CFM Calculation

It is better to under carb than over carb Bruce. How much time are you going to spend wide open?

Could be true,maybe like HP,advertised cfm is more than actual? What did Reid's partner Matt do for PCM? You need to hear from someone who actually delt with its design

Pete, I think the YH's are rated at 128 CFM, so 2 are more like a 2 barrel, which is fine as you can under carb and still run great.

If you go by street numbers, the marine engines I've noted are over carbed by 20%-78%. The 240 hp 351 is way over carbed in theory, but without doubt runs great with the 600, so it's hard to figure any rhyme or reason for carb selection.

I'm trying to figure out if a 750 may be a good selection for a 300 hp Interceptor.

Gary S wrote: 8122pbrainard wrote: It's probably due to the mussel car era when more carbs or bigger ones were better! Must have taken a lot of "clams" to get one of those------

Opps!!    I'll blame it on the spell check!

One interesting note: the Pro-Tech throttle-body fuel injection that was on my 290 HP Pro Boss engine was a 900 CFM. I got this number from PCM when I was making the painful decision to retro-fit mine back to carb. Based on that 900 cfm number, I originally looked to a 750 cfm carb. However, after performing a cfm evaluation, I went with the standard 600 cmf Holley 4160.

(If I had to do it all over again, I would probably go with the QuickFuel 600 cfm unit)

JQ

Not true.

Cfm is always the measure of air ingested by the engine (or the max amount of air that can flow through the throttle plate), it has nothing to do with fuel. The fuel metering on a carburetor can be adjusted just like tbi- it just can't be done in real time.

Unless you want to limit an engines performance, you will always want to have a carb capable of flowing more air than the engine is capable of processing (basically a factor of its displacement and how fast it spins). This is not the definition of "over-carbed". That would imply that the carburetor is oversized to the point where performance is harmed. That doesn't mean it runs poorly, just less than optimally (so the "my boat runs just fine with a carb that is too (big/small/whatever)" is moot). Sizing a carb can be tricky, and it depends on a lot of factors- trial and error included. It is much easier to identify an "under-carbed" engine than the inverse.

What Tim said. You have to watch closely using those estimators because they are set up usually for optimal usage on a drag car (ie top rpm crossing the 1/4 mile traps).

Example: Joe's boat only runs a 650 Demon (Correct Joe??). This boat makes nearly double the horse power of most of our stock boats but still lives a very healthy and productive life with only 50 more CFM than our 240-285 hp boats.

Like Tim said, when upgrading engines or trying different things there are a lot of trial and error processes especially with carburetors and the different types of carbs when you get out of the realm of stock style carburetors and move more towards down leg boosters, primary and secondary air bleeds, etc.

I would be completely fine with a 600 CFM carb on a 300 HP 427. Mark's footer loves the 600 CFM QF marine carb and runs hotter than it did with the brand new holley 650.

Plus, nobody really makes any good carbs in the less than 600 cfm flavor for performance applications.

Carb ratings are a bit wonky… First you have the marketing factor, was it actually 650 cfm or was it 630 or 670 simple rounding gets you the same place with either result and when there is marketing group involved you can expect rounding, plus a finger on the scale. Then of course there is the relation to the rating method to how it will actually be used. In this case that rating method is tenuous at best, by convention 1 and 2 barrel carbs are usually rated differently than 4 barrels, 1 and 2 barrels were rated at the amount of air that would flow through them with 3 inches of mercury pressure drop across the venturis. Later 4 barrels were typically rated at 1.5 inches of mercury pressure. The only time either of these numbers can be applied is under full throttle circumstances where all throttle plates are Wide Open.    One can move more or less air through the venturis by changing the pressure drop across them, so a carb rated at 650 cfm at 1.5 inches of mercury of pressure may move produce 919 cfm at 3 inchs of mercury pressure drop. What is the difference between that 1.5 in of mercury pressure drop to the big air pump that is your engine? It will likely mean that your engine sucks in a charge approx. 5% less dense. So let us say that you are really requiring 700 cfm out of that 650 cfm carb, the additional pressure drop as a result of the carb is going to be considerably smaller maybe enough to decrease the density of the charge by 1%. That may translate directly into engine hp if both charges are equal in quality and there are not exhaust limitations…

Generally pressure drop is not your friend, but neither are poor quality fuel mixtures due to low flow velocities in the venturis.    The carb is of course not the only point of pressure drop between the cylinder and the atmosphere, valves, intake, spark arrester, bilge ventilation all play a role.

Realistically a properly designed 4 barrel carb does a very good job at keeping the fuel flowing and reasonably well mixed at a large range of flows/velocities so they don’t need to make a bunch of different sizes to insure minimal pressure drop and good fuel mixing.

IMHO, 450 is generally sufficient for a 302 with reasonable head room, it is just a tad too small to never be a significant restriction on a 351 (although you would likely still have a very useful 351).    So they jump to 600 which was the next relatively available size, depending on rpm that 600 will take you up to most 400cu inch engines before you start looking for larger bores, you might squeak by a 427 with a 600, but a 650 is probably as low as I would go. And if it was me that would even depend on the intake manifold/cam/exhaust, with a dual plane intake manifold but good exhaust I would probably go bigger on the carb. But again all 650’s aren’t equal, even within a brand a 650 with downleg boosters will have less pressure drop at a particular rpm than a 650 with larger annular boosters. The quality and design of the carb also comes into play, a smooth venturi without a choke horn on it can be expected to cover a larger range more effectively, not simply work at its rating point.   

If you are just looking to try out a 750 cfm edelbrock I can lend you one, its automotive in nature but I could swap in the jets and rods to make it the same as the marine tune and it would give you an idea as to whether you wanted to spend money on a marine version for day to day use.

I have a Quadrajet in my jet boat right now, stock 454. Before I had it rebuilt I had thought I'd switch to a Holley with mechanical secondaries, seems like I'm into the secondaries so often in that application that the efficiency of vacuum was negated. The guy I talked to about this suggested I stay with the Rochester, that they performed very well as long as the engine was mostly stock, providing a lot of vacuum.

In reading about these carbs, which have noticeably small primary bores and oversized secondaries, the claim is that though they can flow 750-800 cfm, they can work very well on smaller engines as well because the small primaries will flow less, and the large secondaries will only flow what the engine "asks" for.

I'm wondering if any vacuum secondary carb will work this way, only open up enough to flow what the engine needs, so that there isn't a reason to limit carb size to 600 cfm if there's any thought that it needs more. Makes sense as I think about it.

Perhaps noteworthy is a true dual plane setup will require a larger cfm carb than an open plenum, for the same reason IDF setups require larger CFM throttles.

As David suggested, no way would these boats would be as precise at 30-36 mph if they were into the vac secondaries due to cutting the cfm too close.

This 800 cfm list 9022 is what PCM put on the LH HP454 engines.
It is also sometimes listed as an 850cfm depending which holley document one finds.

My friend has a spare he keeps in rebuild-rotation for his boat.
Tim if it ever becomes available, i'll let you know!

https://www.holley.com/products/fuel_systems/carburetors/specialty_carburetors/marine_carburetors/parts/0-9022

8122pbrainard wrote: 63 Skier wrote: I'm into the secondaries so often in that application that the efficiency of vacuum was negated. David, I'm sure the carb pros will speak but I have a feeling you're into the secondaries so much because of the higher RPM's the jets run in. Yes, they do go through the fuel and you almost need to own a gas station to run them!

Yes, that's exactly why. They build rpms instantly and at low planing speed, in my boat around 24-26 mph, I can maybe stay on primaries but anything more on the throttle and I'm sucking down fuel. Not a problem, part of the fun! But, it does bring into play the discussion of mechanical vs. vacuum, I lean toward mechanical for jets.