Likely the first FA2/4 hybrid incoming

[Irace86.2.0]

[PulsarBeeerz]

Originally Posted by Lantanafrs2

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

I don't think that is true. The FA24D wouldn't include its larger intakes ports if that was the case.

[Irace86.2.0]

In general, they definitely don’t flow more than needed. There could always be more flow.

The intake manifold is said to flow well enough that it isn’t the bottleneck for NA power, nor is the throttle body, but the heads could definitely use improvement in flow by a port and polish, and they could use larger valves for sure. Doesn’t the FA24 head have larger valves? I know the intake ports and intake manifold is larger.


Will it fit?

https://motor--fan-jp.translate.goog...&_x_tr_pto=nui

[Lantanafrs2]

Then why doesn't port work show any gains on a fa20 (na)?

[Tatsu333]

Originally Posted by Lantanafrs2

Then why doesn't port work show any gains on a fa20 (na)?

Apparently, the FA20 port size and design is pretty good from the factory, so there's not much room for improvement / gains through porting with the stock valve size, but going to bigger intake and exhaust valves sees substantial gains in power, according to Element Tuning.

From their site (https://elementtuning.com/competitio...r-wrx-fa20dit/):

The factory heads are pretty good from Subaru but the valve size holds it back. When too much power is not enough look no further than adding our competition “Big Valve” head modification to your engine. Through years of testing on the track and on the dyno the key to maximum power is unlocking the potential within the Subaru heads. Element Tuning has discovered that the majority of the gains from the Subaru head are attributed to the valve size itself and not the intake and exhaust ports. Often “Big Valve” heads can be $4000 plus dollars due to the labor required to port the heads. The first step is to fit +1mm Ferrea stainless steel intake valves. The modifications to your supplied head (complete heads are available also at a higher price) start with a 3 angle intake valve job and it finished off with a bowl blend to maximize the flow through the intake port to the larger valve. On the exhaust side we also install a +1mm inconel valve and hone the guide but a 2 angle with radius valve job is performed. The exhaust port is again blended to maximize the potential of the larger exhaust valve.

Also:

Many may ask why don’t you port the intake ports fully? We don’t do this because almost 100% of the gain is realized by just fitting the larger intake valve and performing a bowl blend. (HP/value/compared to porting) On top of this the head retains 100% of it’s reliability due to how thin the cast walls are. It’s all too common to see highly stressed, high HP, road race engines blow through over ported head walls. The FA motors have nice and very large ports from the factory but extremely thin castings! With the big valve heads fitted to the motor we dynoed more than 60 whp gain at the same boost level with a 60 trim Garrett turbocharger.

That said, HP gains boosted do not necessarily mean similar gains NA since flow velocity can be much more important when you don't have a turbo / supercharger cramming in tons of air! You may gain peak power at the expense of the low-mid range with "too much" flow.

The pics posted of the FA20 vs. FA24 pistons definitely suggest larger valves on the latter judging by the size of the valve reliefs cut into the tops of the pistons...

[Opie]

The cylinder head / piston matching, clearance is definitely a concern that will be addressed once all parts are in hand. There are several options, each with pros & cons;

Use FA20 head "as-is": Pros - cheap, Cons - compression ratio, flow, combustion issues, etc.
Use "machine matched" FA20 head: Pros - better compression ratio, flow, combustion, Cons cost, proper design.
Use FA24 heads: Pros: will be a perfect match, Cons: Much increased cost, compatibility with Gen 1 intake manifold & components questions.

[NoHaveMSG]

Originally Posted by Tatsu333

Apparently, the FA20 port size and design is pretty good from the factory, so there's not much room for improvement / gains through porting with the stock valve size, but going to bigger intake and exhaust valves sees substantial gains in power, according to Element Tuning.


That said, HP gains boosted do not necessarily mean similar gains NA since flow velocity can be much more important when you don't have a turbo / supercharger cramming in tons of air! You may gain peak power at the expense of the low-mid range with "too much" flow.

The pics posted of the FA20 vs. FA24 pistons definitely suggest larger valves on the latter judging by the size of the valve reliefs cut into the tops of the pistons...

Former forum member 86IT called Element about doing a BVH NA build and they told him it wasn't worth it. I'd dig up the post if I had time but I remember him talking about it. I could probably ask him on FB and see if he remembers what they exactly told him.

[Irace86.2.0]

Originally Posted by Lantanafrs2

Then why doesn't port work show any gains on a fa20 (na)?

Why do you say that? Wouldn't you have to see a port and polish as the only modification for the dyno session with a before and after, and then you would need to start adding modifications to the car in combinations, with and without the port job to see if there was a synergistic effect? Have you seen this data?

There clearly are gains in cfm to be seen by doing a port job. Assuming that doing so hasn't messed with some optimization in the system, then there should be gains. Maybe there would be other bottlenecks in the system like valve size or lift/duration from cams or raising the rpms where porting would be seen on the dyno. Maybe a header would be needed to take advantage or something else, but my point was that the heads aren't perfect, and they will be a bottleneck before some other components.

https://www.ft86club.com/forums/show....php?p=1630770

[Tatsu333]

Originally Posted by NoHaveMSG

Former forum member 86IT called Element about doing a BVH NA build and they told him it wasn't worth it. I'd dig up the post if I had time but I remember him talking about it. I could probably ask him on FB and see if he remembers what they exactly told him.

I could totally see that being the case on the 2.0L since it would likely compromise the low-mid range for small gains up top, but it *might* be worth it if using the FA20 heads on a 2.4L block, particularly if the FA24's heads use bigger valves from the factory (I did a quick search and haven't been able to find any concrete info on the valve sizes for the FA24).

Presumably, using the smaller-valve head on the 2.4L block would give more low-mid range punch at the expense of a little less top end.

Would the difference be worth the $2K+ US it would cost to have the big valve heads done vs. the gains from other stuff (headers, etc.) for the money? Probably not.

If you're going to be in there anyway to clearance the heads, etc. for the bigger bore, though, maybe it would be worth paying the difference.

It's a shame you can't just use the FA24 heads, but according to the OP's information, it's not possible without at least using the FA24's intake manifold plus a LOT of other small changes (due to the various bits and pieces that connect to things on the heads) that would be a major pain and quite costly.

Kudos to the OP for taking on this project! A pioneering effort that will no doubt teach the community some valuable info.

[NoHaveMSG]

Originally Posted by Tatsu333

I could totally see that being the case on the 2.0L since it would likely compromise the low-mid range for small gains up top, but it *might* be worth it if using the FA20 heads on a 2.4L block, particularly if the FA24's heads use bigger valves from the factory (I did a quick search and haven't been able to find any concrete info on the valve sizes for the FA24).

Presumably, using the smaller-valve head on the 2.4L block would give more low-mid range punch at the expense of a little less top end.

Would the difference be worth the $2K+ US it would cost to have the big valve heads done vs. the gains from other stuff (headers, etc.) for the money? Probably not.

If you're going to be in there anyway to clearance the heads, etc. for the bigger bore, though, maybe it would be worth paying the difference.

It's a shame you can't just use the FA24 heads, but according to the OP's information, it's not possible without at least using the FA24's intake manifold plus a LOT of other small changes (due to the various bits and pieces that connect to things on the heads) that would be a major pain and quite costly.

Kudos to the OP for taking on this project! A pioneering effort that will no doubt teach the community some valuable info.

It could have likely been the gain wasn't worth the price of admission. I haven't had a chance to reach out to him today.

[Tatsu333]

Originally Posted by Opie

The cylinder head / piston matching, clearance is definitely a concern that will be addressed once all parts are in hand. There are several options, each with pros & cons;

Use FA20 head "as-is": Pros - cheap, Cons - compression ratio, flow, combustion issues, etc.
Use "machine matched" FA20 head: Pros - better compression ratio, flow, combustion, Cons cost, proper design.
Use FA24 heads: Pros: will be a perfect match, Cons: Much increased cost, compatibility with Gen 1 intake manifold & components questions.

I guess one other thing to consider is the positioning of the valve reliefs on the FA24 pistons vs. the FA20 head's valves. It doesn't look like they will line up when you look at the side-by-side shots of the two pistons, so there may be a clearance issue there as well.

As crappy (and costly) as it may be, there may be little option other than either using the FA24 heads (and all the added complexity that brings), or having custom pistons made with the FA20's shape in the middle 86mm plus an extra 4mm radius around the edge (if you can find someone to do that).

I guess another option might be spacers between the heads and block for clearance valve to piston clearance, but then you'll lose some compression.

EDIT: One other thought: what about some kind of machined adapter plates to use the FA20 intake manifold on the FA24 heads, like the Crawford Billet Power Blocks or a phenolic thermal spacer, but adapting the one shape to the other? Of course, there's still all the other stuff connected to the heads to deal with, but that piece at least could be not too crazy difficult.

It's awesome that you're taking this challenge on, man - best of luck!

[Irace86.2.0]

Probably would need to do a static interference test, and then do a valve interference test, and then maybe limit timing if there is no static interference and just some valve interference or grind down the piston surface at the expense of compression. Depending on the interference, a thicker or doubling up on head gaskets might create clearance, but again, this is at the expense of compression, and it isn't ideal.

[Opie]

Originally Posted by Tatsu333

I could totally see that being the case on the 2.0L since it would likely compromise the low-mid range for small gains up top, but it *might* be worth it if using the FA20 heads on a 2.4L block, particularly if the FA24's heads use bigger valves from the factory (I did a quick search and haven't been able to find any concrete info on the valve sizes for the FA24).

Per the tech docs, FA24 heads use the same size exhaust valves (diameter & length), intake valves are the same diameter, but the length it a few mm longer. I assume to open a little more and allow slightly more air into the cylinder.

[Opie]

Originally Posted by Irace86.2.0

Probably would need to do a static interference test, and then do a valve interference test, and then maybe limit timing if there is no static interference and just some valve interference or grind down the piston surface at the expense of compression. Depending on the interference, a thicker or doubling up on head gaskets might create clearance, but again, this is at the expense of compression, and it isn't ideal.

Lots of measuring and testing to be done before assembly...as I said, using the FA24 heads may end up being the answer...but I'm sure going to see if the FA20 heads are a possibility first.

Short block did not arrive as expected yesterday...still waiting