Likely the first FA2/4 hybrid incoming

[sato]

Originally Posted by Tatsu333

Interesting... So, flat piston with just a minimal blending of the combustion chambers in the heads to allow for the bigger bore (removing the stepped profile). It'd be interesting to see how close to the factory 12.5:1 C/R you could achieve with a flat-topped piston.

Could work, but is there a performance (E.G. combustion control) reason for the pent-roof / domed profile of the FA20 and FA24 pistons, or is it just to increase compression? My guess from the fact that the peak of the dome on both pistons is offset to one side is that it also helps with combustion control, although that *could* just be due to the angles of the intake vs. exhaust valves.

For that matter, I guess if you were going with flat-topped pistons it may even be less expensive to go to aftermarket forged pistons.

Blending something like this?

[NoHaveMSG]

Originally Posted by Tatsu333

Interesting... So, flat piston with just a minimal blending of the combustion chambers in the heads to allow for the bigger bore (removing the stepped profile). It'd be interesting to see how close to the factory 12.5:1 C/R you could achieve with a flat-topped piston.

Could work, but is there a performance (E.G. combustion control) reason for the pent-roof / domed profile of the FA20 and FA24 pistons, or is it just to increase compression? My guess from the fact that the peak of the dome on both pistons is offset to one side is that it also helps with combustion control, although that *could* just be due to the angles of the intake vs. exhaust valves.

For that matter, I guess if you were going with flat-topped pistons it may even be less expensive to go to aftermarket forged pistons.

More efficient and even combustion, improved knock resistance. The turbo FA heads have quench pads in them for even more knock resistance.

[Irace86.2.0]

Originally Posted by Opie

May have just found something useful....the FA24DIT pistons are a flat top design...they are not domed like the FA20/24 N/A engines.....possibly a fix for 2 issues....FA20 N/A cylinder head clearance and not increasing compression ratio......

But aren’t you buying an assembled shortblock? Wouldn’t it make more sense to use the FA24F shortblock, or is the shortblock too different? And I’m sure the FA24F rods would be preferred too. I thought they looked beefier. The FA20F pistons might be a bit beefier or heavier because of the lower redline.

I wonder how much this would change compression. I think you will still want to machine the head. If the piston is perfectly flat then the 4mm lip might be a ring of increased risk of preignition. Not only could the corner get hot, but there could be a diesel effect where the air/fuel mixture gets compressed too much, especially at higher rpms where the air doesn’t have time to squeeze out of the space. At minimum, it might cause some weird flow patterns in the chamber, good or bad. Even if the piston is flat, I would chamfer the edge of the head, but I think you are at risk of losing a lot of compression with chamfered edges and the piston change.

[Lantanafrs2]

Dont want to lose squish area. Super important but who knows? This may work just fine.

[Four_wheel_drifts]

[QUOTE=NoHaveMSG;3479913]I'd still call using the FA24 heads a possible win when you consider what a K swap costs. If it all still works that is, which I believe it will. I would be over the moon happy if this thing does at least 220whp with bolt ons. I am not looking for big power.[/QUOTE]

But doesn’t the FA24 still have the same oiling issues and RPM limitations of the FA20? So wouldn’t Element Tuning’s secret machining, killer b oil pump, baffled oil pan, running a quart extra, oil cooler, etc. be required for reliable, high g track driving?

I’m optimistically hoping the OP gets this to work reliably and power and torque is what we expect.

[FrickingReallySlow]

Originally Posted by Opie

May have just found something useful....the FA24DIT pistons are a flat top design...they are not domed like the FA20/24 N/A engines.....possibly a fix for 2 issues....FA20 N/A cylinder head clearance and not increasing compression ratio......

Yeah I was thinking use the WRX STI FA24 short block! 400 rated HP! Unlike the previous gen which didn’t use fa20 but RA based instead. Also lower compression to start. Probably also around $2400

[Irace86.2.0]

Originally Posted by FrickingReallySlow

Yeah I was thinking use the WRX STI FA24 short block! 400 rated HP! Unlike the previous gen which didn’t use fa20 but RA based instead. Also lower compression to start. Probably also around $2400

Will the starter/transmission bolt up?

[PulsarBeeerz]

Originally Posted by FrickingReallySlow

Yeah I was thinking use the WRX STI FA24 short block! 400 rated HP! Unlike the previous gen which didn’t use fa20 but RA based instead. Also lower compression to start. Probably also around $2400

Where did you get 400hp? The car isn't out and there are no part numbers or prices for the engine parts... The FA24D would already have lower compression from having to open up the combustion chamber...

[FrickingReallySlow]

All speculation of course see here.

https://www.motortrend.com/cars/subaru/wrx/2022/
https://www.torquenews.com/1084/new-...ation-revealed

But for sure it’s gonna be 310+ (Gotta be higher then current gen) and FA24 based instead of RA based like the last generation ( don’t recall where I saw this but I can dig it up) so not unreasonable to assume it can handle 400HP from 310+ stock. RA shortblocks are only $2K

[FrickingReallySlow]

Originally Posted by Irace86.2.0

Will the starter/transmission bolt up?

No idea really since no diagrams are listed yet for wrx or sti but since its going to be FA24 based maybe it could be semi-compatible to a 2022 86 tranny. Maybe custom bell housing? who knows but if it avoids fucking around with cylinder head volume and equalizing it across all 4 we'd probably all take that.

[Irace86.2.0]

Originally Posted by PulsarBeeerz

Where did you get 400hp? The car isn't out and there are no part numbers or prices for the engine parts... The FA24D would already have lower compression from having to open up the combustion chamber...

I think 350whp or less is more likely, just so they can get reliability into a warranty, but it could be a beast. It sounds like it could be a D4S version of the FA24F in the Outback and Ascent. If that is the case then that would be potent. Even if it wasn’t, if all they did was do what Honda did with the K20C1 for the CTR and C4+ variants for the Accord/etc, which is remove some balancing shafts, change a few internals around and add a top end turbo for the CTR, then the FA24F variant in the STI should still be a beast. If they add some forged internals or other things to boost the strength then watch out. Check this out:

https://forums.nasioc.com/forums/sho....php?t=2907007

Tuned by JR: 537 whp & 490 lb-ft @ 22 psi on E56. They continued pushing past 560+ whp but ran out fuel leaning out to ~15:1 AFR.. ultimately melting a piston. They believe the stock block should be capable of 600+ whp with auxiliary port injection (aka dual injection).

[Irace86.2.0]

Originally Posted by FrickingReallySlow

No idea really since no diagrams are listed yet for wrx or sti but since its going to be FA24 based maybe it could be semi-compatible to a 2022 86 tranny. Maybe custom bell housing? who knows but if it avoids fucking around with cylinder head volume and equalizing it across all 4 we'd probably all take that.

I think you would need to either do a custom adapter, custom bellhousing, or you would need to machine a relief into the block or cut and weld the bellhousing swapping the starter relief to the other side. Most likely a custom adapter plate and just go with a CD009 or BMW transmission for the torque.

You can see here that the starter is on the driver’s side and not the passenger’s side like in the 86.

[Goingnowherefast]

I'm not going to speak for the OP, but if you're looking for huge power numbers this probably isn't the solution for you..

In my eyes, this appeals to the HPDE/TT/Misc Road Course market. A cheap, same manufacturer solution that adds a good bit of power and torque.

[Opie]

Originally Posted by Irace86.2.0

But aren’t you buying an assembled shortblock? Wouldn’t it make more sense to use the FA24F shortblock, or is the shortblock too different? And I’m sure the FA24F rods would be preferred too. I thought they looked beefier. The FA20F pistons might be a bit beefier or heavier because of the lower redline.

I wonder how much this would change compression. I think you will still want to machine the head. If the piston is perfectly flat then the 4mm lip might be a ring of increased risk of preignition. Not only could the corner get hot, but there could be a diesel effect where the air/fuel mixture gets compressed too much, especially at higher rpms where the air doesn’t have time to squeeze out of the space. At minimum, it might cause some weird flow patterns in the chamber, good or bad. Even if the piston is flat, I would chamfer the edge of the head, but I think you are at risk of losing a lot of compression with chamfered edges and the piston change.

Yes, shortblock comes assembled from Subaru. Haven't research thoroughly, but there are some significant differences in the 2 blocks....

Contacted two machine shops regarding cylinder head matching and a piston manufacturer regarding a forged piston developed for this swap that maintains CR....awaiting replies...shortblock still has not arrived...there will be solutions....