Things I'm learning about Holley carburetors

[1987cp]

Care to clarify that point?

[import convert]

you know how many guys come into work and if carbs comes up and i say i got a 600cfm carb theyre like "smart kid". sometimes i think people turn the idea of having a big carb on a motor as a cool factor thtough they dont need it.

[P72Ford]

The Monte Carlo has a single 750 DP on it. It is a basic 350, but even with it occasionally tickling 6800 rpm, it is overcarbed.

But, that carb is fan-fucking-tastic. Aside from idle adjustment, its in its 'out of the box' tune. I don't think it could run any better, honestly. Idle is fine, but most importantly, it pulls from off idle to 6500 without hiccups, dead spots, etc.

We had 2 Holley 450s on a tunnel ram (I had nothing to do with that), and it obviously ran like shit. I was set against it from the onset, and wasn't even interested in trying to tune it to run right. A single carb is the way to go for our application.

[tamalpass]

great dicussion!

I figure if our 440 beast only had a 750 and ran as hard and fast as it did, it makes since that a 351 would need something smaller. 500 CFM should be fine for my 83' 351.

I'm learning a lot on this forum, and remembering quite a bit I'd forgotten.

[import convert]

well on a six-pack 440 arent the 2 secondary carbs 250s and the center is a 350 i think
(pobably wrong) and they made decent power in stock form. ive seen them done with heads cam and intake and use the setup i just said and make over 400hp easy

[1987cp]

I'm almost certain that's correct, and even that is 850cfm! That one truck Ivan (smoke'n'rattle) had for a while had a Edelbrock 500cfm on a 429 which I *think* he said had highish compression, and he liked that carb very well on that engine.



Other factiods to add to my list above:

- Holleys have a nice provision for much more precise acclerator pump actuation than I'm used to with my Edelbrock AFBs. There are only two sizes of pumps, 30cc for most applications and 50cc for the serious gas hogs, but the selection of seven different pump "cams" for the 30cc pump plus two more that fit only the 50cc pump, each of which can be installed in two different orientations for differerent pump actuation rates, allows theoretically for up to 18 different possible pump-actuation profiles!

- In addition to having accelerator pump squirters in different sizes, they are available in at least three different styles: "straight type", "tube type", and "anti pull over".

- There are three common types of power valve. Later style valves have 4 large holes below the base of the threads, early style have 4 small holes, and current style ones have oblong "windows" in leiu of drilled holes. Different power valves are identified by two digits stamped somewhere on the valve, as in "50" for 5.0 inches Hg or "65" for 6.5 inches Hg.

- As of the time of publication of this book, Holley offered three sizes of Viton-tipped needle and seat assemblies, which in addition to a letter stamping (D, J, or H), can be identified by inserting a drill shank into the drilled hole in the seat.

- Non-petrol applications must generally use a steel needle and seat with no Viton in it.

- Aftermarket suppliers produce needle and seat assemblies that are generally very close to a "D" assembly in flow capacity, but these are of a design that cannot be easily be measured with a drill shank but must instead be referenced to manufacture advertised flow rates or equivalencies or else suitability determined by road testing.

- All Holley four-barrels have idle feeds in all four venturi bores. Most models have mixture adjustment screws for only the primary side, but some have idle mixture screws on the secondary side as well.

- All metering blocks have a place to install a power valve, but the hole is not drilled out and tapped on many blocks intended for the secondary side.

- All mechanical-secondary carburetors have two accelerator pumps ("double pumper"). No vacuum-secondary carburetor is equipped with a secondary accelerator pump.

- Even if you are saddled with a secondary metering plate, as in a 4160, there are a variety of metering plates available with different combinations of orifices, and it is easy to obtain a kit to switch to using a metering block instead.

- If the block you acquire has a drilled and tapped hole for a power valve but you don't need or can't use one on the secondary side, it's easy to install a pipe plug and still use that metering block on a carb that has no provisions for a secondary power valve (as when the carb body has no power valve recess or the power valve recess isn't drilled with a vacuum transfer hole to allow a power valve to operate).

- Earlier reference to swappable air bleeds is a completely diferent matter than the power valve channel restrictin (PVCR), which is effectively the "jet" actuated by the power valve when it opens. Changing the air bleeds changes the amount of emulsified air present in the fuel as it goes to the boosters. For most applications this is not considered a desirable way to change your air/fuel ratio; primary mixture changes should almost always be done by changing jets instead of changing or modifying air bleeds.

- Holley main jets are NOT numbered according to the size of the orifice. For numbers 40 through 66, the number is very close to the orifice size in thousandths of an inch, but outside that range the numbers diverge dramatically (for example, a "100" jet has a 0.128 inch orifice).

- Except for instances where it would be excessively expensive or inconvenient to purchase a large number of jets for tuning purposes, it is not recommended that Holley main jets be drilled to a larger size. The biggest reason for this is that they are actually numbered according to a nominal flow capacity, measured on a flow fixture to a 3% tolerance, with a 4.5% step between nominal sizes. (There is a special "close-limit" series of jets that is flow-rated to a 1.5% tolerance.) Aside from concerns for burrs or swirls that may be introduced into the orifice by drilling, the actual flow rate is said to take into account the shape of the orifice entrance, as each is counterdrilled to a certain depth that varies with different jet numbers. Therefore, the only reason to drill a Holley main jet is if you know your engine needs to run richer and wish to establish a ballpark range before purchasing and testing correctly-sized jets.

[1987cp]

Another factiod not expressly related to Holley-style carburetors is a calculation for fuel flow requirements. Taken from "Holley Carburetors, Manifolds, and Fuel Injection" by Mike Urich and Bill Fisher, c. 1994, this sample calculation works from a previously established example of an 8000-rpm 350ci engine that requries 811cfm at 100% VE. The authors calculate the max fuel requirement thusly:

cfm * 4.38 = airflow in lb/hr

cfm * 4.38 / 13 = fuel consumption in lb/hr

811cfm * 4.38 / 13 = 273 lb/hr


Assuming fuel with a mass-density of 6.25 lb/gallon, this equates to 43.68 gph. The authors' recommended 6 lb/gallon gives a flow of 45.5 gph.

For comparison, my example of a 5000rpm 306 yields:

391cfm * 4.38 / 13 = 132 lb/hr, or about 22 gph.

Aside from the appearance from my current combo in the Vic that a 30gph pump is insufficient, this suggests the question, why do typical performance fuel pumps free-flow at least 80gph? My kneejerk reaction is that: 1) actual flow as installed is less than rated flow; 2) above calculation is probably for steady-state conditions, and rapidly varying conditions could well produce peak demand at the carb inlet in excess of what the engine can actually consume.

[GM_Guy]

Care to clarify that point?

Not really, but I'm not talking drastic changes (ie: 550cfm to a 1050 double pumper, but to a 650/750 type jump). But next time your at a drag strip inquire about carb usage. Particularily those who spent too much time trying to dial in their by-the-book sized carb only to find stepping up one or two "sizes" improved their track times considerably.

Alex.

[1987cp]

I've never been to a dragstrip. All I know for 100% certain is that on the mild 351 and the 302s I've worked with, a 500cfm Edelbrock drove lots better on the street than a 600cfm, and people here are running carbs as big as 650cfm on milder engines.

[88Vic]

My Silverado has the 260hp crate GM Performance motor with a Lunati Vodoo cam and Flowtech longtubes. I would guess with those 2 upgrades it puts out 240hp to the wheels through the Th350, it has an Edelbrock Performer 1405 carb with electric choke, it's a 600CFM 4bbl. I rarely if ever spin it past 5K, actually i've never gone past 5K except revving it that high a couple times, so it sounds like i'd be better off with a 500CFM carb.

Going by that formula in the first post, did I do that right?

[1987cp]

I got 506, assuming 100% VE, so actual requirement according to the formula is probably between 380 (75%) and 430 (85%). But, the author probably wouldn't have any objection to running a EDL-1403.

BTW, not that it matters, but the 1405 is a manual choke model; I assume you started with that and bought the electric kit to convert it to a 1406? I do keep misspeaking when I say I converted a 1403 to a 1404 with the electric choke kit instead of the other way around.

[88Vic]

Yeah it started as a manual choke but is now electric. It was done by the previous owner, most likely because the truck came stock with an electric choke or no choke as I don't know where the choke would be in the cab.

[ss_mini14]

this makes fuel injection look like a cake walk. personally i'd rather have an edelbrock due to my limited experience with weber carbs but i could be easily persuaded to use a holly if the price was right

[P72Ford]

Not really, but I'm not talking drastic changes (ie: 550cfm to a 1050 double pumper, but to a 650/750 type jump). But next time your at a drag strip inquire about carb usage. Particularily those who spent too much time trying to dial in their by-the-book sized carb only to find stepping up one or two "sizes" improved their track times considerably.

Alex.

This is true.

The difference between theory and the real world can be vast.

[gadget73]

this makes fuel injection look like a cake walk. personally i'd rather have an edelbrock due to my limited experience with weber carbs but i could be easily persuaded to use a holly if the price was right

indeed, there is a reason why I don't own carb cars, and I'm planning to remove them from my boat when budget allows. Carbs are all black magic and guesswork.