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500, 472, 425, 368 Discussion, 425 upgrades in Cadillac Engine Technical Discussion; i just got a 1978 deville with 425 any thing cheap to beff up hp...
  1. #1
    1978425deville is offline Cadillac Owners Member
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    Red face 425 upgrades

    i just got a 1978 deville with 425 any thing cheap to beff up hp

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  3. #2
    davesdeville's Avatar
    davesdeville is offline Banned
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    Re: 425 upgrades


  4. #3
    VicVega is offline Cadillac Owners Member
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    Re: 425 upgrades

    The 425 is not a horsepower engine it is primarily a torque engine (heavy car) and all the torque the engine builds is in by 2500 rpm. Beyond 2500 rpm what little horsepower the engine produces (far less than 200hp) is what is moving the car.

    What's cheap?

    The tune up information provided in the link is useful to put the edge back on the stock set up, except I'd skip for the cat plug step.

    To improve performance in a 425 beyond stock performance that will require a few changes to the hardware. The Qjet may need overhauled (some say this is a good option, some don't, (I don't) you will need to decide this for yourself).

    If you work around the intake take some time to remove the carb then carefully re-torque the intake, these bolts come loose. If you have the ability and can replace the stock intake swap to a 472 intake. The 425 intake is a single plane intake and the 472 is a dual plane intake giving you better off idle throttle response...you will like the difference. If you plan to run a/c and re-install your compressor you MUST get the 472 a/c brackets the 425's will not work. Not an expensive upgrade for nice a idle and a little better throttle response. A 2.5 inch open air cleaner that sits up slighty, not down over the carb works well here too and has plenty of hood clearance.

    If you work around the exhaust change the stock cat to a high flow cat then run 2.25 exhaust pipe to the rear into a resonator. No need for a muffler the cat and the resonator do the job. Sounds good at idle not noisy on the road and now your 425 will breath as good as it ever will without switching to duals and or headers and not as expensive.

    Good luck.

    VV

  5. #4
    Benzilla's Avatar
    Benzilla is offline Cadillac Owners Connoisseur
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    Question Re: 425 upgrades

    Quote Originally Posted by VicVega View Post

    If you work around the exhaust change the stock cat to a high flow cat then run 2.25 exhaust pipe to the rear into a resonator. No need for a muffler the cat and the resonator do the job. Sounds good at idle not noisy on the road and now your 425 will breath as good as it ever will without switching to duals and or headers and not as expensive.

    Good luck.

    VV
    That's actually pretty interesting to me, how is the sound different? if you go WOT will it growl nicely? What size is the stock exhaust? I haven't looked really closely, but it seems pretty big. Would a regular exhaust shop be able to do that? About how much $$$ are we looking at?

    Is that enough questions?

  6. #5
    VicVega is offline Cadillac Owners Member
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    Re: 425 upgrades

    I purchase a high flow Dynomax cat (p/n 289-15146) from Jegs. This is the 2.5" unit 18" long unit without the air tube. I buy the 2.5" unit since most Y pipes are 2.5 inches and most exhaust shops can handle 2.5 inch exhuast piep. I purchase the 18 inch long unit because the longer the cat or the muffler lower the quieter or mellower the exhaust note. These are brand name (walker I think) universal large engine cats and I have never seen a better price anyhwer else. They are monolithic and not bead cats and they will flow great for along time. Cost is $120.

    The resonator I use is a 2.24 inch Pacesetter from Summit, this is dual outlet resonator with chrome tips. Part number is PSM-95-9905. Looks great best tone. This attached just forward of the rear bumper and just extends past the bumper. Cost is $47.

    You can use a single 2.25 glass pack, but be sure its all the way in the back and its the 17 inch unit for best out the back sound. Works just like the resonator since that's really all a glasspack is anyway, but just does not look as fancy.

    I use the 2.5 cats becuase most exhaust shops can redcude the outlet from 2.5 to 2.25 without any problem. I have 2.25 exhaust pipe installed all the way to the back becuase most exhaust shops can handle 2.25 over the axle. I have had this combo installed on many differnet vehicles at more than one exhaust shop with few problems. The problems are alway the skill level of the pipe bender. If the pipe bender is using the exhaust set up cards to bend the pipe this is the place you will have problems becuase they are not really customer pipe benders. If you get a shop that knows what the are doing they will have it all bent and installed in less than 60 minutes and it will look great. The guys who can't really do custom bends have problems getting over the rear axle, the bends look funny.

    The cost to do this has ranged from $100 if I removed everyhting and they just installed my parts to $150 if they remove everything then install my parts.

    Stock pipe is between 2 and 2.25 inches. I have seen both. 2.25 is really all you need and just happens to fit the best. Much larger, say 2.5 and your shop will have trouble with the rear axle bend, but it depends on the shop. 3 inch will probably really create install problems, plus you run the risk of the pipe being too large and reducing exhaust flow, but that's another story.

    This is what I have done, keeps old cars legal, sounds good out the back, quiet inside, not expensive and helps perfromance.

    Good luck.

    VV

  7. #6
    Benzilla's Avatar
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    Re: 425 upgrades

    Thanks, that was very informative.

  8. #7
    Wheelman322's Avatar
    Wheelman322 is offline Cadillac Owners Enthusiast
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    Re: 425 upgrades

    Was wondering if anyone has tried the 472 intake and retained there a/c compressor with the 472 brackets. Just want to be sure it works before I make the swap.

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