carb. size

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kenny38
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carb. size

Post by kenny38 »

Does size really matter? My bikie mechanic suggested the other day that perhaps "unlike your dick your carb is a probably too big. Maybe a 600 would do. Gives yous more to the gallon and with the sort of driving these days on nanny roads yous will not notice difference" Should I have him over for dinner? perhaps to discuss a business venture he has in mind with huge returns. What, if you can be bothered, do you think.Kenny38 :D
kenny38
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Re: carb. size

Post by kenny38 »

I should have told you that am cuurently running a 770 holley street avenger kenny38 :D
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Chris_R
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Re: carb. size

Post by Chris_R »

Size does matter. The size of the throat or venturi is the biggest constraint. If you have a bigger throat you can get more in, you'll get a bigger gulp on the suction stroke and get a bigger bang as a result.
The venturi is the biggest restriction on getting your fuel-air through to the engine. Easy answer to that, reduce the restriction, use bigger venturis and more of them to let the cylinders get a bigger gulp on the suction stroke. The snag about that is you can have too much carburation. Big venturis only really help at high engine speeds when there is a lot of air being sucked into the engine. They hurt you at lower engine speeds where the velocity of air being sucked through is slower and cannot so effectively atomise the fuel dribbling out of the fuel nozzle. Some of the fuel will settle out and puddle in the manifold so you won't get equal amounts to each cylinder, some will run rich and others lean. Throttle response becomes sluggish and the engine will cough and stutter if you hit the loud pedal at low speeds. So it's a vicious conflict. We need smaller venturis for slower speeds and bigger ones at higher speeds.
The four-barrel spread bore carburetor fitted on V8s was a clever attempt at a compromise. Smaller primaries for lower speeds and cruising, bigger secondaries that come into operation when you want instant power and acceleration. The triple carburetor setup on the SP does the same. The centre carburetor gives you your small venturi for low speed and for cruising and the outer 2 carburetors give you all the extra venturi you need when you want that full power exploited.
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kenny38
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Re: carb. size

Post by kenny38 »

Thank you , Chris for your very comprehensive reply. I learn some thing every day. Either I do what the mechanic suggests or fit a six pack back on. I suggested this a while back to him but he has this silly idea that it will need tuning and it's not something he wants to see "every %^$# 2 weeks". Now the famous Tim SP reckons he only needs tune up every 20K. I hope i got that right.Kenny38
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Chris_R
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Re: carb. size

Post by Chris_R »

Don't you have an SP there Kenny? If so it deserves a six pack!
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bkbridges
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Re: carb. size

Post by bkbridges »

The 440 at full tilt can use a lot of air, but since the carburetor is a vacuum computer designed to deliver fuel from idle on up, the vacuum signal to the carb is critical for driveability. The progressive design of the Carter Thermoquad carb is about as good as you can get for smooth delivery and WOT power. The 440 unit was approx 990CFM WFO. It has tiny primaries that create a large signal through the venturis at lower speeds and a huge set of secondaries with an adjustable "trigger" based on a spring loaded air valve. If I was not considering racing my car and did not want to go back to the six pack (which is what I would do) Id put the 440 spec thermoquad on it with an Edelbrock performer manifold and be done with it. There are more extreme measures to assure correct A/F ratios throughout the entire rpm range, but cost goes up dramatically...
Bruce with 6bbl EFI 8)
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RayR
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Re: carb. size

Post by RayR »

Agree with Bruce, I went from a 750 cfm carter back to a non-original Thermoquad with a Performer manifold and very mild head work. Much better, light throttle response is good and the secondary punch is great. AFR is very good across almost all of the range, the only issue I have not yet sorted is an inconsistent transition from primaries to secondaries - no stumble or flat but AFR is inconsistent. I thought the TQ was ""only"" 850CFM though?
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RockyUSA
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Re: carb. size

Post by RockyUSA »

Kenny -

If you have a 6-Pack (either carbs or abs) you need to be showing them off, and running them.

They should not be holding up your table.

Let me know if you need more pictures.

Rocky
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Re: carb. size

Post by bkbridges »

Ray,
The thermoquad was shipped in 2 different cfm ratings, both subject to some speculation. The "small" unit found on the 318 and 360 "cop" motors was rated somewhere between 750 and 850 cfm while the big unit found on 440 was 850-1000cfm "advertised" rating. It doesnt really matter as its all about the way the air is delivered for the most part during normal driving. If I still ran a carburetor, Id go with a built thermoquad for the street and road course and a big Holley for the track (dragstrip)...
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Basil McKinley
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Re: carb. size

Post by Basil McKinley »

Ken,

You have received some very good advice in this thread including the recommendation to use an Edelbrock Performer inlet manifold which is a dual plane manifold. Replace your single plane inlet manifold!

Cheers,
Basil
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RayR
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Re: carb. size

Post by RayR »

Bruce, thanks for the clarification - and a lovely SP FI.

On the dual plane; not sure if it applies generally or if I have an issue, but my mixture distribution across the cylinders is not consistent from plug readings. So if you are tuning to an AFR you should be aware that there are probably some lean and some rich cylinders, to quite a degree. For this reason I don't let my cruise AFR get any higher than 14.1 or 14.2
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Chris_R
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Re: carb. size

Post by Chris_R »

Another important point to bear in mind in relation to carburetor sizing is that the quoted CFM for a carburetor may not match the real world. Carburetor CFMs are calculated and tested under laboratory conditions by sucking dry air under a specific vacuum. For a 4-barrel carburetor that is 1.5 inches of mercury and for single or 2-barrel carburetors that is 3 inches of mercury.
At 4,900rpm a 440 engine will consume at maximum 624cfm. On the face of it therefore a 625cfm carburetor would be big enough. However, the 625 cfm of the carburetor assumes that the engine will be generating a vacuum of 1.5 inches of mercury and also petrol soaked air behaves differently to the dry air of the testing laboratory and so a 625cfm carburetor is unlikely to be big enough since in the real world you can never match laboratory conditions.
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JACB
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Re: carb. size

Post by JACB »

Another good option is the 625 CFM STREET DEMON CARBURETOR Vacuum secondaries and electric choke Black ceramic finish Black COMPOSITE fuel bowl that helps to keep the fuel cool. The great thing about this carb is it is very like the Thermoquad, but new. The old TQ’s are all old and in a lot of cases very worn.

Here is a link, have a look: https://www.holley.com/products/fuel_sy ... rts/1901BK

Another easy to use option is the Edelbrock AVS Series 2
http://www.edelbrock.com/automotive/mc/ ... avs2.shtml

From experience I can tell you that the choke system on the Demon range of carburettors is poor (no matter how you adjust them) and unlike some Holley chokes they dont have the vacuum choke pull off. The new K&N ECI Plate could be the answer if your heart was set on a Demon. I don’t know if these comments effect the new STREET Demon, that would need some research. I have a speed Demon which over chokes the engine in some conditions.
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kenny38
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Re: carb. size and a reasonably long trip

Post by kenny38 »

Hi everyone. Thank you for your advice. I have just come back from a national rally (Jensen) and ran on the 770 holley to Melbourne and back which included downhill a lot from northern NSW and long stretches of excellent road. I also did some city work in traffic (of course). On the way back a lot of those nice hills had suddenly reversed and my missus who has a very lead foot was really trying out her luck at 150+ up the hills. In all I measured we had travelled near 2000 MILES according to the clock and after changing 702 litres into gallons by dividing by 4.5 (i hope that is right) I reckon I got 15 miles per gallon overall. 19MPG on way down and 14 on way back with strong head wind. Reverse on way down. Car ran beautifully with nothing except a blown fuse effecting radio/sat nav/interiour lights. A helicoptor mechanic,Steve, travelling with our good mate "Sentimental Journey" found a very obscure fuse where none should exist and replaced 5 amp with a 10 amp fuse. Told me that fag lighters are just that and were never intended to be used as a constant source of supply for sat navs including whatever past sparkies have hooked up to it. Appears Sentimental Journey had to make a choice between taking wife or mechanic so he did the right thing. Basil Mc Kinley everyone was slagging off about you at the rally :D Kenny38
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Chris_R
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Re: carb. size

Post by Chris_R »

Sounds like a good run was had.
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