ok this is purely hypothetical but i figured i would toss this around to see if it is even worth considering. first and foremost does subaru even make an engine that has a 5 valve head design or are they all 4 valve? second if said 5 valve does exist would it bolt on to a standard/ built ej205or 207 block? the reason i ask is after thinking about it how feasable would it be to have an engine that would make reliable horsepower through a let's say 10,000 rpm limit. i understand that turbo selection and bottom end are factors but i also realise that the main choking point is how well the engine breathes which requires valvetrain upgrades. so i ask what would be needed to make said heads either 4 or 5 valve (which ever is actually available) to make the engine run reliably at this limit. please include any thoughts or potential probems you think might arise. also remember that this is hypothetical so cost is not really an objection but no "just design a 5 valve head and be done with it" answers. thanks in advance and hopefully we might find something interesting.

the only 5 valve head I know of is the VW/Audi 1.8T 20V motor, not the most efficient design in the world, the valves are about nickle and dime sized and has a lot more going on to it. There is no reason that our 4 valve per cylinder design can't handle a high rev limit. There are just preperations that need to be taken to be able to do so such as Ti valve train, balanced assemblies, so on and so forth. There is no said reason that the 4 Valve heads can't rev to that high of RPM, stock a ej207 heads can rev to 8500 RPM, so if the head assembly was built there is no reason you can't squeeze another 1.5k RPM out of it.

5 valve heads suck for performance. VW/Audi got some great efficiency out of it but in terms of ultimate flow it's not that great. There is a ton of added valve train mass as well.

Yamaha, Toyota and Ferrari all to name a few used 5 valve heads for awhile but fell back on 4 valves after they found it to be a superior design.

the reason i orginally asked the question was i was actually thinking of the race prepped 1.6l toyota/yamaha motors with the 5 valve heads that reved to 11,000 rpm. anyway its good to see that it is not unreasonable to use a 4 valve head to make power at that rpm. as far as preperations go what would rally be needed to rev that high? i would assume cams, titanium valves and springs, solid bucket lifters, port and polish maybe but really anything else as far as mechanicals go? also what motor would you go with? jdm 2.0 wrx 2.0 or 2.5 sti or a built and stroked block from somebody like (insert favorite shortblock builder here). thanks in advance.

Longer stroke is not necessarily the way to go if you are shooting for stratospheric redline. Strong bottom end, good oiling and a valvetrain that can keep up without floating the valves are what you need.
This is assuming you have the airflow to make power at those RPM's. Frictional losses increase with RPM, as well.

I used to have an Audi A4 1.8T with an EVO Motorsports Garrett T28 turbo kit. The car went 12.92@106 with a 1.7 60ft. They say the kit is goood for 270+hp on their site, although I believe it's at the crank. That kit (no IC upgrade) was pretty close to as much as you could do without block or headwork or cams on that motor. Those who point out that it's just not a great power design are correct. Also, stock redline was 6500 RPM, and with the chip in the kit is was raised to about 7100 RPM. Stock crank HP from the factory was 155 just to give you an idea.

I used to have an Audi A4 1.8T with an EVO Motorsports Garrett T28 turbo kit. The car went 12.92@106 with a 1.7 60ft. They say the kit is goood for 270+hp on their site, although I believe it's at the crank. That kit (no IC upgrade) was pretty close to as much as you could do without block or headwork or cams on that motor. Those who point out that it's just not a great power design are correct. Also, stock redline was 6500 RPM, and with the chip in the kit is was raised to about 7100 RPM. Stock crank HP from the factory was 155 just to give you an idea.
That's cause you had a tiny t28 turbo. I had a gt2871r on my 1.8t gti and it made well over 300whp and that was all stock motor from head to oil pan. There has been guys that have made well over 400whp on that motor. I do agree that the head design is not the most efficient. That's why they went back to a 16v design.

first off the head isn't our restriction on high rpm's...oiling is...second problem with us screaming is VE's fall off because of our heads low flow.
Five valves do offer nice gains and can be found to have better flow values in a lot of cases...cylinder wall size being a big issue with packaging. The velocity port profiles are then looked at as to which flows more air at less restriction and restriction of flow do to distance between valve and cylinder wall; poor for high flow values and air fuel mixture can mechanically seperate.

so really hard to tell if it would be better...we typically do have big fists in our engines, so it could pay off...

wow you guys never cease to amase me with the weath of knowlege on this forum thank you every much. as far as our engines go assuming you raised oil pressure either through a dry sump system or shimming your oil system to overcome oiling inneficencys and assuming the motor has the mechanical bits to do so then air flow is the only real restriction? so that would require im guessing some sort of intake/exaust valve porting to improve flow or a redesign of the intake manifold?