Carb size ???? 383

[kenm78]

Hey yall,

I got a 71 C10 LWB 3.42 rear gears 383 stroker XE268 Comp cam stock chevy 76cc heads with port polish 2.02-1.6 valves, forded pistons Edelbrock EPS intake headers ect. TH350 with TCI 2400 stall. My inteded power band is somewhere around 1000-5500,or as high as 5800 RPM. Im a realist gas mileage isn't on my agenda but I want performance without fouling my plugs every other drive. What size carb should I go with? Never tried Holley always used Edelbrock I got a 600 and a 750 edelbrock at the house.B ut what is everyones opinion were I might get my best performance through should I stick with edelbrock or go anouther route and what size should I use???

[slugish]

Ihave a similiar engine & am running a 650 cfc Holley double pumper. Was tempted to go larger 9maybe a 750, but according to the formula used to determine which carb. is best suited to your engine the 650 was the one . However , Iwas advised that the 750 with a vacuum secondary would be a good choice also. Since you have a 750 already Iwould try that first. Should be fine as is , but if a little too lean you could rejet it.

holley.com has a website that will match your particular engine with the best carb. Hope this helps. good luck

[1968SWBBigBlock]

Thier is another thread that is a day or two old that discuss' which carb folks like brand wise.
To choose your card go to Barry Grants web site and or Holleys web site, they both have interactive tools to help you choose your carb. Edelbrock may have the same on thier web site
http://www.holley.com/
http://www.barrygrant.com/

[kyle4381]

The 750 should work great just jet it up or down as needed.

[68CHEBY]

I don't know what formula you are using ( the size you are suggesting sounds right for a formula) but they are mostly exact and give no correction factor. Most of the ones I know of say to use the next size bigger to be safe. A 750 Edelbrock sounds good to me, although you may have to play with the jets and or the metering rods a little, maybe not though. You just pretty much have to try and see.

[crazy longhorn]

3310 Holley on mine, # 71 jets primary, & stock secondary plate. The sec diaphragm has a purple spring......specs on the 383 in my sig. L

[Mudder]

Tried a 650 Holley on mine but after about 3 jet changes went with a 750. Seems good now.

[ghackett1]

I'd say a street avenger 670 from holley would give the best throttle response or the 770. The 3310 (which is a vacuum operated secondary 750)is the old standby for your rpm range and although you have a small block you still have the cubes. They used it alot on entry level big block cars. Or you could spend big money and get into a demon or I really like the Holley HP series.

[PanelDeland]

From my experience a 650 will probably give better streetability but the 750 will help overall performance.From what I can tell the smaller keeps the vacuumsignal up which helps street style driving but the 750 flows more air.I think this means you should also take into account the way you will mostly be driving it.

[kenm78]

Does anyone know any secret tricks on adjusting edelbrocks I have played with them and switche step-up springs, metering rods, and jets time and time again. I have read the edelbrock book but the one thing it doesn't tell me is how to determine what step-up spring to use it tells me to put the stiffiest one in and decrease it until problem goes away. Any one know any tricks for edelbrock or should I just file it in the round filing cabnet???? Thanks

[68Stepbed]

Don't have alot of experience tuning Edelbrocks. I would say as mild as your engine is, I would recommend a 670 Holley Street Avenger or a 650 Speed Demon VS. A 750 would work really well if you plan to drag race alot. If you plan to mostly street it with occasional strip use, stick with something around 650.

If you're determined to stick with the Edelbrocks you have, I would try the 750 first.

[piecesparts]

ALL I can tell you, is about my 383 experiences and then you can decide. First off I have an 84 SWB with the 383 in it and short of body styles the trucks will be about the same weight. This is where the impotance to the right carb comes into play. The weight of a vehicle creates the mass that the engine has to move. For instance a 69 Camaro with the 383 is very light vehicle and it will respond a lot different than your truck. That means that you can put a 750 CFM carb on top of the 383 and get a reponsive drive out of the car. However with the truck the weight is it's enemy. The 750 CFM on the truck will have a "SLuGGISH" effect when driving on city streets, especially since you are running 3.42 gears, and this will cause plating out of deposits on your valves and hurt performance more. The 350 tranny of yours is limited on the highway speeds, due to no overdrive and it is ALSO limited on the city streets, due to the fact that the 1st gear is not quite as low in gearing as the 700R4 trannys. (You LOOSE on both ends, to some degree). This sluggishness is removed somewhat by keeping the airflow (VELOCITY) up into the intake manifold. That is done by using a smaller CFM rated carb. (example: 650 rather than a 750 carb) I did a lot of talking to Edelbrock, Holley, and Barry Grant, (HotRod Power Tour) and they all said the same thing.

I had put a 750 carb on my truck (383 stroker, Comp Cams roller # XR264HR-10, 2200 RPM stall, 700R4 tranny, 3.42 gears) and it drove around town like a PIG. After my discussions with the suppliers, I went back to my 600 CFM carb and that solved a lot of problems. The 750 gave me a fantastic top end, but I couldn't get there without punishing the truck to get it to accelerate. By going back to the smaller carb the acceleration improved greatly and the top end curbed some, but I still had a strong engine at 4000 RPM and with the overdrive tranny, I had 130 MPH top end available. I have since improved on that and went up to a 650 CFM carb (Edelbrock Thunder AVS) and changed the stall to a 2400 unit for my cam and the rear end gears are now 3.73s. The motor responds really strong and the 4000# truck responds surprisingly well. Ask the Mustangs in town..

As for fuel mileage, the 750 will hurt you there, unless you are going for performance only and don't care about the mileage. I get 15 MPG with my truck and still work it hard on the street if needed. I am satisfied with the final layout.

Maybe this will help you on your adjustments:

First of all the Edelbrock and the Carter AFBs are the SAME EXACT DESIGN CARBS. So I got a Carter carb tune up manual from Carter and it is very thorough, compared to the Edelbrock handbooks. However the parts from Edelbrock are cheaper than the Carter parts, so stick with them. I purchased a large amount of needles and jets along with the spring kits, so I could play with them at will. I took the jets and needles sizes and made a chart that shows the difference in each and every combination in thousandths to an inch. That way I know what different combinations would give me a small or large change in the fuel flow. The springs work as a reactor to what your vacuum is doing. So when you reach a certain level of vacuum, your spring starts to push the needle open to make up for the need for fuel. The description in the Edelbrock and Carter tuning manuals are pretty good about this. If the motor surges at a certain level, then the spring is definitely to strong and it is "SNAPPING" the needle open at the wrong time. This takes experimentation and driving (accelerating) on a back road until you are satisfied. I believe that I have found the combination with my truck and my son's truck (350 , 700R4, 3.73 gears, and Edelbrock 600 ) as to what works well with them. I still have a strong transition point , when the secondaries come into play, since they are mechanical and not vacuum actuated, but I have set my jets to the point that it is controlled to my benefit.

If you are going with Edelbrock consider the Thunder AVS series to get the 650 CFM over the 600 CFM and the ability to adjust the secondary air flow characteristics.

[wolfthing2000]

750 cfm

[piecesparts]

Originally Posted by wolfthing2000

750 cfm

Anything to support your choice? Am curious to know what you have experienced.

[kenm78]

Thanks Piece Parts

[Billla]

The basic formula for carbs is: CFM = ((CI x RPM)/3456) x VE

Using a VE of 90% - which is a bit optimistic - we find that at 6000 RPM the engine can use a maxiumum of 598 CFM. At cruise RPM of 2500 you'd need 249 (again, optimistic - unlikely your engine is running that VE at that RPM) but this tells us that in a square-bore carb we'd be right on the edge of the secondaries in cruise, and for a spread bore almost certainly would be with a 600 CFM carb - so we know we need something bigger than 600 CFM.

There are a few other factors here, including:

• Venturi size: As pieceparts noted, a venturi that's too big just won't give you the throttle response you expect. In other carbs like Webers, you can change the venturi size...in Holleys and BGs, venturi size is based on CFM although you can adjust to some extent.
• Secondary type: Vac secondaries for sure unless you're going drag racing

Something around 650-700 CFM with vac secondaries is right on. I know this definitely sounds too small...and it is leaving some HP on the table above 5500 RPM...but this will give you the best all-around driveability and throttle response as pieceparts suggests. Invest in a good dyno tune - most of the problems people complain about aren't problems with the carb, just problems with sizing and the tune. IMHO, without an Innovate or a dyno tune you're just guessing.

[Billla]

What problem are you trying to fix? It sounds like you're working to get rid of a bog when the secondaries come in...?

Originally Posted by kenm78

Does anyone know any secret tricks on adjusting edelbrocks I have played with them and switche step-up springs, metering rods, and jets time and time again. I have read the edelbrock book but the one thing it doesn't tell me is how to determine what step-up spring to use it tells me to put the stiffiest one in and decrease it until problem goes away. Any one know any tricks for edelbrock or should I just file it in the round filing cabnet???? Thanks

[crazy longhorn]

Originally Posted by piecesparts

ALL I can tell you, is about my 383 experiences and then you can decide. First off I have an 84 SWB with the 383 in it and short of body styles the trucks will be about the same weight. This is where the impotance to the right carb comes into play. The weight of a vehicle creates the mass that the engine has to move. For instance a 69 Camaro with the 383 is very light vehicle and it will respond a lot different than your truck. That means that you can put a 750 CFM carb on top of the 383 and get a reponsive drive out of the car. However with the truck the weight is it's enemy. The 750 CFM on the truck will have a "SLuGGISH" effect when driving on city streets, especially since you are running 3.42 gears, and this will cause plating out of deposits on your valves and hurt performance more. The 350 tranny of yours is limited on the highway speeds, due to no overdrive and it is ALSO limited on the city streets, due to the fact that the 1st gear is not quite as low in gearing as the 700R4 trannys. (You LOOSE on both ends, to some degree). This sluggishness is removed somewhat by keeping the airflow (VELOCITY) up into the intake manifold. That is done by using a smaller CFM rated carb. (example: 650 rather than a 750 carb) I did a lot of talking to Edelbrock, Holley, and Barry Grant, (HotRod Power Tour) and they all said the same thing.

I had put a 750 carb on my truck (383 stroker, Comp Cams roller # XR264HR-10, 2200 RPM stall, 700R4 tranny, 3.42 gears) and it drove around town like a PIG. After my discussions with the suppliers, I went back to my 600 CFM carb and that solved a lot of problems. The 750 gave me a fantastic top end, but I couldn't get there without punishing the truck to get it to accelerate. By going back to the smaller carb the acceleration improved greatly and the top end curbed some, but I still had a strong engine at 4000 RPM and with the overdrive tranny, I had 130 MPH top end available. I have since improved on that and went up to a 650 CFM carb (Edelbrock Thunder AVS) and changed the stall to a 2400 unit for my cam and the rear end gears are now 3.73s. The motor responds really strong and the 4000# truck responds surprisingly well. Ask the Mustangs in town..

As for fuel mileage, the 750 will hurt you there, unless you are going for performance only and don't care about the mileage. I get 15 MPG with my truck and still work it hard on the street if needed. I am satisfied with the final layout.

Maybe this will help you on your adjustments:

First of all the Edelbrock and the Carter AFBs are the SAME EXACT DESIGN CARBS. So I got a Carter carb tune up manual from Carter and it is very thorough, compared to the Edelbrock handbooks. However the parts from Edelbrock are cheaper than the Carter parts, so stick with them. I purchased a large amount of needles and jets along with the spring kits, so I could play with them at will. I took the jets and needles sizes and made a chart that shows the difference in each and every combination in thousandths to an inch. That way I know what different combinations would give me a small or large change in the fuel flow. The springs work as a reactor to what your vacuum is doing. So when you reach a certain level of vacuum, your spring starts to push the needle open to make up for the need for fuel. The description in the Edelbrock and Carter tuning manuals are pretty good about this. If the motor surges at a certain level, then the spring is definitely to strong and it is "SNAPPING" the needle open at the wrong time. This takes experimentation and driving (accelerating) on a back road until you are satisfied. I believe that I have found the combination with my truck and my son's truck (350 , 700R4, 3.73 gears, and Edelbrock 600 ) as to what works well with them. I still have a strong transition point , when the secondaries come into play, since they are mechanical and not vacuum actuated, but I have set my jets to the point that it is controlled to my benefit.

If you are going with Edelbrock consider the Thunder AVS series to get the 650 CFM over the 600 CFM and the ability to adjust the secondary air flow characteristics.

Just a thought on vehicle weight......a 69 camaro weighs 3500-3600 lbs, in stock form. My longhorn(very stripped, & 1/2 ton suspension), tips the scales @ 4040, less driver with 1/2 tank of fuel! Now some of the shortbed trucks are a good 400-500 lbs less than my curb weight, & that puts them right on line with a Camaro, or Nova I do agree that you will have better throttle response & mileage , with a slightly smaller carb tho. I think it really comes down to what you want from your combo(intended useage). I tryed the 750 Edelbrock, & dont think it is near as tunable as a good old Holley......but I grew up with the Holley! I did not like the Edelbrock carb on my 383, but my brother has a 600 cfm on a mild 305, & it works great! Most of the speed shops in my zipcode, say edelbrock(stock to mild), but go Holley, or Demon on the hotter setups. A couple things that help with throttle responce & big carbs are smaller primaries (Q jet), or vacumn secondaries. IMO, the squarebore carbs will not rival the spreadbore for mileage, given same size anyway. If you really want the best of both worlds, run a prepped Q jet, or spreadbore Holley..... L

[piecesparts]

Originally Posted by crazy longhorn

Just a thought on vehicle weight......a 69 camaro weighs 3500-3600 lbs, in stock form. My longhorn(very stripped, & 1/2 ton suspension), tips the scales @ 4040, less driver with 1/2 tank of fuel! Now some of the shortbed trucks are a good 400-500 lbs less than my curb weight, & that puts them right on line with a Camaro, or Nova I do agree that you will have better throttle response & mileage , with a slightly smaller carb tho. I think it really comes down to what you want from your combo(intended useage). I tryed the 750 Edelbrock, & dont think it is near as tunable as a good old Holley......but I grew up with the Holley! I did not like the Edelbrock carb on my 383, but my brother has a 600 cfm on a mild 305, & it works great! Most of the speed shops in my zipcode, say edelbrock(stock to mild), but go Holley, or Demon on the hotter setups. A couple things that help with throttle responce & big carbs are smaller primaries (Q jet), or vacumn secondaries. IMO, the squarebore carbs will not rival the spreadbore for mileage, given same size anyway. If you really want the best of both worlds, run a prepped Q jet, or spreadbore Holley..... L

You are right about the Q-jet and the spread bore Holley, but many people either don't have the cash for the Q-jet to build or the resources for one to be built for them. The Lower level Holley and Edelbrock carbs put you into the game at a reasonable price, not to mention that a lot of the intakes today are centered on the square bore design. So unless you happen to have the dough or you are aligned to the right supply house, many will end up looking at the market share of the square-bore world.

As for weight, my 84 GMC SWB sets at 4000 or just under it (w/o the hard tonneau cover) and if you put that up against a camaro or Nova of earlier design that is still 400 to 800 # more depending on what they have in the car. It is amazing what that much weight will do on acceleration. Holley is a good carb, I haven't run one since the 70s (the day of the blown power valve diaphragms) but I would consider one again, if I didn't already have all of the necessary linkage set ups for my cruise and tranny in the other brand. I put in the info for a consideration of the fact that a 750 in the design that Edelbrock and Holley now sell the most of, WILL create a problem with the daily driver, unless the owner looks at lower gears, or a better air flow in the operation. I didn't create this the suppliers did and I believe them after what I experienced with mine.

[crazy longhorn]

Originally Posted by piecesparts

You are right about the Q-jet and the spread bore Holley, but many people either don't have the cash for the Q-jet to build or the resources for one to be built for them. The Lower level Holley and Edelbrock carbs put you into the game at a reasonable price, not to mention that a lot of the intakes today are centered on the square bore design. So unless you happen to have the dough or you are aligned to the right supply house, many will end up looking at the market share of the square-bore world.

As for weight, my 84 GMC SWB sets at 4000 or just under it (w/o the hard tonneau cover) and if you put that up against a camaro or Nova of earlier design that is still 400 to 800 # more depending on what they have in the car. It is amazing what that much weight will do on acceleration. Holley is a good carb, I haven't run one since the 70s (the day of the blown power valve diaphragms) but I would consider one again, if I didn't already have all of the necessary linkage set ups for my cruise and tranny in the other brand. I put in the info for a consideration of the fact that a 750 in the design that Edelbrock and Holley now sell the most of, WILL create a problem with the daily driver, unless the owner looks at lower gears, or a better air flow in the operation. I didn't create this the suppliers did and I believe them after what I experienced with mine.

All good info! I havent seen a 750 cfm carb cause any problems in daily driving .....I may be "cheatin" now with 6 gears in the box, but the old 400 T/ 2400 stall/ 3.42 gears , was not a slug in town......yes, she did like the 4.10 gears better tho As far as load -up, or soggie perf in town.....I have not seen it with my 383. its a pretty wild ride for 4000 lbs @ the curb L

[piecesparts]

Originally Posted by crazy longhorn

All good info! I havent seen a 750 cfm carb cause any problems in daily driving .....I may be "cheatin" now with 6 gears in the box, but the old 400 T/ 2400 stall/ 3.42 gears , was not a slug in town......yes, she did like the 4.10 gears better tho As far as load -up, or soggie perf in town.....I have not seen it with my 383. its a pretty wild ride for 4000 lbs @ the curb L

At the time I had the 750, I was still running th 2200 stall and that was hoding up against my cam a little to much. That probably attributed to some of my issues, but the 600 CFM cleaned that up. THEN i put a rod throught the motor. broke a rod just above the crank and had to rebuild. while it was odwn for them otor, the tranny and convertor was upgraded also. Now the combo is dang freaky in performance.....Sit down and Hang on---scream if you need to.

[corn]

When I had my old chevelle with a 406, I stopped by a shop where the guy did a lot of headwork & tested parts on his flowbench. He ran all my motor specs through a computer program he had & it came up as 630cfm as optimum. Id say try the 600 & go from there. Im famous for over carburating...

[Schralper]

I'm using a Holley #3310 750 Vac Sec w/ jets instead of secondary plate on a 388. No problems, runs great, 15K miles. 10.5:1, Alum heads, Edelbrock RPM manifold, XR276HR cam, Supercomp headers, 2.5" to flowmasters. Idles @ 700 in drive.
85 Burban 4x4, T700, 35" tire w/ 4.56 lockers, daily driver/tow rig.
Drives from 400' - 7000' elev. 2700rpm=55 in drive.
I'm guessing you're running kinda low comp w/ those 76 cc heads. Ya might wanna go w/ a Holley or Demon 650 or 700 DP for best power. These strokers produce a pretty good vacuum. You won't need a whole lotta timing either. I've got 15 deg initial, 36 total. If you're fouling plugs you're WAY off on your tune or there's other problems cuz I just haven't seen any issues w/ Holley vac. sec. or DP carb.

[kenm78]

Outstanding information!!! had to read it a couple times for it to fully sink in. Im in the process of rebuilding my 383. The machine shop that performed all my machine work about 2 years ago didn't do what they charged me for. I originally had the RPM air gap with the Comp Magnum 280H cam originally used the manual choke 600 cfm carb and tried every jet and rod combo on the edelbrock chart had a bad dead spot off the line and felt weak at top end. I never went over 5600 rpms. So I have never dealt with Holley and I guess I am timid when it comes to new things. So I bought a edelbrock 750 cfm and again it took alot of time and effort to get it somewhat tuned in and I either had a little bog off the line or I fouled my plugs out, becouse I love to get on it all the time but I guess I didn't do it enough to keep it burning clean enough. Due to the lack of vacume on my engine 11-12 pounds in drive at 800rpms I am switching cams during this rebuild I am going to go with the XE 268 or the Voodoo 60102 PN and the Edelbrock EPS intake. It is a square bore intake, I have often wondered if a Q-Jet would be great but again never tinkered with one. From what I have read Holley or Q-Jet is were I might want to head. Should I spend the money and dyno it before or after I put it in my truck??? Does anyone know a good Dyno shop in or around Oklahoma city??? If I do go down the Q-Jet road what should I look for or should I just purchase a reman from edelbrock or holley????

[piecesparts]

Do you know what you were running for timing on the 383, before you tore it apart for this rebuild? The experts will tell you that timing is also another way to smooth out the HP blues in an engine. If you were running on the bottom side of the performance curve, then you were wasting energy and making the motor run hot and providing for that "DEAD SPOT that you were getting on acceleration. As you advance timing, you start going towards the point of pre-ignition, but there is a stopping point. Looking at Schralper's input, I agree that you may have been really out of tune. The 750 CFM on top of the motor was also giving you fits because of the tuning and the fouling of the plugs gives hint to the over gassing. When you get this next engine assembled and are ready to run it, I would look at setting the timing at a "TOTAL ADVANCE" number of around 36 degrees. I would also look at the idle settings and adjust them using a vacuum gauge. Once you have those basics, then go for the adjustment of whichever carb you decide on. I will stand behind the thought of maintaining a 650 CFM carb rating for a daily driver/street beater. If you decide that you want a spread-bore carb, then you will need to get a manifold to support the mounting of such a creature, so the Edelbrock EPS may have to go. There are specialty shops that build those carbs for every day use. You can make a vacuum carb or a mechanical carb work ineither area, just whatever you want to do.

My 383 runs a vacuum signal close to yours, but I have had no issues with the dogging, since I did the re-assembly. The thought is to get the parts to align with each other.