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Thread: Let The M73 Swap Begin!

  1. #51
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    I dont have any experience with these engines, but it looks like the M73 has a higher compression ratio than the M70, 10.0 vs 8.8. If the combustion chambers aren't radically different between the two heads where you can maintain the higher compression ratio using the M70 heads on the M73 block then I would definitely run the M70 heads with the Shrick cams. I wonder what the piston tops look like in each engine? Did they change the piston design to up the compression ratio or did they reduce the combustion chamber dimensions?

    Maybe there is some improvement to gain with porting, polishing, and a valve job.
    Last edited by JDP530; 12-30-2017 at 02:22 PM.

  2. #52
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    Quote Originally Posted by adwebinc View Post
    And you have already jumped way out in front! Hoping to make some progress this weekend.

    Still really need to decide on which way to go with the heads. @dragon850 - do you have any performance numbers on the tune for the M73 heads? I now have a set of M70 heads and Schrick cams in my garage along with the full M73 donor.

    Still planning on the leak down test on the donor to confirm I have a good one (I suspect it is).


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    Absolutely no point to do the m70/73 combo. You can get aprox 360hp out of M73...

  3. #53
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    Quote Originally Posted by dragon850 View Post
    Absolutely no point to do the m70/73 combo. You can get aprox 360hp out of M73...
    And all I want to know is how?
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  4. #54
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    Quote Originally Posted by 72tpik5 View Post
    And all I want to know is how?
    DYNO, Imulators, INPA+Tuner Pro

  5. #55
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    Quote Originally Posted by dragon850 View Post
    DYNO, Imulators, INPA+Tuner Pro
    Well that’s definitely over my head! I can open up a DME and swap a chip out though.


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  6. #56
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    Quote Originally Posted by dragon850 View Post
    DYNO, Imulators, INPA+Tuner Pro
    Standing by. Looks like I will need a shopping list. Maybe you are talking about a M73 being run by M70 electronics?
    Last edited by 72tpik5; 01-01-2018 at 01:25 PM.
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  7. #57
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    Quote Originally Posted by 72tpik5 View Post
    Standing by. Looks like I will need a shopping list. Maybe you are talking about a M73 being run by M70 electronics?
    Yes, the entire M73 block (save a few bits from the M70), but running on the stock M70 electronics. But still need a proper tune to get the most out of the m73.


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  8. #58
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    Let The M73 Swap Begin!

    Trying to perform a leak down test with donor engine. Is it even worth it on the stand though? I obviously can’t get it up to operating temperate and besides that it’s freezing here. While I have a heater in the garage, it’s still cold.

    Also, any suggestions on the best way to find TDC per cylinder. It’s not as easy as the 6s because of the angle of the spark plug hole to the piston top. so the screwdriver trick doesn’t seem to be very accurate.


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  9. #59
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    Quote Originally Posted by adwebinc View Post
    Trying to perform a leak down test with donor engine. Is it even worth it on the stand though? I obviously can’t get it up to operating temperate and besides that it’s freezing here. While I have a heater in the garage, it’s still cold.

    Also, any suggestions on the best way to find TDC per cylinder. It’s not as easy as the 6s because of the angle of the spark plug hole to the piston top. so the screwdriver trick doesn’t seem to be very accurate.


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    A bent piece of wire in the spark plug hole should get you close, and if the valve covers are off, you will see when both valves are closed. The leak down test will be consistent since all the cylinders are cold. A compression test is quicker and a reasonable way to compare the cylinders condition.
    Last edited by 72tpik5; 01-01-2018 at 04:01 PM.
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  10. #60
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    Quote Originally Posted by 72tpik5 View Post
    A bent piece of wire in the spark plug hole should get you close, and if the valve covers are off, you will see when both valves are closed. The leak down test will be consistent since all the cylinders are cold. A compression test is quicker and a reasonable way to compare the cylinders condition.
    I may just go ahead and pull the intakes and VCs off since I will be doing that any way.

    How would I run the compression test? Use a power drill to spin the engine? Sounds scary not knowing what the lubrication situation is inside.


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  11. #61
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    Quote Originally Posted by adwebinc View Post
    I may just go ahead and pull the intakes and VCs off since I will be doing that any way.

    How would I run the compression test? Use a power drill to spin the engine? Sounds scary not knowing what the lubrication situation is inside.


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    Personally I would not worry about doing the compression test or leak down. Start dissasembly

  12. #62
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    Quote Originally Posted by adwebinc View Post
    I may just go ahead and pull the intakes and VCs off since I will be doing that any way.

    How would I run the compression test? Use a power drill to spin the engine? Sounds scary not knowing what the lubrication situation is inside.


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    I assume it has oil in it, and the oil pump will pump oil as you rotate the engine. Use a battery and cables on the starter. You could take the plugs out of the other cylinders to make it turn easy.
    Last edited by 72tpik5; 01-02-2018 at 12:12 AM.
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  13. #63
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    are you aware that the M73 bell housing bolt pattern is different to the M70 and thus will not bolt directly to the M70 gearbox/auto

    I believe it has the same bolt pattern as the M50/2/4 range of motors so a E46 or E36 s6s420g will fit or perhaps a GS6-53DZ from a Euro 330 or 325
    I am 95% certain of this fact and i would like confirmation of this before i get too involved in the project i have in mind
    which is i have an 850csi with a seized motor in the meantime i intend on inserting a M73 with a manual trans which to all intensive purposes will look,go and preform nearly as well as the original csi with a small side benefit of the s6s420 having the better gear change than the csi 6 speed and without the worry of the csi box self destructing
    brian

  14. #64
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    Quote Originally Posted by adwebinc View Post
    I need all the help I can get, so come on down!!

    Santa came a little early and brought me this fun stuff:

    Not specifically related to the engine swap, but I am going Steffan’s route with my back boxes:



    And goodies for the engine swap. I found a set of NOS cams for a very reasonable price from a forum member. So, that complicated things on my decision. I’m very keen to understand the tune possibilities for the M73 heads. If I can get to the same place with the stock heads and a tune, then I’ll go that route.

    I do plan to perform a leak down test to see what I’m working with. That may help dictate which way to go.




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    Those Schrick cams will most likely not fit. To my knowledge Schrick only produces cams for M70 engines (and heads).

    - - - Updated - - -

    Quote Originally Posted by merlin840 View Post
    are you aware that the M73 bell housing bolt pattern is different to the M70 and thus will not bolt directly to the M70 gearbox/auto
    definitely NOT TRUE. The 4HP24 and / or 6-speed getrag fit to the M73 block without any problem. I've done it several times.
    The large oil pan has a slightly different bolt pattern on the M73, but not the bell housing. Maybe you are mixing up something here.



  15. #65
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    Yes it did seem a little strange but a friend who works at a bmw wreaker/repair shop tried to fit a 5hp30 from a m60 to an e38 750 e38 750il ams couldn't he said it was the 6cyl bolt pattern which would be awesome if it were true mainly because of the multitude of gearbox options that it would create but logically why would bmw do that
    I suppose I will find out for sure when I get the cheap 750il I bought delivered

  16. #66
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    M60 is different from M70 / M73. Not only in terms of cylinders. 😁

  17. #67
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    Quote Originally Posted by adwebinc View Post
    Trying to perform a leak down test with donor engine. Is it even worth it on the stand though? I obviously can’t get it up to operating temperate and besides that it’s freezing here. While I have a heater in the garage, it’s still cold.

    Also, any suggestions on the best way to find TDC per cylinder. It’s not as easy as the 6s because of the angle of the spark plug hole to the piston top. so the screwdriver trick doesn’t seem to be very accurate.


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    TDC is not super critical, I just stick a long plastic stick down cyl and observe valves to ensure on compression cycle. If you are off TDC the air pressure will rotate the engine. Expect to have to rock the crank a little for best results.

    Starter motor is the way to turn motor for a compression test.

  18. #68
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    Quote Originally Posted by wokke View Post
    Those Schrick cams will most likely not fit. To my knowledge Schrick only produces cams for M70 engines (and heads).
    Yes sir. Totally clear on that. I’m still trying to decide whether to go straight swap with M73 heads or the Wokke special M73 bottom end and M70 heads with the Schrick cams. Pros and cons both ways, so trying to make the best long term decision as this is a keeper car for me.

    If I can get into the same performance ballpark with the straight M73 swap, then leaning towards that. Just trying to understand how to get the tune on the M70 electronics for the M73 bits.


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  19. #69
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    Let The M73 Swap Begin!

    Quote Originally Posted by BMSman View Post
    TDC is not super critical, I just stick a long plastic stick down cyl and observe valves to ensure on compression cycle. If you are off TDC the air pressure will rotate the engine. Expect to have to rock the crank a little for best results.

    Starter motor is the way to turn motor for a compression test.
    Going to give the leak down test another shot with the VCs off so I can see when the valves are closed.

    Don’t have a flywheel or an extra starter to run the compression test.


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  20. #70
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    Quote Originally Posted by adwebinc View Post
    Yes sir. Totally clear on that. I’m still trying to decide whether to go straight swap with M73 heads or the Wokke special M73 bottom end and M70 heads with the Schrick cams. Pros and cons both ways, so trying to make the best long term decision as this is a keeper car for me.

    If I can get into the same performance ballpark with the straight M73 swap, then leaning towards that. Just trying to understand how to get the tune on the M70 electronics for the M73 bits.


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    I will make the tune available not a problem just a little dyno time.

  21. #71
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    Let The M73 Swap Begin!

    Quote Originally Posted by dragon850 View Post
    I will make the tune available not a problem just a little dyno time.
    Awesome. Well. Took your advice last night and started tearing in. Long story short, engine hoist I bought was missing critical parts... super frustrating. So don’t have it on the stand yet. Started tearing into top end.



    Most of the calves were pretty clean, but some carbon build up on a few. Recommendations for cleaning up with heads still on?







    I need to take better pics of the cams as this angle is bad. But heads look pretty clean. Cams look good. No visible scoring or unusual wear. The oil spray bar on these heads is certainly way better than the M70!





    Did some additional research on the exhaust down pipes for the M73. It does NOT look as though the later E31 M73 downpipes will match up to the ‘91 stock exhaust. So, will still need to address that piece.


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    Last edited by adwebinc; 01-02-2018 at 04:14 PM.
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  22. #72
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    Quote Originally Posted by adwebinc View Post
    Awesome. Well. Took your advice last night and started tearing in. Long story short, engine hoist I bought was missing critical parts... super frustrating. So don’t have it on the stand yet. Started tearing into top end.



    Most of the calves were pretty clean, but some carbon build up on a few. Recommendations for cleaning up with heads still on?







    I need to take better pics of the cams as this angle is bad. But heads look pretty clean. Cams look good. No visible scoring or unusual wear. The oil spray bar on these heads is certainly way before than the M70!





    Did some additional research on the exhaust down pipes for the M73. It does NOT look as though the later E31 M73 downpipes will match up to the ‘91 stock exhaust. So, will still need to address that piece.


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    CRC gasket remover works great on melting carbon.

    Sent from my SM-G925T using Tapatalk

  23. #73
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    Quote Originally Posted by dragon850 View Post
    CRC gasket remover works great on melting carbon.

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    Perfect. Will go buy a case. LOL!


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  24. #74
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    Quote Originally Posted by adwebinc View Post
    Perfect. Will go buy a case. LOL!


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    Buy 2 cans, spray on and let it sit for 15min. Then you can brush with a small plastic brush, it should wipe away then.

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  25. #75
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    I don't know about where you are (I'm in San Diego) but I was able to buy CRC gasket remover (red can) at my local O'Reilly's for ~$6.50/can. Only a few stores had it in stock but all can get it in a few days. This is much cheaper than buying online (no shipping).

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