Thread: Third gen, built 350, what carb should I look for?

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Hey guys, ive got a third gen 83 trans am, 70's 350 built pretty fresh, rods, pistons, everything. I'm looking for a new carb. Some people tell me to go all the way to 750 cfm. Id like to stay with a Holly/demon, I like their products. I'm a young guy and I'm definitely still learning about carburetors, tweaking etc and I have little knowledge of what size and type of carb I should start with my setup.

I hope this is the correct area to post!

Right now I have a Holly 600 vacume secondary that was given to me, its pretty old but it has a rebuild on it and runs good. Just wondering if there is anything that might run better with my combo.
Id be looking to gear swap for a little higher set and edelbrock heads in the future.

Thanks in advance for any replies!


-357
-10 to 1 compression
-Comp cams Xtreme energy cam 12-262-4
-69 Corvette double hump heads
-Edelbrock performer Rpm air gap intake
-1.5 rockers
-2500 stall Hughes torque converter
-Hooker Super Comp Headers 3 in.
-Hooker Y-pipe 3-2.5 in.
-Custom 2.5 exhaust, small cat, y-pipe to 2, 10" glasspacks rear.
-3.23 gears

Originally Posted by Tredeclaw

Hey guys, ive got a third gen 83 trans am, 70's 350 built pretty fresh, rods, pistons, everything. I'm looking for a new carb. Some people tell me to go all the way to 750 cfm. Id like to stay with a Holly/demon, I like their products. I'm a young guy and I'm definitely still learning about carburetors, tweaking etc and I have little knowledge of what size and type of carb I should start with my setup.

I hope this is the correct area to post!

Right now I have a Holly 600 vacume secondary that was given to me, its pretty old but it has a rebuild on it and runs good. Just wondering if there is anything that might run better with my combo.
Id be looking to gear swap for a little higher set and edelbrock heads in the future.

Thanks in advance for any replies!


-357
-10 to 1 compression
-Comp cams Xtreme energy cam 12-262-4
-69 Corvette double hump heads
-Edelbrock performer Rpm air gap intake
-1.5 rockers
-2500 stall Hughes torque converter
-Hooker Super Comp Headers 3 in.
-Hooker Y-pipe 3-2.5 in.
-Custom 2.5 exhaust, small cat, y-pipe to 2, 10" glasspacks rear.
-3.23 gears

For max horsepower, multiply the cubic inches times 2, then go to the next available size. In your case, that would be 357 X2 =714, rounded up to 750. If you like Holleys, then buy a Holley. If the torque converter will truly stall up to 2500, then you could use either a vacuum secondaries or a double pumper model, but if the motor will not stall up that high, you might be better off opting for a vacuum secondaries to prevent overloading the motor at low rpm's. On a Holley, I would limit fuel pressure at the bowl to no more than 6 psi, and if I were building it for you, I would limit the pressure to 5 psi at the bowl even though some fellows will tell you that you can run higher pressure. Let me tell you this, from 60 years of experience fooling with this stuff, MORE PRESSURE WILL NOT MAKE MORE HORSEPOWER ON A CARBURETED MOTOR. It works on fuel injected motors, but not on carbureted motors. What will happen is the the excessive pressure will overpower the spring that controls the needle and seat in the bowl and will allow the fuel pump to blow raw fuel into the intake manifold, creating horrible fuel mileage and a tuning nightmare for you that will make your life miserable until you lower the pressure. Now, it is true that you will want to build in all the VOLUME that you can, using the largest diameter fuel lines from the tank to the carb that you can comfortably afford to build. 1/2" diameter works well, here's an example......
Moroso 65340, Moroso Aluminum Fuel Line | Moroso

Use insulated Adel clamps every 12 inches to secure the line to the inside of the frame, where it will protected from things like floor jacks.
Mr. Gasket Hose Mounting Clamps 3773G - Free Shipping on Orders Over $99 at Summit Racing

I congratulate you on your matching of camshaft intake duration to the static compression ratio. Perfect, although you will want to address keeping the motor out of detonation by using a premium fuel at all times. 9.5:1 is generally considered to be the limit with iron heads, but with the right fuel, right cam and good tuning, you should be able to skate by. Usually what I suggest to prevent detonation is the time honored practice of narrowing the squish/quench distance between the top of the piston and the bottom of the cylinder head. The generally accepted measurement is 0.035" to 0.045". Do you know how far the piston crown is down in the bore with the piston at top dead center? Do you know the thickness of the head gaskets that you used?
A fellow can engineer this easily before he ever does any work or buys any parts. What you do is measure the block deck height using a 12" dial caliper. Then measure the stack of parts that you are going to use........ add the crank stroke radius (1.74"), the rod length (5.7") and the piston compression height (spec calls for 1.560"), but if you're not careful, you can buy a shorter piston that will increase the squish/quench and make the motor more susceptible to detonation. From the factory, the Gen I small block Chevy block deck height was 9.025" and the stack of parts used in the block was 9.000". You can see that the piston crown would be down in the bore by 0.025" (twenty five thousandths) with the piston at top dead center. Using a stock shim gasket the compresses to 0.020" would put the squish/quench at 0.045", within the recommended range to prevent detonation. Problem crops up when, like I said, a shorter piston is used that leave the crown down in the bore farther and makes a "too wide" squish/quench. Same thing happens with a too-thick head gasket.

I know I'm yakking more than you wanted to read, but the idea is to prompt you and others to ask questions. That's how all of us on this forum learn, by asking questions and learning from the answers.

Now, there are 2 things that you did that I would not have done.

I would not have used an Extreme Energy cam and I would not have used an Air Gap intake manifold unless the motor was in a pure drag race car that was not driven on the street. I like a more gentle action to the valvetrain than the Extreme Energy camshafts offer, something on the order of 55 to 60 Hydraulic Intensity instead of the 44 intake and 46 exhaust of your cam. Unless you're going for every last horsepower at the track, it just doesn't make any sense to me. I'd rather give up 4 or 5 horsepower and have the valvetrain live a long and healthy life.

For the intake, I have read and heard of several fellows who have had drivability problems using the Air Gap in cooler weather. See, on a street car, the fuel needs to be heated in order to turn it into a vapor that can be easily burned. The air gap allows cold air to circulate around the manifold and keep the fuel in a liquid state for longer than it should. Like I said, on a race car this it not much of a problem, but on the street, with stop and go driving, it can be a pain in the ass.

The heads will be the cork in the whole system, so begin saving your money for a set of aftermarket aluminum heads......
Eight Budget Small Block Chevy Heads Tested - Car Craft Magazine

As far as gears, best overall in my opinion would be 3.73's with an aftermarket differential of some kind, like the Auburn Limited Slip unit....and I'd do them before doing anything else. Gears and traction are the cheapest and best money you can spend on a car.

.

X2 on the air gap manifold.

I'm in the deep south, and unless the car was in direct sun, and plus 90 ambient, it ran like kaka for ten miles.

Originally Posted by techinspector1

For max horsepower, multiply the cubic inches times 2, then go to the next available size. In your case, that would be 357 X2 =714, rounded up to 750. If you like Holleys, then buy a Holley. If the torque converter will truly stall up to 2500, then you could use either a vacuum secondaries or a double pumper model, but if the motor will not stall up that high, you might be better off opting for a vacuum secondaries to prevent overloading the motor at low rpm's. On a Holley, I would limit fuel pressure at the bowl to no more than 6 psi, and if I were building it for you, I would limit the pressure to 5 psi at the bowl even though some fellows will tell you that you can run higher pressure. Let me tell you this, from 60 years of experience fooling with this stuff, MORE PRESSURE WILL NOT MAKE MORE HORSEPOWER ON A CARBURETED MOTOR. It works on fuel injected motors, but not on carbureted motors. What will happen is the the excessive pressure will overpower the spring that controls the needle and seat in the bowl and will allow the fuel pump to blow raw fuel into the intake manifold, creating horrible fuel mileage and a tuning nightmare for you that will make your life miserable until you lower the pressure. Now, it is true that you will want to build in all the VOLUME that you can, using the largest diameter fuel lines from the tank to the carb that you can comfortably afford to build. 1/2" diameter works well, here's an example......
Moroso 65340, Moroso Aluminum Fuel Line | Moroso

Use insulated Adel clamps every 12 inches to secure the line to the inside of the frame, where it will protected from things like floor jacks.
Mr. Gasket Hose Mounting Clamps 3773G - Free Shipping on Orders Over $99 at Summit Racing

I congratulate you on your matching of camshaft intake duration to the static compression ratio. Perfect, although you will want to address keeping the motor out of detonation by using a premium fuel at all times. 9.5:1 is generally considered to be the limit with iron heads, but with the right fuel, right cam and good tuning, you should be able to skate by. Usually what I suggest to prevent detonation is the time honored practice of narrowing the squish/quench distance between the top of the piston and the bottom of the cylinder head. The generally accepted measurement is 0.035" to 0.045". Do you know how far the piston crown is down in the bore with the piston at top dead center? Do you know the thickness of the head gaskets that you used?
A fellow can engineer this easily before he ever does any work or buys any parts. What you do is measure the block deck height using a 12" dial caliper. Then measure the stack of parts that you are going to use........ add the crank stroke radius (1.74"), the rod length (5.7") and the piston compression height (spec calls for 1.560"), but if you're not careful, you can buy a shorter piston that will increase the squish/quench and make the motor more susceptible to detonation. From the factory, the Gen I small block Chevy block deck height was 9.025" and the stack of parts used in the block was 9.000". You can see that the piston crown would be down in the bore by 0.025" (twenty five thousandths) with the piston at top dead center. Using a stock shim gasket the compresses to 0.020" would put the squish/quench at 0.045", within the recommended range to prevent detonation. Problem crops up when, like I said, a shorter piston is used that leave the crown down in the bore farther and makes a "too wide" squish/quench. Same thing happens with a too-thick head gasket.

I know I'm yakking more than you wanted to read, but the idea is to prompt you and others to ask questions. That's how all of us on this forum learn, by asking questions and learning from the answers.

Now, there are 2 things that you did that I would not have done.

I would not have used an Extreme Energy cam and I would not have used an Air Gap intake manifold unless the motor was in a pure drag race car that was not driven on the street. I like a more gentle action to the valvetrain than the Extreme Energy camshafts offer, something on the order of 55 to 60 Hydraulic Intensity instead of the 44 intake and 46 exhaust of your cam. Unless you're going for every last horsepower at the track, it just doesn't make any sense to me. I'd rather give up 4 or 5 horsepower and have the valvetrain live a long and healthy life.

For the intake, I have read and heard of several fellows who have had drivability problems using the Air Gap in cooler weather. See, on a street car, the fuel needs to be heated in order to turn it into a vapor that can be easily burned. The air gap allows cold air to circulate around the manifold and keep the fuel in a liquid state for longer than it should. Like I said, on a race car this it not much of a problem, but on the street, with stop and go driving, it can be a pain in the ass.

The heads will be the cork in the whole system, so begin saving your money for a set of aftermarket aluminum heads......
Eight Budget Small Block Chevy Heads Tested - Car Craft Magazine

As far as gears, best overall in my opinion would be 3.73's with an aftermarket differential of some kind, like the Auburn Limited Slip unit....and I'd do them before doing anything else. Gears and traction are the cheapest and best money you can spend on a car.

.

Hey, thanks for the reply.

Another member from the third gen board suggested using this formula, and rounding up to a 650 carb.
(CID x RPMs)/3456. (357 x 5300)/3456.
What would be the difference between using a 650 and 750?

I have about 7-8 psi on my carb now, and it does run through fuel pretty quick. 10 mpg on average. I use 91 non oxy fuel. How much re adjusting would I have to do after adjusting fuel pressure?

I dont know he thickness of my head gaskets, or any of the other engine bits unfortunately.

I am currently looking for a new rear end to replace my broken posi 3.23.

Haven't had any issues driving the car with the intake manifold as of yet. I basically daily drive this during the summer. However I have never had any other manifold.

Heads are definitely what I want, the double humps were given to me.

Thanks for all the information.

Great advise from these guys, I too would spend the money on regearing and a posi unit; again the 3:73 to one gears work great with these third gens. The engine sounds like a really nice build, except for the heads, I would have spent the money on aluminum high flow heads; they simply can't be beat and they help shed some heat and avoid detonation issues. If money was the deciding factor on not going with aluminum heads, I would have gone with a good set of vortec iron heads, they flow much better than the old double humps.

As a third gen owner myself (1986 IROC Z) I'm partial to them, but it's a great car to make a streetrod out of, great handling and parts are still readily available. Lets see a pic or two of it.........