choosing correct carb size for smallblock

pazcarguy · Aug 15, 2008

I just wanted to ask opinions on correct carb sizes out there. I have seen the calculators and read the materials and they seem to contradict. The only thing that does not is that most people put on too large of a carb in regards to CFM. I had a 289 punched out to 292, large cam, heavily ported, balanced and able to spin to 7500 rpm max. I used a 650CFM mech secondaries Mighty Demon (4 speed car) and it was way too rich unless I ;leaned it way out, then it hurt performance. Books say 525~600 cfm for this engine assuming the VE is 80% minimum. I am building an Edelbrock packaged engine (#2091 top end kit with 9.5~1 CR) and spoke to their tech people who said they made 350hp numbers using a 750 cfm carb. This is not a streetable setup using this, just a method to get good HP numbers My question is I was going to use a 575cfm carb for this same package and was wondering how much this would hurt HP. It will definetly help driveability so that is the pro.I am not a fan of vacuum secondaries as I like larger cams so I will be putting a mechanical secondary carb on. Any feedback on what you guys are running and pros/cons of each? Brian

65fastbackresto · Aug 15, 2008

I`m running a 600cfm edelbrock

on a 302 with big cam and mechanical secondarys and I really dont like it much. It bogs a little on takeoff, but wide open its pretty fun. Like you I have to lean it out to get it to run right, about 1 1/4 turns on the air mix screw. If I was a little better at tuning it I might like it more, but maybe that will come with time.

woodsnake · Aug 15, 2008

Before you spend any money go over to S B F tech dot com and read the myths and rumors section, and the carburation thread specifically. I'd run a 750 Holley on that in a heart beat.
The motor will only draw as much fuel as it needs, so what kind of heads you have will impact your combo as well...

zookeeper · Aug 15, 2008

I'm running a pair of Holley 390cfm four barrels on my mild 306ci motor. Pros? It looks bitchin', drives every bit as well as any single 4 barrel car I've ever owned, and fires right up even with no choke whatsoever. Cons? Although I haven't checked the mileage, I'm sure it's a little worse than it could be with less carb.

D.Hearne · Aug 15, 2008

If you want better driveability, go smaller with the carb. If you want to get the most out of it on the topend, go bigger, like a 650-700. 750 will work, but a 650-700 will be optimum for what you want and not give up too much bottom end. My choice for daily driver? A 570, 600 or 650 Holley with vacuum secondaries. Want the wow factor and money's no object? Either a repop 3x2 or 4x2 webers.

65ShelbyClone · Aug 16, 2008

For a street car, I would personally go with a Holley 600(either vacuum secondary or mechanical, but I like mechanicals with manual transmissions). I have a 650 pumper in my '68 and it's a touch on the large side considering my 302 noses over at just 6000rpm.

I would only go larger than a 600 if your engine is going to be making power up to 7500. A stock 302 can be spun to 7500, but the power is going to belly flop far sooner. That means that the airflow starts dropping off.

red302 · Aug 18, 2008

I would recommend a 650 Holley. If it's a stick car go with mech. secondaries. If it's an auto go vacuum, unless you're running a high stall converter with low gears. You're correct in saying that most folks over carb. My '85 has a very healthy 302 with a 150 shot on top of it, and a slightly modified 650 with mech. (stick car) is more than enough carb for it. I also run a Holley 670 Street Avenger carb on the 331 stroker in the '68 with vac. secondarys (C-4 car). Very happy with it.

rbohm · Aug 18, 2008

for stock to mild builds o recommend a 500-600cfm carb. in your case however, since you are going for top end power, i suggest using a 700-750 cfm carb with vacuum secondaries.

htwheelz67 · Aug 18, 2008

what is your combo? cam specs? compression? heads?gears? converter? intake?

Usage for the car? drag race? street? street strip?

Got to know all those to recommend a carb or carb tuning.

blown65 · Aug 18, 2008

http://www.prosystemsracing.com/index1.html

Little more money but you get a carb specific to your combo and what you plan to do.

Theslicknick6 · Apr 3, 2020

Ive also got a question on this. Ive got a 260 with a 4 barrel intake and was wondering on sizing for a carb. Its just a simple street car, nothing fancy so not looking for top end performance but would like more zip in the low range. From what ive read most folks seem to point to the 600-650 cfm for 289-302s but given the engine size of mine should i aom lower? Say 450-500cfm? Other issue ive run into is ive done a front disc conversion that takes a good bit of vacuum to run, so the idle has had to be bumped up to accommodate. As a result it has stalling issues on hills or when i try to give it a bit more gas off the line. Appreciate any recs in advance!

Billm · Apr 4, 2020

Find a 1.08 Auolote 4100 original carb or the small Summit 500cfm carb. Your car should run great with either carb. For the vacuum issue, install a vacuum reservoir from Summit. I had low vacuum with my cam and the canister solved the problem. Run a hose to the reservoir and then one to the booster. Mount on front left inner fender.

Theslicknick6 · Apr 4, 2020

Thank you for the advice. Any particular combination between a certain carb, say the summit 500 and a particular vacuum reservoir or should any work?

Realmongo · Apr 4, 2020

Go on over to Holley and try their carb size selctor:

Carburetor Technical Support - Holley

www.holley.com

Billm · Apr 4, 2020

I bought a reasonably priced one. I used it first with an Edelbrock carb, than aSummit carb and now an Autolite 4100. Works great. The can keeps storing vacuum as the engine revs up and down. Used it on a 302 with the Edelbrock top end kit. Vacuum was low. The reservoir solved the problem. No need for an expensive vacuum pump. Buy a small gauge for the can and it will tell you how much vacuum is stored.

Adieu · Apr 4, 2020

Theslicknick6 said:
Ive also got a question on this. Ive got a 260 with a 4 barrel intake and was wondering on sizing for a carb. Its just a simple street car, nothing fancy so not looking for top end performance but would like more zip in the low range. From what ive read most folks seem to point to the 600-650 cfm for 289-302s but given the engine size of mine should i aom lower? Say 450-500cfm? Other issue ive run into is ive done a front disc conversion that takes a good bit of vacuum to run, so the idle has had to be bumped up to accommodate. As a result it has stalling issues on hills or when i try to give it a bit more gas off the line. Appreciate any recs in advance!

A 500cfm Edelbrock or a 390-475cfm Holley should make you pretty happy with it. (500cfm is the smallest Edelbrock makes, and their carburetors are more forgiving when it comes to tuning thanks to their secondary design). You might want to put a vacuum reservoir under the hood, or run an electric vacuum pump for the brake booster if adding power brakes introduced your issue.

choosing correct carb size for smallblock

pazcarguy

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Billm

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Carburetor Technical Support - Holley

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