Small Block & Big Block Chevy Specific - how big of a carb does my motor need?

hey guys, i need a little help w/ my set up. I just built a motor and put it in my 84 3/4 ton truck. It is a 355 w/ tfs twisted wedge heads, around a 230 duration cam ( cant find specs ), 1.5 roller rockers, hooker long tubes with dual flowmasters, holley intake and a holley 600 carb. it isnt a double pumper. the motor is strong but it stumbles when i smack it and i know i need a bigger carb, but how big should i go. I need all the low end tq i can get with the heavy truck.I have a 650 dp coming but i think i will need a bigger one than that. i almost want to take the cam out and put my truck cam back in but i was coing to try the carbs first. thanks guys.:)

If your engine ws producing 100% volumetric effiency then all that a 355 would need is a 600 cfm carb. Most engines run between 85-90% at sea level. If you had a supercharger, then it would go over 100%. However, if you are winding the engine up to 6000 rpms then a 650 would help the upper rpms. I have seen dynos of 750's on 355 with a 230/236 cam producing some good numbers. However, I have also seen a 383 with AFR heads with a 600 cfm carb make some outstanding numbers as well in the 450hp range. Tuning the carb will help in all areas of the power range. Hope this helps.

I had a 650 double pumper on my 355 combo with canfield heads, and sometimes a bigger (heavier) vehicle can use a vacuum-secondary carb, but I think the 650 double might work good. If not try a 750 double, although I think that might be a tad much for the 355. Just my $.02, good luck!

Derek

750 double with the 30cc acc pump and smaller squirters

thanks guys, ill try the 650 and hope it works. i might need my truck cam back to wake it up on the bottom side. I kept telling everyone i needed around a 650 and they were saying 750 atleast. But these guys will over carb a motor bad. They were running a 750 dp on a 310 ci. mustang. I wish i knew how to tune a carb but ive never messed w/ one enough to try it. thanks again Charles

I always love this discussion.

A motor will only use what it wants from a carb. The smaller the carb, the richer it runs per cfm, a bigger carb allows you the option of tuning it in with jetting and squirters.

We always run 750s on our healthy 350s, and generally need to add squirter to eliminate stumble, and occaisionally need to add jetting over the factory settings.

Now I don't have access to unlimited dyno time, but we use the track and e.t.s as well as mph to tune our carbs.

As far as streetability, as long as you gear to the power range, and properly size the converter, you will not have a slug under part throttle driving.

This is strictly my opinion and andecdotal experience, but I would rather run a bigger carb then a small carb, then I can tailor the carb to the engine in question, obviously, this assumes the motor has some power to need a decent carb in the first place.

An example, I had an '81 'Bu, with a 355, TFS Twisted wedge heads, a 507 lift cam 3.73 gear 28" tire, T-350, and a 10" coverter, which btw was a little too tight.

I was running a 650 Dbl Pump, and it ran farly well, like 13.0someting, but it was hard to get it to hook, so listening to conventional wisdom, I thought, well maybe if I "Overcarb" it it may be a little soft off the line, hence slow down the spinning.

So I borrowed a 800 Db P., and took it out to drive it, well, the motor really came alive, it was almost impossible to even launch the car, and I had 2 other guys with me and we were all over 200 + pounds.

So I went to the track, and it spun as bad if not worse than before, but ran a 12.8something. So you see, that blew the big carb poor low end power theory out of the water!!!!

thanks for the info! ive always heard that also and i used the carb calculater and it said my motor would be fine with the 600. But i know it needs a little more. the truck has 4.10's and it is a 4x4 3/4 ton. oh its got a 4 speed w/ granny low also. i just need to learn how to tune the carb to make it work best for my set up. i cant take it to the track and run it because i cant speed shift it or anything so my times would be to inconsistent.thanks again

Those calculators tell you the minimum amount of cfm needed to run that VE at that rpm with that motor. So if you ever wanted to be more efficient, turn more rpms, etc, you would need a bigger carb. The carb would be maxed out at that rpm, ve, etc. I want the motor to be able breathe easy at redline, not be maxed out.
I have a similar setup- 350, trick flow g2's, xr288hr cam, holley street intake, and I run 2 different 750's- one vac, one mechanical. I have to say that it runs best with the mechanical secondary holley 750. Almost any built 350 will run better with a 750, regadless of what the calculator says. They have even done magazine dyno tests (f.w.i.w.) where the calculator said 600 and the motor ran better and maee 10 more hp with the 750.

Now that you mention it, I read an article where they wanted to see if the effects on a "Too" big of a carb would hurt power, they started off with a 750 on a 383, and went up to I believe a HP1000, now there was gain up until the 900 cfm and only with the 900 and 1000 did they see any loss of torque.

And torque is king at the stoplight.

So again my opinion, I think it is very hard to over carb a sbc chevy, again the assumption is there is some ass in the motor.

I went from a very good 850 to an 1100 Dom, and the throttle response was insane, I really had to watch to rpms on the burnout, which I never really had to with the 850.

I got a pretty mild 454 in my camaro. It came with a 600 cfm edelbrock. What type of carb do you think I shold go for, I read a 750 would be good.

I got a pretty mild 454 in my camaro. It came with a 600 cfm edelbrock. What type of carb do you think I shold go for, I read a 750 would be good.

Are you being serious, or trying to be funny??

after reading all this i guess i will be looking for a good 750 carb instead. thanks for all the info guys. Charles

a 750 double pumper worked great for a buddy's 454 that ran mid-low 12s. Tried an 850 as well but track testing showed no gain, or in some cases a loss.

Derek

I was being serious, why the hell wouldn't I be? Anyway thanks for the info Villain. Anyone else think 750 is enough?

There's no doubt that the 750 will make more hp in the upper range but what about the low end torque that a 43/4 ton truck needs. Plus you have a 454 with a 750 cfm carb. This guy is trying to run a 355. Now it really is personal preference of what he wants to do with the engine but a 600 cfm carb is all he needs unless he runs some kind of forced induction.

I was being serious, why the hell wouldn't I be? Anyway thanks for the info Villain. Anyone else think 750 is enough?

I just can't imagine putting a 600 cfm carb on a BB, thats all.

The factory ran 780s on their HiPo BBs "In the day", so for your purposes a 750 should be fine.

Factory small block comes with more than a 600 cfm carburator, i changed from my q-junk to a edelbrock 600 it(this was on a freinds note so thats why i went 600 first) lost power so would definatly run a 750 iam saving for one now, i would like to know how the 750 does i have a 3/4 ton with a th 400 and a 355 small block so lemme know please.

i am going to try the 650 dp first since i have already traded some stuff for it. ill let u know what i think about it. the motor is very strong once it gets enough fuel so i cant wait to get the carb switched out.

Holley 3310 vac sec 750 would probably be what I would go with. You can pick those up pretty cheap.

Holley 3310 vac sec 750 would probably be what I would go with. You can pick those up pretty cheap.
:werd:

That's exactly what I would do. ;)