The reason I recommended the 670 over the 770 wasn't it's extra cfm capacity. You don't understand carbs I see. Better to go slightly smaller than larger when it comes to carbs. With a too large carb on any engine, you get to a point where there's no pressure differential and that's the point where the engine stops pulling fuel from the bowls. With one that's slightly on the small side, you never get to that point. And the engine will feel a little snappier on the street than one that's larger (you don't go around with the secondaries open all the time anyway) And PLEASE, don't try jetting it down. All you'll do there is lean out the mixture ( bad for pistons) Only way you can tune it smaller is to put a heavier secondary spring to keep the engine's vacuum from pulling the secondaries fully open.
spacers..worth it ? or get a new carb ?
- Thread starter Stangninjak
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- Apr 16, 2004
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Hmmm ok. Well when I spoke with the guys where I ordered from, I spoke with their carb guy to order. I sent my sheet to them with engine specs, because they had some sort of survey going. I ask for the 670, and I was told from them that my build would benifit more from the 770. I explained that I did not want to go to big, that I recieved advice to go a little smaller, but He explained to me that going with the 670 would be good, the 770 would be better as it could be dialed in with the engine better. He also said that the SA's were more tuneable then the double pumpers, so I would still be ok.
Well you are right, I suck at knowing carbs and how they work, but I also bought with a deal of being able to switch out, if the application failed. I spoke with the carb rep/mechanic, and I got a guarantee that it would work, or they would send me a 670. Guess we will see if it gets honored.
So you think the 770 is to big for my app ? Is there that much difference between a 750cfm, and a 770 ? In your earlier post you recomended that I used a Holley 750, before I mentioned the 670.
Well you are right, I suck at knowing carbs and how they work, but I also bought with a deal of being able to switch out, if the application failed. I spoke with the carb rep/mechanic, and I got a guarantee that it would work, or they would send me a 670. Guess we will see if it gets honored.
So you think the 770 is to big for my app ? Is there that much difference between a 750cfm, and a 770 ? In your earlier post you recomended that I used a Holley 750, before I mentioned the 670.
LMan
Founding Member
Just remember that if you dont ask a Ford-specific guy what to use, the 'default' is someone with Chevy experience...and the rule-of-thumb is that Chevy motors need/like more carburation (sp?)/CFM than Ford. A Chevy app that is a no-brainer for a 750-800 CFM carb will call for a carb 100 CFM less in a Ford app.
Do I have tech for this? Um, no. But Ive always heard it from the dragrace people and I think there is truth in it. Just a data point to consider when you are sizing your carb, for what its worth.
In your case, if I had stock-to-mild heads I would go 650. If I had/planned on large heads (2.02 intake) I would go 750. JMO.
Do I have tech for this? Um, no. But Ive always heard it from the dragrace people and I think there is truth in it. Just a data point to consider when you are sizing your carb, for what its worth.
In your case, if I had stock-to-mild heads I would go 650. If I had/planned on large heads (2.02 intake) I would go 750. JMO.
- Apr 16, 2004
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Well, yea he's right too, the 670 was simply my personal choice. After running the six pack setup on my 331 with it's three small 250's and seeing how the engine responded on just the primary center carb, and on the previous 600 I had on my spare 5.0 (which is back in the Ranger now, but with a too big 750) I've been leaning toward "slightly" smaller carbs. Run the 770 with the sec. spring it comes with, then try a stiffer secondary spring and see how it responds. Let me know if it's better or worse. Also run it awhile with the jets it's got, then pull the plugs and see if if needs more or less fuel.Stangninjak said:
I think that's really an out-dated formula for carb sizing. That was true back in the day when Fords were severely handicapped with crappy heads ( I'm just talking about the small blocks here) They didn't need, nor could effectivly use a bigger carb. The Chevy small and big block heads were far and away superior to stock Ford small block heads. Also the reason, the small block chevy guys hated to go up against a 351 Cleveland, the tables were turned against themLMan said:
I've also seen so many Chevy guys dumping their big carbs on ebay after learning smaller was better in some cases.
- Apr 16, 2004
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D.Hearne said:Well, yea he's right too, the 670 was simply my personal choice. After running the six pack setup on my 331 with it's three small 250's and seeing how the engine responded on just the primary center carb, and on the previous 600 I had on my spare 5.0 (which is back in the Ranger now, but with a too big 750) I've been leaning toward "slightly" smaller carbs. Run the 770 with the sec. spring it comes with, then try a stiffer secondary spring and see how it responds. Let me know if it's better or worse. Also run it awhile with the jets it's got, then pull the plugs and see if if needs more or less fuel.
Ok
whew..I was worried I might had screwed up..Well it will be here on weds..and my block should arrive today or weds as well. My intake should be here on thurs. My mechanic bud is real excited about the project, and we are ready to dig in. I am actually feeling real good about this myself, as I luv building a project up, and seeing the results. Heh from a computer systems engineer point of view...But this is sort of the same thing. when I had my cobra, it was sort of a no brainer...chip, pulley, exhaust, hello big power. With this project, I had to take time and research different options, make choices, and the end results will be seen. I feel more "Involved" with Operation "Lift her Skirt" (15years of MP/CID makes you name projects like this....)and I am ready to get this puppy to the track
- Apr 16, 2004
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Did not want to get crazy with it..The stage 1 cam will have some nice kick to it with my other combo's..The car will be a cruiser/driver/streeter/strip. In that order..so I went with that for some compromise. I should still get some good power from this setup, and being that the car is well under 3grand, I should get some decent track times with good driving.
750 is too big.
D Hearne had it right, speed = efficiency which comes down to math and physics, not bench racing stories.
(351cuin x 6000rpm)÷3456=609 CFM
So running a 351 up to 6000 RPM (and that's assuming 100% efficiency) (which ain't gonna happen on an NA motor) will demand 609 CFM. A 600 will work, 650 would give you a little extra capacity, and a 750 is too much.
Your not on a circle track running a constant 6000 RPM. You have to get from idle to 6000 and if your carb is oversized and not working efficiently (as D Hearne was explaining), the guy with the smaller carb is gonna beat you every time.
D Hearne had it right, speed = efficiency which comes down to math and physics, not bench racing stories.
(351cuin x 6000rpm)÷3456=609 CFM
So running a 351 up to 6000 RPM (and that's assuming 100% efficiency) (which ain't gonna happen on an NA motor) will demand 609 CFM. A 600 will work, 650 would give you a little extra capacity, and a 750 is too much.
Your not on a circle track running a constant 6000 RPM. You have to get from idle to 6000 and if your carb is oversized and not working efficiently (as D Hearne was explaining), the guy with the smaller carb is gonna beat you every time.
Your original question. Yes a 4 hole spacer will help. It will give you more torque. Think single plane (high rpm) vs dual plane intakes. Or think of the old Chrysler cross ram intakes where the carb was actually located over the valve cover on the opposite side of the motor. Long individual runners help torque.
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