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carb mods

 
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rascal1911
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Joined: 12 Jun 2011
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PostPosted: Sun Jun 12, 2011 10:04 pm    Post subject: carb mods Reply with quote

Whats gonna work on my SBC? Its a 377 incher. Its got ported eagle heads, a mechanical roller cam, and I am running a nash 4+1 with 3.08 gears in a 12 bolt. The car is a 67 nova, that weighs in at 2800 lbs. I run leafs and ladder bars, with comp t/a drag radials. Whats the real deal regarding carbs? I hear so much contradictory info on these. I have an out of the box holley 850, it runs okay, but the response is crap. WOT seems fine, but anywheres less than WOT is soft and mushy. Any recommendations on a good carb builder?
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10sec.et
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1969 Oldsmobile Cutlass

PostPosted: Sun Jun 12, 2011 11:46 pm    Post subject: Reply with quote

IMO, that 850 is too big. i would start with an "out-of-the-box" 750 DP and tune from there. may want to put some more gear in the car too. 3.08 is for grandmas grocery getter.

welcome to SMOKEmUP Cheers

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SMOKEmUP
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1979 Chevrolet Camaro

PostPosted: Mon Jun 13, 2011 9:50 am    Post subject: Reply with quote

A 377 cu-in motor at 7000 rpm with 95 VE will flow 725.42 cfm.
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Big Dave
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PostPosted: Mon Jun 13, 2011 9:58 am    Post subject: Reply with quote

You have discovered why there is so much variation in your responses that you get when you ask this question. The answer is where do you want the car to work best (at what RPM range)? The cfm demand of your carb increases with RPM. So your 850 works great on a 377 at wide open throttle; WOT (actually at WOT it is a little small). But on the street where you rarely get above 4600 RPM your carb is lazy. There isn't enough air flowing through the venturi to pull the gas out of the fuel bowl and efficiently atomize it.

This is why Holley offers annular boosters with the 850 carb. Annular boosters adds $134 dollars to the price of the carb (which is probably why you didn't buy it when you bought your carb), but it allows a carb that is too large to work well on the street; by more efficiently atomizing the fuel, for better throttle response in the lower RPM's. It also does a better job of atomizing the fuel at high RPM which is why they are used on all Dominator carbs, as gas fumes burn, but liquid gas doesn't.

You can buy annular boosters to install in your carb but they cost more than $134 and the tool required to press them in costs over $200. I suggest you take yours back and trade it in on an annular booster carb or bolt on a smaller cfm carb to give better throttle response on the street and let the smaller carb be a restriction on the top end when racing. Or buy a smaller carb and swap carbs when you go racing.

By the way the Rochester QuadraJet doesn't have this problem. It is a variable venturi carb and the carb flows anywhere from 540 cfm to 870 cfm depending upon the engine's RPM. Just shows you that those Chevy engineers know what they where doing when they designed it. They offered all the power of the Holley 850 and the fuel economy of a 500 cfm carb.

Big Dave
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af2
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1933 Willys Coupe

PostPosted: Mon Jun 13, 2011 6:24 pm    Post subject: Re: carb mods Reply with quote

rascal1911 wrote:
Whats gonna work on my SBC? Its a 377 incher. Its got ported eagle heads, a mechanical roller cam, and I am running a nash 4+1 with 3.08 gears in a 12 bolt. The car is a 67 nova, that weighs in at 2800 lbs. I run leafs and ladder bars, with comp t/a drag radials. Whats the real deal regarding carbs? I hear so much contradictory info on these. I have an out of the box holley 850, it runs okay, but the response is crap. WOT seems fine, but anywheres less than WOT is soft and mushy. Any recommendations on a good carb builder?


That DN with 3.08 is ok for freeway driving. I would run a 3.55 minimum.

You never gave a rundown of the engine and I can't even start until.
Who knows what cam you are running. There are a ton of Mech rollers out there and a ton that try to run wimpy spring pressure. The timing advance on the thing is an unknown also.

I run a 1050 on mine and it is responsive as soon as you hit it.

I think there are other tuning problems is what I am saying. Wink
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rascal1911
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PostPosted: Tue Jun 14, 2011 11:59 pm    Post subject: Reply with quote

The doug nash 4+1 has a "granny low" and close ratio 2nd rd and 4th that gears the car as though it has 4.56 gears. the 5th gear is 1:1, so it ONLY a 3.08 car on the highway. I guess thats not as common of knowledge as I thought. So the car effectively has 4.56 gears.
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squeeezer
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1991 Chevrolet Camaro

PostPosted: Wed Jun 15, 2011 11:08 pm    Post subject: Reply with quote

a good carb spacer (hvh super sucker ie 4 hole) may be good too
for signal response assuming the engine needs it

just a thought

im sure you may need some work as well to idle ,both enrichment circuits, and maybe timing first

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clay
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1972 Chevrolet Nova

PostPosted: Wed Jun 15, 2011 11:58 pm    Post subject: Reply with quote

I could see a 850 being a little soggy on your combination. What does your timing curve look like? A slow curve with a decent duration camshaft can give the same symptoms you are describing. If you have a pretty agressive 377 I don't exactly thing it's to big of a carb. Some booster work on the carb would most likely make a noticable difference. I have had a blow through built by CSU and was very happy with the carb and the service and support I received after the purchase for a few tuning questions. A really good friend of mine had an 850 reworked by SMI (Sean Murphy) and was very happy with his too. Clay
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