FB series slant connecting rod. What do you think?
 

I've read countless posts in this sub-forum. Not many have touched on this design much. FB originally designed to allow compactness for the Forester and Impreza and more stroke for more torque down low.

I want to know what you think about the idea of this sports car having a high output, high revving, n/a engine that may need to rev high to make power with the bigger stroke, smaller bore and what looks like skinny slant connect rods.

Are they titanium? Are they at least forged? I have a feeling owners running at the track and autox will blow them up. Although I just don't see a company making a sports car that can't withstand the punishment it knows it's going to receive and what's it built for.

On an engine only producing 148hp used for daily driven task by normal people seems good, but for enthusiast level ownership? What do you think? :iono:

http://blog.edoperformance.com/tag/subaru-fb20/
http://blog.edoperformance.com/wp-co...ewSubar135.jpg

I know there is an engine spec thread and head specific thread, but I want this to be about the slant connecting rods and how they would fair against high rpm or boost with the big stroke and small bore setup.. :thanks:

right now we get less info on the engine more on the chassis so we can't really say much about what the FB on the FT86/FR-S compare to the FB of the forester, specially what Yamaha people have in stored for this chassis. we dont even know what will be the redline on this motor so right now its all speculations.

From what I remember on the subject the slant helps with the forces on the connecting rod bottom end by positioning the connecting rod bolts/cap-screws at an angle the vertical forces won't cause as much stretch between the cap and the rod.

Not 100% sure on it, I can't recall any of the other benefits.

I believe the FB rods are cast like most cheap mass produced engine connection rods, they sure as hell wouldn't be titanium, I don't expect titanium components for the stock FT-86 or even STI engine. Forged would be my hope at the moment.

Creating a high revving reliable engine is no easy feat. In layman's terms: Decrease inertia via weight and stroke, reduce friction, increase air flow into combustion chamber.

the engine is going to be a reworked FB20. this is the only logical answer. i doubt they will mess with the bore x stroke that much. imo you can pretty much kiss an oversquare design goodbye at this point. they will use the D4-S system for a broader tq curve. they will rely on high compression, made possible by direct injection, not high revs to make power. the 200 hp figure thrown around by the press is a good figure.

dont expect a high revving engine. i just cant see them going this route... remember subaru is the one who gave us the EJ25 sti while japan and europe enjoyed the EJ20....

But I thought they SAID it would be high-revving at some point, no?

Either way we just need to wait.

^ They said it would be 'free-revving' which generally means low internal friction/weight, so you have more control over the engine (much like changing out a flywheel to a lighter one)

I cant see the FB20 getting into Yamaha's hands and not be nice and high revving tho

I think the OP is a bit baseless in his fears. What draws you to the conclusion they'll fail? The con-rods would obviously have been FEA'd prior to getting out of the concept stage to withstand forces much greater than any driver can impose on them in stock power output. Engine manufacturers subject the engines to marathon WOT/Redline sessions for days at a time, subjecting the engine to stresses and temperatures far beyond it would ever see even at 24 Hours of Le Mans. They're likely forged steel, though that's just an educated guess based on prior Subie engines. Subaru's press releases have stated the slant shape is primarily to quicken assembly and maintenance of the engine. It's a way to save money without sacrificing strength.

Now, you want to talk about aftermarket boosted and/or high-RPM setups? That's another story. The con-rods will have limits, like any other con-rod, but that limit will have to be found.

the reason they're using a slant connecting rod is so you can install the piston/rod assembly after the block halves are bolted together around the crankshaft. on EJ series engines, you bolt the rods (without pistons) onto the crankshaft, then bolt the block together around the crank & rods, and then you install the pistons onto the rods.

with the slant rods, the FB goes together similar to a "normal" engine.

I thought it had to do with clearance issues, as the halves were kind of offset or something so they could make it narrower with it's new, much longer, 90mm stroke.

looks like it's for both reasons. http://www.caranddriver.com/news/car...-four-car_news

And just because the engine is likely to be undersquare doesn't mean it won't be high revving: the 2zz, B18, K24, and the Lambo/Audi V10 are all undersquare.

^Yup!

And we know it's capable of 225BHP(230PS). If we're lucky it will be as simple as raising the rev limit and tuning for the high RPM range to get all of the brake horse power. But depending on how much design they put into it, anymore than that is guesswork at this point :iono:.

If Toyobaru is keeping the slant rods design on the FB20/FB25 for high performance operation, i think it would be kind of a duh to assume that they will be Forged. Over the cast slant rods of the normal FB engines found in the Forester and future N/A Subaru models.