425 CI build options and opinions
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500, 472, 425, 368 Discussion, 425 CI build options and opinions in Cadillac Engine Technical Discussion; While most of the attention is given to big block cadillac engines is for 472 and 500 CI, there are ...
  1. #1
    Allante North *'s Avatar
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    425 CI build options and opinions

    While most of the attention is given to big block cadillac engines is for 472 and 500 CI, there are a few of us interested in the 425 CI. I have picked up a 425 CI and Th 400 for some cheap $ and decided to work with it since the other powerplants aren't weren't available and I already have a 425.

    I was able to pick up a dual plane intake manifold from a member on the MTS board that I'm told will help it breathe. The heads are at the machine shop and new larger valves and stiffer springs are on the order. The block and pistons looked new, so I can't see doing anything there except new rings I may be ready to glue it all back together.

    I guess I'm looking for additional ideas and or opinions to finish this motor. I have a 1976 El Camino to put it in, but have had some thoughts about selling it and maybe buying an older Cutlass(68-72 or 83-87) or similar body style. The El Camino would likely be a strip car just for fun, but I kinda want a sleeper for the street/strip. Decisions Decisions.

    I know Angela Desmond is back in the fold and I would love to hear her views as well as input from the rest of the big block crowd. Lets hear what y'all have to say, and don't hold back. You won't hurt my feelings and I'll be the first to admit I'm a novice at best. Thats why I decided to do this. I want to learn a bit on a budget and if I have some fun, I would like to shoot for a bigger project with a bigger price tag.

    Thanks for lookin and let me know what y'all think.

    Don

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    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: 425 CI build options and opinions

    Hi Allante,..I wanted to go with bigger valves on my 425 but Al at MTS said the bore itself would end up shrouding the valves, so even though I opened up the chambers to unshroud the valves, it would be a moot point.What intake did you get from someone on this site; the Edelbrock 2-plane? I asked Lux Hauler in another thread what he thought of the Petronix HEI conversion, 'cause I'm using the same one and I just wanted to hear a little feedback on it, but he hasn't answered yet...If I had to buy the motor, I probably would have gone with the 500 or 472 but I got the 425 for free, so the price was right..You didn't mention what carb you're going to use.I have 2 Q- jets. One was a Holley rebuild that I rebuilt again with new gaskets and needle and seat, and the other I did all the trick Hi-Po modifications mentioned in the Haynes Rochester manual.I'll probably try to start the engine with the stock carb, as I don't have the trick rebuild dialed in perfectly to the motor and I don't need starting problems with a new cam in there...

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    Allante North *'s Avatar
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    Re: 425 CI build options and opinions

    Hello Angela,

    I got my intake from someone on the other Cadillac board. It is a 500 CI dual plane and it weighs a ton. It would make a good manhole cove, but I hope it helps the 425 breathe a bit better. I haven't decided on a carb yet, I am leaning towards a Holley 750 Double pumper, but even that is not carved in stone.

    I didn't want to bore it out because it looked so good when I tore it down. This is a learning project for me, and I don't want to spend big bucks on this motor. I do want to learn as much as I can so that if I decide to build another it should be easier and parts easier to find. I haven't decided on a cam yet either, but I would like to consider options there. The 425 has very few aftermarket performance parts available. I hope some members here can offer some input.

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    lux hauler is offline Cadillac Owners Connoisseur
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    Re: 425 CI build options and opinions

    Maybe someone who knows will chime in but from what I've read, the 425's big problem is it's lack of air flow. The heads just can't flow what the motor needs.
    I've wondered.....people have used the 425 heads on 472/500 engines to boost compression but found that top end suffered because of the lack of air flow. Can the 472/500 heads be used on a 425 to get more flow?
    Anyhow, I say put it in......run it and have fun. If you find later that you want more, drop in a bigger motor.

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    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: 425 CI build options and opinions

    The ports are smaller than the 472/500's...I think for your cam, Allante, you shouldn't go with too long of a duration if you have the stock compression ratio,,and I found out the hard way that compression can't be boosted beyond 11.1 without smacking the piston into sparkplugs or valves..I wish I had big bucks to experiment with connecting rod lengths and stroke on the 425s to see if there is a better ratio between the two.I'm lucky I had the money to pay the machine shop P.S. if you have a Desktop Dyno, Allante, try running the different cams offered with the set-up you have and see what results you get..if you don't have D.D. I'd be happy to run them for you.

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    JWalker is offline Cadillac Owners Member
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    Re: 425 CI build options and opinions

    Quote Originally Posted by Angela Desmond
    The ports are smaller than the 472/500's...I think for your cam, Allante, you shouldn't go with too long of a duration if you have the stock compression ratio,,and I found out the hard way that compression can't be boosted beyond 11.1 without smacking the piston into sparkplugs or valves..I wish I had big bucks to experiment with connecting rod lengths and stroke on the 425s to see if there is a better ratio between the two.I'm lucky I had the money to pay the machine shop P.S. if you have a Desktop Dyno, Allante, try running the different cams offered with the set-up you have and see what results you get..if you don't have D.D. I'd be happy to run them for you.

    Any 472-500 intake is better than whats on a 425.
    seems to pick up a 368 even.
    spend your money on headwork
    you dont need as big a valve as you think to get air thru the 425 heads
    there are several 2.08 and 2.10 valves that work in a 425 that will pick up the airflow on the heads alot.
    dont concern yourself with compression either it , can be done without it

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    Allante North *'s Avatar
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    Re: 425 CI build options and opinions

    Thanks for the reply's. I spoke with the machine shop today and we agreed to proceed with head work. He suggested we use 2.08 intake and 1.625 exhaust valves that are stainless steel. He advised that he would use springs from a Z-28. I priced valves from MTS and these are cheaper and he feels comfortable with doing the work. Didn't plan on decking the heads too much, but am open to suggestion if anyone wants to chime in. The actual work wont beging for a week or so.

    I am considering shaft rockers if anyone know of a source thats affordable. Are other applications likely to work on a Cadillac? Cam and Carb will be the next considerations.

    I plan to drop it all in an El(Hell) Camino for the present, but it would likely be just a toy for the strip. I want a street strip car and am considering an older Cutlass sleeper. Thanks for all the replys and keep em comming. Not many 425 builds out there and information shared here can benefit several others.

    If all this works out well, I would like to strart thinking about trying a 500 next, and will spend the bucks on it. This one is a semi econo build just for fun so please keep that in mind, but don't hold back on the suggestions.

  9. #8
    JWalker is offline Cadillac Owners Member
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    Re: 425 CI build options and opinions

    first off dont use z28 springs
    99% of them are junk and lose a plie of installed pressure within months of installing them.
    the 1% that are good cost too much for a 1.26 diam stock replacement spring.

    if your going to shaft rockers dont use the stamped ones unless you want to lose lift....stick with the stockers for what your doing save the shafts for a big engine with some spring pressure.
    try the cast ones from rockerarm specialist
    same place everyone buys rockers from

    the 2.08-1.6 combo works fine

    i can suggest some springs and a cam for you but youll have to contact me off the board as they think im advertising or something.

  10. #9
    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: 425 CI build options and opinions

    I'm using the shaft system that MTS sells; not the roller system, the stamped system. I think JWalker is right about losing lift with these guys. They are advertised at 1.7:1 but my initial readings came out to less than that. I'm waiting for a different attachment for my dial indicator 'cause the way I had it set up was throwing the geometry off on the readings, then I'll know for sure.The springs I used are for the MT15 cam that Al sells and they have a bit too much pressure for my liking but that cam has a good deal of lift without a great deal of duration, so the ramps are pretty steep and need a spring that will keep the lifter in contact with it, but I know this is going to create a lot of were on the valve guides.Thats one reason for going with bronze inserts. With larger valves you should definetly do some pocket porting to open up that area just under the valves. That spot is choked to death on the 425.Also, unshroud the valves in the chamber. The stock valves are right on top of the sides of the chamber as it is and kills flow.But then you'll have a larger chamber than you started with.They are listed at 96cc's and mine measured 116 after opening up the valves area.

  11. #10
    lux hauler is offline Cadillac Owners Connoisseur
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    Re: 425 CI build options and opinions

    Here's the rocker set-up that JWalker is talking about.

    You can find the part numbers and a little info at their web site.... http://www.rockerarms.com

  12. #11
    Angela Desmond is offline Cadillac Owners Fanatic
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    Re: 425 CI build options and opinions

    That IS a sweet looking set-up !!!

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