FB25 VS FA20

[8RZ]

Funny, neither are ours.

[nikitopo]

Originally Posted by 8RZ

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.

[celek]

Originally Posted by fika84

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?

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

[celek]

Originally Posted by nikitopo

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

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

[NoHaveMSG]

Originally Posted by celek

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

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.

[fika84]

Originally Posted by celek

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

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.

[celek]

Originally Posted by NoHaveMSG

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.

Displacement

[celek]

Originally Posted by fika84

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.

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

[Voltwings]

Originally Posted by celek

Displacement

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.

[nikitopo]

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:



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

[Spuds]

Originally Posted by nikitopo

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:



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?

[nikitopo]

Originally Posted by Spuds

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.

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

[celek]

Originally Posted by Voltwings

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.

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.

[Baramzisyan]

Can i use fa20 exhaust manifod for my fb25 outback?