The Stovebolt Site Forums The Shop Area Engine Shop Rochester 2GC and Rochester 4GC

Hello All,

I have two easy questions that I cant seem to find answers for anywhere.

Can anyone tell me what the CFM rating was for a factory 2GC tripower from the late 50's to early 60's that came on 348s.

Also I have read that there were these 4GCs that came in 553 cfm. Does anyone know what they came installed in?

I would figure it at about 250 cfm/carb or 750 cfm. I do not have bench flow data to back this up. Maybe someone like carbking can give a more accurate answer. I am not familiar with the 348 tripower setup. I assume it uses 3 small base rochester 2g carbs like the 57 and 58 olds and pontiac v8s. Pontiac went on to use 1 small base 2g in the middle and 2 large base 2g carbs on the end from 1959-1965. Pontiac changed again to 3 large base rochester 2g carbs for 1966, the final year of pontiac factory tripower.

READ THIS LINK FIRST!!!:

http://www.thecarburetorshop.com/Carbshop_carbsizesandCFM.htm

A PARTIAL chart of Rochester CFM ratings was published in the book "Rochester Carburetors" authored by Doug Roe:

In that chart one finds:

2G from 278 to 435

4G from 486 to 692

Remember that CFM numbers, TO HAVE ANY VIABLE MEANING, must specify the test criteria.

Mark Twain probably knew more about CFM tests than anyone. He wrote "Figures don't lie, but liars figure"!!!

Also remember that CFM of multiple carburetors is NOT an additive property. (eg. two 600 CFM 4-barrels probably will flow about 900 total CFM). Tripower is even worse (at least on a V-8).

Jon.

Thanks Guys for the info!

Jon your site is one I my favorite places to visit packed with info and I learn a lot there every time I dig in. I have a few rochester books but I find myself in the 73 version of Doug Roe's book more so than the rest. I feel like I am a rochester guy but not that great with the numbers yet. I am missing something in the theory and it just won't all click.

I don't mean to turn this into a which carb threat but that is my ultimate goal with this info. I was given an orphan 64 300hp 327 for helping someone out. It has a manifold for a carter and I know carter was correct for that engine. Based on your site the carter was rated around 570 cfm and this elusive mid level rochester 4gc was rated around 553. Since I am more familiar with rochesters now it would be easier that learning about carters and I definitely don't want an edlebrock for this engine.

So should I stick with what GM engineers put on that engine, go with a rochester 4gc rated at 553 cfm or the ever popular tripower. I have a tripower already so it won't cost anything to use.

Thanks again

Unless you have a Rochester 4GC which is specifically calibrated for a 327, you may find it easier to learn about the Carter than to recalibrate the Rochester. Yes, Rochesters CAN be recalibrated, but the design makes re-calibration less easy than a Carter or possibly a Holley.

(Opinion) - there is NO BETTER CARB for a basically stock 300hp 327 than the Carter AFB 3720sb/3721sb. The 3720sb is a factory service replacement for the 300/327 with autotrans; the 3721sb for the manual trans. Since both of these are service replacements, they are available at less cost than originals.

And either of the Carters or the original Rochester will outperform the tripower in all areas except looks.

Jon.

Jon,

Thanks a lot for the info, I can do a rebuild but definitely won't attempt a retune. I will take your advice and go with the carter.

Thanks again!

Angelo