what carb is everyone running?
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1st Gear Member
Joined: Jan 2008
Posts: 58
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RE: what carb is everyone running?
thanks for all the responses. now, edelbrock says that for my application, a 650-750 will work just fine. i do infact have a race 289 that does rev to 8K+. no i havent reved it that high, bt at a half throttle tap it goes to 6000 and does so quickly. my engine is no where near stock. it was meant for a 9 second sprayed ford falcon that my dads friend owns, but he decided to go bigblock b/c he got a good deal on one. i bought this engine from him, but i dont intend to run 9's b/c im not gona spray it like that if i spray it at all. my mech. says is sounds and runs like its goingto be a solid high 11 sec car all motor. my sig only says "worked 289", i say this becasue i noticed a member said that by my sig, its too much. im not pointin fingers lol beasue ido value opinions. there is always someone out there that knows more then the next guy, which is why i asked to see what everyone was running. it seems i got everything fromstock, to even and 289 with an 850 with no problems.my car is 100% responsive with the slightest flick of the throttle. iwish some ofyou lived in my area so you couldcomep personally to see the car for yourself and be the judge of how it runs. all my ford friends with 289's,302s',331's,347's all say they never heard a 289 sound like mine does.id like to continue this post, beause i think it will help others who have carb selection problems. thanks for all your input!! Nick[8D]
6th Gear Member
Joined: Apr 2007
Posts: 10,468
From: California
RE: what carb is everyone running?
If it's a fully built 8k+ winder then yeah, a 750 would prolly work fine. Liek I said at first, it's too big unless you have an engine that turns that kind of rpm. If you plan to drive it on the street on a semi-regular basis, then going on the slightly smaller side may help to bring the bottom end up if it's lacking from cam/head/intake combo. It depends on what you want it to do.
As far as dual vs single plane intakes, yeah that has an effect on carb size as well. A dual plane intake with a certain size carb, is almost like a single plane with half the carb size(though not really because it's more complex when you deal with charge robbing etc). Dual planes CAN run bigger carbs more easily, but don't need to. Half the engine drawing from half the carb or the entire engine drawing from the entire carb gets the same amount of air into the engine. In either case though, the engine needs to have the right carb for the right application.
As far as carb ratings, 4bbls are rated at 1.5" Hg and 2 bbls are rated at 3.0" Hg(iirc on the 2bbl rating standard), so the 2bbls show an artificially high cfm for comparison purposes. Demon's carbs flow more than they're rated because others often flow a dry rating, sometimes without the obstruction of the booster. Demon and others rate the WET cfm flow, with the booster in place delivering fuel, so it's the actual cfm the carb will move at a given pressure differential when in actual operation. That's rather significant since different boosters affect cfm differently. Annular boosters like those used in the Autolites atomise better and with less signal, but are larger and more obstructive, so they reduce the dry cfm rating, and additionally annular boosters tend to deliver a wider fuel cone into the venturi, and since fuel has more mass than air, the fuel cone tends to divert air around it rather than through it, acting almost like a liquid "plug" in the venturi, restricting the flow. Annular boosters are a tradoff between atomisation/metering capability and flow. You could run a much larger carb with a very sensitive annular setup(though annulars have their own problems sometimes of trying to over-meter at higher rpm), they're also more expensive. Downlegs are a nice mix of atomisation/signal and flow at a reasonable cost. Again, it comes down to how the carb is setup visa-vis the engine.
Where people get into trouble with larger carbs is when they put a 750 on a stockish smallblock for example, that uses downleg or straight leg boosters and it's not able to develop enough signal to atomise effectively at lower rpm. Interestingly though the amount of air an engine pumps is also dependant to a degree on how much power it makes since that power has an effect on how the engine will move air. More power from more combustion creates more exhaust gas under higher pressure, which leads to higher velocities in the exhaust tract which means there's a stronger signal during overlap for better scavenging and so on and so forth. The ACTUAL cfm an engine moves can be calculated as... CFM = BSFC * HP * 2.723 where BSFC is brake specific fuel consumption(0.45 is a good factor for BSFC of a well tuned n/a engine). And then of course actual cfm divided by theoretical 100% VE cfm gives you the actual volumetric efficiency(for those who care). A combined calculation for that is actual VE = (9411*HP*BSFC)/(CID*RPM) which is a shorter version of (2.723*HP*BSFC)/(CID*RPM/3456). The interesting thing is if you calculate the actual cfm your engine moves based on fuel consumption(more accurate if you know what it is accurately) and power, you'll find that it's often not nearly what you may think it would be at peak horsepower. If you calculate VE at peak torque(using the cfm flow at the rpm it occurs) then you can find what the peak VE of the engine is(approximately). Again, another interesting phenomena is that as rpm increases above the peak torque rpm moving towards peak hp rpm, the VE is dropping....so while the increasing rpm may require a larger cfm flow the decreasing VE is trying to reduce the required cfm flow, so depending on how the engine is setup it may require more, the same or even less cfm at peak hp than it does at peak torque. It's going to depend on how efficient the engine remains after peak torque. When we calculate carb cfm the old fashioned way, the VE factor we multiply by to get the required cfm is usually based on peak VE that occurs at peak torque, but not the VE at the max rpm we use for carb calculations. When you calculate a carb cfm based on the VE at max horsepower it comes out to be a lot smaller than you think you might need(in a performance engine it's still usually higher at peak hp than peak torque).
I've seen dyno testing of differing cfm's on the same carb manufacturer, model and with the same boosters(to try to get the best apples to apples comparison). They used a range of carbs on a well built street Chevy 350, from a way undersized 400cfm carb all the way up to a way oversized 950cfm carb. The engine lost power everywhere with too small of a carb, even the lower cfm rated carb was restrictive at lower engine rpm(where the VE was rather high). But once they got to 650cfm the power remained about the same. The 950cfm carb only made like 10 more peak hp over the 650(which arguably was a tad on the small side for the engine they tested) but "tilted" the power band so that while the peak hp was a tad higher, it lost as much power only a couple thousand rpm lower, and lost a bit of bottom end everywhere. They also had hooked up a flow meter before all the carbs to measure the actual cfm the engine was ingesting. Basically what they found in testing was that too small a carb restricted flow into the engine everywhere and caused power to be lost from idle to peak rpm, but at maximum power the 950cfm carb was negligably less restrictive than the 650cfm carb(the the tune of like 10-20cfm) but the larger carbs lost measurable bottom end. And again it gets back to why spend money on a high cfm race carb so that it CAN work on the street, when the engine will run just as well with a much less expensive carburetor and be easier to tune?
Carburetion isn't NEARLY as restrictive as many people think it is, which has lead to a lot of overcarburetion and lack of signal. It's about metering fuel properly without being restrictive, but too many people view it exclusively from the restriction standpoint. Giving the engine the cfm it wants, without compromising metering is what it's all about. How that's done varries from one engine design to the next and from one carburetor design to the next. One of the reasons for example that Edelbrocks tend to do well when overcarb'd is that their booster design while an older lower cost design, tends to do quite well at metering and shearing with low signal. They also use more of a spread bore than a sqaure bore design, so the primaries always have high velocity.
As far as dual vs single plane intakes, yeah that has an effect on carb size as well. A dual plane intake with a certain size carb, is almost like a single plane with half the carb size(though not really because it's more complex when you deal with charge robbing etc). Dual planes CAN run bigger carbs more easily, but don't need to. Half the engine drawing from half the carb or the entire engine drawing from the entire carb gets the same amount of air into the engine. In either case though, the engine needs to have the right carb for the right application.
As far as carb ratings, 4bbls are rated at 1.5" Hg and 2 bbls are rated at 3.0" Hg(iirc on the 2bbl rating standard), so the 2bbls show an artificially high cfm for comparison purposes. Demon's carbs flow more than they're rated because others often flow a dry rating, sometimes without the obstruction of the booster. Demon and others rate the WET cfm flow, with the booster in place delivering fuel, so it's the actual cfm the carb will move at a given pressure differential when in actual operation. That's rather significant since different boosters affect cfm differently. Annular boosters like those used in the Autolites atomise better and with less signal, but are larger and more obstructive, so they reduce the dry cfm rating, and additionally annular boosters tend to deliver a wider fuel cone into the venturi, and since fuel has more mass than air, the fuel cone tends to divert air around it rather than through it, acting almost like a liquid "plug" in the venturi, restricting the flow. Annular boosters are a tradoff between atomisation/metering capability and flow. You could run a much larger carb with a very sensitive annular setup(though annulars have their own problems sometimes of trying to over-meter at higher rpm), they're also more expensive. Downlegs are a nice mix of atomisation/signal and flow at a reasonable cost. Again, it comes down to how the carb is setup visa-vis the engine.
Where people get into trouble with larger carbs is when they put a 750 on a stockish smallblock for example, that uses downleg or straight leg boosters and it's not able to develop enough signal to atomise effectively at lower rpm. Interestingly though the amount of air an engine pumps is also dependant to a degree on how much power it makes since that power has an effect on how the engine will move air. More power from more combustion creates more exhaust gas under higher pressure, which leads to higher velocities in the exhaust tract which means there's a stronger signal during overlap for better scavenging and so on and so forth. The ACTUAL cfm an engine moves can be calculated as... CFM = BSFC * HP * 2.723 where BSFC is brake specific fuel consumption(0.45 is a good factor for BSFC of a well tuned n/a engine). And then of course actual cfm divided by theoretical 100% VE cfm gives you the actual volumetric efficiency(for those who care). A combined calculation for that is actual VE = (9411*HP*BSFC)/(CID*RPM) which is a shorter version of (2.723*HP*BSFC)/(CID*RPM/3456). The interesting thing is if you calculate the actual cfm your engine moves based on fuel consumption(more accurate if you know what it is accurately) and power, you'll find that it's often not nearly what you may think it would be at peak horsepower. If you calculate VE at peak torque(using the cfm flow at the rpm it occurs) then you can find what the peak VE of the engine is(approximately). Again, another interesting phenomena is that as rpm increases above the peak torque rpm moving towards peak hp rpm, the VE is dropping....so while the increasing rpm may require a larger cfm flow the decreasing VE is trying to reduce the required cfm flow, so depending on how the engine is setup it may require more, the same or even less cfm at peak hp than it does at peak torque. It's going to depend on how efficient the engine remains after peak torque. When we calculate carb cfm the old fashioned way, the VE factor we multiply by to get the required cfm is usually based on peak VE that occurs at peak torque, but not the VE at the max rpm we use for carb calculations. When you calculate a carb cfm based on the VE at max horsepower it comes out to be a lot smaller than you think you might need(in a performance engine it's still usually higher at peak hp than peak torque).
I've seen dyno testing of differing cfm's on the same carb manufacturer, model and with the same boosters(to try to get the best apples to apples comparison). They used a range of carbs on a well built street Chevy 350, from a way undersized 400cfm carb all the way up to a way oversized 950cfm carb. The engine lost power everywhere with too small of a carb, even the lower cfm rated carb was restrictive at lower engine rpm(where the VE was rather high). But once they got to 650cfm the power remained about the same. The 950cfm carb only made like 10 more peak hp over the 650(which arguably was a tad on the small side for the engine they tested) but "tilted" the power band so that while the peak hp was a tad higher, it lost as much power only a couple thousand rpm lower, and lost a bit of bottom end everywhere. They also had hooked up a flow meter before all the carbs to measure the actual cfm the engine was ingesting. Basically what they found in testing was that too small a carb restricted flow into the engine everywhere and caused power to be lost from idle to peak rpm, but at maximum power the 950cfm carb was negligably less restrictive than the 650cfm carb(the the tune of like 10-20cfm) but the larger carbs lost measurable bottom end. And again it gets back to why spend money on a high cfm race carb so that it CAN work on the street, when the engine will run just as well with a much less expensive carburetor and be easier to tune?
Carburetion isn't NEARLY as restrictive as many people think it is, which has lead to a lot of overcarburetion and lack of signal. It's about metering fuel properly without being restrictive, but too many people view it exclusively from the restriction standpoint. Giving the engine the cfm it wants, without compromising metering is what it's all about. How that's done varries from one engine design to the next and from one carburetor design to the next. One of the reasons for example that Edelbrocks tend to do well when overcarb'd is that their booster design while an older lower cost design, tends to do quite well at metering and shearing with low signal. They also use more of a spread bore than a sqaure bore design, so the primaries always have high velocity.
Joined: May 2008
Posts: 3
RE: what carb is everyone running?
i ran a 318 with a 650 edelbrock went low 15s 4200 lb car i now have a 3400 lb car 440 ci with a 980 carb 11.20s theres no reason a 289 spinning 8000 cant handle it if your 302 ,289 cant take it then keep the trap door closed and quit hating the kid for your short coming.if it works then it works.
2nd Gear Member
Joined: Oct 2005
Posts: 320
From:
RE: what carb is everyone running?
I ran a race modified 930 on my 302. It ran great. I launched at 5500 rpms and shifted at 8k. For your application I would suggest the 670 Street Avenger. I ahve run many of these on 289,302, 327, 350. They work great out of the box.
