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  1. #1
    Registered Gold Member
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    looking for 420 megablower info

    I can't really find any info online and I know there are a few here that run these.

    What is the capability of these, max boost and HP on a BBC?

  2. #2
    Platinum Member Platinum Member JRider's Avatar
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    I dont think they are being produced anymore. You may be better off with a 671 or 871 as they are very plentiful and parts are readily available. 420 is the displacement, a 671 is 411cid and a 871 is 436cid.

  3. #3
    Registered Gold Member
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    The problem is I already have it and it is being rebuilt right now. Can't afford to upgrade this year but that is planned for in the future. Just trying to figure out how hard they can be ran. Somewhere I saw someone say they had 1070hp. Thanks though.

  4. #4
    VIP Member VIP Member MILD THUNDER's Avatar
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    I have them on a pair of 468's, good heads/cam combo, 6psi of boost, 805hp, 7 percent underdriven. Mercury used them on a 800SC, 18 percent overdriven. The 800sc was pretty much a 572CI station wagon engine with a blower. Very reliable, and very mild.

    Ive seen builds with them supporting 900+hp. According to the info pre-programmed into my desktop dyno, they show the 420 B&M to be more effecient than a weiand and BDS 871. If that means anything.

    Ive seen lots of guys write the B&M stuff off, as being lame technology, etc. B&M probably produced more blowers than anyone from the early 80's till the turn of the century. They cast and designed their own superchargers, not copied a GM diesel compressor. There was some engineering involved.

    So, go for it. Its a good piece. Just keep an eye on the splined coupler in the snout, make sure to keep it greased. The snout isn't as strong as a GMC blower, so try to stay away from the cogged belt setup if possible. The ribbed deal is just easier on things.

  5. #5
    VIP Member VIP Member MILD THUNDER's Avatar
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    also, the original question is kind of a loaded one. MAX HP and Max BOOST on a BBC is very vague.

    Are we talking a 396CI with GM oval port heads, or a 632CI with Big Chiefs???

    Do you have a engine that would make 750+ HP on just the motor, or a engine that makes 500HP and your looking to boost it heavily to make 1000HP.

  6. #6
    Registered Gold Member
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    Thanks Mild Thunder,
    Your numbers sound good. what heads are you running?
    As for mercury that sure was mild and mild boost they were running too. In the chart I have the blower can go to %27 over.

    As for the belt I am using the 16 rib.

    I guess that was kinda vague, I am running a 540 with AFR heads 8.2 comp and Bob Madara cam.

  7. #7
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    dj,
    I did a 540 with one, probably at least 12 years ago now. It made close to 1000 on the dyno at about 8 lbs boost if I remember correctly. The dyno I was using was being pushed to the limit so I am not real certain how accurate the hp was, but according to the jet pump builder, it was at least 1000, determined by the rpm it would turn the pump to and the speed of the boat it was in.

    The engine was actually pretty mild, pump gas hydraulic roller cam. World products Merlin block and cast iron heads. 2 750 Holleys that were built for the application. I have the build info, dyno sheets and details at the shop.

    This was a 540 so a lot depends on the cubic inches of the engine too.

    Bill Koustenis
    Advanced Automotive Machine
    Waldorf Md

  8. #8
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    They were great blowers but Colt Industries had already acquired Weiand, then B&M... to many similar products and when it all sold B&M blowers died.

    I think this is the progression of change... long time ago

    In any case we ran the 420s during the mid 90s on average of 15PSI and up to 300% over drive with chillers on T/E and M/E drag boats. They worked great.... better than the GM styles.

  9. #9
    Registered Gold Member
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    Thanks Bill,
    I was told with this blower I wouldn't see over a 1000 and that is what I am shooting for. This too is a 540.

    Tunertech,
    the better than GM seems to be what I am hearing now, but still so many say even the 8-71 is better yet the efficiency is better with the 420. The place I read the 1070hp from I think also mentioned the 15psi but that was the only place I have seen those numbers. Good to see others have experience in that range. The guy rebuilding mine was not sure of certain numbers but thought that they could spin like 16000 rpm max. So that goes with your 300% over.

    Here is all I have as for charts.
    Attached Files Attached Files

  10. #10
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    DJ

    I was a good unit. The only thing you have to use caution in is the installation of the intake.. make sure torque is accurate, check flatness of the SC mating surface to the blower and torque the 4 .500 bolts as specified. Other wise you can run into issues caused by a cold warp install. re-check torque after 1 or 2 full heat cycles.


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