miche25

How can we know what is the size of our carburetor? Where are the indications? 550 - 650 ???

sofakingdom

iTrader: (1)

What kind of carb?

GeneralDisorder

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Q-Jet's are 750 or 800.... and those are the only carbs generally seen on 3rd gens and really the only one's I'm mildly familiar with.

Based on OP's previous posts, he seems to have an 84 Z28.... which should be a 750 CFM Quadrajet if I'm not mistaken.

GD

sofakingdom

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Which is of course, precisely the problem... when somebody asks "what CFM is my Q-Jet" (which he hasn't told us yet that he's looking for info on a Q-Jet), it is INVARIABLY the wrong question to ask. Usually they're fishing for something like "what aftermarket carb size should I buy to replace my Q-Jet", which is a COMPLETELY different thing altogether. Or, "is my Q-Jet too small for a 350", "I put a Summit K1102 cam in my 305, does my Q-Jet have enough CFM", etc. In EVERY such case, "how many CFM" is simply NOT a useful thing to be concerned with. Much like "I bought this rebuilt motor and the block stamping is XX00XXX, how much horsepucker do I have?" Wrong.

Most electronic Q-Jets on 305s have a "feature" in the casting that limits how far the air valve opens. As-is, it limits the CFM to something between 650 and 700. Not an issue for a 305 unless it's going to be turning 9000 RPM or something, instead of merely a puny 7500 RPM. Once the casting detail is removed, allowing full opening of the AV, the CFM is the usual 750ish.

The best answer there is for "what CFM is the Q-Jet on my 305", is, "enough".

GeneralDisorder

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One interesting aspect of doing a lot of Fuel Injection tuning, is that with MAF cars you can actually quantify the grams per second..... which with a rough approximation can be converted to CFM (assuming a lot of variables like air density) by multiplying by 1.76....

Grams per second is also very close to rear wheel horsepower in real world testing. So 300 RWHP is still only about 528 CFM or so.... round it up to 550 because density of air and reasons. In theory the stock Q-Jet, appropriately modified, ought to be good to 400 RWHP, which is about 500 HP at the crank. To say the stock carb has plenty of overhead is nearly an understatement. It would take a lot of cam and RPM to get a 305 to the point of maxing out the stock carb I should think.

GD

sofakingdom

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Very true: energy is obtained by burning fuel; the amount of fuel burned, determines the energy released; the amount of air required to burn the fuel properly, therefore, is a direct measure of the energy available.

The one single primary thing that can cause engine output to vary, within that framework, is compression... which determines the portion of the energy that comes from the fuel, that gets turned into useful work. The old "rule of thumb" that at an engine's "max HP" conditions a third of the fuel energy gets turned to useful work, a third goes into the cooling system, and a third goes out the exhaust (6% ALONE goes out the exhaust in the form of the heat of vaporization of the water produced by combustion, and there is NO WAY to recover this in a conventional ICE) still stands. But if you can move it just a few percent - let's say, 40% into useful work, 30% into the cooling system, and 30% into the exhaust - you can make a YUUUUUUUUJJJJJE improvement in power output. Easier said than done however. High compression, well insulated combustion chambers, etc. can only go just so far, within the larger environment of pump gas, idling in traffic, etc.

But yeah... a 305 would require 9442 RPM at 90% VE at that RPM (not too shabby for a street motor at ANY RPM) to consume 750 CFM.

The Q-Jet is "enough" for any 305 that exists, in the RW.

GeneralDisorder

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Hey Sofa - curious what your thoughts are on these:

https://www.ebay.com/itm/1910-Remanu...C/263929321747

It's an Edelbrock 850 CFM Q-Jet.... they don't make them anymore but this seller seems to have quite a few.... reasonably priced for the size carb it is. I rather enjoy the Q-Jet myself. Thinking of trying one out for my 69 GMC truck......

GD

sofakingdom

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Wouldn't be my first choice. It's the M4MC model, so it's got the mid-70s emissions crippling aspect to it, unless Vic changed those aspects of it somehow. I have no knowledge of any such details of that particular Edelbrock model. Better than some in stock GM trim but not what I'd consider ideal.

My all-time fave Q-Jets are 7029202 and 7029208; 69 350 & 396.

I wonder if you could find a Holley 6211... just like the 6210, except 800 CFM. Spreadbore, manual secondaries, the early versions came with a sec metering block, maybe all did. Divorced choke. Unbelievably easy to put on a 60s Chevy motor. I had one for a long time on a 74 454 Caprice convertible that I had put a 69 intake on. Ran WAAAAAAAAYYYYYYYYYY better than the M4MC Q-jet and upside-down ("Vette") intake that it came with, and got the same or better gas mileage.

GeneralDisorder

iTrader: (1)

Interesting - I looked up those numbers - I keep seeing the 7029202 listed along with the 7029207 number (as it you may get either one). Is the 207 the same?

I don't see a single 6211 for sale. Just rebuild kits.

You can see the specs on the Edelbrock made "1910" 800 CFM Quadrajet here (the "Performer RPM Q-Jet"):

https://www.carburetion.com/Rochester.asp

It was billed as a high performance Q-Jet by Vic.

GD

sofakingdom

iTrader: (1)

Odd numbers are (were...) for stick cars, even ones for auto. Or maybe the other way around? My old-timers disease is in effect. In any case they often came in pairs; 201 & 202 were the same car auto or manual, etc.

Then of course, each carb # was set up for a different motor and option package. So there are tiny detail differences, and sometimes not so tiny. I just remember those were the 2 cores I'd look for in the buzzard nest, back 30-40 yrs ago; they were the easiest to do a performance tune on, and not get limited by some one or more of the internal passage drilling and orifices. Plus, they were common enough that there was a good chance of coming across one.

207 is probably 208's transmission sister, and therefore probably a good core too.

GeneralDisorder

iTrader: (1)

Yeah the 202 keeps coming up as a 1969 with automatic transmission. But auctions list 202/207 as both being auto 69 models which is odd..... and even.... and that's odd.... given your information about numbering.

GD

sofakingdom

iTrader: (1)

Could be. I'm no "authority" or "expert". And as we all know, rules are like pie crusts, made to be broken. (excuse the paraphrase of a sinister historical figure)