I'm at the stage of rebuilding my m20 turbo. i know with the late spec (low comp) pistons my compression ratio will be 8.8:1. Has anyone machined the tops off their pistons in order to reduce the CR? if so what sort of cut am i looking to take in order to hit the 8.0:1 or 8.5:1 region? (bottom end is standard)
cheers.
machined m20b25 pistons
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Gunni
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Why do you want lower compression ratio?
8.8:1 is actually GREAT for turboing.
8.8:1 is actually GREAT for turboing.
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HairyScreech
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^^ as above, the 8.8 provided by the stock pistons has been proven to be VERY effective when turbocharged.
The chamber design is very fast burning for a 2 valve design and even 8.8:1 has been combined with a lot of boost without detonation.
personally i would say changing the geometry of the chamber on a b25 head and pistons could result in the chance of detonation increasing above that at 8.8 with the chamber intact due to the creation of crevice volumes that produce late combustion of the mixture held in that area.
if you really want to do it then the best way to go about it would be to leave the bowl the same diameter so the squish bands are unaffected and lower the center of the bowl to create a larger but more spherical chamber.
The chamber design is very fast burning for a 2 valve design and even 8.8:1 has been combined with a lot of boost without detonation.
personally i would say changing the geometry of the chamber on a b25 head and pistons could result in the chance of detonation increasing above that at 8.8 with the chamber intact due to the creation of crevice volumes that produce late combustion of the mixture held in that area.
if you really want to do it then the best way to go about it would be to leave the bowl the same diameter so the squish bands are unaffected and lower the center of the bowl to create a larger but more spherical chamber.
2.8 development thread http://www.e30zone.net/modules.php?name ... c&t=170822
m3.3.1 m20 thread - now running, chip needed - any volunteers?
http://www.e30zone.net/modules.php?name ... =viewtopic&
m3.3.1 m20 thread - now running, chip needed - any volunteers?
http://www.e30zone.net/modules.php?name ... =viewtopic&
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b12rlw
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Gents, thats all the info i need!! It was a thought that was planted by my brother. on with sticking it all back together as is!! Thanks.