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Thread: M73 Heads Vs M70 Heads

  1. #1
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    M73 Heads Vs M70 Heads

    sure i posted this already but i cannot find the thread
    but apart from the obvious roller rockers what is the differences
    like can these heads be modified to flow better than the M70 heads or are they still the limited by the exhaust ports

  2. #2
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    Not sure but I looked at the M73 headers and they were more restrictive than the M70

  3. #3
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    The manifolds are definitely smaller diameter, I measured them side by side.

  4. #4
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    Page 22 of the M73 Engine Training Reference Manual suggests otherwise.
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  5. #5
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  6. #6
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    Merlin840, there is a local E31 owner that has done m73 head swap into a m70 and it is a very fast car. You might touch base him.

    Alex Molavi

    Quote Originally Posted by merlin840 View Post
    sure i posted this already but i cannot find the thread
    but apart from the obvious roller rockers what is the differences
    like can these heads be modified to flow better than the M70 heads or are they still the limited by the exhaust ports

  7. #7
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    Quote Originally Posted by 72tpik5 View Post
    Page 22 of the M73 Engine Training Reference Manual suggests otherwise.
    which is exactly why I purchased a set to install on my car. Then once I had the standard ones removed I could see that was definitely not the case.

    - - - Updated - - -

    https://www.bimmerforums.com/forum/s...-850CSi/page12 see post #300

  8. #8
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    Quote Originally Posted by Omega man 1969 View Post
    which is exactly why I purchased a set to install on my car. Then once I had the standard ones removed I could see that was definitely not the case.

    - - - Updated - - -

    https://www.bimmerforums.com/forum/s...-850CSi/page12 see post #300
    I would say hands on experience win over the factory publication any day. Thanks for the info.
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  9. #9
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    Why not go all the way and put the M73 engine in the car by slightly modifying it like WOKKE has done many times. You get larger displacement, roller rockers, better oilbar design and much more HP. Added bonus: you can run everything in M70 electronics. There is a great write up here about it if you haven't already read it.
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  10. #10
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    Quote Originally Posted by Omega man 1969 View Post
    The manifolds are definitely smaller diameter, I measured them side by side.
    Quote Originally Posted by 72tpik5 View Post
    I would say hands on experience win over the factory publication any day. Thanks for the info.
    The M73 stainless steel manifolds are larger internally, they are "necked down" at the inlet and taper off at the outlet. I have measured these has well, correctly. We had to pull this (M73) motor apart in BMW NA class back in 96' and measure then confirm the differences against the M70.

    Mark.

  11. #11
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    Quote Originally Posted by TxGR8White View Post
    Why not go all the way and put the M73 engine in the car by slightly modifying it like WOKKE has done many times. You get larger displacement, roller rockers, better oilbar design and much more HP. Added bonus: you can run everything in M70 electronics. There is a great write up here about it if you haven't already read it.
    I am pulling the trigger on a used m73 engine this week for this very project. I posted on another thread, but didn’t get an answer. Is it possible to use the M73 (from e38) manifolds and stock downpipes from the M73 850 to address the issues with mating up to the rest of my ‘91’s stock 850 exhaust?


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  12. #12
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    Quote Originally Posted by mandd View Post
    The M73 stainless steel manifolds are larger internally, they are "necked down" at the inlet and taper off at the outlet. I have measured these has well, correctly. We had to pull this (M73) motor apart in BMW NA class back in 96' and measure then confirm the differences against the M70.

    Mark.
    Being a 4 stroke a larger internal with smaller inlet will restrict power. Only 2 strokes gain from expansion pipes.

  13. #13
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    Quote Originally Posted by Omega man 1969 View Post
    Being a 4 stroke a larger internal with smaller inlet will restrict power. Only 2 strokes gain from expansion pipes.
    The difference in the inlet outlet ports is not enough to make considerable gains with this motor. The goal with this engine was linear power delivery and driving comfort. Even when fitted with headers the gains are minimal unless you are using larger cams, modified heads etc, even then the gains are small.

    Mark.

  14. #14
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    Quote Originally Posted by mandd View Post
    The difference in the inlet outlet ports is not enough to make considerable gains with this motor. The goal with this engine was linear power delivery and driving comfort. Even when fitted with headers the gains are minimal unless you are using larger cams, modified heads etc, even then the gains are small.

    Mark.
    The only way it would make more power compared to the bored out M70 pipe is if the M70 pipe was too big and reduced exhaust velocity. It was too small and the M73 was smaller still... at the inlet, which in turn will cause more back pressure. The only advantage the M73 would have would be in the low to mid range where velocity will be increase and volumetric efficiency increased as a result. At wide open throttle and max power the M73 pipe will be less effective......than a bored out M70 pipe.

    Both heads are restrictive enough as is, they don't need any additional back pressure to make things worse.

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