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The Classic Cadillac Forum Discussion, replacement for a rochester 4 jet in Past Cadillac Vehicle Discussion; Having built my fair share of mid-60's Chevy 283/327 engines in various stages of tune I can testify that "1 ...
  1. #16
    Submariner409's Avatar
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    Re: replacement for a rochester 4 jet

    Having built my fair share of mid-60's Chevy 283/327 engines in various stages of tune I can testify that "1 cfm for 1 hp" is ummm....... off the mark. Even a stock 350 hp 327 c.i 1965 smallblock came with a vacuum secondary Holley 4 barrel - rated at 585 cfm. I used heads ported by Mondello and others, as well as my own work, and quickly learned that there is a very real relationship between carb, intake, combustion chamber, port, and exhaust sizing - it's a fine tightrope to keep it all in the right power band.

    rksm, For the '66 Deville carb work, I'd stick pretty close to the stock induction setup if you have no other "speed parts" ideas. Talk to the guys at www.allcarbs.com. They have built some pretty impressive QJets for some of my Olds 455 boat engine modifications. Find a 8.5 X 11 paperback book by Doug Roe - "Rochester Carburetors". HP Books #014.

    This low rpm/high torque marine 455 (bored to 461) dynoed out at 465 hp and uses a QJ set to pass 785 cfm at wot/vacuum.

    Are we mistaking 1 hp to ~1 cubic inch as nominal NA "stock" output ??

  2. #17
    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    No, but, because your 327 is running a 585 CFM vacuum secondary unit and producing 365 hp in 1965 - on 1965 pump gas - in factory state it would NOT dyno that amount on 2014 pump gas - does not mean that the 327 is using all of the 585 CFM. In fact, if working properly, pulling open those secondaries when and how it needs them, if you were to measure actual flow, you will find that it is not utilizing all 585 CFM but in fact somewhere south of that.

    Your Olds 455 marine engine (nice looking powerplant, btw) producing 465 hp at the dyno, with a Q-jet at W.O.T., may not be using all 785 CFM (again, it has vacuum secondaries) just because the unit is rated to flow that much. You would have to measure the total amount flowing through and probably would find out that the engine can only breath about 500 CFM or so at WOT.

    Unless you are scientifically measuring the amount of air being pulled through under W.O.T. conditions, it would be difficult to accurately state that the Olds powerplant is using all 785 CFM that the Quadrajet is capable of flowing.

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    Re: replacement for a rochester 4 jet

    Still waiting to see that 1CFM=1HP formula.

    Your words:

    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.
    I proved this conclusively wrong. Care to comment?

  4. #19
    aussiejohn is offline Cadillac Owners Member
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    Re: replacement for a rochester 4 jet

    G'day,

    A very interesting read, I hope that no-one is banned or removed from this Forum just for saying what they think. Just to put another perspective on all of this, let me tell you about a couple of Holden engines of which you may not have heard. Holdens are cars made in Australia since the post war merger of Holdens Body Works and General Motors, the product being General Motors Holden. The first Holden was built in 1948 with a 132 ci inline six. Inline sixes of varying capacity up to 202 cubic inches were the norm until 1968 when the HK model was introduced. With a much wider engine bay to suit both the 307 and 327 Chev and Holden's own V8, at last we had a V8 option. The 202 had a 3+5/8" bore and a 3.25" stroke.

    The Holden V8 had several features of the GM V8s of the day, such as a SBC bore spacing of 4.40" and was offered in two capacities, 253 and 308 ci. Both had a 3+1/16" stroke and bores of 3+5/8" (same as the 202 six) and 4.000", same as 327 and 350 (plus others). On the HK Holdens, the 253 V8 had a two barrel Rochester, while the 308 had either a two barrel or a Qjet. With the fuel crisis of the times, Holden released a new smaller model car, the VB Commodore, in 1978 and it could be had with a 161 or 202 six and a 253 or 308 V8. By the time the VH Commodore was released in 1982, both V8 engines came equipped with the Qjet. That's right, a 253 cubic inch V8 with a 650 cfm carburettor.

    If the engineers at General Motors really thought that 1 ci needed 1cfm, then you have to ask why they fitted a 650 cfm carb to a 253 ci engine. What a lot of you seem to miss is that the Qjet is a really wonderful carb and will not flow 650 cfm every time you floor the pedal. Its air valve above the secondaries will only open if there is sufficient vacuum to allow it to open and at the same time raise the fuel rods to allow sufficient petrol to flow through the secondaries. Most of the time, a Qjet operates ONLY as a two barrel carb and on a 253 ci engine, I doubt if the secondaries ever have any fuel ( and air!! ) flowing through them.

    On another matter, I have e-mailed the Edelbrock foundry asking them if they had considered making an inlet manifold for the 390/429 engines, but so far have received no reply. I wonder how many of you would be interested in a "performance" aluminum manifold for the 390/429 engines? If enough of us contact them about it, maybe they'll pull their fingers out.

    Let me know if I need to start up another thread about this, I'm new here and don't know all the rules.

    Regards from Down Under.

    aussiejohn

    ----------

    Quote Originally Posted by DouglasJRizzo View Post

    Your Olds 455 marine engine (nice looking powerplant, btw) producing 465 hp at the dyno, with a Q-jet at W.O.T., may not be using all 785 CFM (again, it has vacuum secondaries) just because the unit is rated to flow that much.

    .
    Douglas,

    You are totally wrong here. The Quadrajet IS NOT a vacuum secondary carburettor. It is a MECHANICAL secondary carb. Turn one upside down and fully open the throttle linkage if you don't believe me! Now, the vacuum operated air valve up top is another matter...................

    Regards from Down Under.

    aussiejohn

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

    Finding performance parts for Cadillac 390/429 engines is extraordinarily difficult. At one point in the 50s or so, Offenhauser made some things as did Iskendarian. I doubt they do now.

    John, emailing Edlebrock is a good idea, but I think a response will be slow coming. Not for any reason other than they are probably swamped.
    One thing not in anyone's favor is the fact that these engines have been out of production for almost 50 years, 1967 being the last of the 429s, and with so few in circulation, it might take a small "boutique" parts builder to make some induction parts. Ditto for others.

    Jay, when I get back to the lab, I will post my data.

    John - As taken from Wikipedia - "Most Quadrajets use a vacuum operated piston to move the primary metering rods to control the air-fuel ratio, allowing the mixture to be lean under low load conditions and rich during high load conditions. A less-common version uses a linkage driven off the primary throttle shaft to mechanically move the power piston. "E" (Electronic Control Module controlled) series of Quadrajets use a computer controlled Mixture Control Solenoid that responds to electronic signals from the throttle position and oxygen sensors via the computer, ideal for precise fuel metering and allowing additional fuel under load."

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    Re: replacement for a rochester 4 jet

    The QJ uses mechanical secondary butterflies and a vacuum operated swing valve over the secondary bores. The valve opening rate can be modified with a small clockspring adjustment.

    No, that engine does not fully open the air valve - and that's one of the "tuneable" features - using secondary jet needles, needle hangers, and clockspring adjustments.

    Screw with that air valve indiscriminately and you introduce a Pandora's Box of stumbles and poor performance.

    Late edit: I still recommend that the OP seriously consider rebuilding/having rebuilt his present carburetor. It's already jetted and tailored to THAT engine. Regardless of aftermarket carburetor fitted, it takes time, expertise, black magic and testing to get an "out of the box" carb to perform accurately on an engine that it was NOT specifically set up for.
    70eldo likes this.

  7. #22
    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    Quote Originally Posted by Submariner409 View Post
    The QJ uses mechanical secondary butterflies and a vacuum operated swing valve over the secondary bores. The valve opening rate can be modified with a small clockspring adjustment.

    No, that engine does not fully open the air valve - and that's one of the "tuneable" features - using secondary jet needles, needle hangers, and clockspring adjustments.

    Screw with that air valve indiscriminately and you introduce a Pandora's Box of stumbles and poor performance.
    Absolutely.

  8. #23
    aussiejohn is offline Cadillac Owners Member
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    Re: replacement for a rochester 4 jet

    Quote Originally Posted by DouglasJRizzo View Post

    John - As taken from Wikipedia - "Most Quadrajets use a vacuum operated piston to move the primary metering rods to control the air-fuel ratio, allowing the mixture to be lean under low load conditions and rich during high load conditions. A less-common version uses a linkage driven off the primary throttle shaft to mechanically move the power piston. "E" (Electronic Control Module controlled) series of Quadrajets use a computer controlled Mixture Control Solenoid that responds to electronic signals from the throttle position and oxygen sensors via the computer, ideal for precise fuel metering and allowing additional fuel under load."
    Douglas,

    Thanks for the explanation of the operation of the PRIMARY METERING RODS, but this is related to the SECONDARIES how......? We were talking about the secondaries and I stand by my statement that the Quadrajet is a MECHANICAL secondaries carburettor.

    Regards from Down Under.

    aussiejohn

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    Not sure to whom or how to make this request; as a longtime active and enthusiastic participating forum member I kindly request in writing the removal of the warning against me. I also still request consideration of electing a moderator who is wanting and willing to be a regular participant and cheerleader for the forum and its members and the integrity of the hobby.

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    Re: replacement for a rochester 4 jet

    Bro-Ham, It's a pretty safe bet that none of us is 100% right or wrong in this carb/induction sizing argument. That's gearhead work, not moderator territory. Too bad the banner gets in the way of advice at times.

    For you to jump in, out of the blue, with a curve ball attack on someone's "frequency" of cheerleading - or wrenching experience - is a bit much. Heck - I'm not in this forum very much - there's enough to do elsewhere - but I like big engines and carburetors so thought I'd throw in my 2¢. Do I now qualify for a "blowhard" tag ?

    EDIT at 1124: I looked at your warning. It automatically expires in about 26 hours. No records kept.

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    Hi Sub, Thank you for responding. My reaction is more to the pattern I have noticed. The long passages of time of disengagement followed by bursts of participation in which I feel accuracy seems often clouded in favor of ego, and this time we have bullying. I pointed this out not siding with who may be technically correct but more to illuminate the disservice to the forum and hobby. I blew the whistle; I don't feel it is right to be shackled with the punishment. Please remove the warning.

  12. #27
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    Re: replacement for a rochester 4 jet

    Please understand that I will not go behind another Site Staff member's back and rescind an infraction/warning. As posted earlier this morning, your warning auto-expires tomorrow early afternoon.

  13. #28
    DouglasJRizzo is offline Cadillac Owners Enthusiast
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    Re: replacement for a rochester 4 jet

    Ok. My notes from the work on the flow bench -

    1.25 CFM /hp for street motor.
    1.45 for Street/Strip with manual transmission/compression/cam
    1.65 for normally aspirated strip with gears/cam/compression.

    Therefore, a 429 cube Cad motor, at 10:1 compression, with stock transmission/rear on pump gas, at 340 hp (1966) should make well with about 400 or so CFM. Any more and you're just blowing dollars. Now - jack compression, stall speed, cam, axle ratio, and the requirements change.

    Remember that fuels have changed, and that is a factor. The old points ignition systems don't throw a lot of fire compared to new systems. Depending on how much originality one wants will determine whether the old Delco ignition gets boxed in favor of something else.

  14. #29
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    Re: replacement for a rochester 4 jet

    I'll throw this in for comparison, and that's the end of my involvement in this.

    Back in the late 90s Joe Mondello did several sets of Olds heads for me (iron Ga series) - marine engines again - "street porting" - and his supplied flow bench figures agree almost exactly with your second (1.45) set of figures. Thus the 780 QJ on a 461 engine.

    I might throw in that the best, cleanest, ported, polished NA street engine has a VE of about 0.85, and that has to be factored into airflow and jetting.

    As always, don't go too big on a carb airflow rating.

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