FB25 VS FA20
 

hello! fa20 exhaust manifold to fb25 , will it fit?

Yes same block height the F series are modular.
you can bolt a set of BRZ heads on a FB25 block, they have the same head bolt pattern.
Unknown at least to me is the front cover bolts and how they line up. between the 2 engines.
I should pop the hood on the 2018 Legacy 2.5i and take a peek at the cover to compare.

thanks! im interesting how brz exhaust manifold better then rxhaust manifold fb25 engine

Problem is the exhaust routing and engine cradle location differ in the 2 chassis. Not sure everything will line up the same.

Can i use FB bottom end with FA top and will increase the compression?

In theory yes. I have been trying to locate a FB25 since about 2013 to confirm fitment. I have not validated the setup yet.

Wrong thread @celek ! Keep your eye on the prize! Back to work! :lol:

If the FB have stronger bottom end than the FA i think would be worth to do the search/test.

The FB25 is actually worse shorter rods crappy rod to stroke ratio and smaller thinner rod bearings

:bellyroll::bellyroll::bellyroll::bellyroll:
I really want to switch it up for a 2.5 build if someone will by my 2.3

I was just about to reference your build as I had remembered reading that.

Wouldn't you still use your FA heads off your build? PS - what did you do to your heads at this point? Or has it all been bottom end work thus far?

The FB25 wasn't supposed to be a performance motor.

We will not be oppressed !!!

:burnrubber:

Funny, neither are ours.

I have both of them. What do you want to know? In fact, I bought the BRZ because I was too disappointed with the FB series. It is an excellent motor for a daily driver (e.g. no torque dip at all), but try to drive it really hard and bad things are happening. It'll start burning a lot of oil and have excessive knock. The FA series is much more focused for performance driving (although it has at the same time a Toyota dna - D4S ... ) and it has been tested already in many professional races. It was not perfect in the early years, but there were revisions and the issues are solved.

Yes I would still use my heads, I want to sell the shortblock and build a 2.5 No one wants to pay double a cheap turbo kit price though

Neither was the 97-01 CRV engine 2.0 liter 140 HP
Or the 1.8 VW Beetle 80HP
Or the 92-00 1.6 SOHC Honda 109HP
Or the 2.4 liter CRV K series 160HP
ect....ect...
But the industry made those work.
Thinking outside the box

I have read your whole build thread, is their a reason you would prefer the 2.5 over your current setup? Whatever the outcome I await the completion of that monster.

And doesn't the FA still have better oiling system? I remember Element talking about how Subaru had updated the oiling systems with bigger oil ports, etc.

Displacement

Would still use the FA20 pump and front cover most likely.
bigger ports are great for flow but produce less pressure

Just for the sake of engine theory, are you hoping to get more mid range with the larger displacement at the potential cost of top end performance, or are you convinced the 2.5 will be able to rev as high and there will be no downsides? I'm not familiar enough with the bore / stroke of these engines to comment on how high one car rev vs the other, just curious is all.

In general bore/stroke engines is known to be stronger in medium ranges. For example a specific 2.3lt FA20 engine project is known to give more power until the 6k rpm range and then start falling. If you see a dyno of the FB20/FB25 there is no torque dip at all (forget the nonsense trying to fix it with headers) and that's why I said these engines are better for a daily driven car and maybe in a few racing applications, but overall I would consider a purebred racing engine one that works better higher.

FB20 dyno:
http://i64.tinypic.com/x4p5zs.gif


PS: Apologies for the German. Torque is the blue color :)

The reduction in torque at higher rpms is a factor of maximum airflow restrictions in the head. There's an article on FA20 development on here where they apparently dedicate A LOT of attention on improving the heads over the FB series. That and careful intake and exhaust tuning is what allows the FA20 to make good torque up to 7krpm. Now, there are bottom end differences to let the FA20 do that reliably that the FB20 doesn't have, but Im thinking it's going to be easier and probably less expensive to compensate for those than lack of displacement, even if the end result is a little jankey.

Increase in displacement increases maximum potential torque. Better flow characteristics reduce the gap between potential and actual output torque.

@celek, was there a reason why you didn't specify fb25 sized sleeves and use a fb25 head gasket to begin with?

Yes sure. My comment was relevant for an out of the box solution. If you are building an engine you have many options.

I can make a 2.5 with the same rod to stroke ratio as the FA20 I have already done it at 2.3 liters capable of 9500 RPM
the limitation is in the valve train not block.

Can i use fa20 exhaust manifod for my fb25 outback?

Most probably, no.

Probably not as state previously the exhaust routing is placed differently on the Outback. Engine sits farther forward to allow AWD front axels. Engine cradle is different as well. This is speculation without seeing one first hand. Maybe a custom over pipe depending on how the Outback exhaust is routed.