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5.0 and 5.7 Discussion, Carburettor Spacer in Cadillac Engine Technical Discussion; My Cadillac has the Olds .307, and I'm looking to increase it's performance (I'm tired of people passing me on ...
  1. #1
    sven914 is offline Cadillac Owners Connoisseur
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    Question Carburettor Spacer

    My Cadillac has the Olds .307, and I'm looking to increase it's performance (I'm tired of people passing me on the highway while I'm trying to get up to speed). I recently read that adding a one inch spacer between the carburettor and the intake manifold would increase horsepower, by about 50hp, and increase the fuel mileage. Would this be true of the .307 and if so would I need to do anything else to the engine such as adjust the timing, change the fuel trim, or increase the gasoline's octane rating?

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    Submariner409's Avatar
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    Re: Carburettor Spacer

    A simple carburetor spacer cannot increase horsepower by that amount, not even in one of my Olds 455 boat engines. Take a look at www.mondellotwister.com . Although Joe Mondello, father of Olds engine mods and one of the greatest cylinder head porters of all time, is retired, his business carries on under new ownership. Get their catalog and Technical Manual. More Olds info than you can ever digest. Also look for Dick Miller Racing (DMR). Haven't seen them around for a while, but he used to have scads of Olds parts and a part number interchange booklet.

  4. #3
    RocketFast321's Avatar
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    Re: Carburettor Spacer

    Quote Originally Posted by sven914 View Post
    My Cadillac has the Olds .307, and I'm looking to increase it's performance (I'm tired of people passing me on the highway while I'm trying to get up to speed). I recently read that adding a one inch spacer between the carburettor and the intake manifold would increase horsepower, by about 50hp, and increase the fuel mileage. Would this be true of the .307 and if so would I need to do anything else to the engine such as adjust the timing, change the fuel trim, or increase the gasoline's octane rating?
    No way a spacer can do that. You have a 307 they are not power houses. Trying to make a 307 fast is like throwing money into a fire. But the good side is they last a long time. The best way to make your car faster is to get a olds 350, and swap over everything off your old 307. Don't mess with the timing and make the computer mad. Some small things you can do is give it a good tune-up. Plugs and wires, replace any bad vacuum lines. Also cheack to see if the vacuum lines are going to the right place. Replace the O2, yes these cars have a O2 on the pass. side mainitfold. Check the PCV vavle on the pass. side of the motor, and replace the oil breather on the driver side. Seaform it, and with the motor cold, give the carb a bath with some carb cleaner. Make sure the bolts are still tight that hold the carb down.

    That's all i can think of that i did to my 307

  5. #4
    sven914 is offline Cadillac Owners Connoisseur
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    Re: Carburettor Spacer

    Thanks. What gave me the idea was the Ram Air engines used in the 70's Pontiac Trans Am's. Essentially it's the same principle as a carb spacer, in that the Ram Air engine's carburettor sits on a taller intake manifold which forces more air into the intake plenum (a.k.a. the Ram Effect). Another thing that got me thinking that the .307 had a little more to give was the fact the the same engine was used in the 83-84 Hurst/Olds and in 86-87 Buick Grand National (used with same transmission as the Brougham), both of which are know as the last American muscle cars.

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    Re: Carburettor Spacer

    Two different principles. A simple spacer increases total under-carb volume to tune the manifold to a specific power band. Ram-Air or any variation uses each individual runner in the manifold to "tune" the intake pulses in order to supply a denser fuel/air charge to each cylinder.

    Take a look at some of the later V-8 fuel injection air manifolds and how they wrap 8 individual runners for maybe 10" or so.....same principle as tuning the size and length of exhaust headers for maximum gas extraction.

  7. #6
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    Re: Carburettor Spacer

    Quote Originally Posted by sven914 View Post
    Another thing that got me thinking that the .307 had a little more to give was the fact the the same engine was used in the 83-84 Hurst/Olds and in 86-87 Buick Grand National (used with same transmission as the Brougham), both of which are know as the last American muscle cars.
    Stock 307 has 145hp and 255Tq. The HO 307 was 180HP. The 2004r has many different designs. The B'ham and the GN tranny are the same on the outside but different on the inside.
    Note this page, it shows the different models of the 2004r.
    http://web.archive.org/web/200701281.../thm2004r.html

  8. #7
    sven914 is offline Cadillac Owners Connoisseur
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    Re: Carburettor Spacer

    Again, thanks for your inputs. I got in touch with a friend of mine, an old GM master tech, and he corrected me. A carb spacer would only increase horsepower by 5-10. He did say that a one inch spacer would create a Ram Effect and increase the volumetric efficiency of the engine. The engine would then have to be re-timed and the air/fuel mixture fattened up. He suggested that I either rebuild the carburettor with larger jets to accommodate the increase in air flow or replace it entirely with a Holly. And with it being a feedback carburettor, he said to disconnect the ECM and change the distributor (to one with vacuum and centrifugal advance).

    I'm not going to do that. Why should I care if I piss off little old ladies. I drive a Cadillac.

  9. #8
    RocketFast321's Avatar
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    Re: Carburettor Spacer

    Their is a pice on the carb you can grind off to make the secondaires open up all the way. I will see if a can get a pic of it. By default it only opens about 75%. No since putting on a IE 650 holly since the q-jet is either a 750 or 800cfm. But still the best thing to do is give it a good tune up. Leave the computer in it.

  10. #9
    Submariner409's Avatar
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    Re: Carburettor Spacer

    Don't forget that a wide-open Quadrajet, even the later electronically controlled unit, is WAY too big for a stock 307. Fool with secondary opening angles and air valve opening rates and you'll wind up with a huge stumble in the midrange.

    Find Doug Roe's Quadrajet book (HP Books #014) at a speed shop, Jeg's, Summit, or Performance Automotive Warehouse.

    A bigger carb is usually not the answer.....the original 1965 350 HP Chevy 327 came with a Holley vacuum secondary 585 cfm which was all the carb it could handle. Bigger is NOT better in carburetors for daily drivers.

  11. #10
    N0DIH's Avatar
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    Re: Carburettor Spacer

    Yup, sub speaks the truth. I did some nice tweaks to my 307 carb (I had a 350 in my 85 Cutlass) and even with CCC, it was darn snappy and crisp power. 2.14's and 2.56's. I put in a 204/214 cam (too big, yes, very much so!) and wasn't happy with it. Missmatched combo, the heads were not enough to make it run right (77 350 3A's)

  12. #11
    Submariner409's Avatar
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    Re: Carburettor Spacer

    I run Mondello (Engle) 204/214 (at .050) cams in my 455 boat engines, advanced 4 degrees with a Cloyes double roller timing set. Built up like a high torque RV install. Very powerful. This 461 has a pair of Mondello street ported Ga heads with filled exhaust riser passages and Comp steel roller tip rockers.

  13. #12
    N0DIH's Avatar
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    Re: Carburettor Spacer

    Sweet..... drop that in place of a Northstar and have fun!

  14. #13
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    Re: Carburettor Spacer

    Guaranteed to break the front motor mount.......

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