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Discussion Starter · #1 ·
Hey guys, i just purchased a b21a1 for my 89 lude wich had a b20a5 in it originally. I actually thought i was getting a b20a5, but i didn't check the engine code before i bought it and low and behold its a b21. :shock: I am going to do a complete rebuild on this motor. so here are a few questions for ya

what ecu will i need for the b21?
will the tranny bolt up?
what if any are the best mods for the b21?
should i build to turbo?

any help is greatly appreciated!
 

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i would stay away from turbo...you cannot use forged pistons in a b21 block due to the FRM sleeves(unless you resleeve it with cast iron sleeves). as long as you have the same clutch kit for that year tranny it will bolt up and work properly. as for mods: lsd intake,full exhaust(header,cat,cat back) and then talk to motoxxxman about a tunable ecu(he can tune it if you want him to),cam gears, etc.

search around for info! there is much more than what i have touched on!
 

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I think you have to use all the external parts from the 20A5 if you are going to stay OBD0. Someone will tell us the whole story! SNOKING
 

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what year is your car??? you want to use the same ecu for whatever manifold you have. example: if you have a b21 intake manifold use a b21 ecu, people tend to have problems when using a b21 ecu for a b20a/a5(unless they swaped a b21 head then they are ok)

so if you are going from b20a5 to b21 i would recommend using a b21 computer...if your car was made before 1990 then in order to run a b21 ecu you have to do a conversion to the harness to make it from obd0 to obd1.

wait i just remembered, all b21's are obd1 so in order to run a b21 ecu you will have to do a conversion to the harness. HERE LOOK AT THIS http://www.preludepower.com/forums/showthread.php?t=287169&highlight=b21+conversion
 

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the only way to do the swap and have everything work as best as possible for the lowest cost possible, is to only use the b21 block and head.
use the b20a5: flywheel, tranny, exhaust manifold, intake manifold, wire harness, ecu, accessories, motor mounts, basically use all the b20a5 parts bolted to just the b21 block and head.

and no, building a b21 for boost is not a good idea at all unless you get the block sleeved as mentioned above. for boost you're better off just using a b20 block as it already has steel cylinder walls
 

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the only way to do the swap and have everything work as best as possible for the lowest cost possible, is to only use the b21 block and head.
use the b20a5: flywheel, tranny, exhaust manifold, intake manifold, wire harness, ecu, accessories, motor mounts, basically use all the b20a5 parts bolted to just the b21 block and head.

and no, building a b21 for boost is not a good idea at all unless you get the block sleeved as mentioned above. for boost you're better off just using a b20 block as it already has steel cylinder walls
as for easiest less expensive way to do it this is the best and only way to do it....but moto, will using the b21 block and head with all the b20a5 stuff bolted to it rob the b21 of its low end torque?? just figured i would ask since he is trying to get some added power.
 

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Discussion Starter · #9 ·
cool... well i have alot of options here. so low cost would be to bolt the b20 head on the b21 block and run it as is. could i not just use the intake and exaust maniflods form the b20 on the b21 head and block?

the head from the b21 is in great shape. but i have 2 good b20 heads sittin in the other garage. my car is an 89 si to answer the question above.
 

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Discussion Starter · #10 ·
sorry i didn't read moto's statement correctly. he is telling me to use the BLOCK AND HEAD from the b21 which is what i would like to do...
 

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Discussion Starter · #11 ·
as far as the build goes... i'm tearing it all the way down. matter of fact i'm almost there. are there any mods i need to do to the low end internals for dependability or power purposes? I know people do alot of mode in the b20 like b21 pistons and h23 rods... is there anything i can or need to do to this b21 while i have it down?
 

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A compression test would have been a good idea, but at this point, don't touch the bottom end.

The furthest I would go would be to pull the oil pan and be sure nothing looks abnormal.
 

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Discussion Starter · #14 ·
the cylinders need honing. If my camera wasn't dead i would post some pics. i need to go get a battery for it. I'm gonna go ahead and put new rings and bearings in as well.
 

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the cylinders need honing. If my camera wasn't dead i would post some pics. i need to go get a battery for it. I'm gonna go ahead and put new rings and bearings in as well.

really? if you have an FRM block that needs honing, you have a core motor that isn't worth anything. take it back, and get your money back.


your block is worthless and practically unrebuildable.
 

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yes it can be done i had mine done but it didnt clear all the taper so it needs bored over...there is a certain stone and honing process that has to be done. i think you have to use a 600 grit stone and only do like 15-20 passes on each cylinder with the hone spinning at a slow speed,dont quote me on it but there is a process. but i know you can not use a ball hone for sure!
 

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ok...let me rephrase that.

it can be honed.

it has be to honed with tools your normal local machine shop who is used to dealing with iron walls probably won't have.

it has to be honed with processes your normal local machine shop who is used to dealing with iron walls probably is not used to doing.


finding the machine shop with the specialized tools and who is willing to use the specialized process is possible....but not commonly financially feasible.

who knows...in your town, you might have the right shop.

in my state.....we don't.
 
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