Early proposal: FA20 with a FB25 shortblock, .5l more displacement anyone?
 

This has already been proposed a few times in the forum, most recently by coyote, but I figured theres enough reason to start a complete thread about this.

The basic idea is something that Subaru folks have been doing for awhile with early EJ motors (and other motors through history of course, but this specifically relates to Subaru boxer motors), using a larger displacement shortblock with a smaller displacement head to get more power out of your motor through an increase in displacement and also compression ratio.

Anyway, the premise is this:

IF and I mean a big IF the FA20 and the FB25 share the same bolt pattern and coolant channels on the shortblock allowing you to use the FA heads on the FB block, then you can instantly get .5 liters more displacement and an even higher compression ratio for more power.

Here are photos of the two motors that we have seen so far:

http://i15.photobucket.com/albums/a3...en/comparo.jpg

Its IMPOSSIBLE to know for sure at this point, especially in terms of whether the accessory bolt locations and such will even be the same between the motors, or if the shortblock was significantly modified on the FA20 (from everything we've seen, it seems the heads are heavily modified while the block might not be).

But there are several possible issues with this at this point (please chime in if you have more to add):

1) The FA20 crankshaft and rods MIGHT have to be used with the FB25 block in order to get this to work.
2) The bolt patterns need to match up (I'm 99% sure this will not be an issue, Subaru doesn't like to change their bolt patterns).
3) The headgaskets need to match up rather well, and the FB25 will probably be the gasket that would need to be used.
4) Accessories from the FA20 will have to bolt up properly to the FB25 shortblock.
5) ??? (space to add more later)

One thing that does need to be noted for those not familiar with the new Subaru motors, but they use a cockeyed conrod that may or may not be used in the FA20:

http://i15.photobucket.com/albums/a3...e-inline-1.jpg


Now this is all VERY preliminary and pure speculation at this point, but if this does work or can be made to work fairly easy, I think this might be the best, quickest way to get a significant increase in power with this car. And the best part is it will be all but invisible from the outside.

Thoughts, opinions, more info anyone?

great write up. hope we can do this. pretty easy power bump.

I'm hoping whenever the service manuals come out for this car, which is usually a month or two before the cars hit the lot, we can figure this out for sure.

In the mean time, I think I need to put together the stats on these motors... Bore x stroke, etc.

FA20 is 86mm bore, FB25 is 94mm, how would you be able to put the head from the FA20 on? That seems unlikely...a huge difference in diameter.

Maybe I'm just a complete noob, but perhaps what you're getting at is more along the lines of taking the FB25 pistons and block and putting it onto FA20 connecting rod and crankshaft. That would cut the stroke a little, but give an FB25 more room to rev. But then the head would need modification too...

Its big, but not that big. A common modification for the WRX 2.0 liter engines is to use the 2.5 shortblock from the STI's.

Fits just fine, HUGE increase in power.
Head shouldn't need modification.

Here is the FB25 head, note how flat the surface is and how small the coolant channels are:

http://i15.photobucket.com/albums/a3...ubaru-fb20.jpg

IF the FA20 uses modified heads but with the same size/bolt pattern, then its possible those coolant channels might still match up and the compression chamber is actually smaller with the 2.0 so that will fit within the diameter of the larger piston bore no problem.

This thread makes me really happy. I wouldnt really have wanted a complete different motor. but having essentially a fa25 sounds awesome. Pay the car off in 2-3 years and then when the warrenty runs out do this.

I see an issue here, we are assuming that the FA20 is even remotely the same block as FB engines. If so..why FA? why not FB201 or FB207? Why would subaru give a new engine designation code if the FA is just an FB with different heads? See what im saying?

I have a feeling that the FA block is essentially a re-engineered EJ block not using the new slanted connecting rods, but using a traditional design rod.

I just don't know..and i wont dare try to answer those questions until i see the bastard engine in pieces.

I wish they would make a FA25(240hp/185tq I bet) and price it like 2k more than the regular car.

And the tuning options are...?

I hate to rain on everyone's parade, but imagining anything more complex than bolt ons is kinda pointless until we hear more about potential tuning options.

I'm hoping for Cobb accessport reflash capability, but everyone with big plans may get stuck buying an aftermarket ECU.

Or are you thinking about ripping out the whole drivetrain and replacing engine, trans, ECU and just using the chassis?

That's a big project, especially on a brand new car...

If you look at the leaked tech brochure, you can see that the rods are similar to the new FB type (if the pic is accurate).

That being said, Pretty sure that 86 vs 94 mm is a quite a big a difference to just drop the head on there (not even accounting for coolant channels etc). Putting the FB25 pistons in the FA would be a no no as well.. cause then... you'd have to overbore the FA bock..by 8mm :confused0068:. However, I think the subbie guys have had some success and the 2.0 EJ engines have a huge bore difference with the 2.5s... So it probably is possible, but it wouldn't be a bolt up affair. Far from it.

Perhaps you could put the FB crank and rods into the FA, but that'd hardly be worth it, it'd go to 2.1L only (2088cc)..and this is assuming you end up with the same deck height.

Time to start playing Lego engine builder.

FWIW, the big bore delta likely means the bore spacing is different, meaning FA and EJ heads-n-blocks will have to stay in their own family.

Now, the FB20 has a 84mm bore with a 90mm stroke. Since Toyota/Subaru have admitted the FA is a derivation of the FB. We might be able to drop-in an FB 90mm stroke crank into the 86mm bore and end up with a whopping 2.09L and the reduced ability to rev.

The FB25 also has a 94mm bore, just like the EJ25.

Just swap the h6 for more NA power.

Or wait for a stroker kit.

In before thread turns into "turbo with 300hp version"

Just swap a Quad-Turbo 8L W16 and we'll be making 320 bhp/ton!