Carb Size
March 24th, 2015, 08:25 AM
#1
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Thread Starter
Join Date: Aug 2011
Location: Flint, Michigan
Posts: 156
Carb Size
Hello Everyone.
I was curious about what would be a good carburetor size for my build:
400G Block
Edelbrock Aluminum Performer Intake Manifold
C-Heads
Everything is pretty much to stock specs.
Obviously it doesn't really see much time over 5000RPM very often, however I do kick it in the butt every once in a while.
It's not a full race motor, but I do want something that will be a good balance of power and street application.
Thank you!
I was curious about what would be a good carburetor size for my build:
400G Block
Edelbrock Aluminum Performer Intake Manifold
C-Heads
Everything is pretty much to stock specs.
Obviously it doesn't really see much time over 5000RPM very often, however I do kick it in the butt every once in a while.
It's not a full race motor, but I do want something that will be a good balance of power and street application.
Thank you!
March 24th, 2015, 09:34 AM
#2
CH3NO2 LEARN IT BURN IT
Join Date: Jul 2010
Location: Land of Taxes
Posts: 4,821
650-700 CFMs by the math. The original Q Jet was 750CFM.
Engine Size (c.i.d.) X Maximum rpm/3,456 = cfm at 100-percent Volumetric Efficiency (VE)
Example: 400 c.i.d. X 5500 rpm = 2,200,000/3,456 = 636 cfm.
But keep in mind this is at 100% VE. The argument could be had that 750 is too much. But thats what the factory spec was rated at and it worked just fine with the factory jet/rod set up
(Stock Engines VE = 0.75 - 0.85, Mild Built VE = 0.85 - 0.90, Racing Engines VE = 0.90 - 1.0)
You will do well with a 650-750cfm street carb from say Edelbrocks AVS series or a well tuned Q-Jet. Keep in mind most Qjets are 750cfm. The Ebrocks are very user friendly and can be tuned on the car with a rod/spring/jet kit. Tech support is great.
A properly tuned Q-Jet is hard to beat as well. And no throttle modifications needed.
Anything more than 750 the engine will be over carbureted causing drive ability issues.
There are other parts that can sway carb selection too such as intake design, head and cam design, converter stall, rear gears etc...but for the average mild street car the above math will be fine.
Engine Size (c.i.d.) X Maximum rpm/3,456 = cfm at 100-percent Volumetric Efficiency (VE)
Example: 400 c.i.d. X 5500 rpm = 2,200,000/3,456 = 636 cfm.
But keep in mind this is at 100% VE. The argument could be had that 750 is too much. But thats what the factory spec was rated at and it worked just fine with the factory jet/rod set up
(Stock Engines VE = 0.75 - 0.85, Mild Built VE = 0.85 - 0.90, Racing Engines VE = 0.90 - 1.0)
You will do well with a 650-750cfm street carb from say Edelbrocks AVS series or a well tuned Q-Jet. Keep in mind most Qjets are 750cfm. The Ebrocks are very user friendly and can be tuned on the car with a rod/spring/jet kit. Tech support is great.
A properly tuned Q-Jet is hard to beat as well. And no throttle modifications needed.
Anything more than 750 the engine will be over carbureted causing drive ability issues.
There are other parts that can sway carb selection too such as intake design, head and cam design, converter stall, rear gears etc...but for the average mild street car the above math will be fine.
March 24th, 2015, 09:36 AM
#3
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Location: Poteau, Ok
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If your going back to a Rochester then you'll get a max of around 750-800 cfm for stock. If your going to run anything else 750-780 is my rule of thumb for a BB. I think anymore than that would be over kill. For normal everyday driving a 650 would be adequate.
March 24th, 2015, 02:27 PM
#5
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Join Date: Nov 2008
Location: Central Fl
Posts: 7,824
No less than a 780. I've done multiple 455's on the dyno, at 4000 and above they start pulling vacuum with 850's. What does that tell you. Forget the formulas, go bigger than what they recommend and take the time to tune it right. You won't be sorry.
March 25th, 2015, 02:00 AM
#6
Does any know if a edelbrock 750 carb use much fuel, or is it the trottle response Who deside it if the carb use much fuel anyway if it big or small? im thinking on just cruising around wiyh carefull trottle respons. Or how big the engine is? Thinking more a stock/street engine.
I have a q-jet carb (i dont know what size it is) on a 307 and it use very low fuel as long im carefull with the trottle response.
I will use a 750carb on my g block too, with same setup as the first post says. But im thinking a BBo will use more fuel than a SBO anyway?
I have a q-jet carb (i dont know what size it is) on a 307 and it use very low fuel as long im carefull with the trottle response.
I will use a 750carb on my g block too, with same setup as the first post says. But im thinking a BBo will use more fuel than a SBO anyway?
Last edited by Oldsragger; March 25th, 2015 at 02:11 AM.
March 25th, 2015, 06:35 AM
#7
Out of Line, Everytime😉
Join Date: Dec 2006
Location: Melville, Saskatchewan
Posts: 8,911
Get your Qjet retuned or a Street Demon 750 if you want fuel economy for your 407. A lot of people have had issues with the Edelbrock 750 and it won't match the fuel economy of the carbs above.
March 25th, 2015, 07:39 AM
#8
if i will replace the carb i will go for a computer injection carb i guess.. Its expencive but maybe i will find economy and power with rigth setup. Like an modern injected v8, lol
Last edited by Oldsragger; March 25th, 2015 at 07:44 AM.
March 25th, 2015, 11:26 AM
#10
Beer Connoisseur
Join Date: Aug 2008
Location: Daly City, California
Posts: 2,090
Not sure if you noticed that OP is from Norway. Sometimes things are lost in translation.
English is probably his 2nd or 3rd language. When looking at his posts, his english is better than half of the American people that post here.
English is probably his 2nd or 3rd language. When looking at his posts, his english is better than half of the American people that post here.
March 25th, 2015, 12:19 PM
#11
Rocket Renegade!
Join Date: Dec 2014
Location: Vancouver Island
Posts: 4,392
March 25th, 2015, 12:56 PM
#12
(was it right)..like this i mean, just dont remember what it names, actually i like pump carbs best,and the oldschool way,,, but im just thinking about our gasoline prices these days
so maybe its a good replacement if a guy want economy and power when he want it.. (unless you have a good normal carb with rigth setup then)..If i fill up my gastank in our prices here for 95octan or 98oct (when i find it) it cost me about 128 U.S each time... maybe a little lesser.
the carb from this site cost 2010 U.S but i guess i will fill up my gas tank more than 15-16 times in the future,so i guess i can save some money if buy a carb like this in the future gasoline use i mean... just a tought from my side..
Link for infos
https://www.holley.com/products/fuel.../parts/550-405
Sorry LoganMiller68 for talking about this.
Last edited by Oldsragger; March 25th, 2015 at 01:22 PM.
March 25th, 2015, 04:46 PM
#14
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Join Date: Feb 2013
Location: Phoenix, AZ
Posts: 13,738
March 26th, 2015, 08:13 PM
#16
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Join Date: Nov 2012
Location: Munkfors, Sweden
Posts: 55
2.59 a gallon is a joke to us, with prices x3 at home!
And just to make it clear, our 95 & 98 equals to 91 & 93 over here.
And snow, fourbangers, taxes and my mother in law makes it even better....
March 27th, 2015, 05:48 AM
#18
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Join Date: Nov 2012
Location: Munkfors, Sweden
Posts: 55
Does any know if a edelbrock 750 carb use much fuel, or is it the trottle response Who deside it if the carb use much fuel anyway if it big or small? im thinking on just cruising around wiyh carefull trottle respons. Or how big the engine is? Thinking more a stock/street engine.
I have a q-jet carb (i dont know what size it is) on a 307 and it use very low fuel as long im carefull with the trottle response.
I will use a 750carb on my g block too, with same setup as the first post says. But im thinking a BBo will use more fuel than a SBO anyway?
I have a q-jet carb (i dont know what size it is) on a 307 and it use very low fuel as long im carefull with the trottle response.
I will use a 750carb on my g block too, with same setup as the first post says. But im thinking a BBo will use more fuel than a SBO anyway?
March 27th, 2015, 07:30 AM
#19
Hmm ok, Well i have 2 x 650 holley carbs too, but they need a rebuild anyway, but maybe they use a little more fuel than other carbs i have heard. They have manual choke too. Or i can ask my friend if he want to sell his holley doublé pumper carb.. But again i guess its not really economic choice,
im Just use the car in the summer thimes anyway so i can live with some fuel drinking that days.
im Just use the car in the summer thimes anyway so i can live with some fuel drinking that days.
March 27th, 2015, 10:59 AM
#20
I'm not saying that the ebrock 750 is a bad carb, but i had one on my mild 461 w 4speed, and i put around 1 year of tweaking with rods and jets in that one. Never got i to run smooth in transit between idle and part throttle. Off it came and a rebuilt and recalibrated q-jet was installed. world of difference, runs a lot smoother. The edelbrock 750 sits on a shelf, and i think thats the best place for it.
March 27th, 2015, 11:11 AM
#21
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Join Date: Nov 2012
Location: Munkfors, Sweden
Posts: 55
Hmm ok, Well i have 2 x 650 holley carbs too, but they need a rebuild anyway, but maybe they use a little more fuel than other carbs i have heard. They have manual choke too. Or i can ask my friend if he want to sell his holley doublé pumper carb.. But again i guess its not really economic choice,
im Just use the car in the summer thimes anyway so i can live with some fuel drinking that days.
im Just use the car in the summer thimes anyway so i can live with some fuel drinking that days.
March 28th, 2015, 12:36 PM
#23
Hmm ok, so why they sell edelbrock carbs if they cant work as they should? I remember my friend have a edel carb on his 454 chevy and he have only problem with it, tryed to replace needels and stuff like that many times, he installed a holley doublé pumper and problem solved and the engine wakr up.
Im exited to try this 750 carb i have, cause i guess its setup allready for the engine, but i have only the dyno papers, no info about setup of carb or how it runs,but i will find out..
I have a q-jet and it works great, but its on a sbo, and its automatic chocke on it and does not fit the edelbrock intake i have on my bbo, only i know from specs that the edelbrock 750 is the carb that should be used with the 2151 performer intake.
But again, i Will find out how it will work when i will fired it up.
Ok, so the Cfm numbers is the size of air Flow in the carb? Not needle setup sizes? Maybe i have missunderstand something about it.
Im exited to try this 750 carb i have, cause i guess its setup allready for the engine, but i have only the dyno papers, no info about setup of carb or how it runs,but i will find out..
I have a q-jet and it works great, but its on a sbo, and its automatic chocke on it and does not fit the edelbrock intake i have on my bbo, only i know from specs that the edelbrock 750 is the carb that should be used with the 2151 performer intake.
But again, i Will find out how it will work when i will fired it up.
Ok, so the Cfm numbers is the size of air Flow in the carb? Not needle setup sizes? Maybe i have missunderstand something about it.
Last edited by Oldsragger; March 28th, 2015 at 12:38 PM.
March 28th, 2015, 02:56 PM
#24
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Join Date: Nov 2012
Location: Munkfors, Sweden
Posts: 55
A #2151performer has dual patterns for both spreadbore and squarebore. The q jet on your 307 is at least a 750, maybe a 800, you can tell them apart by looking at the venturi castings from above. Google it, there is a lot of good info around.
What differs your qjet from a bigblock one is basically the opening angle on the secondaries. Put some money on this and you'll be a lot happier, unless its one of those newer electronic controlled ones, in that case you'll have some electric connectors on top of it in front of the primaries. Mine is from a '77 350 and is an 800, had it rebuilt and the secondaries modified, an adjustable power valve piston installed and runs very good with stock 350 jetting.
What differs your qjet from a bigblock one is basically the opening angle on the secondaries. Put some money on this and you'll be a lot happier, unless its one of those newer electronic controlled ones, in that case you'll have some electric connectors on top of it in front of the primaries. Mine is from a '77 350 and is an 800, had it rebuilt and the secondaries modified, an adjustable power valve piston installed and runs very good with stock 350 jetting.
Last edited by Backstrom; March 28th, 2015 at 02:58 PM.
March 28th, 2015, 04:48 PM
#25
A #2151performer has dual patterns for both spreadbore and squarebore. The q jet on your 307 is at least a 750, maybe a 800, you can tell them apart by looking at the venturi castings from above. Google it, there is a lot of good info around.
What differs your qjet from a bigblock one is basically the opening angle on the secondaries. Put some money on this and you'll be a lot happier, unless its one of those newer electronic controlled ones, in that case you'll have some electric connectors on top of it in front of the primaries. Mine is from a '77 350 and is an 800, had it rebuilt and the secondaries modified, an adjustable power valve piston installed and runs very good with stock 350 jetting.
What differs your qjet from a bigblock one is basically the opening angle on the secondaries. Put some money on this and you'll be a lot happier, unless its one of those newer electronic controlled ones, in that case you'll have some electric connectors on top of it in front of the primaries. Mine is from a '77 350 and is an 800, had it rebuilt and the secondaries modified, an adjustable power valve piston installed and runs very good with stock 350 jetting.
But if its a 750 or 800, how can i know it will deliver fuel enought for the BBO? but anyway it have this heating thing, that goes down the exhaust crossover on the 307 intake, i cant use it on the performer intake.. maybe its possible to built it to manual choke?
I can try to google the number on the carb if i can find out how big it is.. here is a pic of it..
PA150567898094.jpg
PC090795.jpg
March 29th, 2015, 03:36 AM
#26
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Join Date: Nov 2012
Location: Munkfors, Sweden
Posts: 55
You can convert it to electric choke, mine is and works very good.
try www.carbguy.com. I would say the most competent carbrebuilder on this side of the atlantic, or Kents qjet, he made my last carb. It aint cheap, count on nearly 4000 sek if the throttle shaft needs new bushings, and it probably will. Any of these guys will tell you if your carb is a good start and take it from there. Pm me if you want info on Kents qjet.
try www.carbguy.com. I would say the most competent carbrebuilder on this side of the atlantic, or Kents qjet, he made my last carb. It aint cheap, count on nearly 4000 sek if the throttle shaft needs new bushings, and it probably will. Any of these guys will tell you if your carb is a good start and take it from there. Pm me if you want info on Kents qjet.
March 29th, 2015, 09:33 AM
#27
Navy P-3 FE (Ret.)
Join Date: Dec 2009
Location: Great Mills, MD
Posts: 759
The primary venturi size for the 800 cf. is 1-7/32 in and 1-3/32 in for the 750. If you look down the primary venturi the 800 cf. model will be smooth all around while the 750 will have a raise area around.
March 29th, 2015, 01:04 PM
#28
I can check inside the carb primary tomorrow and take pic, on that pic above, i see the 800cfm have an extra plate or something down there? It looks like, and the 750 have nothing, i see its darker there.. Do you mean that Who sticking out a little in the wall?
March 29th, 2015, 01:23 PM
#29
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Join Date: Dec 2011
Posts: 7,286
Yeah, that baby bump there
PS how is one supposed to measure the venturi ID- by what means? And, can we PLEASE FOR THE LOVE OF GOD AND ALL THINGS SACRED do that in mm or decimal inches like a digital calipers might read? Please and thank you. Fractions are so 19th century. A really bad idea since the advent of the machine age. Pointless.
How about this- it appears the larger venturi means a larger GAP between that wall with the bump and the OD of the booster venturi. How about we come up with a handy way of measuring this gap that anyone can easily perform? Such as, a drill bit of size x will fit into this gap readily on the 800 carb but not so on the 750 carb. Now that would be handy useful info.
PS how is one supposed to measure the venturi ID- by what means? And, can we PLEASE FOR THE LOVE OF GOD AND ALL THINGS SACRED do that in mm or decimal inches like a digital calipers might read? Please and thank you. Fractions are so 19th century. A really bad idea since the advent of the machine age. Pointless.
How about this- it appears the larger venturi means a larger GAP between that wall with the bump and the OD of the booster venturi. How about we come up with a handy way of measuring this gap that anyone can easily perform? Such as, a drill bit of size x will fit into this gap readily on the 800 carb but not so on the 750 carb. Now that would be handy useful info.
Last edited by Octania; March 29th, 2015 at 01:26 PM.
March 30th, 2015, 12:34 PM
#32
Navy P-3 FE (Ret.)
Join Date: Dec 2009
Location: Great Mills, MD
Posts: 759
1 7/32 minus 1 3/32
39/32 minus 35/32
1.21875 minus 1.09375
equals .125
or 1/8
39/32 minus 35/32
4/32
1/8
Mines closer all kidding aside.
Point well taken in your post, yeah I could have been more clear.
39/32 minus 35/32
1.21875 minus 1.09375
equals .125
or 1/8
39/32 minus 35/32
4/32
1/8
Mines closer all kidding aside.
Point well taken in your post, yeah I could have been more clear.
March 31st, 2015, 06:27 AM
#33
Out of Line, Everytime😉
Join Date: Dec 2006
Location: Melville, Saskatchewan
Posts: 8,911
More than likely it is 800 cfm. Just get an electric choke coil from a late 70's to 80's chevy, it will fit right in. There is a lttle you can block with a tiny dab of jb weld behind your choke coil. If you buy one or two larger primary jet sizes, start one jet size larger, your carb should run fine, depending on your cam size. What duration is your cam?
March 31st, 2015, 08:28 AM
#34
Ok, i Will look for an Electric choke convert if i find it.
I dont know what camshaft it is inside, i guess it is orginal, i have only this pic, can anyone see if this cam have orginal shape on the "heads"? Maybe hard to see.
I dont know what camshaft it is inside, i guess it is orginal, i have only this pic, can anyone see if this cam have orginal shape on the "heads"? Maybe hard to see.
March 31st, 2015, 09:39 AM
#35
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Join Date: Dec 2011
Posts: 7,286
If you can tell the specs of a cam by looking at the shape of the lobes like that...
I bow to you.
I am lucky if I can get the specs by examining the chamfers and notches [factory] or the PN stamp in the aft end [aftermarket].
Other than that, it's degree wheel time.
I bow to you.
I am lucky if I can get the specs by examining the chamfers and notches [factory] or the PN stamp in the aft end [aftermarket].
Other than that, it's degree wheel time.
March 31st, 2015, 12:12 PM
#36
Maybe i can see the nr on the rear of the camshaft if i stick a small mirror and light down the disturbutor hole? Im not sure now
Lol, Well the point was if the orginal cam have sharper lobes than a upgrade cam. But i dont know. In my engine build paper it stands nothing about cam replace or specs.
Lol, Well the point was if the orginal cam have sharper lobes than a upgrade cam. But i dont know. In my engine build paper it stands nothing about cam replace or specs.
April 1st, 2015, 03:50 PM
#39
OK, thats good then, guess i should Just get an Electric choke and just swap it over the bbo And hope for just turnkey startup without any other problems. I dont wish to modify it if its not nessesary.. Maybe i need to take a gasket swap again, cause i can see the gasket are a little "wet" that part sticking out from carb..Im not sure if its normal. 2 yrs ago from last gasket replace. Will check the bolts if its tight enought again.
Last edited by Oldsragger; April 1st, 2015 at 03:54 PM.
