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Old 04-17-2004, 11:42 AM
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600cfm Carb question

i was told that for my 289 ford mustang engine which is pretty much stock that i should run a 500cfm carb because if i dont i will need to upgrad to aftermarket notched pistons and a 3,000 RPM stall torque converter.

Is this true, should i not go with the 600 and stick with the 500.

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Old 04-17-2004, 11:54 AM
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The best advice I can give you.........never take advice from someone who tells you that notching the pistons is required when going to a larger cfm carb.

There is a formula for carb matching.


cubic inches X maximum rpm divided by 3456 = recommended cfm


289 ci. X say 4500 divided by 3456 = 376


The 500 should be fine. When you start adding different things such as camshaft and compression then you should be more concerned with upgrading the carb, IMO. Even at that point notching the pistons and adding a higher stall is not necessarily required. So if your bent on using a 600 cfm carb go for it. Good luck with your project.


357ford
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Old 04-17-2004, 02:47 PM
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Agreed, assuming this 289 wont be running past 5000 RPMs, 500 cfm is more than adequete. notching the pistons have nothing to do with carb size.
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Old 04-17-2004, 10:12 PM
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Re: 600cfm Carb question

Quote:
Originally posted by calimstng66
....... i will need to upgrad to aftermarket notched pistons and a 3,000 RPM stall torque converter........
I'm surprised the person who told you this didn't mention the need for chrome muffler bearings with the bigger carb.

Next time you run into that gasbag let him know you took his advice and things are running real good now!

BTW, here's a link to a good article on how to properly choose a carb size:

http://www.truckpulls.com/Carbs_How_Big_Is_Too_Big.htm
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Old 04-17-2004, 10:27 PM
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Hey Joe! Some GOOD information at your link, worth a look for someone trying to pick a carb!!!
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Old 04-18-2004, 05:21 AM
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Arrow

If you are considering future power upgrades, a 600cfm (preferably HOLLEY) should be installed. This has been the norm since the sixties.
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Old 04-18-2004, 11:40 AM
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Quote:
Originally posted by OddRodder
Hey Joe! Some GOOD information at your link, worth a look for someone trying to pick a carb!!!
Thanks. I think so too!

It's okay to use the old "equation" for a stock grocery getter. You can certainly get by sizing a carb that way, but for peak performance there are many factors to consider.
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Old 04-19-2004, 08:17 AM
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I don't get it??

The link states "Carb formulas are useless?"........

then goes on to say...Yet dyno-tests show the formula is an accurate reflection of an engine’s airflow needs.


I agree, though the forumla is only good for a general carb choice, as they only come in certain variations 500/600/750/etc. cfm and a couple variations in between so your not gonna find a 376cfm carb. If your at the point of running a high rpm engine, then your probably tuning/ have adjusted the carb in some way, anyways.



rustyford
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Old 04-19-2004, 08:25 AM
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actually I have always found that formula to be the work of a quack!

for a 289, a 370 CFM carb would be seriously restricting it.

for a 289 I would say 600CFM would be right around the mark.

289x2= 578 so 600 is right around there.

Did the guy remind you to grease those muffler bearings as well?
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Old 04-19-2004, 09:16 AM
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Quote:
Originally posted by 357ford
........"Yet dyno-tests show the formula is an accurate reflection of an engine’s airflow needs"...........
I think there may be a distinction between "airflow needs" and giving an engine what it wants relative to optimum performance.

The "equation" was formulated to help the average guy put a carb on his engine that would work okay and get the job done without too much tweaking.

I don't know anyone running a high performance engine that makes a decision on carb size based solely on the equation.
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Old 04-19-2004, 04:26 PM
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I agree & I dont use that formula, everyone has there own opinions on what works. In my opinion(my formula) a 289 that never sees past 5000 RPMs needs 434 CFM... so 500 should be more than plenty. I personally couldnt justify 600 CFMs on a 289 unless its going to be run to 7000 RPMs. Now, thats not to say that a 600 wouldnt work, Im sure it would, I just feel that thoratle response, overall performance & MPG will be better with 500 CFMs. If you dont feel 500 CFM is enough, the 570 Holley Street Avenger surely well be & its a great carb. John
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Old 04-19-2004, 04:41 PM
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Carb size

Hello 66; Consider the following and think for yoursef abut it.

How Much Airflow Does an Engine Need?

CASE #1: Street Engine with a dual-plane intake manifold--

The induction system should be able to draw in 120% to 150% of the engine's total air requirement. In other words, how much air can the engine pull in at its top RPM? Take that number, and multiply it by 1.2. Then multiply that same number by 1.5. The engine needs an induction system that will breathe between those two numbers.

In my case: 1097 x 1.2=1316 & 1097 x 1.5=1646, so: Range=1316-1646 cfm. I am a little above the middle of this range: 1316 + 1097= 1481 which is good. This figure is also confirmed by my Dyno 2000 programd which claims a 5% accuracy. Dyno 2000 is in widespread use and apparrently solid as a baseline for race engine building.

Personally, I am confident that I need only 2 x 750 cfm = 1500 cfm to more than meet my needs. If anyone out there has better information that is supported by scientific fact via research and experience I would like to hear about it myself!

Bigger is not better with carbs. To get all the engine has to offer the CFM must match the engines demand. To exceed the required figure will simply hurt overall performance, particularly at the lower end and also each time you shift into a higher gea and the rpm,s drop.

With an oversize carb, as the airflow drops (putting around and rpm drops when shifting), the VE takes a beating and throttle response falls off. You want to keep the air flow through the carb venturi up there so the fuel does'nt start dropping out of the air stream and puddling in the runners...if it puddles then you are running rich and performance suffers.
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Old 04-19-2004, 09:34 PM
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To over-simplify carb selection, TRY THIS, estimate your engines H.P. but don't over-estimate!!! Now, double the h.p. figure to get a close c.f.m. requirement for a RACE application!! For a street application drop an additional 10 -- 15% from that figure!!


400h.p. X 2 = 800c.f.m. for a race engine

minus 10% = 720c.f.m. for a street engine

Try it, you'll Like It!!!

P.S. This doesn't necessarily hold true for super--turbo charged engines!!!
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