choosing the right carb

[Mrturner1]

How do you guys choose the right carb for your builds? How do you know how many cfm you need? Right now I'm running a 600cfm 1406 edelbrock carb, and my engine has edelbrock performer intake, mild cam, stock heads, LT headers and flow masters, and msd distributer.

Im getting ready to install a set of 180cc Pro SR-T heads with 64cc chambers, and I'm wondering if my 600cfm edelbrock will be enough or if I could benefit from an upgrade. looking at a nice 1806 edelbrock 650cfm

[toolboxchev]

Someone told me at one time that a quadrajet, regardless of jet and needle size is capable of 850 cfm.

What does that make you think? Can anyone confirm this?

[geezer#99]

Originally Posted by toolboxchev

Someone told me at one time that a quadrajet, regardless of jet and needle size is capable of 850 cfm.

What does that make you think? Can anyone confirm this?

Some stock big blocks had 850 quads.
Chevy, Buick and Olds.

[trac209]

You may have to step up jetting but a 600 cfm should be ok. I have a similar engine and my 600 does fine with it.

[engineer_gregh]

If you trust Summit here is their calculator.

http://www.summitracing.com/expertad...cfm-calculator

[Skinny Kid]

I personally love Holleys. I'm not a fan of Edelbrocks. I like q-jets too in stock or mild performance setups. I have the stock q-jet on my truck now and daily drive it. On my last three builds I ran Holleys. My last car, 66 Chevelle with a 383, mild cam, 3:36 gears, 700r4, stock converter, 26 inch tires had a 750dp which is not the right carb for that at all, but it loved it. I tried a 650dp, 670 street avenger (vac secondaries) and the 750. Drivability was the same with all three, but the mpg was the best with the 750. 18.6 freeway, 15 combined. Go figure. Just have to find what it likes.

20 years ago back in high school I had a 66 Chevelle with a 468 and a 4 speed. It was a mild build, but still ran high 11's. 850 Holley double pumper. All my friends ran Edelbrocks. The used to tease me that I was always working on my car and I was. Every day after school I was under the hood trying to make it run better and faster. They said Holleys never keep their tune blah blah blah. One day I said to my friend with a 69 Chevelle with a 454 and a 750 Edelbrock, "Ok, let's swap carbs and you tune mine and I'll tune yours then we'll race." He had never beaten me before. His car ran consistent 12.20's and mine ran 11.80's all day. We did and when we raced we were fender to fender. Never got time slips, but mine just felt slow and he was shocked how fast his felt. He got a Holley the next week never looked back. Oh, and he ran 11.90's next trip out.

[jhwkns]

This Holley 390 works for me. Most of the time when it comes to carburetors, bigger is not better. It's not very realistic when the only part of your engine that's capable of 8,000 rpm is the carb.

[leftybass209]

Well, picking a carb isn't as simple as "i like Holley" and throwing it on there. The CFM rating can be directly related to the Hp/Tq that the engine is being designed to produce. In your case, you should either computer dyno, or guestimate the power your engine is going to provide.

Everyone has a preference, but the fact is they all work well. Some better than others in certain situations.

As far as the big three,

Tons of people use Quadrajets and have no issues with them. They get called Quadrajunk by people that don't know how to tune them, so they're pigeon-holed unfairly.

Edelbrocks are great carbs as well. They get slammed a lot because people mis-diagnose the heat soak/carb boiling/vapor lock issues and flood the carb by using too much fuel pressure. In reality, they're great carbs. They're also unfairly represented as the "fuel economy" carb and as such, people think they can't produce power, which is also wrong.

Holley carbs are great. Though, they are probably the most overused carb to fix issues with the other two. Edelbrock or Quad doesn't work? Many suggest, just throw a Holley on it, that'll cure your issue. Why? Holley carbs tend to work (albeit not well) in situations where the other carbs tell you something isn't right, and they don't know how to adjust the other carbs or diagnose their issues.

I've also seen dyno results on similar CFM carbs where the Holley was tuned to produce BETTER gas mileage than the Edelbrock. Of course that's the magical dyno, and not real world but it does indicate your level of tuning knowledge with all carbs is more advantageous than someones' opinion on which is considered junk, or better in one area than another.

I can't tell you your carb will be fine for your application, even with a jet change, because I don't know enough about the power output of your motor. I can tell you that hardly any street driven motor will struggle with 600 CFM at idle or around the town cruising. It's the full throttle demand that will cause lean conditions. You should assess how you plan to drive your truck and what you expect from your carb.

[davepl]

Originally Posted by leftybass209

Tons of people use Quadrajets and have no issues with them. They get called Quadrajunk by people that don't know how to tune them, so they're pigeon-holed unfairly.

^^^ This

A Q-Jet properly tuned for the application will make as much power as any Holley, but they're harder to set up. I've seen that comparison done about every 5 years in every car magazine. Usually within 2-3 horsepower either direction.

The difference is that a properly configured Q-Jet will drive like fuel injection, cold start like a dream, idle, and do it all. But they're complicated systems, and not that many people are qualified to set them up correctly.

The factory may have had thousands of hours - maybe more - invested in tuning a Q-Jet to a particular vehicle.

Then someone "rebuilds" a q-jet, only has a few parts left over, it bogs, and so it's a Quadrajunk.

I can rebuild and tune a Holley 3310 (780 cfm) with my eyes closed. But I've never achieved the driveability of a factory-calibrated Q-Jet with all of the driveability components (heat stove, choke, etc) present and working.

Get in, pump twice, drive to other coast. They're that good, done right.

[GRX]

^^^ that. Well said. "CFM rating" is not really a good way to judge a Q-Jet. The rating is the max the carb will flow. Q-jet for smaller cube engines like 305s have a tab on their air valve that limits its travel, thereby lowering "CFM rating". In the right hands one can be modified to provide up to and over 1,100 CFM. Then you have the fact they require different jet sizes and metering rods depending on the application. Main reason inexperienced re-builders get into trouble.

To answer the original question ... I would go a for a Q-jet, but I have been working with them for a couple decades. Many would benefit from the more simple, and less tunable, Edelbrock plug & play AFB clone carb.

[Redline novdog]

Originally Posted by engineer_gregh

If you trust Summit here is their calculator.

http://www.summitracing.com/expertad...cfm-calculator

If you are wanting baseline. This is it
Sounds like you are (building) a small block. I have ran a 454 on 500cfm. If you are looking for streetability and mileage go small CFM. If you want G'force then go big cfm! on your combo I would say a 750 double pump Holley would set you back in the seat!!

[GASoline71]

For reference to all the "CFM" stuff...

I have a very snotty 355 that I have used a Holley 3310 (750 vac sec) on for years. I have used 3310's on most of my builds. But had my carb guy swap out the 750cfm main body of the 3310 for a 600cfm main body from an 1850.

My truck is currently in a bazillion pieces so it will be aile until I could report how I like it. But I think it's gonna be waaaaayyyyy better on the street than my 750 was.

I have a buddy with a '72 Monte with almost the exact same engine combo as me that I might see if he will let me bolt it up to test it out. But until then... it won't be until early summer until I get the truck back on the road.

Also... even some of us that love the ol' Quadrajet... still call them Q-pukes. I cut my teeth on those carbs and have the scars to prove it.

Gary

[BCOWANWHEELS]

nothing like a stock q jet

[Mrturner1]

Originally Posted by leftybass209

Well, picking a carb isn't as simple as "i like Holley" and throwing it on there. The CFM rating can be directly related to the Hp/Tq that the engine is being designed to produce. In your case, you should either computer dyno, or guestimate the power your engine is going to provide.

Everyone has a preference, but the fact is they all work well. Some better than others in certain situations.

As far as the big three,

Tons of people use Quadrajets and have no issues with them. They get called Quadrajunk by people that don't know how to tune them, so they're pigeon-holed unfairly.

Edelbrocks are great carbs as well. They get slammed a lot because people mis-diagnose the heat soak/carb boiling/vapor lock issues and flood the carb by using too much fuel pressure. In reality, they're great carbs. They're also unfairly represented as the "fuel economy" carb and as such, people think they can't produce power, which is also wrong.

Holley carbs are great. Though, they are probably the most overused carb to fix issues with the other two. Edelbrock or Quad doesn't work? Many suggest, just throw a Holley on it, that'll cure your issue. Why? Holley carbs tend to work (albeit not well) in situations where the other carbs tell you something isn't right, and they don't know how to adjust the other carbs or diagnose their issues.

I've also seen dyno results on similar CFM carbs where the Holley was tuned to produce BETTER gas mileage than the Edelbrock. Of course that's the magical dyno, and not real world but it does indicate your level of tuning knowledge with all carbs is more advantageous than someones' opinion on which is considered junk, or better in one area than another.

I can't tell you your carb will be fine for your application, even with a jet change, because I don't know enough about the power output of your motor. I can tell you that hardly any street driven motor will struggle with 600 CFM at idle or around the town cruising. It's the full throttle demand that will cause lean conditions. You should assess how you plan to drive your truck and what you expect from your carb.

Man that is a lot of good info, thanks for the read. This website is full of so much knowledge its amazing.

[swamp rat]

Originally Posted by GASoline71

For reference to all the "CFM" stuff...


Also... even some of us that love the ol' Quadrajet... still call them Q-pukes. I cut my teeth on those carbs and have the scars to prove it.

Gary

We called them Quadraflush back in the day.

[BCOWANWHEELS]

I,ve run q-jets for 45 years with 0 problems

[Eddie H.]

When I worked for a Chevy dealer back in the 80's I think I rebuilt 2 or 3 Quadrajets every day. It was considered part of a major tune up back then. They're great carburetors when they are set up correctly. I've also ran Holleys, Carters, Edlebrocks, and Rochesters without any problems. It is mainly just a matter of personal preference.

[chopnchaneled]

Check with National Carb. in Jacksonville Fla. They will help you.

[Skinny Kid]

It's true. My q-jet in my 71 is the best driving carb I've ever had. I've had Holleys that drove awesome but, all around the q-jet takes it. 30 degree cold start, one pump start and go. No issues at all.

You can take a 2 barrel and tune it on a 6 cylinder the take it right off and drop it on a (mild or stock) big block and other than setting idle settings it will be pretty close. Too small of a carb won't cause you to run lean, it will just limit power and rpm potential. For a lean condition to occur the engine has to have inadequate fuel for the given air flow. If the air/fuel ratio is correct, regardless of CFM, you won't run lean. A properly sized carb will perform well all around though. Too small and the power and rpm are limited (way too small there will be other issues), too large and it will have poor drivability and be lazy due to a weak signal. Correctly sized and it will have a strong signal, be responsive and run well up top. That's another area the q-jet shines. Small primaries for a strong signal and great drivability, large secondaries for power and the air door only delivers what the engine can use. An 850 q-jet for a big block will run just as well on a stock small block if it's set up right.

CFM is not the end-all though. I ran my 750 on the Chevelle 383 because that's what it liked best. It didn't need that much at all. For instance, I run a 1050 Dominator on my 422 small block in my 67 Nova. It turns 8,200 rpm and runs in the 9.10-9.20 range in the 1/4. I run the car in c/gas which is a 9.60 index class so to slow it down I run a Dedenbear adjustable restrictor plate underneath and you would be shocked to see how much I have to close the plate to run a 9.60. I've never flowed the plate to see, but it has to be in the 800 cfm range or less. I can't insert a pic of it for some reason, but it looks like a cat's eye.

[Big Kev-O]

Originally Posted by chopnchaneled

Check with National Carb. in Jacksonville Fla. They will help you.

I bought a rebuilt carb from those guys on ebay $270, Holley 670 Street Avenger, worked excellent right out the box, no adjsutments. They do nice work.

[Mrturner1]

Thanks guys for all the info, I love learning about all this stuff. Ive come to the conclusion that the Edelbrock 600 I have now served its purpose, was very reliable, but isn't enough anymore (especially when the new heads go on). Ive also figured out that the next carb I get wont be my last, because I want to see what my engine really responds well to.