I'm currently using a Holley 600cfm on my 383, and it runs good but it looks like I'm likely leaving some horsepower on the table, so I asked Holley what they recommeneded for my specific setup and they replied back with their Avenger 770 carb. However, using another formula, I came up with the 670 being a better choice. My question is, if I go with the larger 770 carb, I've heard I could lose some throttle response....What does that mean exactly? Could the 770 make it feel like a dog in the low rpm"s? (fyi: I'm running the GM ZZ3 cam with Patriot Freedom heads in my 383).

There is a 28 horse difference between the 670 and the770 on my 355 small block on the dyno. The 770's response was is fine as far as I can tell.

I think you'll be better off, maybe a lot better off with the 670 all around. What percentage of your driving is above 5000 RPM?

There you have it.....two different angles of thought to give you different answers. Both good.

Jerry - If you go 770, you are not leaving hp on the table.

Tom - If you go 670, you are not giving up street manners in lieu of the pursuit of all hp possible.

The answer is actually what is best for your current combo, and what you are wanting to do with it. Either way you go, it is a compromise. If you think of it, a street/strip combo is a compromise. Not the best of either world.

If you have a loose converter, TUNE the 770, max timing, you can mask or overcome any throttle response loss you may notice. 670 if no to any of these.

Just trying to help you think it through. Sometimes your choice is not the popular thing, but correct for your intentions and combo.

I say get one off of ebay, try it, and see what you think...do some back to back testing. I bet you'll like the 770.

If you have a loose converter, TUNE the 770, max timing, you can mask or overcome any throttle response loss you may notice. 670 if no to any of these.

Just trying to help you think it through. Sometimes your choice is not the popular thing, but correct for your intentions and combo.



I have a very tight converter. Its a 2000rpm stall TCI Saturday Night Special. That would hurt me if I went with the 770?

However, using another formula, I came up with the 670 being a better choice.
The best "formula" for predicting carb size for a street-driven car that I've found:

[(CID X Max RPM) / 3456] X VE X "manifold factor"

WHAT is your max intended RPM? WHAT is your intake manifold?
"manifold factor" ranges from ~1.5 to 1.1; more efficient manifolds use a LOWER correction factor. So a stock dual plane might require a correction factor of 1.4--1.5; while a single-plane Victor manifold might use 1.1.

All of this was shamelessly stolen from the Edelbrock web site years ago; but I have trouble finding a link to the web page.

For a 383 turning 6000 rpm and using a Performer RPM--or similar manifold:

383 X 6000 = 2,298,000
2,298,000 / 3456 = 665
665 X .90 = 598
598 X 1.3 = 778 CFM

A properly-tuned carb in the 750--800 range looks pretty good to me.

The best "formula" for predicting carb size for a street-driven car that I've found:

[(CID X Max RPM) / 3456] X VE X "manifold factor"

WHAT is your max intended RPM? WHAT is your intake manifold?
"manifold factor" ranges from ~1.5 to 1.1; more efficient manifolds use a LOWER correction factor. So a stock dual plane might require a correction factor of 1.4--1.5; while a single-plane Victor manifold might use 1.1.

All of this was shamelessly stolen from the Edelbrock web site years ago; but I have trouble finding a link to the web page.

For a 383 turning 6000 rpm and using a Performer RPM--or similar manifold:

383 X 6000 = 2,298,000
2,298,000 / 3456 = 665
665 X .90 = 598
598 X 1.3 = 778 CFM

A properly-tuned carb in the 750--800 range looks pretty good to me.

Thats exactly the equation I found except it didn't include the intake factor which is why the 670 looked like a better choice. Actually the 600 I'm currently using looked right. I had even used the same .90 efficiency number and the 6000rpm. Might explain why Holley is recommending the 770 as well.

All of this was shamelessly stolen from the Edelbrock web site years ago; but I have trouble finding a link to the web page.
I went looking again; I can't find it. I think it's been removed during a "remodel" of Edelbrock's web site.

This seems to be their "new" version:
http://edelbrock.com/automotive_new/misc/tech_center/install/tech_disc_cfm_rules.shtml

Overall, not so different except they suggest a manifold correction factor of 1.1 to 1.3 for a single-plane manifold; 1.2 to 1.5 for a dual-plane manifold.

Maybe you ought to see if anyone has developed a carburetor with small primaries for good low end performance accompanied by large demand actuated secondaries for covering the top end.

Maybe you ought to see if anyone has developed a carburetor with small primaries for good low end performance accompanied by large demand actuated secondaries for covering the top end.

ummm the quadrajet or spreadbore come to mind....... :D

ummm the quadrajet or spreadbore come to mind....... :D

I would be open to that. My original 350 that came with the car when I bought it about 5 years ago had a quadrajet. It was great until my friend tried to rebuild it and then the accelerator stuck on the first test drive....that was scary. He tried to re-rebuild it and it still would stick. Then I took it to a auto parts store and traded it in for a rebuilt quadrajet and that one wouldn't stop leaking gas...also scary! Bought the Holley and haven't looked back. How are the rebuilt ones typically? Maybe I just had some bad luck.

By the way, anyone noticed summitracing has all the Avengers on sale for $80 bucks off the regular prices. That with the $50 off coupon I have sure is tempting!

How are the rebuilt ones typically?
Production-line rebuilt carbs are ABSOLUTE JUNK. One-at-a-time rebuilds done by someone with a brain and caring attitude can be excellent.

Although the 770 makes more power on my small block I use the 670 for better all around driving.