2012 Subaru WRX STi

[LaszloT]

While we’ve seen the all-new Subaru Impreza earlier this year, we have yet to hear much about the ultimate high-performance version of the car, the WRX STI. Subaru has been very secretive about the car, not allowing any information to get out to the media, but our colleagues in Japan were able to uncover some important details.

The STI’s launch is still about a year away—we hear that it’s scheduled to reach dealer showrooms in the summer of 2012. The car will first appear as a 5-door hatchback, with perhaps a sedan version to follow. It seems that Subaru has plans to compete in various motorsports events with the WRX STI, so expect to see the hatchback at rally events, with the sedan competing in touring-car races.

We’re still unsure how the U.S.-spec WRX STI will differ from the home-market version, but we do know that both will produce more than 300 bhp. The Japanese model will come powered by a new version of the FB flat-4 with a turbocharger. Our sources say that the 2.0-liter engine will produce 330 bhp and 324 lb.-ft. of torque, which is 8 bhp and 12 lb.-ft. more than the current domestic model. Despite the additional power, we expect fuel economy to improve.

The price is expected to be around $35,000, but we’re hoping Subaru will be able to bring that down to the $30,000 mark. With the high value of the Japanese yen to the American dollar, however, that might prove impossible. But we do know that the car will be one potent machine. The product general manager of the Impreza, Akihide Takeuchi, said that although he can’t comment on the progress of the car, it would be safe for all of us to have very high expectations.

What do you think?

[JoeT]

I wonder if they made the engine bay wider to accommodate the better rod/stroke ratio of the FB engine. The elongated rod stroke ratio amongst other small items is what contributes to better fuel efficiency (reducing friction) and reducing the extreme angle of the con rods to crank. This makes the engine a bit more durable and gives it a "rev happy" feel.

Drawback is it'll need to be wider in the engine bay.

[LaszloT]

High performance engines tend to have larger bore-to-stroke ratios, to lower piston speed and improve mechanical efficiency (cylinder filling with larger valves). The FB engine will have a smaller bore-to-stroke ratio than the current engine, and this is better for fuel efficiency and emissions but not so good for improved performance.

Here's the article :

http://www.caranddriver.com/news/car/11q1/examining_subaru_s_new_fb-series_flat-four-car_news

The FB engine has a narrower valve angle and twin-cams (not bad), and those cams are chain-driven (not sure if this is good), has the same packaging (size) and weight (good) and the intake manifold is plastic and there is no direct injection.

As long as the new WRX and WRX STi continue to evolve as seriously high-performance engines I'll be happy, but I'm concerned that the FB engine won't be quite as capable as the current twin-cam turbo powerplant.

[JoeT]

Hey Lazlo,

You do understand that one of the BIG problems with our engines are "Rod Stroke" ration right?

My point above is the 134.5 mm rod vs 79mm stroke giving us a rod stroke ratio of:

1.7 ish..

BTW: This rod stroke ratio = LOTS of piston side loads on the cylinder walls.

I do agree that large bore (piston area) short stroke is good for high performance engines.

[JoeT]

Wow, I just read that article, the stroke is increased to 90mm, I wonder how long the rods will be? EEEK let's hope that they thinned out the heads to accommodate longer rods to minimize the sideloads..

EEEEK.

[Lloyd]

The motor in the Prelude is pretty rev happy with a 1.58 RS ratio...

[JoeT]

Hey Lloyd,

What length are the rods, and whats the actual stroke on the prelude, pretty please. (just found the specs on the H22A1) on: http://en.wikipedia.org/wiki/Honda_H_engine#H22

Subaru FB25: Rod = 134mm X 90mm stroke (Guessed on Rods)
Honda H22A1: Rod = 143mm x 90.7mm Stroke

Rod / Stroke: 1.48 for the Subaru FB
Rod / Stroke: 1.58 for the Honda H22A1
Rod / Stroke: 1.69 for the EJ25

Interesting.... Hummmmmmm

[LaszloT]

For the same displacement, I thought longer stroke meant higher piston speeds (lower redline) and MORE piston side loading (greater Con rod angles).

So I'm a little confused about the points made on piston side loading.

To me the EJ motor with the shorter stroke would create a "Rev happy" feel, and the FB series motors would have a lower redline to keep piston speed the same, and not seem so freely rev-ing. They could get better power with this new engine with higher turbo boost, and use a lighter racing flywheel to simulate that "Rev happy" feel.

Enligthen me, please?

[Lloyd]

Yeah, that's true of all the H22s.

http://honda-tech.com/showthread.php?t=1232811

Most of the performance Honda motors are in that range (B18C, H22, K20, K24, F22C). The really extreme RPM motors did change the formula a bit (B16, F20C)

[alf]

Let's say the picture is accurate. It's hard to see and extrapolate since one of the picture is not complete. the 104mm measurement given cam distance, that would make the had about 150-170mm think (not precise at all but that's an idea). The head of the ej is about 200mm (not measure but I could probably measure a set at home tonight). If this is about right, they would have gained ~20-40mm on each side.

The new front rail/ cross-member are suppose to remove some of the previous engine restriction. If they gain 20mm on the engine and 10-20mm on the chassis, that's getting to be a lot of room to make this happen.

Joe, can you measure the space between frame rail of the Legacy and new impreza?

[LaszloT]

For you Honda/Acura fans:

http://www.team-integra.net/forum/14-performance/70914-bore-stroke-ratio-piston-speed.html

"many of you ask me about oversquare and undersquare engine layouts.

Well at least you know how that fits into the puzzle.

Be aware that "most" successful racing superbikes and racing engines have Bore:Stroke ratios ranging in the 2.0-2.2 range.

Our Tegs are undersquare (D series = 0.83 bore to stroke, Bseries = 0.91 to 0.93) or square (K series = 1.0).

You don't necessarily have to have a bore:stroke of 2. That's the "ideal". We really can't get that in our engine dimensions. What you want to do is to improve it (i.e. increase it if you can within the limits you can tolerate for reliability).

So to increase the bore:stroke, you can either increase the bore OR decrease the stroke OR do both.

The B16a is the only engine that is oversquare with a bore to stroke ratio of 1.05 (upper limit for a street B16a 1.1). "

[dubya_rx]

The 2012 STI will be the same as the 2011. You are probably speculating about the 2013.

I thought the engine was narrower than the outgoing. And they used those connecting rods so the engines could be easier to assemble by machine and thus reduce manufacturing costs.

Timing chains are much better than belts in my opinion.

[LaszloT]

Correct Walter. We won't get the new STi until the 2013 model year, even though the articles are referring to it as a 2012 design.

It's a completely new vehicle too.

If I'm due to turn over my 2009 STi in Feb 2013, and assuming that I can afford the new STi, would it be better to go with a 2012 big-wing sedan, or get into the new design STi at the front-end of it's introductory year?

This is the big question, ha ha.