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

[NESW20]

i'd caution stroking the FA20 until we know a lot more about the engine. i know on my 3SGTE, the rod:stroke ratio wasn't ideal to begin with, and when i stroked it, i made it worse. this coupled with my higher compression ratio has made tuning difficult and annoying on pump gas, to say the least. just something to consider...

[EJ20]

I guess we will end up with totally different engine, where nothing is sharable with the fb25 hope I'm wrong

[Kostamojen]

Originally Posted by WingsofWar

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'm concerned about that too, but if the heads alone are different enough they might label it something else, especially with the DI in there as well.

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.

Thats not the case for sure, EJ's don't have oil filters on the top or timing chains. Everything about this motor screams modified FB.

Originally Posted by Jordo!

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.

Its not as early as you think. We probably will have everything released information wise within a couple months, and the service manuals (at least in Subaru's case) are out a month or two before the car even hits lots, so we should have very detailed information available before the car is even out.

[coyote]

Originally Posted by mkivalex

In before thread turns into "turbo with 500hp version"

Most sensible idea.

[old greg]

Originally Posted by Kostamojen

I'm concerned about that too, but if the heads alone are different enough they might label it something else, especially with the DI in there as well.

Unfortunately, that has never been the case with the EJ's. They've had everything from SOHC heads to DOHC heads with variable cam phasing, and that never changed the engine family.

[Dimman]

Also, if the power is flow-limited (but bigger bore could result in a bit more flow from de-shrouding the valves), the result would be nothing more than a 200 hp FB25 with correspondingly lower rev limit. May have less of a usable power curve as well.

But the extra low-end may be what some people are looking for (SUB this may be your lucky mod...).

[serialk11r]

In that case, wouldn't it be easier to just take an FB25, tune it, raise compression, and call it a day?

[Kostamojen]

Originally Posted by Dimman

Also, if the power is flow-limited (but bigger bore could result in a bit more flow from de-shrouding the valves), the result would be nothing more than a 200 hp FB25 with correspondingly lower rev limit. May have less of a usable power curve as well.

No, the flow should be good for well over that, especially if the heads have been modified for better flow as the articles have mentioned. Freeing up the intake piping and exhaust of course will help with flow, but either way it will be a big bump in power.

Originally Posted by serialk11r

In that case, wouldn't it be easier to just take an FB25, tune it, raise compression, and call it a day?

No way, the FB25 doesn't have the heads or compression. And the BRZ/FRS will surely have a different ecu and wiring harness for the DI that won't be compatible with the FB25 heads.

[ichitaka05]

Screw FB25! Give me 2.2L ver w turbo so I can call it 22BRZ

Put the joke on the side interesting topic

[Dimman]

Originally Posted by Kostamojen

No, the flow should be good for well over that, especially if the heads have been modified for better flow as the articles have mentioned. Freeing up the intake piping and exhaust of course will help with flow, but either way it will be a big bump in power.

No way, the FB25 doesn't have the heads or compression. And the BRZ/FRS will surely have a different ecu and wiring harness for the DI that won't be compatible with the FB25 heads.

Why do you say this? The implication being that the FA20 is power limited by its rpm then?

Airflow=hp. Pulling more air from bigger cylinders doesn't change the limit of what can get through the valves.

[Kostamojen]

Originally Posted by Dimman

Why do you say this? The implication being that the FA20 is power limited by its rpm then?

Airflow=hp. Pulling more air from bigger cylinders doesn't change the limit of what can get through the valves.

If you are implying that somehow the FA20 heads have limited airflow via intake ports and valve size and that they need to be bigger, there is no way to know that at this point.

But thats always been the last issue with EJ motors in the past, the head flow was usually the last in the line of modifications you needed to do in order to get the most power out of the motor.

The "frankenstein" motors with EJ18 heads on EJ22 blocks, or EJ22 heads on EJ25 blocks, or even the turbo EJ20 heads on the STI EJ25 blocks, all of them never needed head work to get those significant gains in power, and none of them had lower peak power numbers either.

[Dimman]

Originally Posted by Kostamojen

If you are implying that somehow the FA20 heads have limited airflow via intake ports and valve size and that they need to be bigger, there is no way to know that at this point.

But thats always been the last issue with EJ motors in the past, the head flow was usually the last in the line of modifications you needed to do in order to get the most power out of the motor.

The "frankenstein" motors with EJ18 heads on EJ22 blocks, or EJ22 heads on EJ25 blocks, or even the turbo EJ20 heads on the STI EJ25 blocks, all of them never needed head work to get those significant gains in power, and none of them had lower peak power numbers either.

What you are implying is that with nothing more than larger cylinder volume, that this motor will magically suck more air through the valves/ports. This thing would need super-aggressive cam timing to take any real advantage of that, aggressive to the point where the bottom-end of the stock FA20 would be terrible (ie the stock ports are either oversized or the cams have enough duration/late closing intake to take advantage of super high speed intake flow that can keep filling the cylinder as the piston starts up for compression.)

If we work with the assumption that there are cam upgrades and intake upgrades and exhaust upgrades as well, it has the potential to make more power, despite fighting heavier internals. But all of those mods would also be making similar results on the base FA20.

[Kostamojen]

Originally Posted by Dimman

What you are implying is that with nothing more than larger cylinder volume, that this motor will magically suck more air through the valves/ports. This thing would need super-aggressive cam timing to take any real advantage of that, aggressive to the point where the bottom-end of the stock FA20 would be terrible (ie the stock ports are either oversized or the cams have enough duration/late closing intake to take advantage of super high speed intake flow that can keep filling the cylinder as the piston starts up for compression.)

Don't forget there is also a compression ratio increase involved with this, not just displacement.

And also don't forget the FA has full variable timing PLUS the Direct Injection. I'd be interested to see what you can do with the cams for these heads seeing as I don't know of anyone making or modifying them yet for FB motors, but I doubt it would be "required" to run the FB25 shortblock (if this even works at all).

If you want more information on what people have done before, there is a good thread about it here:

forums.nasioc.com/forums/showthread.php?t=631527

[Dimman]

Originally Posted by Kostamojen

Don't forget there is also a compression ratio increase involved with this, not just displacement.

And also don't forget the FA has full variable timing PLUS the Direct Injection. I'd be interested to see what you can do with the cams for these heads seeing as I don't know of anyone making or modifying them yet for FB motors, but I doubt it would be "required" to run the FB25 shortblock (if this even works at all).

If you want more information on what people have done before, there is a good thread about it here:

forums.nasioc.com/forums/showthread.php?t=631527

There are some aspects that could make more power, but just assuming that they will because they did on 30 year-old non-optimized tech is silly.

Potential gains in flow can be had by de-shrouding valves (this would be achieved by the cylinder wall of the bigger bore being further from the valves). The CR increase would be part of it, but also the same thing that is probably causing the CR bumb in the EJs would also create a more favourable combustion chamber shape (more squish area).

But in this much more optimized motor (de-shrouding can sometimes lose flow on some highly developed ports) if these are feasible at all the gains will likely be much smaller. And they are likely able to be offset by things like the heavier rotating assembly.

Also the valve timing is optimized for the rpm range of the FA20's intake and exhaust pulse tuning. These would be different for the longer stroke, slower revving 'FA25', and could screw up the power curve (spikes and flat spots).

Last is something I'm not certain on, and that is I think the DI/D4-S engines have different shaped pistons, due to the nature how the direct fuel injection spray works. So another wrench in the machine there.


An FB25-swap with cams (if they come out) and bolt-ons may give what some of the members here want (200ish hp at lower revs, with more low-end torque).