Carburetor advice

[Paul A.]

Looking for some advise on the Holley Carborator that’s on the Vette when I got her. 66 427/390, manual with a roller cam. No information on the cam unfortunately. The carb is a Holley Street Avenger #4150 770 cfm. Would that carb be considered to big for my engine combination? Car will definitely not be used as a racer. Just for local car shows.

[Mark E.]

Re: Carburetor advice

The original Holley is 600 cfm, replaced by a 750 cfm Quadrajet in 1968.

[Owen L.]

Re: Carburetor advice

The original Holley is 600 cfm, replaced by a 750 cfm Quadrajet in 1968.

Just some anecdotal feedback: I have the original style (holley replacement 3811) for my '67 as-factory 390 and it runs perfectly fine and very strong with the 600 cfm. In my opinion, unless one is driving flat-out (and even then) the engine is not going to be able to use 750 or 770 cfm; it just doesn't have that amount of air flow requirement.

[Mark F.]

Re: Carburetor advice

Just some anecdotal feedback: I have the original style (holley replacement 3811) for my '67 as-factory 390 and it runs perfectly fine and very strong with the 600 cfm. In my opinion, unless one is driving flat-out (and even then) the engine is not going to be able to use 750 or 770 cfm; it just doesn't have that amount of air flow requirement.

Agreed, Owen...

Numbers in this chart created using the calculator on this site...

https://racingcalcs.com/cfm-cubic-feet-per-minute-calculator/

CFM at RPM 427 Cu In_001.jpg

[Paul A.]

Re: Carburetor advice

Sorry all for just getting back to this post. I was thinking along the Holley 600/650 tops cfm. Very useful chart Mark. Ecklers offer this model. https://www.ecklerscorvette.com/1966...-113510-1.html

[Larry M.]

Re: Carburetor advice

If it has vacuum secondary, then the (properly adjusted) secondary will only open as far as the engine requires for air/fuel flow. If it doesn't bog the car when you floor the gas, it is likely just fine.

The factory Z/28 back in 67-67 time period with the 302 engine had a 700-780 CFM carb on it from the factory. It ran around town just fine.....with plenty of capacity for 7000 RPM shifts.

Larry

[Duke W.]

Re: Carburetor advice

The defacto industry standard for measuring four barrel carburetor air flow is 1.5" Hg depression and 3.0" for two-barrels, and flow varies with the square root of depression. As a general rule of thumb a high performance road engine carburetor should provide the engine's maximum air appetite at no more than about 1.5" Hg depression and 1.0" or less on a racing engine.

Figure on a stock vintage high performance engine volumetric efficiency is about 85 percent at the redline so:

CFM = 427/1728(5300/2)(0.85) = 557, which is pretty close to what the graph shows in post #4, and the 600 CFM Holley offers adequate air flow for a production L-36.

Duke

[Mike T.]

Re: Carburetor advice

Paul - When I bought my NOM 65 396 Roadster a couple years ago, it had a warmed over 454 in it. It had been opened up to 467 cubic inches and had a fairly warm hydraulic cam in it and around 9.2 compression. The carb that was on it was a Holley Street Avenger 770. The stock jetting on that carb is 72 Primaries and 75 Secondaries. I didn't have it running long before I did my usual, which was to tear it apart...and I'm 'still' working on it, but the plugs looked a little black and sooty but most of that is probably due to living here in North Central Arizona at the 5500 foot altitude. I changed the jets to 69 on the primaries and 72 on the secondaries and it ran fine, except for one small issue. It had this annoying bog when you gave it gas and the carb was practically brand new, accelerator pump arm was adjusted properly, timing had been double checked, no vacuum leaks, etc. What I found was the squirter on the 770 is quite small so I ordered up a larger one and the bog went away.
Having a 770 on your 427 may not be a bad thing, maybe evaluate it first before looking for a replacement. Plugs don't 'color' like they used to in the past but they will still indicate if you have an fuel delivery issue, both too much, and too little. Let us know what you decide.

[Gene T.]

Re: Carburetor advice

Paul, I have called Holley Tech Support on numerous occasions when I had an unique question. Their number is 866-464-6553

[Paul A.]

[Mark E.]

Re: Carburetor advice

With vacuum secondaries, or with a vacuum controlled air door like the Q-Jet, the engine will run fine without bogging even if the carburetor's capacity is greater than needed. For example, the mild 1969 passenger car 350/250hp runs fine with its 750 CFM Q-Jet. This design is essentially CFM on demand.