cfm Holley carburetor with a modern carburetor (such as a Holley HP, Quick Fuel or AED). In this example, when only requiring 1in/Hg (or less) of vacuum pressure drop it results in being able to install a 20 - 25% larger carburetor that at wide open throttle (WOT) has a 1in/HG pressure drop. In this case, a 750 cfm carburetor is appropriate having in lower air flow resistance resulting in providing further performance potential.

Applying this knowledge to the earlier example of the 351 engine, the air requirement was calculated to be 520 cfm at 6000 RPM hence using the guide of 20 รขโฌโ 25% larger flow requirement results in a recommendation of a 650 cfm Holley HP or equivalent carburetor. This is assuming the engine is under low vacuum during WOT. If vacuum is higher than 1in/Hg then a larger carburetor can be used unless other restrictions exist. For example, the manifold port design is restrictive in which case potentially minimal benefit will be gained from a larger carburetor.

In summary recommend guide to apply is;
1. if installing on a single plane manifold use a mechanical secondary carburetor with a 25% factor and
2. if installing on a dual plane manifold use a vacuum secondary carburetor with a 40% adjustment factor.

Note: A mechanical secondary will provide greater power while a vacuum secondary in this case would provide drive-ability flexibility where a smaller mechanical secondary carburetor would be more responsive in the lower to mid rev range.

The post has removed any mystery to the process of carburetor selection. It has shown selecting a correctly sized modern Holley carburetor involves calculating the peak HP air flow required then applying the appropriate size adjustment factor. The resulting carburetor will result in a balanced fuel delivery system while minimising pressure drop for enhanced overall performance.

There is definitely a to big size and there is definitely a to small size - you need just right.
The Flow formula is accurate though as I stated the context does not match with the ratings of the carbi's. Which is the issue as many do not understand and so they will install a carbi that is to small. To big is harder as at WOT you want to minimise the pressure drop preferably to sub 1in/Hg more like 0.7in/Hg . Above I am suggesting to work to 1in/Hg as we are talking street driven cars but lower is needed for maximum peak RPM HP. For example, a 347 with good heads (such as TFS TW 205) and decent camshaft (such as 226/230@50 600 thou lift) using a ported Vic Jn intake will run well mid range on a 650DP best all round on a 750DP but will produce the highest HP (6500rpm) with a 850DP or bigger. Dynos show this all the time - as long as the heads and intake are not a restriction the pressure drop can be minimised and HP maximised. Now this will be harder to tune for low RPM and idle which is why I like to suggest you aim for 25% above the suggested formula. So an engine as defined would have a VE of .95 peak 6500 = 620cfm (unrestricted) so we are looking to finding the flow at 25% higher = 775cfm carbi.
A good choice would be a 750DP for all round usage or 850DP for higher peak HP.

So Boof, let me understand what you are saying. If Summit have the 1050 dominator on sale you are saying that might be too big for the 302? What about even if I can get it for 25% off, is it still too big?

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So Boof, let me understand what you are saying. If Summit have the 1050 dominator on sale you are saying that might be too big for the 302? What about even if I can get it for 25% off, is it still too big?

If your 302 pulls 8000 rpm it could well be a very good choice for maximum HP.
Calc shows 700 cfm unrestricted this then is expanded 25% (to make it equivalent to 1in/Hg pressure drop) which equals 874 cfm. A little more for the extra low pressure drop - sure it would work well for a top end 302.

Then again why calculate it - discounted so yep its the one to get.

I will someday think of something clever to say.

Last edited by boofhead on Thu Dec 15, 2016 11:10 pm; edited 2 times in total

Sorry Boof, but all your wonderful calculations mean diddly when a bunch of blokes are staring into the engine compartment going ooh ahh. It's bling and size that count! Shiny is cool and bigger is always better!!

My Motto - Don't get Caught! If you do, Blame Someone Else!