my friends got a 91 lude with a b21a1 didnt know they made. whats the differnts in the b21 and b20? and are the older b20 blocks and same as newer ones?

 

Basically the motors in that car suck. The b21a1 only came in 90-91SI, and the years before that it got a b20a5. There was also carbed version of the b20a5. They both have 95mm cranks, but to get to the 2056cc of the b21 honda used a 83mm bore oposed the b20a5s 81mm. They are both closed deck blocks. The one thing that kills the b21a1 is its sleeves they are prone to blow by no matter what ya do. If your looking for a new motor look for a jdm b20a. Unless your gonna boost it dont bother doing anything to it.

 

The difference between the B21A1 and the B20A5 is the B21 is obviously 2.1L's and has a slightly larger bore at 83mm, as triple_s1x said. The B21A1 also has unique cylinder sleeves called FRM, or Fiber Reinforced Metal. This FRM contains basically Aluminum Oxide Alloy and Carbon Fibers to make it real strong, and lighter than the Iron-sleeves usually on engines. The B20A5 has iron-sleeves. The B21A1 has higher compression, from the taller pistons and larger bore, but not due to the combustion chamber. The combustion chamber on the B21A1's are larger than the Chambers on the B20A5's, at 51cc's per chamber vs the B20A5's 47.3cc's per chamber. The B21A1 also has a single runner intake manifold, vs the B20A5's dual-runner. The B21A1 has a slightly more agressive exhaust cam, most likely due to the extra displacement it has, more lift and duration to get the exhaust out of the engine easier. The B21A1 also has 1 4-wire heated oxygen sensor on the downpipe, vs the B20A5's 2 single-wire non-heated oxygen sensors on the exhaust manifold runners. I dont think there is much else to mention.
Oh the B20A3 is the dual-carberated SOHC 12-valve engine. And about the FRM sleeves, they are not known to "have blowby no matter what". What happens with these engines is the FRM sleeves are so hard and abrasive that they end up eating the piston rings and when these piston rings wear down, then you lose compression, start burning oil, and THEN you will get blowby. Usually FRM sleeves dont even need to be bored, or even honed. Usually these sleeves come out looking new and the piston rings end up looking like total ****, so you just replace the rings and seat them and you should be ready for another 80k miles of no oil burning, and good compression.
That's about it in a nutshell.