Forget rebuilding a 307! Get a 350!
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5.0 and 5.7 Discussion, Forget rebuilding a 307! Get a 350! in Cadillac Engine Technical Discussion; 1968 OLDS 350 4 BARREL Motor Cheaper to rebuild and much more rewarding. AND 95% compatible with all emissions devices. ...
  1. #1
    N0DIH's Avatar
    N0DIH is offline Cadillac Owners Master
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    Forget rebuilding a 307! Get a 350!

    1968 OLDS 350 4 BARREL Motor

    Cheaper to rebuild and much more rewarding. AND 95% compatible with all emissions devices. Stuff in a 307 HO cam and you have a great combo that will blow nice numbers.

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  3. #2
    joe_padavano is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    Depends on what you want and how rigorous the local emissions test laws are. All Olds small blocks (260, 307, 330, 350, and 403) are externally identical, so by transferring the emissions equipment, you can easily pass the visual part of the emissions test. The electronic Qjet only controls the primary side - the secondaries are purely mechanical just like the old Qjets so rejetting for a larger motor is easy. So long as you keep the O2 sensor and EQjet, the computer should keep the idle and low speed clean enough to pass. If it were my time and money, I'd opt for the 403.

    Now, if you're really feeling adventureous, the 455 Olds looks like a 307 (same valve covers, same water pump, same general geometry) but the deck is about an inch taller. This makes for a wider intake and of course the casting numbers on the heads and block are different. Also, the small block accessory brackets won't bolt on in some cases (like the alternator, for example), so you'll have another visual difference. You can probably get the 455 to pass tailpipe (I know someone who did with the EQjet) but the visual inspection might be a problem, depending on how sharp the inspector is.

    OK, stop laughing...

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    caddydream is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    Thanks guys for more good info.

    Here's a story: A couple of months ago a young guy I know at the local auto parts store bought a 77 Buick Electra with an Olds 403. Ran pretty good. I went there a few days ago and he had a different car. I asked about the Buick. Well..... He said he was putting headers on it and cracked one of them. He got so mad he took the car across the street to the fire department and donated it to them for training.

    Guess what? Yup, they trashed the Buick, 403 and all. Aw sh_t!

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    N0DIH's Avatar
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    Re: Forget rebuilding a 307! Get a 350!

    I made my 350 look like a 307, as close as I could. Only an expert would have known. I almost ran the 307 heads to have uber high velocity heads, but figured it would hurt too much. So ran the stock 77 3A heads and intake. Ran fine. I think I could have got better mileage out of the 7A heads, but top end would have been less. Being I spend so little time in the high end and low end power was the important thing I probably should have.

    I hope to get another and do it again. I might go with a set of heavily modify 7A heads to concentrate on velocity and throttle response. Couple it with a set of 2.56 gears and OD, it should be a great mpg daily driver.

  6. #5
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    0to60n4.5 is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    I have a fuel injected 403 in my 79' Seville with 10.5 c/r and a big cam .
    I also put in a 2004r trans but the motor made enough power to finish the trans off. I will put back in the stocker TH400 trans back in. I have 3.42 gears instead of the 2.24 stock stuff.

    I still have bugs to work out with the fueling from the ECM . The old fashioned ECM used manifold vaccume as part of its feedback. And with this cam as soon as you crack the throttle a little bit the vaccume drops. I haven't worked on it in a while , but I haven't given up yet either.

    Back in the day when I started the project ( 15+ years ago ) it was going to be a fast Sedan. Nowadays they make tons of Sedans that most likely will run circles around whatever mine might have become.

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    joe_padavano is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    Well, give us details. I assume you're not using the analog Seville EFI system, right? Do you need to pass emissions where you're located?

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    Re: Forget rebuilding a 307! Get a 350!

    You assumed incorrect. The same EFI that ran the 350 oldsmosqueel .No emission testing where I am ( Mid Michigan ).
    Car runs good @ idle and runs strong WOT, but I'm thinking it runs very lean also. You have to feather the throttle to accell to cruising speed to keep it smooth.

    Haven't used it for a while as I need to replace the trans.

    I'm guessing there are no programmers for the ancient stuff , so I planned on
    adding a touch of fuel with slightly bigger injectors . Or perhaps a 9th injector and play with fuel pressures.

    Its all in fun as the car hasn't owed me anything for the past 15 years anyway.

    Too many projects , too little time.

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    joe_padavano is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    Thanks for the info. The consensus in the Olds community is that the 70s EFI intake flows worse that a stock 2bbl intake, but I don't have any real numbers to prove that. The analog computer is not serviced by anyone that I'm aware of. You may try an aftermarket digital unit like a Megasquirt or F.A.S.T. controller. Are the Seville injectors compatible with any of the later units? Also, you may want to try an adjustable fuel pressure regulator, since higher fuel pressure will richen the mixture (across the board, unfortunately). I know you can get regulators that are referenced to manifold vacuum so that the system leans out at idle and richens at WOT.

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    Re: Forget rebuilding a 307! Get a 350!

    Yea , If I remember correctly the manifold is very short and a single plane design so it has no runner length. The throttle body is a 2bbl config, with the 76' and 77' being alot bigger than the 78'-79'.

    I had already replaced my 79' one with the bigger unit.

    And the FPR is manifold vaccume controlled but I don't remember the pressure ranges that it had.

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    Re: Forget rebuilding a 307! Get a 350!

    IIRC the 350 didn't get hardened valve seats until 1972. But if you aren't racing, likely won't be a real issue until much higher miles.

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    boatman is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    383 stroker .........man!!

  13. #12
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    Re: Forget rebuilding a 307! Get a 350!

    Not with an Olds 350....

  14. #13
    3141592654 is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    For a 1977 vehicle, I'd look into getting a Chevy 350 out of an old Impala or something. I gotta' believe that this engine would have the same or better emissions than the Olds.

    The only thing might be the bolt-pattern on the tranny. On the later model Broughams, Caddy put in the Unicase THM 2004R transmission. Any GM V-8 will bolt up.

    Do a little research on the tranny and go with the Chevy engine, if you can. If you can't replace the tranny while you are at it. Then, put the Chevy in.

    Please...no "Chevy ain't a Cadillac" comments. I should want to remind everyone that Caddy engineers designed the HT4100 engine in the first place.

    Caddy refinement with Chevy drive-train experience...perfect combination! Would that be a Chevrolac or a Cadillet??

    pi-

  15. #14
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    Re: Forget rebuilding a 307! Get a 350!

    So is there room in a N* to stroke it to 5.7L or larger? That would be SWEET.... Make a RWD Fleetwood truly Cadillac powered again.

  16. #15
    joe_padavano is offline Cadillac Owners Member
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    Re: Forget rebuilding a 307! Get a 350!

    Quote Originally Posted by N0DIH View Post
    So is there room in a N* to stroke it to 5.7L or larger? That would be SWEET.... Make a RWD Fleetwood truly Cadillac powered again.
    Nope. Olds DID make a 5.0 L version of the block for use in IMSA racing. I've been trying to find one of those.

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