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Engine, Exhaust, Transmission Discuss the FR-S | 86 | BRZ engine, exhaust and drivetrain.


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Old 05-13-2020, 07:23 PM   #15
Spuds
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Originally Posted by Fa20Toybaru View Post
I am also building my FA20. I am currently researching cylinder heads. The recommendations on here are pretty good. Going +1 has shown good results. I was looking at the piper and HKS cams. I did not know what the difference was until I asked my tuner. If we were to look at this as an option. We have to consider "LIFT" and "DURATION". This from my understanding is what gives the increase in performance. If anyone has further explanation on this please let me know. I have also heard that the stock cam are reliable and good.
Question: If one uses the HKS cams, are we stuck only using their motor upgrades?
I have also posted a thread that I need help on. It's about transmissions. I do need help on that, at this point . Thanks in advance, good luck on the build
I'm just going to start at the beginning here...

Cams operate the valves that let air into and exhaust out of the cylinder. Cams operate in a cycle fixed to the rotation of the engine, turning 1/2 turn per engine rotation in a 4-stroke engine.
The intake valves open on the intake stroke, the exhaust valves open on the exhaust stroke, with no valves open on compression and combustion strokes. NA gasoline engines are typically oxygen-limited, meaning the more air you can get in the cylinder, the more power you get.

Lift is the amount of distance a given valve opens. More lift, more airflow, more power (or less power lost in the case of exhaust flow). The valves will only lift as much as the profile on their cam forces them to.

Duration is the amount of time the valves are open. More time open means more total amount of air gets into or out of the cylinder at a given flow rate (determined by, among other factors, the lift).

There are other considerations such as how 'sloped' the profile is because you can't go fully open to fully closed instantaneously. Greater slope means you need stronger valve springs, which in turn means you lose more energy to valve spring compression each engine cycle, so you have to balance that too.

Now we bring in timing, which is the offset between top-dead-center of the engine and that of a given cam (every other rotation). AVCS makes timing almost infinitely adjustable, so we really don't need to worry about that in terms of hardware.
Instead it is just tuning parameters.

Based on the lack of cam products available, it looks like Subaru did a pretty good job with the cam optimization to begin with. There isn't much room to grow with lift because of physical can clearance (it's a boxer after all), and increasing duration would likely only help at higher RPMs, and would probably hurt at lower RPMs. Timing can be adjusted however you want with tuning.
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Old 05-14-2020, 12:42 AM   #16
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I was researching this topic for some time. Our cars don't have an external EGR system, but it looks that a similar functionality was implemented in the factory cams. Specifically, in the exhaust cam which retards the closing time of the valve. This was mentioned in an official engine paper (file attached). The Piper Stage 2.0 cams (272 266) are particularly good to eliminate this setting and enhance the scavenging effect. The HKS cams don't do anything to eliminate this setting. Does it really matter? Are there any considerable gains if the rev limit is not raised? No one really knows, because we don't have any dyno results.

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Last edited by nikitopo; 05-14-2020 at 02:33 AM.
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Old 02-03-2022, 02:25 AM   #17
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Originally Posted by nikitopo View Post
I was researching this topic for some time. Our cars don't have an external EGR system, but it looks that a similar functionality was implemented in the factory cams. Specifically, in the exhaust cam which retards the closing time of the valve. This was mentioned in an official engine paper (file attached). The Piper Stage 2.0 cams (272 266) are particularly good to eliminate this setting and enhance the scavenging effect. The HKS cams don't do anything to eliminate this setting. Does it really matter? Are there any considerable gains if the rev limit is not raised? No one really knows, because we don't have any dyno results.

.

EGR is not something implemented physically in the exhaust cam,
it is achieved though VVT.
The around +40 of retard in the exhaust cam combined with -10 of advance (so retard actually) in the intake cam that you can see in the avcs tables low rpm/low load is what gives an EGR effect
You change cams and keep those Avcs tables, you still get and EGR effect, it's the combination of exh retard + int retard at low loads anyway
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