[Kosak]

Ive got a B21a prelude 91 motor for free and I was thinking to swap only the block in my LS just for fun.

I know you will tell me that the B21a suck, but I have a LOT of time to waste so, I think that ill like working on this project.

I want to know if motor mounting point are similar to the b18b ? If they are different, do you know if there is a kit available for the swap ?

Also, do the b18b tranny fit on b21a block ?

I know ill have to match the head to the block with custom head gasket. I think I should be able to get this thing working.

Thank a lot,

Kosak

 

[MichaelDelaney]

mounts same: yes

tranny ok: yes

the head is the same as your b18b and port it, use a skunk2 IM for the b18b and port it out, not your b18b stock IM or the B21's.

 

[MichaelDelaney]

for future reference about swaps that need mount adapters please go to the Hasport site.

 

[cluelessmale]

is that block better then a b20_?

 

[Kosak]

Thank a lot MD, I was very distressed from reading thread in the www.preludepower.com forum since they were somewhat negative about swap by saying B18B/B18A are totally different from B20A/B21A. They were saying that no B18 can fit into their 3g prelude. But they were never getting into detail about this. Also, I found that they like to pretend things they never experimented, so now, comming from you im a lot more confident about this project !

cluelessmale:
I dont know much, but from what I heard; B21A are worst than B20A are worst than B20b/B20Z. The reason given is, the poor rod ratio 1.49 in the B21A and B20A. B21A/B20A seems to have problems of oil leaking from poor sleeve and piston rings material.

Dont take this as fact, this is only informations ive collected from others on the net.

B21A : 83mm bore, 95mm stroke
B20A : 81mm bore, 95mm stroke
B20B/B20Z : 84mm bore, 89mm stroke

MichaelDelaney:

the head is the same as your b18b and port it, use a skunk2 IM for the b18b and port it out, not your b18b stock IM or the B21's.

The B21A head is the same as B18B head ? I though they were different, because B18B make 140 hp from 1.8 liter and B21A also make 140 hp but from 2.1 liter.

Can you get into more detail ? Into difference and similarities, and the why of your previous statements ?

 

[MichaelDelaney]

personally if you're not popping on a VTEC head with this and using the proper machine work to make it fit, I wouldn't bother. But you specifically asked if you could use the stock mounts without having to weld on new mounting points (or use a hasport aftermarket mount kit) and hook up the tranny from the 3rd gen 1990-91 Prelude Si motor: I answered those questions.

the B21A has a worse rod ratio than the B18B.:

141.7 mm rod / 95 mm stroke = 1.49

vs. the B18B 1.54.

the B21A makes the same peak power 140 whp and powerband location (low).

So why do it?

The head itself is larger in chamber volume at 51 cc compared to the B18B's 45cc but the flanges should align for the IM. The B21A's IM is like the B18B's in that it's a long narrow runner IM oriented for the low rpms.

This is why you want an aftermarket IM that has shorter and wider runners which shift the powerband up if you are doing serious performance work with it.

Your other alternative is to just only use the longer stroke crank from the B21A (stroking the motor) into your B18B block and add a deckplate with a longer rod to correct the rod ratio into the low 1.5's. Or you can get an Endyn long rod and piston kit which moves the piston pin up to the oil ring to allow for a longer rod but still keep the deck clearance inside your B18B's deck height . The B21A is the crank Brian Crower used here with a deck plate on a B18B block.


I think we covered that here (common topic) already many times before (article page) (also search under forum body and use B21A crank, or deck plate, or stroker for more recent threads on the topic like this one last month about the 155mm rods).

But you were keen to go ahead and plop this in and asked is it doable. You didn't ask if it's any better than a B18B.

 

[Kosak]

... and hook up the tranny from the 3rd gen 1990-91 Prelude Si motor...

I meant b18b tranny on a b21a block, is that what you were thinking ?

Thank

 

[sickbling]

wouldnt you be making the teg slower? not trrying to diss or anythin just trying to learn something new i give you props for trying something diffrent though

 

[Kosak]

wouldnt you be making the teg slower? not trrying to diss or anythin just trying to learn something new i give you props for trying something diffrent though

Slower, I doubt. I hope of gaining some low end torque, where I mostly drove on street. I dont think that will yield something really faster than stock if I use the same head/IM/header combo, because the high end torque will just drop faster in the rpm band.

 

[MichaelDelaney]

wouldnt you be making the teg slower? not trrying to diss or anythin just trying to learn something new i give you props for trying something diffrent though

I personally would not do that swap Kosak but if your heart is set on it, that's up to you. It's not a winning proposition especially if you want some serious cams that rev past 7000 rpm ( a 1.49 r/s is trouble waiting to happen at above 7000 rpm).

if you want to do the swap properly, I'd just use the B21A crank (a great deal if you got it for free) and use the right rod and piston to get 1958 cc using your stock b18b block that's been notched.

 

[sickbling]

so why are you doing it? your not gonna gain anything, so wuts the point?

 

[Kosak]

I personally would not do that swap Kosak but if your heart is set on it, that's up to you. It's not a winning proposition especially if you want some serious cams that rev past 7000 rpm ( a 1.49 r/s is trouble waiting to happen at above 7000 rpm).

if you want to do the swap properly, I'd just use the B21A crank (a great deal if you got it for free) and use the right rod and piston to get 1958 cc using your stock b18b block that's been notched.

Your right, my heart is really set on it .
I dont have much money, so I dont plan to buy aftermarket cams with more durations to get the full potential of the 2.1 liter. I'll stick with stock cams and stock 7000rpm redline. Ive been given the block so my plan is to do the swap, have fun doing it and run the block till it blow up since they say its not a long lasting block.

The reason I want to use the b21a block instead of the b21a crank into the b18b block is from the fact that:
I have to notch the b18b block to get it fit

Also for one of your statement in the "Engine Package : Swapping Parts For Power" article :

"After doing these clearance adjustments for the new crank, there is even less lubrication in an engine that needs even more lubricating with a poorer rod ratio."

If I use the b21a crank into the b18b block I must buy custom pistons with relocated pin and get some connecting rods if I want better rod ratio.

If I use same b18b piston, I must buy b16a connecting rod, and that combo will yield even lower rod ratio :
1.41 for b16a rods
1.49 for stock b21a

I dont have much negative fact of using the b21a block into my teg, or I must be missing somethings ! From what facts in doing this swap (complete block into teg) should I be warned before proceeding ?

 

[Louse76]

What is the stock redline for the B21A engine as shown on the stock tach?

Logically it should be lower than the B18B, but I am curious if it is shown as such.

 

[Kosak]

so why are you doing it? your not gonna gain anything, so wuts the point?

Low end torque + fun

 

[onepointate]

this is a bit off topic but i read somewhere that the b21a intake cam was more aggressive than the ls intake cam. is there any truth in this

 

[MichaelDelaney]

I think it'll be more helpful if you get the specs from a dealership parts person. No-one here will know since we aren't a Prelude site. Knowing the B21A block info is one thing but cam specs for cams that are rarely used by us would need people to go out of their way and get the specs (remember to get the max lift, duration @1mm lift, and intake cam's valve closing ABDC spec). How do you think I got to collect and gather the database that I have? It's not from waiting for other people to tell me...usually if I did that then I'd never get the answer. Sometimes on these rare info things you (as the person asking a very important question) has to be the person who goes and gets the info. No guessing. No relying on hearsay and rumor. If you post the specs there's facts and not opinions.

 

[Kosak]

From the preludepower forum :

"I just ordered a head gasket for a B18 and one for the B21 to compare."
"Okay, I just compared the head gaskets to each other. They are not even close. Head bolts are further apart by about 1/4 inch. ... Way too much modding required."

Can someone validate or contradict these statements ?

Kosak

 

[MichaelDelaney]

the headgaskets are different because the bores are different:

B21A1 83mm

B18B 81 mm


even the B20B/Z guys with an 84mm bore go to a different cometic headgasket when they do the B20VTEC's and don't even use the B18B head gasket. In the B20B/Z the head is identical to the B18B.

I made a mistake on my first reply when I said the B18B head was the same as the B21A head. I was thinking about about the IM. The B21A head IS different but the flange bolt holes will accept a B18B aftermarket IM flange. You'll need an aftermarket IM, since the B21A IM is a choke point for an upper rpm powerband which is what you need if you want a performance engine (not a low-mid powerband location which is what the B21A is oriented for). The head volumes (if you know the relevance of that) are completely different whereas if you look at a B20B/Z the 1.3 cc extra head volume comes from the piston dome clearances and not just the head chamber bore. Ask your friend over at the Prelude forum if he's taken those head gaskets over to machine shop to have the 1/4 in. offset on the head bolt holes slotted.


If you want to put this in (because it has a worse piston geometry and less power potential than the current engine you have), then that's your perogative.

The fact that it has 2056 cc displacement doesn't make it better which is what you seem to be focused on . You will make better use of your free time by using only the free B21A crank.

 

[Kosak]

You asked if it can be put in a B18B. You didn't ask if putting it in is an advantage over the current block. You just have to read what I wrote and not skim quickly through it.

Sorry MD, but I do my best to learn. Ive done a hell lot of research before posting this thread on here. And I try to understand what you saying at my best. But english is not my primary language and I can get confused in all the high tech terms you include in your posts trying to teach me. Please dont blame me like this.

Most of the times I understand what you are saying, but sometimes I try to search a direct answer to my questions and cant clearly see it.

But, thnx for the infos, I appreciate your good will to educate us.

 

[MichaelDelaney]

I made a mistake by saying in the first reply that the heads are the same but corrected it in the second reply. So that really screwed up the rest of the answers afterwards on the thread. I was focusing on the B21A IM which sucks. If you want to keep that B21A head, then you should check out the aftermarket IM's to replace it from the other models. Look at your B21A head's IM flange bolt hole pattern and the coolant ducts and compare them to the IM flange bolt hole pattern and collant duct. You'll see what I mean.

Seriously, if I were you, I'd take the crank and ditch the rest. That's the same crank as what they use in the Crower stroker kits. But you'll notice from that Bseries shootout link I gave you on the first page that even Crower uses a longer rod and deck plate to correct the bad rod length to stroke ratio on the B21A to allow him to get a little more safety to rev a little higher (at least to 8500 rpm).

See if you can get the 155 mm rod and pistons that have the piston pin relocated higher from Endyn. Everything can then still fit inside the B18B block as long as you notch it and you won't need to weld on a deck plate. If you just put in the B21A crank and used the B18B piston & rod, the piston would stick up above the block's deck. That's not good. The Endyn kit will change the rod length to stroke ratio from a bad 1.49 (stock 141.7 mm / 95mm) ) to 1.63 (155 mm/95 mm) which allows you to rev to 10,000 rpm without putting the piston through the side of your cylinder wall and less ring wear & vibration.

Put the B21A crank and those rods/pistons in the b18B. You'll end up with your 2L displacement, have more power potential, and be happier from the free time that you spent.