lost rpm with top end change.
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lost rpm with top end change.

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    Senior Member wpmxwb's Avatar
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    Default lost rpm with top end change.

    motor is a 396 with a 454 crank (434). i had small oval port semi-closed chamber heads, 10.5:1, with an edelbrock performer rpm and a 650 holey dp. I came across some iron merlin rectangle port open chamber heads w/ 1.88/2.30 valves, 9.5:1, i put a victor jr and a 750 holey dp on it. cam is a solid flat tapet, 258 duration at .050, .615 lift, 108 lsa. im running logs and 4" risers. pump is a fresch jg with a detailed bowl and b+ stainless impeller, loader and diverter.

    i was turning 5200 rpm now im down to 5000 rpm. im thinking i need to go with a smaller impeller to make up for the bigger ports and loss in compression. i know its running a little rich but not enough to loose 200 rpm in my opinion. any input is greatly appreciated.

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    Did you degree the cam? 2 degrees adv? whats the timing set at?

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    Senior Member wpmxwb's Avatar
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    cam is degreed in ar 106* lobe center and timing is 34* total

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    Senior Member jetboatperformance's Avatar
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    Ran this (similar Combo (434) in my RR boat with 2 600 Holleys on a low rize , MSD , 10 1/2 to 1 , iron rec ports , balanced , solid roller turned a AA 5100 , for comparison for ya Tom


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    Quote Originally Posted by wpmxwb View Post
    cam is degreed in ar 106* lobe center and timing is 34* total
    Bump the timing to 38 total all in by 3000rpm. Your logs are not helping but that should do it for ya!

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    Senior Member wpmxwb's Avatar
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    Quote Originally Posted by jetboatperformance View Post
    Ran this (similar Combo (434) in my RR boat with 2 600 Holleys on a low rize , MSD , 10 1/2 to 1 , iron rec ports , balanced , solid roller turned a AA 5100 , for comparison for ya Tom

    ive been told to go to an 850 maybe that could be part of the problem. but dam that makes my motor sound like its got a big problem.


    Quote Originally Posted by PS&B1 View Post
    Bump the timing to 38 total all in by 3000rpm. Your logs are not helping but that should do it for ya!
    ill try putting some more timing in it. i really wanna get rid of the hei and get an msd at the same time. ive got a set of bassetts that are going on as soon as i get the money to get them coated

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    Just Me snoc653's Avatar
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    I know logs aren't performance pieces. But, woud port matching the logs and heads work? Also are logs thick enough to do any radius blending (like with abrasive fluid blending)?
    So many projects, so little time

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    Senior Member wildstreak's Avatar
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    Just for grins I would try the smaller carb again just to see if the loss was carb tuning. The open chamber heads should work better than the old school heads even with less compression. Wouldnt take a lot of time or money to swap carb.

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    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by wpmxwb View Post
    ive been told to go to an 850 maybe that could be part of the problem. but dam that makes my motor sound like its got a big problem.




    ill try putting some more timing in it. i really wanna get rid of the hei and get an msd at the same time. ive got a set of bassetts that are going on as soon as i get the money to get them coated
    Reading back , you gave up a whole point of compression and went to much bigger heads/valves etc and your under carburated IMO ,could explain some Tom

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    Just my .02 Every move you made either reduced peak torque or moved the torque curve to higher rpm. The bigger heads really reduce flow velocity. I would borrow a larger carburetor and see if you get the rpm back before messing with the pump. Easier to do and less costly. I do not see the logs being the point of restriction necessarily. I have a 540 that turns 5,400 rpm with "A" berkely impeller with logs similar to those you are describing so pretty sure you can get the exhaust out. Sounds like you may be hurting on getting air and fuel in. As I keep reminding my wife, bigger is not always better.

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    Senior Member LOLYFE's Avatar
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    Quote Originally Posted by jetboatperformance View Post
    Ran this (similar Combo (434) in my RR boat with 2 600 Holleys on a low rize , MSD , 10 1/2 to 1 , iron rec ports , balanced , solid roller turned a AA 5100 , for comparison for ya Tom

    Tom, Dont mean to jack thread, but what manifold is that on that bbc, thanks much

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    Senior Member Widetrack's Avatar
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    What was the air like,temp,humidity? Higher air temps will reduce RPM anywhere from 100 to 300 RPM! Just keep that in mind before tearing into anything! Tom and Fastmarlene are on the right track as other that have posted too. Carb size may help some but it does sound like it needs the RPM to use the heads that you have!


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    I don't mean this to sound like I am being a smart ass...but...

    you aren't running NOS, you aren't running huge cubes or compression, and you aren't running big RPM....

    so why the desire to ditch the HEI?

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    Senior Member jetboatperformance's Avatar
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    Quote Originally Posted by LOLYFE View Post
    Tom, Dont mean to jack thread, but what manifold is that on that bbc, thanks much
    Early Offenhauser Josh scored from "Fleabay" sorry to the OP for the sidetrack on your thread

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