Likely the first FA2/4 hybrid incoming

[Brz-123]

Originally Posted by Ashikabi

Shouldn't the FA20 headers fit then?

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Yes they should. Its also stated in one of the posts above. Change to intake side slightly , not to exhaust. I think even Mike at CSG is saying that in his new brz thread

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[Goingnowherefast]

Originally Posted by Brz-123

Yes they should. Its also stated in one of the posts above. Change to intake side slightly , not to exhaust. I think even Mike at CSG is saying that in his new brz thread

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OP is using the FA20 cylinder heads anyways, so FA20 headers will almost guaranteed work. Will they be optimized? Probably not.

[Brz-123]

Originally Posted by Goingnowherefast

OP is using the FA20 cylinder heads anyways, so FA20 headers will almost guaranteed work. Will they be optimized? Probably not.

It is an experiment for sure but not a random one as the OP has put the gaskets on top of each other and verified oiling lines etc. How optimised or not with a minor change, the results of this will tell. We are all keenly watching as this is the first!

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[soundman98]

[Opie]

Originally Posted by Goingnowherefast

The question here is will it use the same CPS (crank position sensor). That's another critical sensor you'll need to determine engine position. If it is different, it may be possible to swap the gen 1 sensor in.

Per the documentation, it's in the same location.

[EAGLE5]

Redline is 6000k. Normally turbocharged. Designed for torque. Lower compression. It's interesting, but is it really going to have more power if not turboed?

[NoHaveMSG]

Originally Posted by EAGLE5

Redline is 6000k. Normally turbocharged. Designed for torque. Lower compression. It's interesting, but is it really going to have more power if not turboed?

This is the 22’ BRZ short block he bought. Not the turbo Acent/Crosstrek engine.

[Lantanafrs2]

The fa20 heads seemed to flow more than needed so maybe they'll be dead nuts for the larger displacement?

[Tatsu333]

Excited to see how this build goes!

Is the plan to machine the heads to match the larger bore, or...?

[Goingnowherefast]

Originally Posted by Tatsu333

Excited to see how this build goes!

Is the plan to machine the heads to match the larger bore, or...?

I'll let OP comment but that's usually not needed. No one does it on the really popular K20/K24 hybrid. It's a good example, because it's 2.0L heads with a overbored 2.4L crankcase.

[Tatsu333]

Originally Posted by Goingnowherefast

I'll let OP comment but that's usually not needed. No one does it on the really popular K20/K24 hybrid. It's a good example, because it's 2.0L heads with a overbored 2.4L crankcase.

This is not *quite* the same as K20 / K24, since the difference in bore between those two motors is only 1 mm (most of the displacement difference there is with a jump from 86mm stroke to 99mm), while the difference between the FA20 and FA24 bore is 8 mm (from 86mm to 94mm) while the stroke stays the same.

That would mean a substantial amount of extra material sitting around the top of the bore, which at the very least would be sub-optimal for combustion, if not downright problematic.

This is closer in concept to an EJ20 / EJ25 combo - I haven't really looked into what they do with those, but I'd be surprised if the heads are untouched with a 7.5mm increase in bore.

EDIT: I looked into the EJ20 / EJ25 combo, and to do it properly, it seems they either machine the heads to enlarge the combustion chambers or use custom pistons to bring the compression ratio back down. Otherwise C/R is too high. That said, some folks DO run it without either...

EDIT 2: Running some quick rough numbers, the combustion chamber volume for an FA24 is 19.47% larger than the FA20's, so with the FA20's combustion chamber volume unmodified that would make the hybrid's C/R about 13.74:1. That is a BIG jump that would be hard to tune for.

The stepped shape at the top of the bore would also make for some funky swirl of the mixture as it compresses, and could create some major heat risers that would trigger detonation. With all that considered, I don't think it would be a great idea to run it without machining the heads to match...

[Irace86.2.0]

Machining the head seems necessary. The pistons could cause interference with the lip of the head, and if the lip of the heads are sharp then they could be a location for preignition, or at best, they could cause weird tumbling of air coming into the cylinder. Personally, I feel like getting the heads machined was already a given.

[Tatsu333]

Originally Posted by Irace86.2.0

Machining the head seems necessary. The pistons could cause interference with the lip of the head, and if the lip of the heads are sharp then they could be a location for preignition, or at best, they could cause weird tumbling of air coming into the cylinder. Personally, I feel like getting the heads machined was already a given.

That's kind of what I was thinking, but it seems some folks have done the EJ20 heads on EJ25 block combo without it.

[Irace86.2.0]

Originally Posted by Tatsu333

That's kind of whatI was thinking, but it seems some folks have done the EJ20 heads on EJ25 block combo without it.

The EJ is a low compression motor. What do the pistons look like? They are recessed.




The FA20/24D has domed pistons to create the high compression. Here are some pictures of my blown engine. The piston height is visible from the side, and the piston in the background shows a piston close to TDC with a dome shape and impedance reliefs in the crown for valves. I’m sure if someone wants to throw some low compression pistons in there then there is no risk of any issues. If Toyobaru changed the valve angle or valve opening height to make it open higher or enlarged the valves then that could cause interference, depending on the tolerances. Regardless, the edge of the cylinder-head combustion-chamber probably will interfere with the crown of the piston without machining.