What internal modifications can be made to a carburetor in order to get the most power from it? I've been reading about the guy's that run pure stock drag racing and since everything has to be as it came from the factory, I'm wondering how they get the power that they do. The cam has to have stock lift but there's suppose to be leeway on the duration but I can't find what it is. Any ideas on how to tune for power will be appreciated. I think that appling some of their techniques on a daily driver would be awesome.
Carburetor tuning
- Thread starter Dan the man
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Mattax
Just the facts, ma'am
The beauty of carburation is it reponds to whatever the engine is doing.
There's really no magic in them, just physics.
Build a better engine (that eats more air and develops higher VE under the exact same condition) and the carb will respond.
Yes there are some techniques that can take advantage of improved fuel/air distribution and vaporization. And there are also some techniques needed to address engines that have worse than factory combustion conditions.
For the street, if its a hot rod, then the secret is to first tune under steady state conditions. Idle - loaded, Slow low rpm, then high speed highway steady. Then tune WOT in top gear (1/4 mile or dyno) for steady AFR and best mph or hp curve. Then fill in the transititions. its a lot of testing and sometimes it requires a few steps back or change of sequence in the testing.
The target for each of these tests will be some sort of performance measurement representing power and efficiency. For example, at idle in gear (automatic transmission) highest vacuum and/or least rpm drop. If its dropping a bunch of rpm, its because the engine is lacking power.
A very experienced tuner or builder can select droplet sizes (or try to) as well as to optimize the fuel distribution for a particular race engine.
A street engine has to handle a wider range of fuels and conditions, and just as important, often is running on the low speed circuit (aka idle circuit). That's the most difficult to tune well with low vacuum typical on high overlap cams.
There's really no magic in them, just physics.
Build a better engine (that eats more air and develops higher VE under the exact same condition) and the carb will respond.
Yes there are some techniques that can take advantage of improved fuel/air distribution and vaporization. And there are also some techniques needed to address engines that have worse than factory combustion conditions.
For the street, if its a hot rod, then the secret is to first tune under steady state conditions. Idle - loaded, Slow low rpm, then high speed highway steady. Then tune WOT in top gear (1/4 mile or dyno) for steady AFR and best mph or hp curve. Then fill in the transititions. its a lot of testing and sometimes it requires a few steps back or change of sequence in the testing.
The target for each of these tests will be some sort of performance measurement representing power and efficiency. For example, at idle in gear (automatic transmission) highest vacuum and/or least rpm drop. If its dropping a bunch of rpm, its because the engine is lacking power.
A very experienced tuner or builder can select droplet sizes (or try to) as well as to optimize the fuel distribution for a particular race engine.
A street engine has to handle a wider range of fuels and conditions, and just as important, often is running on the low speed circuit (aka idle circuit). That's the most difficult to tune well with low vacuum typical on high overlap cams.
Mattax
Just the facts, ma'am
Lemme add this.
In Stock Eliminator 2 bbl classes (T, U, R IIRC) these are generally small displacement engines. So even though yes the cam lobe shapes have been changed to maximize breathing there's legal ways to improve flow at high rpm. In some other restricted racing classes (often in circle track) the best guys really can do some amzing things to deal with the air restriction of a two barrel at high rpm. That's beyond my experience but always enjoy reading about it
Seeing some of your other threads on carbs, it seems like you're pretty interested in learning about them.
Chrysler had several booklets and filmstrips about about carburation.
The one in '66 may be the better one to begin with as emissions were not a concern, so idle mixes were still focused on mechanical efficiency (power and fuel savings) rather than reducing pollutants.
Browse MTSC by Model Year – 1966 – MyMopar
The fuel mixtures need for different loads are described in this post.
Wideband
and touched on here
Large RPM and Vacuum drop when shifting from park to gear
The first page (linked below) summarizes some of my own experience with modifying and tuning carbs. The results are not always what the popular magazines and writers promote as 'better' and worth spending your $$$ on.
100 cfm more with anular and downleg boosters can't beat AFR
In Stock Eliminator 2 bbl classes (T, U, R IIRC) these are generally small displacement engines. So even though yes the cam lobe shapes have been changed to maximize breathing there's legal ways to improve flow at high rpm. In some other restricted racing classes (often in circle track) the best guys really can do some amzing things to deal with the air restriction of a two barrel at high rpm. That's beyond my experience but always enjoy reading about it
Seeing some of your other threads on carbs, it seems like you're pretty interested in learning about them.
Chrysler had several booklets and filmstrips about about carburation.
The one in '66 may be the better one to begin with as emissions were not a concern, so idle mixes were still focused on mechanical efficiency (power and fuel savings) rather than reducing pollutants.
Browse MTSC by Model Year – 1966 – MyMopar
The fuel mixtures need for different loads are described in this post.
Wideband
and touched on here
Large RPM and Vacuum drop when shifting from park to gear
The first page (linked below) summarizes some of my own experience with modifying and tuning carbs. The results are not always what the popular magazines and writers promote as 'better' and worth spending your $$$ on.
100 cfm more with anular and downleg boosters can't beat AFR
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Stock eliminator class carb cheat, AFB 500, use 750 rear boosters
furrystump
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Carbs are rated for flow at a standard pressure differential, because the higher the differential the more CFM they will flow. You just have to be able to flow enough fuel through it. I’m running an 800 AVS Eddy carb. I am getting very close to flowing the maximum amount of fuel that carb can flow. I have run it without rear jets as a test. It was a bit rich, but not crazy rich. Which would indicate that my motor creates significantly higher pressure differential than the standard. I once asked another FAST racer running a thermoquad how much jet/fuel he was running. “All of it” was the response. Funny and truthful at the same time. I believe people view ultimate carb size by CFM. A standard that is not fixed. How much fuel a carb can flow ultimately sets the limit.
furrystump
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None. First the carb should be designed to run correctly on the engine size you want to use it on i.e it should meter fuel correctly for AFR requirements for various conditions and secondly you don't really tune a carb you tune your engine through your carb. Sometimes a smaller carb will make more power simply because it improves vaporization because of the greater shear force at the booster which helps break up the fuel into smaller droplets and the greater vacuum generated at the venturis. Also the greater vacuum generated at the Tslot helps the car accelerate quicker because it improves the quality of the mixture exiting the slot which improves distribution.
Interesting that the smaller carb ET'd quicker:
How to Pick the Best Carburetor for a Street/Strip Car
The plain old 650 DP did best on a 425 CI engine. Idle quality and part throttle response are a dead give away to this fact.
It also wasn't much of a surprise when the 650-cfm carburetor came up 14 hp shy of the 850's peak number. That was as expected. However, looking at the rest of the results, it was clear that if we were more interested in idle quality and part-throttle response, the numbers indicate the 650 would be a much better choice. This might surprise some people, but the smaller-diameter venturi and throttle blades should contribute to a crisper throttle response, especially on a 425ci engine. The 650 delivered the leanest idle and appears that it would also produce the best fuel mileage. Some carb tuners fall back on adjusting the carburetor to run rich to make the engine run properly. However, taking the time to adjust the carburetor so the idle is as lean as possible (short of a light-acceleration stumble) is a much better approach. Of course, with enough tuning, that is also possible with the larger carburetors.
As you can see from the Dragstrip Test Results chart, our guess that the 650 carburetor might run the quickest of the bunch turned out to have some validity. The smaller carburetor delivered the quickest move to the 330-foot mark and the quickest overall e.t. and fastest mph, although the engine popped out the exhaust near the finish line. This was with the lean chassis dyno jetting. Once we richened the jetting three sizes in the secondary, the 650 slowed slightly to a 7.12 at 94.77 mph.
A properly set up 750 DP may do better in the 1/4 mile.
How do the guys that run pure stock drag racing get the power from their combination as they have to be 100% stock? They are allowed a increase in compression of 1.5
Efficiencies.
A bone stock 1973 340 running a 13.83 at just under 99 mph I find hard to believe. I don't see how fine tuning can make that much of a difference when they can't run headers, no aftermarket parts except the pistons and rings. Have to run stock heads with stock valve size, stock cam, etc. Where how can they improve efficiency
Everywhere. There a a lot of places to find improvements but it takes time and effort. If you think you can just bolt parts together and it will run its best you are wrong.
Go look at what 318willrun has managed to run with a stock 360 with a 204/216 crane hydraulic cam, he's into the 13's
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The answer to that would fill a book.
OldmanRick
Well-Known Member
Say a Duster at 3350 lbs, it only takes about 250/260 hp to run 99 mph. Easily done with a well blueprinted engine and add a efficient converter, some good belted tires and you're on your way
Furrystump,
I can think of 3 reasons that your engine could be fuel starved, not air starved because of carb cfm.
- insufficient fuel volume delivery to the carb
- n/s too small. All the Edel carbs I have seen come with piddling 0.093" n/seats. In contrast the original AFB 500 cfm came with 0.101" n/seats. I would be using 0.110" for any performance application with an 800 cfm carb
- some Edel carbs had restricted secondary E tube holes in the base that acted like the MJ. For reasons known only by Edel....
I can think of 3 reasons that your engine could be fuel starved, not air starved because of carb cfm.
- insufficient fuel volume delivery to the carb
- n/s too small. All the Edel carbs I have seen come with piddling 0.093" n/seats. In contrast the original AFB 500 cfm came with 0.101" n/seats. I would be using 0.110" for any performance application with an 800 cfm carb
- some Edel carbs had restricted secondary E tube holes in the base that acted like the MJ. For reasons known only by Edel....
In that article did they do anything to improve the emulsion stack which in a bunch of stuff is dead rich down low and mid range? Change any IFR's or IAB's to clean up the idle/transitions of the bigger carbs?
Were they all tuned to the same spec, RPM, timing, etc?
I can make stuff work and others look like junk if you tweak things ever so slightly or just ignore what you know is an issue OOTB for some carbs. Twisting the idle mix screws is not really completely tuning a carb, except setting up the idle AFR. Pulling from a rich idle is going to compromise the low end, 60' times almost every time.
Were they all tuned to the same spec, RPM, timing, etc?
I can make stuff work and others look like junk if you tweak things ever so slightly or just ignore what you know is an issue OOTB for some carbs. Twisting the idle mix screws is not really completely tuning a carb, except setting up the idle AFR. Pulling from a rich idle is going to compromise the low end, 60' times almost every time.
More than likely as the other carbs are HP's with adjustable bleeds. Too rich down low is usually an indication of too much emulsion.
The thing's that you mentioned here I have no idea what it means. I'm wanting / trying to learn how to tune a carburetor in order to get maximum performance from it as I'm thinking about building a engine to run in pure stock, so the carburetor has to be of the same cfm and secondary operation as it came from the factory when new. I'm sure that it's going to take a lot of reading,etc. I've always liked to see how good a car can be made to run with what it came with.
That would mean about 320 horsepower at the flywheel, as you said can be done. What would be considered a efficient converter.Say a Duster at 3350 lbs, it only takes about 250/260 hp to run 99 mph. Easily done with a well blueprinted engine and add a efficient converter, some good belted tires and you're on your way
Thanks. I'm probably going to do a 360
Do you know what title that build might be under? I'm not real good with computers. I found a cam with those specs at 0.050 but it's a lunati grind. I've been away from the car scene for awhile, is crane even still around? Would you recommend a comp or a lunati cam?
I'm sure you could pm 318willrun and he will gladly share details on his build. Otherwise he'll chime in and answer your questions soon.
Awesome. Thanks everyone for helping me to understand what it takes to get a engine to run at its best without spending a fortune. I thought that it was possible but never had any idea on how to go about it. It takes more than just putting a engine together.
It ran high 13's with stock exhaust manifolds, 2.45 gears, and factory torque converter. It did have a 750 DP on a eddy rpm (not air gap). 2.94's and headers brought it to 13.3's. 3.23's and OOTB speedmaster heads it ran 12.9's. Speedmaster heads were worth about 2.5 tenths over the factory cast due to the small cam not utilizing the heads (we knew this when we did the swap).
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