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Thread: M50B25 block with M50b20 head?

  1. #1
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    M50B25 block with M50b20 head?

    Hi everybody! is there any problem with putting an M50B20 head on to M50B25 block with M52B28 strocker (crank, rods, pistons). and the turbo it?
    I mean will the b20 head work on b25 block?

    up

    anybody?
    Last edited by blink977; 12-12-2009 at 01:35 PM. Reason: Automerged Doublepost

  2. #2
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    It probably would but the m50b20 has smaller valves


    86 325es, 2.8L NV m50, S366, Megasquirt 2, e85, solid rear axle, 4 link, spool, T56 Magnum trans, 28x10.5w slicks, zip ties
    Best = 9.43 @ 149.15 mph, 1.30 60 foot 28 psi boost

  3. #3
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    Look up the valve sizes on www.realoem.com
    And youll see that its not a good idea
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  4. #4
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    Quote Originally Posted by someguy2800 View Post
    It probably would but the m50b20 has smaller valves
    And if I'm not mistaken, the CC is smaller, they use smaller HG (not thickness) as well, so I guess it won't be a good idea.
    Euro M3'87 NogaroSilver/Euro E34 M5 '93/Porsche 993 TT 97' Euro/Porsche 993 Carrera 95' Euro/Skyline R33 GT-R


  5. #5
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    Yes, cylinder bore is 84mm on b25 vs. 80mm on b20, but, what if I'll use b25 headgasket?

  6. #6
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    I don't think that the HG will be a problem as that the resulting CR will be.
    Few years ago I've made some measurements/ calculations for the reversed combo (M50B20 block + M50B25 head) and the resulting CR was 8.4:1 (using stock thickness HG). For your combo, to achieve a decent CR, most probably you will have to use some very thick HG (3.5-4mm) that will lead to other problems, like distribution alignment, etc ..
    Anyway, considering that the B20 head have smaller valves and less flow than B25, i don't see the point to work such a pair
    To help you with the calculations, if i'm remembering well, B25 head is 32cc and B20 - 28cc (you will better check again these numbers)
    Last edited by kaliE36; 12-14-2009 at 01:55 AM.
    ' 99 E36 316i Compact, 83kw

  7. #7
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    I will bring this post alive. Some people in my country put b20 head on the b25 block to have higher CR and power. The question is what CR with stock head gasket will be in this setup. On the E85 gas it could be interesting, high CR project.
    The valves are thinner on m50b20 nonvanos 30mm/7mm vs. 33mm/7mm at m50b25 nonvanos but the question is does it really matter at forced induction?
    You can put some hotcams on to let valves open little bit longer for more air.

  8. #8
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    That is the dumbest possible way to increase compression. This makes no sense at all and is entirely stupid.


    86 325es, 2.8L NV m50, S366, Megasquirt 2, e85, solid rear axle, 4 link, spool, T56 Magnum trans, 28x10.5w slicks, zip ties
    Best = 9.43 @ 149.15 mph, 1.30 60 foot 28 psi boost

  9. #9
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    That was not very constructive of me, let me elaborate. The cylinder head flow is the #1 most important determining factor in the power curve of a motor so any decrease in head flow will affect the powerband especially above peak torque. This is equally true of a turbo motor. If you decrease the head flow it just means you need to increase the boost pressure to move the same mass of air through the engine. When you increase the boost you add more heat to the charge air and put more resistance on the turbine shaft which increases exhaust manifold pressure, which makes the pressure ratio accoss the intake and exhaust worse, which in turn further reduces intake mass flow due to the loss of scavenging and makes the motor more detonation prone. A decrease in head flow is never a good thing. Yes you could make up for it with cams but if your going to spend the money on cams why not just put the cams on the good head?

    If you want more compression the correct way to do it is decreasing the combustion space by using the correct piston. You can have a piston built with the correct compression height and whatever amount of dome you want. If you wanted to there is an easy way to do this on the cheap. The non vanos m50b25 pistons have a shallow dish and they sit about .6mm above the deck at top dead center. However the M50b25tu (vanos) pistons are flat top and sit about .1mm above the bore at top dead center. This gives them a piston to head clearance of 1.65mm with the stock head gasket. If you simply buy a 1mm thick cometic MLS gasket you will net about 12:1 compression with stock pistons. Even cheaper yet you can just shave 1mm off the top of the block and use a stock head gasket to get the same results. There are a couple combinations that would work with an M52 crank as well to raise compression using stock parts.


    86 325es, 2.8L NV m50, S366, Megasquirt 2, e85, solid rear axle, 4 link, spool, T56 Magnum trans, 28x10.5w slicks, zip ties
    Best = 9.43 @ 149.15 mph, 1.30 60 foot 28 psi boost

  10. #10
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    e34 m50b25 turbo
    Of corse you are true with this. But it sounded like a good idea to me.. B20 heads are everywhere, but B25 non vanos head is almost impossible to get in these days. I dont trust vanos heads valves with forced induction. One m50b25 already KOed bcs of broken valve at 7k rpms... Actually Ive been building FI m50b28 stroker, and i will use m52b28 head. But Im still affraid of valve strength...

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