500 head swap question
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500, 472, 425, 368 Discussion, 500 head swap question in Cadillac Engine Technical Discussion; I was reading an article in an old copy of Hotrod magizine the other day and they said that putting ...
  1. #1
    pdoke2003 is offline Cadillac Owners Member
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    500 head swap question

    I was reading an article in an old copy of Hotrod magizine the other day and they said that putting the early high compression heads on a later low compression bottom end was a great way to build power but that it was "like building a time bomb" unless you wanted to upgrade the pistons and rods from cast to forged.

    what I was wondering was how accurate is this? and if it is accurate, why? did the early engines with the higher compression come stock with forged pistons and rods or what? also is there any difference between 472 and 500 heads?

    If any of you read my previous post and are wondering, yes I am still working on swapping the caddy 500 into the wrangler. I'm probably picking up the engine this weekend.

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  3. #2
    Ted in Olympia is offline Banned
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    Re: 500 head swap question

    Early engines (71-73) had the same compression as later 500s (74-76) but the early ones had a big dish in the piston. The later engines had pistons with a very small dish, almost a flattop.

    When you put the early heads on the later engines with flattops you end up with 12 to one conpression that sounds great and has lots of power.

    You also end up with an engine you cannot run on pump gas no matter what you do. I have tried everything and ended up with bent valves and broken pistons.

    Just don't do it. The safe limit is 10 to one and you can do this with custom pistons.

    http://www.msnusers.com/CadTBird/sho...to&PhotoID=120

    TED

  4. #3
    pdoke2003 is offline Cadillac Owners Member
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    Re: 500 head swap question

    ok that makes sense. I read some where that the early ones came with higher compression from the factory and they lowered the compression to allow them to run lower octane gas, I guess that was wrong.

    Ok, now I just need to know if there is a difference between 472 and 500 heads.

  5. #4
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    BluEyes is offline Cadillac Owners Enthusiast
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    Re: 500 head swap question

    Actually, the '68-'70 engines had 10:1 compression, which was then lowered in '71 to run on low octane gas. But, the '68-'70 heads had the same combustion chamber as the early low-comp engines they just used a different piston. The heads were redesigned later to allow the use of the flat top piston instead of a huge bathtub.
    Either way, swapping large chamber to small chamber heads puts the compression up in race gas only territory (or propane, if you're adventurous)

    I'm pretty sure that the 472 and 500 heads are identical, just like the blocks and everything else except the pistons and rods.

  6. #5
    Ted in Olympia is offline Banned
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    Re: 500 head swap question

    472 and 500 heads are the same as long as you have the same series.

    The 68-73 (76cc) is the same and the 74-76 (120cc) is the same.

    Some have smog ports and some do not but the ports and chambers are the same. They quit making the 472 in 74 and this is a odd year for the 472. It is the only year with flattop pistons and the 120cc heads.

    Early 472 (68-70) and 500 (1970, the first year) had higher compression but this was due to a little less dish in the piston, not a different head.

    As a side note the 425 also had the same size ports but 108cc chambers.

    I hope this makes sense.

    TED

  7. #6
    pdoke2003 is offline Cadillac Owners Member
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    Re: 500 head swap question

    thanks, to both of you. I will be reading your other post on the topic ted but do you know what kind of compression and hp/torque increase I could expect by putting 425 heads on a 500 lower?

  8. #7
    pdoke2003 is offline Cadillac Owners Member
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    Re: 500 head swap question

    nevermind, after reading that other post I dont think thats the option for me. I think I'm going to go with the build up described here: http://www.cadillacowners.com/torquepig.html with the exception of swapping the low compression heads because I cant afford the forged pistons and rods right now.

  9. #8
    Ted in Olympia is offline Banned
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    Re: 500 head swap question

    I have that same combo in my Cad-T-Bird now. If I had to do it over again I would go with a little less cam. The MT15 runs great but it a bit lumpy at idle for my taste. It's great for country driving but going through town with lots of red lights gets on my nerves. If I did it again I would go with the MT10 or similar cam from Comp or Crane.

    As a side note the MT15 cam will hit the pistons with the 76cc heads on a later block, I don't know how Hot Rod did it. It may be because I had 2.19 valves and theirs was stock. Also i think that with the stock 8.5 compression, this is a bit low for the MT15 cam.

    If you run the 120 cc heads you can run as much lift as you want and not worry about the valves hitting the pistons.

    I think that I will port the set of 425 heads that I have and give it a try. I currently have all the parts I need to put together a bullet proof bottom end, I just have to take a break from parting out Eldorados and do it. But I have 15 of them to part out right now, about a years worth of parttime work.

    TED

  10. #9
    pdoke2003 is offline Cadillac Owners Member
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    Re: 500 head swap question

    my jeep won't be doing much city driving when I'm done with it so I think that set up will probably be fine for me. I'm thinking about trying to find some of the "bathtub dish" pistons, if I can get a decent price one them, and some 76cc heads to at least bump it up to 10:1 compression. that should give me a decent boost in power with out having to worry about exploding pistons. If I can't find any I'll probably just go with the stock flat top pistons and 120 cc heads.

  11. #10
    caddycarlo is offline Cadillac Owners Member
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    Re: 500 head swap question

    I have run the 76 cc heads on a stock 76 lower end for years and over the years of hard use with blower and nos the only thing that has happened is some small cracks at the pin bosses and I think that is more from heat then pressure ....... the motor has made 808 hp and 1024 ft lbs with 12 to 1 and 8 lbs of boost and nos ......... the stock pistons are a very tough part .... as compresstion goes up you will have to watch timing and no piston will hold up well to a impact with a valve so you have to watch valve to piston clearance and spring pressure .....

  12. #11
    Nick Campagna is offline Cadillac Owners Member
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    Re: 500 head swap question

    To Caddycarlo:
    What does the timing curve look like, I'll assume 91-93 octane pump gas, and will this live on the street without a blower? I know it depends on the dynamic compression ratio, so what cam and/or cam advance are you using to make this live?

  13. #12
    caddycarlo is offline Cadillac Owners Member
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    Re: 500 head swap question

    I think it is like 32 total and I run no vac advance ..... the cam I am running is the largest crane listed at 8 degrees advanced ...... the dynamic compresstion with a stanard cam and VE of 85% would only put the compresstion at just over 10.5 to one ...... the other thing I did was deburr the chambers and pistons and cut back the ground strap on the plug which was also colder to keep the hot spots down to a min ..........

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