replacement for a rochester 4 jet
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The Classic Cadillac Forum Discussion, replacement for a rochester 4 jet in Past Cadillac Vehicle Discussion; Looking the replace the factory carb on my 66 coupe Deville. It's a 729 with a 4 jet (not a ...
  1. #1
    rksm is offline Cadillac Owners Member
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    replacement for a rochester 4 jet

    Looking the replace the factory carb on my 66 coupe Deville. It's a 729 with a 4 jet (not a quadrajet but the one prior)
    Has anyone done a similar switch before. I'm sure I need an adapter plate, hood clearance seems OK.
    Edelbrocks look like a good option, but Holly definitely won't line up. It's too damn cold up here in Canada to tinker with the car now. So if anyone knows the specific parts I need...I will appreciate it...thx
    Ps. Apologies to the purists out there

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    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    That carburetor is either a Rochester 4GC or a Carter AFB. Most likely a Rochester.
    Edlebrock and Barry Grant are good options if Holley doesn't have a replacement for it.

    Avoid the temptation to "overcarb". If the V8 is mostly stock, a 390 CFM is adequate. It takes about 1 CFM to make 1HP, so for a 345 hp engine, a 390 CFM unit is adequate. I've seen people put 700 CFM carbs on smogged, low compression small block Chevies and wonder why the milage sucks and the car bogs on acceleration.

  4. #3
    CaddyGas's Avatar
    CaddyGas is offline Cadillac Owners Fanatic
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    Re: replacement for a rochester 4 jet

    It' s amazing what people can do to their car, because they assumed bigger is better.

  5. #4
    P.Katson is offline Cadillac Owners Member
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    Re: replacement for a rochester 4 jet

    Whats wrong with the 4GC?

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    jayoldschool is offline GM RWD V8 addict
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    Re: replacement for a rochester 4 jet

    If the V8 is mostly stock, a 390 CFM is adequate. It takes about 1 CFM to make 1HP, so for a 345 hp engine, a 390 CFM unit is adequate
    That is so far from correct I don't even know where to start. Please look up what the CFM numbers are for factory GM four barrel carbs, along with the displacement/HP of the engines.

    OP: the 4GC uses a small pattern, and all other four barrels (even square bores like Edelbrock) are too big to bolt on. Either rebuild your stock carb, or swap on a later manifold that uses a Q jet.

  7. #6
    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    Quote Originally Posted by jayoldschool View Post
    That is so far from correct I don't even know where to start. Please look up what the CFM numbers are for factory GM four barrel carbs, along with the displacement/HP of the engines.

    OP: the 4GC uses a small pattern, and all other four barrels (even square bores like Edelbrock) are too big to bolt on. Either rebuild your stock carb, or swap on a later manifold that uses a Q jet.
    Where to start is that you are using outdated and obsolete information.
    The CFM ratings on old cars are based on old fuels - just like the gargantuan ports in the cylinder heads. Using 1966 leaded, high toluene fuel, sure you could use a huge carb. But I stand by my words, and my PhD in engineering is my backup.

    Simple formula - 1hp needs about 1 CFM. Any more "carb" and it's useless. Remember that modern fuels are FAR LESS volatile that what was available at the pump in 1966. One of the reasons Detroit went to higher power ignition systems starting in the 70s was that the no lead fuels that were going to be made mandatory in 1975 needed WAY MORE fire power to ignite than previous fuels. They were made aware of this with time to spare and those systems started showing up - first as optional, then standard in '75. In the decades since, those systems have gone sky high in output.

    If you are aware of this then you know that the huge carbs used on V-8s of prior years along with those enormous, yawning ports in the cylinder heads have little basis for making power today. The current crop of small block Chevy V-8's have ports WAY smaller than in years past yet exhibit vastly superior horsepower and efficiency. The 1CFM/1HP rule stands. If it's good enough for the S.A.E, it's good enough for me.

    Don't believe me? Put a 750 double pumper Holley on that vintage Cad motor running modern gas and run it through the 1/4 mile. You'll be SLOWER than the stock unit and SLOWER than a 390 CFM modern unit.

    Now, IF you were running 1966 fuel, things might be a little different. In case you have noticed, gasoline is far less pungent in its hydrocarbon odor now, than back in the 60s and 70s. There's a reason. There's no longer any lead, and reduced or eliminated MTBE, Toluene, etc. All of this makes the fuel burn differently and far less volatile.

  8. #7
    rksm is offline Cadillac Owners Member
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    Thx for the feedback guys..
    My main concern was finding a carb that fits..
    Jay is right abt the ports...it just won't bolt on...
    And Doug ' s info abt the specific carb seems appropriate.
    I can always get the stock carb rebuild..but for a bit more money I can have something modern which is more efficient and reliable. Unless this assumption im making abt newer carbs is wrong.

    So...I rather not change the manifold..is it possible to find a suitable adapter plate to make the newer carb fit?? Or is that not recommended?

  9. #8
    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    Thank you.

    To answer your question:
    A modern carb would be better if the car is more of a driver. It would handle modern fuels much better than the old 4GC will. Drivability, throttle response, and mileage will all increase. Generally, I recommend putting the original unit in a box, just in case you want to bring her back to 100% original.
    An adapter plate if necessary is not really a bad thing. I'd be careful of hood clearance and make sure there's no binding of the linkages.

    If no adapter plate is available from the induction guys, one could be made easily.

  10. #9
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    jayoldschool is offline GM RWD V8 addict
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    Re: replacement for a rochester 4 jet

    Last time I checked, CFM is a measurement of AIR, not fuel. Also, I don't need anyone throwing their education around to try and prove a point (although, engineers ALWAYS seem like they need to do just that).

    Simple formula - 1hp needs about 1 CFM
    Show me the technical data on this "formula".

    Put a 750 double pumper Holley on that vintage Cad motor running modern gas and run it through the 1/4 mile. You'll be SLOWER than the stock unit and SLOWER than a 390 CFM modern unit.
    Depending on year, the OEM carb was rated higher CFM than the Holley.

    Detroit went to higher power ignition systems starting in the 70s was that the no lead fuels that were going to be made mandatory in 1975 needed WAY MORE fire power to ignite than previous fuels.
    I thought they went to electronic ignition for emission requirements. I'm not an engineer, though

    If you are aware of this then you know that the huge carbs used on V-8s of prior years along with those enormous, yawning ports in the cylinder heads have little basis for making power today. The current crop of small block Chevy V-8's have ports WAY smaller than in years past yet exhibit vastly superior horsepower and efficiency. The 1CFM/1HP rule stands. If it's good enough for the S.A.E, it's good enough for me.
    It looks like I am not the one that is misinformed.

    Let's compare a couple of the most famous, high compression, leaded fuel small block chevy heads from the 1960s. And yes, I have a pair of these heads on my 1965 Impala (that just happens to have a Rochester 4GC on it).

    3782461 – 1964-1966 327, camel hump, no accessory holes, 160cc intake ports, 62cc exhaust ports, 62cc chambers
    Casting number 3795896. Manufactured from 1963-1965 on 283 Cubic Inch engines. Called the “Power Pack” cylinder heads. 1.72˝ intake valves and 1.50˝ exhaust valves. 60cc Combustion Chambers.

    Casting number 3795896. Manufactured from 1963-1965 on 283 Cubic Inch engines. Called the “Power Pack” cylinder heads. 1.72˝ intake valves and 1.50˝ exhaust valves. 60cc Combustion Chambers.

    3782461X – 1960-1963 283 or 327, camel hump,no accessory holes,larger 172cc intake ports, 64cc exhaust ports, 62cc chambers

    Now, let's look at the most modern update of the original small block, the LT1 head from the 1990s. You know, bad fuel times. Oh, btw, I've got a few cars with these heads, too.

    10207643 – LT Small Block V8 1994-1996 350, LT1, aluminum, reverse flow cooling, 175cc intake ports, 68cc exhaust ports

    Hmm, BIGGER ports on a modern engine. Strange.

    What about modern high performance heads? Surely those would have small ports?

    10051101 – V8 Bowtie, aluminum, 55cc chambers, angle plugs, raised runners, 196cc intake ports

    Even bigger.

    Ok, let's compare to some brand new, high tech LS1 heads:

    Stock LS1


    Intake valve size: 2.0
    Exhaust valve size: 1.55
    Intake port volume: 205 cc
    Exhaust port volume: 70 cc
    Chamber volume: 68 cc

    Bigger than ANY production line small block chevy head. Interesting.

    How about some modern aftermarket LS heads:

    AFR 245

    Retail price: $2,699
    Intake valve size: 2.160
    Exhaust valve size: 1.6
    Intake port volume: 244 cc
    Exhaust port volume: 87 cc
    Chamber volume: 64 cc

    Just because they call me "oldschool" doesn't mean I don't play in the modern world. I do as much time on the laptop working on my fuel injected cars as I do playing with points on my old ones.

  11. #10
    rksm is offline Cadillac Owners Member
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    More I kno...less confident I feel...

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    Re: replacement for a rochester 4 jet

    Knowledge is power, my friend. The more you know, the more you can be confident in your final decision being the right one.

    To sum up your choices (in no particular order):

    -rebuild your 4GC. Modern rebuild kits will have updated seals/rubber parts compatible with today's fuels, although, in Ontario, you can easily get (and should be using) high octane 91/93 with no Ethanol
    - go with a 4GC to square bore adapter and use a Carter or an Edelbrock. Despite other advice, the correct size for your engine would be a 750cfm, Edelbrock 1411 "Designed for 402 c.i.d. and larger engines" http://www.edelbrock.com/automotive_...erformer.shtml
    - swap your intake to the later intake and use a Q jet.

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    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    Comparing standard passenger car heads from 1965 with high performance heads of 1994 is not being intellectually honest.
    Also the challenge still stands - run the vintage cad through the traps in stock configuration - then run it through with a large modern carburetor.

  14. #13
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    Bro-Ham is offline Cadillac Owners Connoisseur
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    I am appalled. I find our classic Cadillac forum moderator, DouglassJRizzo, to be a blowhard who is wrong more often than right which does a disservice to both this forum and the hobby. Plus our moderator seems to make only seasonal appearances with his pontificating here on the forum. I think it is time for a responsive and responsible moderator.

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    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    Quote Originally Posted by Bro-Ham View Post
    I am appalled. I find our classic Cadillac forum moderator, DouglassJRizzo, to be a blowhard who is wrong more often than right which does a disservice to both this forum and the hobby. Plus our moderator seems to make only seasonal appearances with his pontificating here on the forum. I think it is time for a responsive and responsible moderator.
    Just because I post very little from time to time does not mean I'm not here.
    Name calling can and will get you banned. Keep it up and find out.
    Wrong more than right? I beg to differ, but then I have no need to defend myself.

    CONSIDER THIS A WARNING. NEXT TIME AND YOUR MEMBERSHIP GETS REVOKED.

  16. #15
    Bro-Ham's Avatar
    Bro-Ham is offline Cadillac Owners Connoisseur
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    You truly lack character. You issue me an infraction yet you lack the interest to even admit you may be wrong, which I have been amazed that you often are despite your position as moderator of this forum, of which you seem only occasionally interested in participating. I wish to request a complaint against you for your disinterest and now dictatoresque action and further request your removal as moderator.

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