What size carb to buy.

[Andy]

Here is my setup:

306 motor
T-5 tranny
3:73 Gears
K&N air cleaner
electric fan
Edelbrock RPM intake
Comp cam with ".500 lift" 224/224 duration
Stock E7 heads
Stock mechanical fuel pump
Holley 600cfm vac Secondary
2.5" H-pipe W/flowmaster dumps
Flowtech headers.

*Futur plans are to buy a set of GT40 or something of that nature Heads w/Roller rockers.*

The carb I have now is about 15+ years old and I just recently rebuilt it. It runs okay but not the best, I wanted to buy a newer carb for now and then eventually save up and get a good set of heads. So should I get a 750 and jet it down or 650?

This is a weekend car and will mostly be street driven, but looking to get as much Horse Power as possible.

Looking for some suggestion,

Thanks,
Andy

[85_SS_302_Coupe]

I'd say a 600 double pumper would be spot on (i prefer mechanical secondaries). You really don't wanna do a 750 jetted down because a big key in making good carb performance is air velocity, and flowing more air if the heads/cam can't really move it fast enough means the air will move slower, which is going to kill throttle response. Even with GT40 heads i would still go with a 600dp. You would probably be ok with a 650...i'm using one right now, but it would be better suited for top end than bottom end, and even with GT40s and a decent cam you won't really need more than a 600 unless you build a stroker.



The best example i've ever heard about carb air velocity is to picture yourself drinking water through a series of straws. Lets say in glass #1 you've got a typical McDonalds straw, and we'll call that the 600cfm carb. You know that it doesn't take a whole lot of suction to pull up a drink. Now lets say glass #2 has a 1 inch wide straw...you might indeed get a lot more drink per suck, but it's going to take a much stronger suction to get your drink. This is basically how your engine and heads relate to the carb. If your engine can't make enough suction to justify the bigger drink it's getting, then it'll struggle to get what it wants....whereas with the properly sized straw it can chug away. On the other hand if you put one of those little coffee straws you know you have to suck your face off to get a sip....that would be like having a carb that's too small. The engine has plenty of suction power but it's starving because it just isn't getting enough. You want that sweet spot right in the middle.

I've seen guys running blow through turbo setups with a typical 600cfm carb. It's not a good idea to go with too much carb no matter what your engine setup is.

[Andy]

I'd say a 600 double pumper would be spot on (i prefer mechanical secondaries). You really don't wanna do a 750 jetted down because a big key in making good carb performance is air velocity, and flowing more air if the heads/cam can't really move it fast enough means the air will move slower, which is going to kill throttle response. Even with GT40 heads i would still go with a 600dp. You would probably be ok with a 650...i'm using one right now, but it would be better suited for top end than bottom end, and even with GT40s and a decent cam you won't really need more than a 600 unless you build a stroker.



The best example i've ever heard about carb air velocity is to picture yourself drinking water through a series of straws. Lets say in glass #1 you've got a typical McDonalds straw, and we'll call that the 600cfm carb. You know that it doesn't take a whole lot of suction to pull up a drink. Now lets say glass #2 has a 1 inch wide straw...you might indeed get a lot more drink per suck, but it's going to take a much stronger suction to get your drink. This is basically how your engine and heads relate to the carb. If your engine can't make enough suction to justify the bigger drink it's getting, then it'll struggle to get what it wants....whereas with the properly sized straw it can chug away. On the other hand if you put one of those little coffee straws you know you have to suck your face off to get a sip....that would be like having a carb that's too small. The engine has plenty of suction power but it's starving because it just isn't getting enough. You want that sweet spot right in the middle.

I've seen guys running blow through turbo setups with a typical 600cfm carb. It's not a good idea to go with too much carb no matter what your engine setup is.

Thats good info:bigthumb
So what all do you have in your setup?

[85_SS_302_Coupe]

Thats good info:bigthumb
So what all do you have in your setup?


Well contrary to my good advice (if you can call it advice..lol) i've got a 650 which is too big, on a nice intake which is a Wieand Stealth, on stock heads with a very light gasket-matched port job all on a high mileage tired ass stock '85 block. Yeah, i need to get some work done :dummy:

A nice healthy combo would be your 306, GT40P heads, a Stealth (or even a Vic. Jr if you want more top end than bottom end but you'll give up a lot down low compared to the Stealth) and a nice Holley 600 double pumper w/mechanical secondaries. If you had the cash to throw around, the Holley HP series carbs are super badass. Also if you have some carb building/assembly knowhow, you can get a Proform main body which doesn't have a choke horn on it (better air flow) and put all your Holley parts onto it.

Alternatively you could do that work yourself and mill the choke horn off, but IMO that's a waste since if you wanna do it right you're probably going to take it apart anyways, so why not just get a new main body and be done with it? They're about $100 on Summitt, but unfortunately you're stuck with only a 650 with either the HP or Proform body, but somewhere down the road you might get better aftermarket heads that would justify the 650.


Like i said though a 650 isn't too oversized. I've ran mine for 3 years with no problems, but i would have a much stronger bottom end on the stock heads/cam with a 600 and MUCH better throttle response. I bought the 650 for a 351 that never happened.

[Andy]

Well contrary to my good advice (if you can call it advice..lol) i've got a 650 which is too big, on a nice intake which is a Wieand Stealth, on stock heads with a very light gasket-matched port job all on a high mileage tired ass stock '85 block. Yeah, i need to get some work done :dummy:

A nice healthy combo would be your 306, GT40P heads, a Stealth (or even a Vic. Jr if you want more top end than bottom end but you'll give up a lot down low compared to the Stealth) and a nice Holley 600 double pumper w/mechanical secondaries. If you had the cash to throw around, the Holley HP series carbs are super badass. Also if you have some carb building/assembly knowhow, you can get a Proform main body which doesn't have a choke horn on it (better air flow) and put all your Holley parts onto it.

Alternatively you could do that work yourself and mill the choke horn off, but IMO that's a waste since if you wanna do it right you're probably going to take it apart anyways, so why not just get a new main body and be done with it? They're about $100 on Summitt, but unfortunately you're stuck with only a 650 with either the HP or Proform body, but somewhere down the road you might get better aftermarket heads that would justify the 650.


Like i said though a 650 isn't too oversized. I've ran mine for 3 years with no problems, but i would have a much stronger bottom end on the stock heads/cam with a 600 and MUCH better throttle response. I bought the 650 for a 351 that never happened.


Thanks for the input, I always like bouncing ideas around with other carb guys, I try to get as much info as I can before I start spending the cash. Ive always messed with EFI in the past so I am still new to the carb world and so far I have been having fun with it. Still learning the tuning stages. Its hard to tune it now though since the weather sucks. Seems like it ran great with 65 jets and 12 degrees timing. I have since jetted it up to 68 jets and moved the timing to 14 degrees and it doesn't seem to run any better than it did with the previous setup...it was smoother and very responsive? I guess I am trying to push as much power out of the current setup as possible. Still trying to find a happy medium:dunno:

[85_SS_302_Coupe]

More fuel doesn't always mean more power....it might just be running richer now. Jetting is all a bit new to me as well. I really wanna get a wide band O2 sensor and an air/fuel meter for this exact reason. That way you don't have to guess about the jetting, you just look at the a/f ratio and go from there. Then if you can afford one with some data logging you can make a couple passes at the track and then see where the a/f was all throughout the pass. You could be rich as hell at idle but leaning out at the end of the track, and the a/f meter would tell you that.