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Old 10-14-2008, 11:21 PM   #1
zggill04
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Default 750 cfm Demon carb too much?

Is a 750 cfm Demon carb too much for my mild built 351W? A member of my club is selling one for cheap and I thought I heard people say that 600cfm Holley's take away from some performance on the 351's.
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Old 10-15-2008, 12:03 AM   #2
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depends on a lot like your heads cam intake etc etc.
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Old 10-15-2008, 06:15 AM   #3
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If it is truely a mild built 351w and you are using on the street, that carb is a bit too much. A 600 or 650 Holley is a better choice, unless you are racing it regularly.
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Old 10-15-2008, 06:21 AM   #4
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Fakesnakes, your one heck of a knowledge base for me and I appreciate it! I'll stick with my 600 Holley then. Now switching gears here for a second. Do you think it would be too much for a 406 Chevy engine? My buddy has one it's got a descent cam in it with some other work, he is going to do some head work here shortly. He has a 625 Demon he was going to put on, I was thinking switch him carbs if the 750 wasn't too big for him.
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Old 10-15-2008, 06:31 AM   #5
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that might be a pretty good idea. you'd love that 625Demon, which will be too tight for him. 750 sounds ok for 408 with decent cam and heads. friend of mine's got a 396 with a 750 and runs good. but don't know what it would be like with a 700cfm ..

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Old 10-15-2008, 06:39 AM   #6
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Glad I can help!

Use this chart, but be aware that your combo is probably only 80 to 90% VE. Not because you have a bad set up, but because that is the normal efficiency of a mild engine. Pay no attention that it is from a Buick site!

http://www.buicks.net/shop/reference/carb_cfm.htm
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Old 10-15-2008, 09:25 AM   #7
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Hiya .. this is getting a cfm discussion again :-)
the link fakesnakes gives matches what about every cfm calculator would suggest.
so for your 351 stock/mild and VE of 75% you should be needing a 380cfm carb at 5000rpm max.
That is a theoretical value of what the engine will need (based on cylindervolume and speed). Fact is that a carb with higher cfm rating will perform better at higher rpms as there is less resistance causing intake manifold vacuum. At wide open throttle there should be none. But if you stay on the lower side of things you will get a much better throttle response. About the same as what people call the straw effect (try drinking water out a straw that is 2" diameter ....)

So that said if you call up holley who manufacture those things they will suggest 650cfm for your engine. If you listen to barry grant then one of the most common problems with carbed applications is overcarbing.

As you have 2 worlds in favour I believe it's a good idea to stay in the middle. For my 289 the 525 demon would have been perfect, but I got that 570 cheap. For your 351 the 600-650 range migt be perfect. A 525cfm might even perform fantastic, but what we all agree on: 750cfm for a mild 351W is wayyyy off :-)

I have no experience on this. What you see above is the summary of reading on this forum for about 2 years worth ... ;-)

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Old 10-15-2008, 09:44 AM   #8
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All good info.

Now this problem leaks over to my buddies car too. It's a 1970 Monte Carlo, and he has the 400 sbc, yes small block, not big block, pushed to a 408. He's been pushing carbs for a little bit, but sticks to around the 600cfm. At the track he tends to brake up in the upper end, and I've always thought it was just starving up there. Would a 700cfm be over carbed for his application? He's got a little bit more build with a final touch of aluminum heads coming sometime in the near future. But he has forged internals, dome pistons, cam, blah blah blah. What do you think trading the 700 to him for the 625 demon? I'm not looking to screw the pooch on this one, he's like a brother to me.
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Old 10-15-2008, 10:26 AM   #9
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I'd say have your buddy join one of the many Chevy forums out there to get a real educated answer.
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Old 10-15-2008, 10:33 AM   #10
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Short answer is yes unless you are using a turbo or blower. If naturaly aspirated, your 351CID need a 650cfm, no more.

On a 400, I would say 700 possibly a 750. I have a 750 on my 383 and it is really too much. I have atomization problems that I could probably cure with a 700. Make and brand don't matter when it comes to carb sizes.

The only thing that matters is how much air is entering the intake and how fast. You want the aire to be as fast a possible which requires, believe it or not, a smaller opening. A big opening has plenty of potential but no velocity. When you over carb, you loose air velocity which causes the carb to not atomize the fuel into the air. You get big drops of fuel entering the intake. Mist burns, drops do not! Thus overcarbing is bad. The trick is to have the smallest hole, or venturi, without creatating a restirction where you engine cannot pull enough air in. The amount of air required by a given engine is based on CID (cubic inch displacment) and VE (volumetric efficiency). CID is easy. You have a 351 and your friend has a 400. The hard part is the VE. That is determined by your intake, heads, and cam. Unless you have a NHRA ProStock car, you do not have more than 100%VE without using power adders. Also the VE of an engine is going to change throughout the rpm band. Most engines max out at about 88-90%. Below is a link to a carb calculator that should help get you in the ball park.
http://www.csgnetwork.com/cfmcalc.html

One other thing. Have you ever wondered why the ProStock NHRA guys rev their motors in neutral? It is "clear" them out. They are overcarbed at low rpm due to low VE at low rpm (high power band) so wet fuel is entering the cylinders which washes the cylinders down and does not burn well. They rev the motor up after idling a while to blow that wet stuff out and prevent floading the engine. At high rpms, the carb works the way it should because their VE comes way up and the engine is happy.

Over carbing is bad, but in race cars you be over carbed at low rpms so you have it right in your racing band. On street cars, we typically do not have the problem.
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Old 10-15-2008, 10:33 AM
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