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.

Exactly. Undersquare is not related to rod ratio.

But 90mm is a long stroke, which will put a TON of stress on the rods at high revs. And for the motor to make good power NA with a 2.0L, it'll need to rev.

2.0L with over 200hp that redlines below 7k, it can happen, it does happen.
http://www.rmsci.com/caterhamusa/zetec.html

Also lets not forget about torque. Even current NA subies are fun because of their torque characteristics :burnrubber:

You might want to read that again.

Hahaha good call... it tops out at 7.5k but the thing peaks before redline.

Correct.

The rod asymmetrical design revving to 7k rpm with that r/s ratio is is what the topic is about.

FB20 and FB25: 90mm stroke with a rod length of 129.3. 1.44/129.3 rod to stroke ratio. You kinda want a minimum of 1.5 r/s ratio for an engine more if it high revving. 1.74 r/s ratio is an ideal r/s ratio for the street. A 2.0 r/s and above is f1 and supercar territory.


Some info for comparison. I gave the r/s ratios and 89-90mm strokes on the Hondas that had them and then the EJ stroke for comparison to FB.

EJ20 1.74/130.5 75mm stroke
EJ25 1.67/131.7 79mm stroke

Every rod to stroke ratio from Honda is better than the FB20/25 since the 80s.
K20A 1.62/139
K24A 1.54/152
B16A: 1.74/ 134MM
B17A1: 1.62/131.87MM
B18A1: 1.54/ 137MM 89mm stroke
B18B: 1.54/137MM 89mm stroke
B18C1: 1.58/137.9MM
B18C5: 1.58/137.9MM
B20A3 1.50/142.75MM
B21A1: 1.492/141.7MM
B20B: 1.539/137MM
H22A: 1.577/143MM 90mm stroke
H23A1: 1.49/141.5MM
F20C: 1.821/153MM
D17A1: /140MM
D17A2: /140MM
D16Y8: 1.52/137MM 90mm stroke
D16Y7: 1.52/137MM **
D16Z6: 1.52/137MM **
D16A1: 1.52/137MM **
D16A6: 1.52/137MM **
D15B1: 1.59/134MM **
D15B2: 1.59/134MM **
D15B7: 1.59/134MM **
D15B8: 1.59/134MM 90mm stroke
ZC: 1.52/137MM

It's actually a pretty good example of how the performance version of the FB20 could be from the original.

Bore and stroke and power are pretty similar between the base Zetec and the FB20.

That being said, the performance version is probably close to the edge. Any more power out of it would likely require massive $$$ or boost.

That is why I'm not too keen on a maxed out 200 hp FB20. There may not be a lot of room for more power at reasonable cost.

Something in the 86mm x 86mm would be much better: 3SG(T)E, K20, SR20 all use it (also 2JZ in L6 form). The square 2.0L 4 seems pretty popular for a performance motor. Less stress on the rods for more rev-friendliness, and a bit more potential valve area for flow.

:word:

I totally agree, that has kind of got me hoping for a turbo release soon after its debut. Usually some good tuning to be had there :evil:

The N/A 2 liter boxers make weak torque IMHO. 140 lb ft of torque can't feel strong in a 2900+ lb car. My RSX has about the same amount of torque and feels weak as shit, especially with two extra passengers.

:word:

I went and test drove a base Impreza a couple months ago and walked away with the distinct impression that I had just driven a brick.

You're not seriously complaining about lack of torque from Japanese stock naturally aspirated 2.0L engines (or 4 cylinders in general) on 3000lb. vehicles???

I think a nice American engine might do, or a Diesel if you want to go nuts...;)

evolution on ej20

http://www.fhi.co.jp/english/news/pr...10_09_23e.html