Actual cfm used vs carb size

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Steve.k
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Actual cfm used vs carb size

Post by Steve.k » Tue Feb 26, 2019 9:33 am

How do you guys determine your carb size for specific build? I currently run two 1050's on 438 cubes that netted 51hp gain over a single 1050 at exactly the same rpm. When talking to holley they recommended two 650 built carbs as they said engine can pump 715 cfm. They were close as single carb used 740. Thoughts??

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Re: Actual cfm used vs carb size

Post by PackardV8 » Tue Feb 26, 2019 11:05 am

Those who do a lot of dyno work will tell you larger carbs usually show an increase in horsepower at the top end. How the engine will actually be used and how much time it spends at max RPM versus how much time at idle and part throttle determine the best carb choice.

Think of a QuadraJet. GM knew an OEM engine spends 99% of running time at idle and part throttle, so very small primary bores. For that 1% of full throttle, they included the huge secondaries.

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Re: Actual cfm used vs carb size

Post by Steve.k » Tue Feb 26, 2019 11:34 am

Jack this car picked up .18 in 1/8 mile with above swap. Not sure if two 650's would better that or not.

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Re: Actual cfm used vs carb size

Post by cgarb » Tue Feb 26, 2019 2:36 pm

In an old book my dad had that was written about Bill Jenkins and his Vega Prostock car he had said similar results with twin 1050's on his small block had gained hp on the Dyno. He had two 660's that were worked on the car prior, but the on track results he had thought did not reflect the Dyno results. He said the car reacted better and was faster with the 660's. I know it's old school stuff, I have always wondered the same thing, is the tunnel ram itself the HP gain and not the larger CFM. I would like to run a 1050 with a well prepped single plane with a tunnel ram and a pair of split dominators on top, basically the same carb CFM just the single plane vs tunnel ram swap and see what gain was there. I'm sure someone has done something similar, I can't be the only one that has thought that way.

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Re: Actual cfm used vs carb size

Post by Steve.k » Tue Feb 26, 2019 2:51 pm

Yes cgarb it makes me wonder. The single 1050 i had was on chi 3v dommy flange 400 intake so a well tuned intake. Obviously the plenum volume alot smaller than tunnel ram. The 650's may offer more acceleration due to velocity? Im not sure though. 1800 bucks to see!

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Re: Actual cfm used vs carb size

Post by David Redszus » Tue Feb 26, 2019 3:21 pm

During dyno testing, has anyone experienced a situation where high partial throttle actually made more power than full throttle? We've seen that more than once.

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Re: Actual cfm used vs carb size

Post by F-BIRD'88 » Tue Feb 26, 2019 3:29 pm

David Redszus wrote:
Tue Feb 26, 2019 3:21 pm
During dyno testing, has anyone experienced a situation where high partial throttle actually made more power than full throttle? We've seen that more than once.
Some of the oem GM qjet applications were set up like this. Better overall fuel distribution for that specific manifold.

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Re: Actual cfm used vs carb size

Post by Truckedup » Tue Feb 26, 2019 3:42 pm

David Redszus wrote:
Tue Feb 26, 2019 3:21 pm
During dyno testing, has anyone experienced a situation where high partial throttle actually made more power than full throttle? We've seen that more than once.
Seen it on bikes with slide valve carbs...Usual cause was a weak main jet...These carbs run on the needle until about 3/4 throttle opening.The needle jet is sized ok but once off the needle the lean main jet comes into play... Hillbilly tuning for carburetor sizing..I would hook up a vacuum gauge to the intake, hang it on the window wiper...Open up the throttle in second gear and watch the vacuum, it fall to zero then will creep up as the engine winds out...If near hp peak it's showing maybe 3 inches, more carb may help...Pretty crude but much better than guessing..
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Re: Actual cfm used vs carb size

Post by digger » Tue Feb 26, 2019 4:53 pm

David Redszus wrote:
Tue Feb 26, 2019 3:21 pm
During dyno testing, has anyone experienced a situation where high partial throttle actually made more power than full throttle? We've seen that more than once.
I've noticed the same with individual throttles at low rpm I think the throttles perhaps reduce reversion due to the IVC event being way off at low rpm

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Re: Actual cfm used vs carb size

Post by Steve.k » Tue Feb 26, 2019 7:09 pm

I know that every time i put bigger carbs on seen gains. From 830 hp series to 1050 then to twin 1050's. Never thought of going smaller.

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Re: Actual cfm used vs carb size

Post by John Wallace » Tue Feb 26, 2019 8:00 pm

I'd go with truckedup's idea.
Except use 1.5" since it's 4 bbl carbs.

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Re: Actual cfm used vs carb size

Post by MadBill » Tue Feb 26, 2019 10:09 pm

The law of diminishing returns applies, but some max effort tunnel ram-equipped engines see as little as 0.5" Hg. at peak revs. :-k

Also, a restrictive air cleaner will throw off a vacuum test.
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Re: Actual cfm used vs carb size

Post by BILL-C » Wed Feb 27, 2019 7:27 am

The trend that i see is a power loss from around peak tq and up if the manifold vacuum at WOT is 1.2" or greater. There are exceptions to this trend when comparing dissimilar carbs. If an "oversized" carb is properly designed, it can perform better at lower speeds that you would expect.
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Re: Actual cfm used vs carb size

Post by Steve.k » Wed Feb 27, 2019 9:38 am

I know when we went from nascar 830 to 1050 quite a difference in throttle response and low end grunt. No comparison actually the dommy ruled!

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Re: Actual cfm used vs carb size

Post by jmarkaudio » Wed Feb 27, 2019 1:14 pm

When discussing airflow with carb size it is misleading unless the carb is significantly restrictive, like a very small 4 barrel or 2 barrel carb. When looking at a reasonably large single versus two carbs the actual airflow gains with two that the engine sees are not much, the HP gains seen are from lowering pumping losses without hurting distribution. Getting relatively equal distribution is one of the more important components to making power, the better distribution is the bigger the carb that you can use. This is where finding the balance on any given engine is key, Using a larger carb lowers pumping losses, what the engine uses for HP to pull air into the engine thru the carb, heads, and intake. However with a larger carb or carbs you lower manifold vacuum at WOT, that vacuum aids atomization, vaporization, and subsequently distribution of the fuel with the incoming airstream. It also helps to create a more homogenous mix of air and fuel, improving combustion efficiency. Higher vacuum does this at the expense of HP required to pump thru the carbs restriction. Two carbs usually work better lowering pumping losses and maintaining more equal distribution with an individual carb barrel over or nearly over each cylinder inlet port. Each barrel can be manipulated to improve the fuel curve needs of each cylinder.

So what can we do to improve power, especially on a single carb intake? Work on anything that improves airflow without significantly hurting even velocity. Work on making the intake more equally distribute fuel as well as air. Textures on the plenum and runner surfaces can also help maintain a more even mix of air and fuel, minimizing fuel puddling or sticking to surfaces. The lighter weight of air makes it less affected by inertia, easier for it to turn corners. Because fuel is heavier makes it harder to turn corners, this can impact equal distribution. Typically the end runners on a single plane tend to be leaner than the center due to the design of the intake, however this is not always the case. I have seen both cases, where a large carb struggles to distribute evenly because of the intake design, both with the end runners lean on one and the center lean on a different engine. In these cases a smaller carb or even one with a different bore to venturi size/shape will improve over another. The smaller carb uses increased WOT vacuum to atomize and vaporize the fuel better improving distribution. Fuel vapor is lighter than liquid and makes the turns easier to improve distribution. In the case of the different bore/venturi size the fuel and air exiting the carb exit at different angles leaning one to be more efficient than another for that engine. In the end it is about finding the balance for the given combination of components used to build that engine. Change the intake or something inside in the plenum or runners, camshaft profile, headers, etc. then you can affect the needs of that engine.
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