2.0 V10 goodness

[Evan]

The EJ20 does in fact rev past 8k pretty easily. The difference is that in FI application, you dont need the revs. And in most motorsports that subaru competes in like WRC, restrictors are used so revving the motors up gets you nowhere.

In Japan there is/was an all Impreza race series that used the NA EJ20. The engines revved to the moon and made 300+ hp NA!

[CaptainHenreh]

The EJ20 does in fact rev past 8k pretty easily. The difference is that in FI application, you dont need the revs. And in most motorsports that subaru competes in like WRC, restrictors are used so revving the motors up gets you nowhere.

In Japan there is/was an all Impreza race series that used the NA EJ20. The engines revved to the moon and made 300+ hp NA!

See, that's what I mean. Sure the turbo motors don't need the revs, and the FI eliminates the cylinder filling "problem" they may (or may not) have...but I just don't understand why 2.5 RS's don't have an 8500ish rpm redline, and make about 180-190 horsepower.

Still, triggered some interesting discussion.

[Andy]

My impression is that oversquare motors VE drops at the upper rpm ranges due to piston speed and a miniscule dwell time. Though FI would appear to remedy the VE issue.

Though what do I know. Honda's K-series is square (86mmx86mm) and it revs like hell too.

[CaptainHenreh]

My impression is that oversquare motors VE drops at the upper rpm ranges due to piston speed and a miniscule dwell time. Though FI would appear to remedy the VE issue.

Though what do I know. Honda's K-series is square (86mmx86mm) and it revs like hell too.

*nitpick*
The K20 is Square. The K24 isn't.

But alot of older hi-po motors seem to have a tendancy to be square or oversquare...but the more research I do the more I find that alot of the "killer" motors I can think of are turbocharged. 3S-GTE, SR20, 2JZ, and that delicious Lotus TT V8 (Quite possibly my favorite motor...ever.) Though the number of undersquare motors has been increasing as of late (All the Nissan and Honda jobs, not to mention the ever-present 4G63) I still think "undersquare" when I think of a performance motor. Maybe it's just my raising.

[Evan]

...but I just don't understand why 2.5 RS's don't have an 8500ish rpm redline, and make about 180-190 horsepower.

well first and foremost, the engine was designed for torque. Gotta have torque if you want to build wannabe SUVs for soccer moms. The EJ25 was first put in Foresters and Outbacks.

Engine design and engineering is definately outside of my area of expertise, butI always assumed that it didnt spin very high due to the displacement. 2.5L is a lot to spread out over just 4 cylinders. My KA24 doesnt rev high at all, and neither does the 2.5L engine in the Sentra specV
I cant think of any high displacement 4cyl that can rev.
All the high revving hondas are under 2L.
honda K20 actually doesnt rev very well. Guys getting those things to rev are fully building the bottom ends to stand the abuse.

[Andy]

h22a revs to 7700, with a stock power peak 7k. The valvesprings were the weak link. Once that was fixed, guys could reliably bring it up to 8k

[ScottyB]

I cant think of any high displacement 4cyl that can rev.

weird but true. the 2.5L S14 in the E30 M3 can do 7000 rpm....thats about the highest i can think of.

[mpg9999]

My impression is that oversquare motors VE drops at the upper rpm ranges due to piston speed and a miniscule dwell time. Though FI would appear to remedy the VE issue.

Though what do I know. Honda's K-series is square (86mmx86mm) and it revs like hell too.

Huh?? Oversquare (or I should say motors with a smaller stroke) have lower piston speeds. I'm fairly sure oversquare motors have higher VE at higher rpm. Look at motorcycle engines, they are oversquare with very short strokes and rev to 14k + rpm. It also has alot to do with rod stroke ratio. A longer rod slows piston acceleration near TDC.

[Andy]

It also has alot to do with rod stroke ratio. A longer rod slows piston acceleration near TDC.

I can't speak to motorcycle engines but oversquare engines tend to have poor rod ratios, thereby affecting dwell time which negatively affects high rpm VE.

But as stated earlier, what do I know.

[.RJ]

Once that was fixed, guys could reliably bring it up to 8k

8k is pushing it on a road race motor, IMO.

[CaptainHenreh]

...but I just don't understand why 2.5 RS's don't have an 8500ish rpm redline, and make about 180-190 horsepower.

well first and foremost, the engine was designed for torque. Gotta have torque if you want to build wannabe SUVs for soccer moms. The EJ25 was first put in Foresters and Outbacks.
...
2.5L is a lot to spread out over just 4 cylinders. My KA24 doesnt rev high at all, and neither does the 2.5L engine in the Sentra specV
I cant think of any high displacement 4cyl that can rev.
All the high revving hondas are under 2L..

I agree with you on the torque thing, but your high displacement examples are all undersquare...dramatically so. The subaru is oversquare, again, dramatically. Furthermore, alot of the four bangers don't rev very high because an I-4 isn't balanced very well, and shit tends to shake itself apart...but this is not true of the H4.

And, looking at the "smaller" honda motors that spin well, like the b18C5, you've got a 5.429 inch rod and a 3.433 inch stroke. That's a 1.58:1 ratio, which isn't *that* far from the subie. I guess. For the H22A, you've got a 1.57:1 rod ratio...which is even better than the B18...(though it revs lower).

Only thing I can figure is, I dunno, piston mass. Muffler bearings. Flux capacitor size? Or maybe the large combustion area makes slower propogating flame, preventing high rpm performance?

I mean, my only point is that we're told this stuff, right? Rod ratio, and oversquare and undersquare and all that crap...and it doesn't REALLY mean anything in the real world.

I wish I were good enough at math to be an engineer.

[Evan]

Maybe the answer is that it can rev, it just doesnt need to for the applications its used in?
Although it sure would be nice if the 2.5L STi revved to 8k like the 2.0L STi does. It uses the 2.0 STi gears, which means it runs out of revs really quick.

I do know that turbocharging the N/A 2.5L rarely turns out well. Not many 2.5RS guys who turboed their cars had their engines last very long

Porsche motors sure can rev to the moon, so the H layout wouldnt appear to have any shortcoming in that department.

Rex, it sounds like you need to build up a N/A 2.5 subaru.
2.5 engines are cheap, impreza shells are cheap, now have at it!

[Maengelito]

All the high revving hondas are under 2L.

<nitpick> s2k
2L = 9k redline
2.2L = 8k redline
may not rev high compared to F1 cars or bikes, but thats still damn high
</nitpick>

[ViPER1313]

I think on a lot of cars much more design would have to go into the intake system to allow the car to breath at high RPMs. Accessories also take a hit from high RPMs. Speaking from my total lack of experience in this area, I will relay what I know about the SHOs engine: redlined at 7000rpm (7300 soft fuel cutoff) from the factory, Car and Driver reported that Yahama spec'ed the engine to 8500rpm (many owners up the redline to 8k+ with no ill side effects). This necessitates installing underdrive pulleys unless shit goes bam, and then you are left with no voltage and heavy steering at idle speeds. Dynos show power drop-off at higher RPMs, and increasing the redline doesnÔÇÖt keep the car in the power-band. More aggressive cams and better flowing exhaust help, but then you are left with a crappy idle.

To me, redlines seem like an area of give and take. Higer and you can make more power and stay in gears longer, but they also equate to:
- more maintenance (in the form of valve lash adjustments)
- stricter tolerances / higher quality drive accessories needed
- drivability issues if maximum power is desired
- decreased reliability overall

Boost, on the other hand

[.RJ]

More aggressive cams and better flowing exhaust help, but then you are left with a crappy idle.

Sounds like a tuning issue rather than a design issue

RPM = ruins peoples motors, to an extent. If you can make the same power at a lower rev limit, thats going to be a simpler solution (all the components are less stressed). However if you can make more power, at a higher rev limit, and keep the engine together, and the engine is designed well enough to handle the revs (strong pistons/rods, proper rod/stroke ratio to keep side loading and piston speeds down) then I'll take that any day. All in theory and bench engine building.

[CaptainHenreh]

Rex, it sounds like you need to build up a N/A 2.5 subaru.
2.5 engines are cheap, impreza shells are cheap, now have at it!

Find me an impreza shell for 50 bucks, no title needed, and I'll build you The Subaretard. Rally machine.

[HAULN-SS]

Crappy idle is in the eye of the beholder. Some guys at cruise ins run shitty performing cams just because of the lope they give. It's not uncommon for these guys to get custom made cams just for the sound