Never "too much" carb???
10-14-2005, 01:30 PM
#1
Burning Brakes
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Never "too much" carb???
OK, guys...I've been reading along here from guys asking is this carb to big...is that too big...and I hear.."nothing is TOO big"
I have a 76 L-48 auto with dual Edlebrock 1405's on it...from the guy I bought it from, the papers indicate there was a "cam" installed at some point...no specs, just a cam, I can only assume a mild street cam...it does has a mild lope to it at idle...
Now, after adjusting timing (the Lars deal) and syncing the carbs ( via instructions from Edlebrock specific to the 1405's)...it is still quite the dog...my 2000 V6 Mustang 4 speed (with some basic performance mods) will blow the vette's doors clean off...I mean I can't even find the doors it's so bad
Stock exhaust, stock tranny, stock rear end, DUI distributor....low loss wires...
I just don't understand why it's so doggy, except that maybe I've got..(here come those words)..."too much carbs" on it???
I'd be interested what the next step would be with some of you guys if you were in my place..
I would love to get this thing to run decent before puttin' her up for the winter (Great White North East)
I have a 76 L-48 auto with dual Edlebrock 1405's on it...from the guy I bought it from, the papers indicate there was a "cam" installed at some point...no specs, just a cam, I can only assume a mild street cam...it does has a mild lope to it at idle...
Now, after adjusting timing (the Lars deal) and syncing the carbs ( via instructions from Edlebrock specific to the 1405's)...it is still quite the dog...my 2000 V6 Mustang 4 speed (with some basic performance mods) will blow the vette's doors clean off...I mean I can't even find the doors it's so bad
Stock exhaust, stock tranny, stock rear end, DUI distributor....low loss wires...
I just don't understand why it's so doggy, except that maybe I've got..(here come those words)..."too much carbs" on it???
I'd be interested what the next step would be with some of you guys if you were in my place..
I would love to get this thing to run decent before puttin' her up for the winter (Great White North East)
10-14-2005, 01:48 PM
#3
Drifting
I have a different opinion...I think Bubba often puts on too much carb. Look at all the very fast and strong 2-barrel engines. My '70 Olds 455 currently has a 2-barrel, very fast, strong, and responsive.
My undertsanding is that huge carb flow is necessary for "big end"...high RPM, and huge displacement engines.
My street rod is running dual carter AFBs, and runs much better with the smallest ones (450 cfm) than the bigger ones (600 cfm) I ran previously.
My undertsanding is that huge carb flow is necessary for "big end"...high RPM, and huge displacement engines.
My street rod is running dual carter AFBs, and runs much better with the smallest ones (450 cfm) than the bigger ones (600 cfm) I ran previously.
10-14-2005, 01:51 PM
#5
Burning Brakes
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Originally Posted by halfmoontrail
I have a different opinion...I think Bubba often puts on too much carb. Look at all the very fast and strong 2-barrel engines. My '70 Olds 455 currently has a 2-barrel, very fast, strong, and responsive.
My undertsanding is that huge carb flow is necessary for "big end"...high RPM, and huge displacement engines.
My street rod is running dual carter AFBs, and runs much better with the smallest ones (450 cfm) than the bigger ones (600 cfm) I ran previously.
My undertsanding is that huge carb flow is necessary for "big end"...high RPM, and huge displacement engines.
My street rod is running dual carter AFBs, and runs much better with the smallest ones (450 cfm) than the bigger ones (600 cfm) I ran previously.
My point exactly...
10-14-2005, 02:16 PM
#6
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...I can taste the words being put in my mouth...
The quote has never been that "no carb is too big." The quote - and the point - has been that any carb can be set up and tuned to not be the cause of drivability problems. I have demonstrated this by running carbs ranging in size from 390 through 1250 on my 357 small block, all without any adverse effect. This is not to say that bigger carbs run better: Once over-carb'ed, the car will not go faster than with the "right" size carb, but a big carb can be set up to run without problems. The common solution to any problem ranging from stumbles, sags, bogs, and poor throttle response is "your carb is too big" (typically when a guy has a little 750 carb...) To this, I say that a "too big carb" is seldom the cause of the problem - you need to tune the car correctly to solve problems rather than immediately blame the size of the carb.
A carb that is much too big can result in a loss in bottom end torque and throttle response. To see if this is your problem, simply disconnect the progressive linkage on your setup and drive it with only a single carb hooked up. If this suddenly makes your car into a head-snapper, you should consider dropping down to a single carb or changing over to 2 smaller carbs.
The quote has never been that "no carb is too big." The quote - and the point - has been that any carb can be set up and tuned to not be the cause of drivability problems. I have demonstrated this by running carbs ranging in size from 390 through 1250 on my 357 small block, all without any adverse effect. This is not to say that bigger carbs run better: Once over-carb'ed, the car will not go faster than with the "right" size carb, but a big carb can be set up to run without problems. The common solution to any problem ranging from stumbles, sags, bogs, and poor throttle response is "your carb is too big" (typically when a guy has a little 750 carb...) To this, I say that a "too big carb" is seldom the cause of the problem - you need to tune the car correctly to solve problems rather than immediately blame the size of the carb.
A carb that is much too big can result in a loss in bottom end torque and throttle response. To see if this is your problem, simply disconnect the progressive linkage on your setup and drive it with only a single carb hooked up. If this suddenly makes your car into a head-snapper, you should consider dropping down to a single carb or changing over to 2 smaller carbs.
10-14-2005, 03:56 PM
#8
Originally Posted by kb2fzq
Stock exhaust, stock tranny, stock rear end, DUI distributor....low loss wires...
Steve
10-14-2005, 03:58 PM
#9
Race Director
Lars has explained it quite well as usual. For years people have been blaming lousy acelleration, bog and few other things on the motor being "overcarbed" when in reality is just a poorly tuned motor and carb.
You can put a big carb on your motor but at some point ( in size ) it does you no good and, as pointed out you will loose low end throttle response and torque with no real gain upstairs either.
You can put 850cfm Holley on a mild 350ci and it will run just fine, you hit the gas and the motor just doesn't use all the cfm available, it still runs fine. It will however probably run better in the lower RPMs with a 650 or if it is done up a 750.
BTW a tunnel ram with two Dominators on stock L48....you are overcarbed
You can put a big carb on your motor but at some point ( in size ) it does you no good and, as pointed out you will loose low end throttle response and torque with no real gain upstairs either.
You can put 850cfm Holley on a mild 350ci and it will run just fine, you hit the gas and the motor just doesn't use all the cfm available, it still runs fine. It will however probably run better in the lower RPMs with a 650 or if it is done up a 750.
BTW a tunnel ram with two Dominators on stock L48....you are overcarbed
Last edited by MotorHead; 10-14-2005 at 04:01 PM.
10-14-2005, 06:30 PM
#10
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kb2 -
Another thought that makes me think that your carb sizing is not directly your problem or issue....
The carbs you have are the 600 cfm Carter AFB re-pops by Edelbrock. These carbs utilize a counterweighted airvalve to control secondary opening rate and opening point. The use of the airvalve effectively makes the carbs "variable cfm" carbs, in that the airvalves will not open until engine airflow demands the capacity. Thus, when you nail it from a standing start, you are not supplying the engine wth 1200 cfm. Rather, if the primary side throttles on both carbs are slammed open, you are exposing the engine to only 600 cfm - the same as if you had a 600 cfm mechanical secondary carb on the car and slammed it wide open.
Obviously, nobody would argue that a 600 cfm carb is "too big" for your car, but that's effectively what you have with the airvalve-controlled secondary carbs.
Rather than a carb sizing/cfm issue with your poor performance, you might have a component parts compatibility issue with the overall layout of your engine. You have stock exhaust manifolds with a dual quad intake... maybe a cam... probably low compression... stock '76 rear end (probably a 3.08:1)... automatic probably with a stock converter... stock heads... these things add up to high performance expectations with low actual output: the modification of dual quads with a mild cam in an otherwise stock engine/drivetrain will result in performance little changed from the stock configuration other than an actual loss of low-rpm torque.
Lars
Another thought that makes me think that your carb sizing is not directly your problem or issue....
The carbs you have are the 600 cfm Carter AFB re-pops by Edelbrock. These carbs utilize a counterweighted airvalve to control secondary opening rate and opening point. The use of the airvalve effectively makes the carbs "variable cfm" carbs, in that the airvalves will not open until engine airflow demands the capacity. Thus, when you nail it from a standing start, you are not supplying the engine wth 1200 cfm. Rather, if the primary side throttles on both carbs are slammed open, you are exposing the engine to only 600 cfm - the same as if you had a 600 cfm mechanical secondary carb on the car and slammed it wide open.
Obviously, nobody would argue that a 600 cfm carb is "too big" for your car, but that's effectively what you have with the airvalve-controlled secondary carbs.
Rather than a carb sizing/cfm issue with your poor performance, you might have a component parts compatibility issue with the overall layout of your engine. You have stock exhaust manifolds with a dual quad intake... maybe a cam... probably low compression... stock '76 rear end (probably a 3.08:1)... automatic probably with a stock converter... stock heads... these things add up to high performance expectations with low actual output: the modification of dual quads with a mild cam in an otherwise stock engine/drivetrain will result in performance little changed from the stock configuration other than an actual loss of low-rpm torque.
Lars
10-15-2005, 01:25 AM
#11
Burning Brakes
Lars is definitely steering you in the right direction. Also, please consider that a '76 L-48 *IS* a dog. Unfortunately you don't know a lot about the engine, so it is hard to say how much over stock the actual engine is. I can tell you that the money was definitely spent in the wrong place. There is no reason you should *NEED* a dual carb setup on that engine under "normal" circumstances.
Some common things that will help performance in the order you should think about them:
1 - Recurve your distributor for optimum performance (normally about 36 degrees mechanical advance all in by 3000 RPM.). There is a document at http://www.thirdgen.org/newdesign/tech/hei.shtml that describes how to do this, but Lars also wrote a very good one that should be over at http://www.corvettefaq.com/ , but it doesn't go into a huge amount of detail for the HEI specifically, though there isn't a whole lot different about it.
2 - Carb tuning. Another one that's very inexpensive/free is to tune the setup you have.
2 - Exhaust. The stock exhaust is pitiful. A set of headers and a true dual exhaust with maybe 2.5" pipes would do a world of help, especially with other mods.
3 - Cam swap. If you think about this route though, consider what else may have been done to the engine (heads, intake, etc). The idea is to find a combination that works well together. If you don't know what kind of condition the engine is in, swapping a cam may just be a silly waste of money. You may be better considering a rebuild, but at the minimum you should try to identify the heads and intake. You could also map the profile of the cam. . .
4 - Maybe not #4 on the priority list, but since it wasn't engine related . . . Rear gears & torque converter. The rear gears can make a huge difference in your perceived performance. Depending on your use the ratio will vary, but generally a 3.55 or maybe 3.73 is a pretty good street compromise behind a 1:1 final ratio transmission with stock sized tires. You could also go with a 2500 RPM stall torque converter, but I wouldn't do that unless you step up to at least 3.55 gears.
-dath
Some common things that will help performance in the order you should think about them:
1 - Recurve your distributor for optimum performance (normally about 36 degrees mechanical advance all in by 3000 RPM.). There is a document at http://www.thirdgen.org/newdesign/tech/hei.shtml that describes how to do this, but Lars also wrote a very good one that should be over at http://www.corvettefaq.com/ , but it doesn't go into a huge amount of detail for the HEI specifically, though there isn't a whole lot different about it.
2 - Carb tuning. Another one that's very inexpensive/free is to tune the setup you have.
2 - Exhaust. The stock exhaust is pitiful. A set of headers and a true dual exhaust with maybe 2.5" pipes would do a world of help, especially with other mods.
3 - Cam swap. If you think about this route though, consider what else may have been done to the engine (heads, intake, etc). The idea is to find a combination that works well together. If you don't know what kind of condition the engine is in, swapping a cam may just be a silly waste of money. You may be better considering a rebuild, but at the minimum you should try to identify the heads and intake. You could also map the profile of the cam. . .
4 - Maybe not #4 on the priority list, but since it wasn't engine related . . . Rear gears & torque converter. The rear gears can make a huge difference in your perceived performance. Depending on your use the ratio will vary, but generally a 3.55 or maybe 3.73 is a pretty good street compromise behind a 1:1 final ratio transmission with stock sized tires. You could also go with a 2500 RPM stall torque converter, but I wouldn't do that unless you step up to at least 3.55 gears.
-dath
10-15-2005, 07:14 AM
#12
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[. You could also go with a 2500 RPM stall torque converter, but I wouldn't do that unless you step up to at least 3.55 gears.
-dath[/QUOTE]
could you explain why you wouldn't put a convertor on say a 3.08 gear rear end?
-dath[/QUOTE]
could you explain why you wouldn't put a convertor on say a 3.08 gear rear end?
10-15-2005, 09:47 AM
#13
Burning Brakes
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Thanks to all with this...as I read a couple of the replies, I see my fears becoming a reality...this is not my first Chevy engine...back when I was younger (much younger 
) I saw guys attempting this type of mod with horrific results...the reply that hit the head...."why would you put dual carbs on a stock L-48 without working the rest of the engine?"...I fear my car was owned by someone that didn't understand the whole picture...and thought by "just" a dual quad swap, he would make a bullet...wrong..
So, at this point, I am trying to keep the budget down, yet have a car that can at the least...beat a V6 Ford...I am toying with removing the dual quad intake, replacing with a stock single quad intake, or a slight mod intake and a carb that you guys might suggest...fact is, I still have the stock carb in a box the guy I bought it from gave me...without a rebuild, I wouldn't trust it, tho...
I guess my last question is..should I try to work out what I have now or go back to a single carb???
) I saw guys attempting this type of mod with horrific results...the reply that hit the head...."why would you put dual carbs on a stock L-48 without working the rest of the engine?"...I fear my car was owned by someone that didn't understand the whole picture...and thought by "just" a dual quad swap, he would make a bullet...wrong..So, at this point, I am trying to keep the budget down, yet have a car that can at the least...beat a V6 Ford...I am toying with removing the dual quad intake, replacing with a stock single quad intake, or a slight mod intake and a carb that you guys might suggest...fact is, I still have the stock carb in a box the guy I bought it from gave me...without a rebuild, I wouldn't trust it, tho...
I guess my last question is..should I try to work out what I have now or go back to a single carb???
10-15-2005, 01:01 PM
#15
Burning Brakes
Originally Posted by dath
[. You could also go with a 2500 RPM stall torque converter, but I wouldn't do that unless you step up to at least 3.55 gears.
-dath
-dath
Originally Posted by fast idle
could you explain why you wouldn't put a convertor on say a 3.08 gear rear end?
I actually have *EXACTLY* this situation on my car right now as I haven't gotten to swapping my gears out yet. I tend to avoid the freeway and don't go on any long drives. Just like engines, one has to think about what they are doing with the drive train before just jumping in and swapping parts randomly; for best performance everything must be choosen to work together.
Of course the stall speed "rule" I just mentioned goes out of the window for people who primarily drag race as they have very high stall speeds many times (5000 RPM in some cases) where there would be no gear setup that could accomodate it well. However, there is not a chance anyone who is that serious would be running a 3.08 rear gear, which is why I said what I did. People with a 3.08 rear gear are pretty much limited to street drivers (not entirely, but normally).
-dath
Last edited by dath; 10-15-2005 at 01:04 PM.
10-15-2005, 01:52 PM
#16
Melting Slicks
What you mentioned about the stall speed and crusing with 3.08 gears is true if driving in high gear at 45mph. At 60 or 65 mph crusing on the highway, the car will taching higher then a slightly higher then stock stall converter (say a 2000-2400rpm stall conv). If driving at 30-45 mph, why couldn't the tranny be in 2nd gear.
10-15-2005, 03:12 PM
#17
Burning Brakes
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Your dual 500's are like a double pumper with another extra 500cfm thrown in just for humor LOL.
Seriously though....each carb's primary side idle circuit is built to deliver enough emulsified fuel to run the entire engine.....and you have TWO of them.
It's overkill.
Seriously though....each carb's primary side idle circuit is built to deliver enough emulsified fuel to run the entire engine.....and you have TWO of them.
It's overkill.
10-15-2005, 03:25 PM
#18
Race Director
perhaps i missed something....your are running stock heads, stock manifolds and 2 into 1 exhaust with a cat? and you are wondering why this is a dog? your current modifications are like being in pancake eating contest while you are constipated. nothing good is to be expected.....start with better heads, better exhaust manifolds or headers and a low restriction exhaust system then your car will be more responsive. your 3spd auto tranny with say 3.08s rear end is not fast off the line in any way shape or form....you will need different rear end ratio or an overdrive trans....this is where all the fun begins.
10-15-2005, 03:27 PM
#19
Burning Brakes
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Originally Posted by bobs77vet
perhaps i missed something....your are running stock heads, stock manifolds and 2 into 1 exhaust with a cat? and you are wondering why this is a dog? your current modifications are like being in pancake eating contest while you are constipated.
10-16-2005, 09:10 AM
#20
Burning Brakes
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Yes...I am painfully aware that I lack in the exhaust side of the formula, and eventually I will remedy that...and the 3:08's make some sense now regarding off the line performance...I wasn't aware what I had back there...and actually, where the car performs the best...if at 50 mph, I put the foot in the carbs, it will almost snap your neck...so I'd say top end works.
I have used Lars timing advise and get 36* between 2500-2800 rpm..it's just a slug off the line, the rear end well explained here, obviously has more then alot to do with that line slug, as well as a pancake eating, constipated exhaust (very clear )....soooooooo...
OK, again, thank you all for the great advise....you guys never let me down..I will take all of your advise and move forward...
I have used Lars timing advise and get 36* between 2500-2800 rpm..it's just a slug off the line, the rear end well explained here, obviously has more then alot to do with that line slug, as well as a pancake eating, constipated exhaust (very clear
)....soooooooo...OK, again, thank you all for the great advise....you guys never let me down..I will take all of your advise and move forward...