Engine technology thread.

[fito68]

This is going to be a noobish question but does anyone know if the engine is going to have to be taken out to install the headers??

Before any flaming occurs the only reason i ask is because i had to do that to install on my Z and it was an expensive pain in the ass.

[old greg]

I'm assuming that the specs you're listing for the Prius are for the intake cam and not for the exhaust cam. The long duration is needed because it is an Atkinson cycle engine. The intake valves stay open during part of the compression stroke, effectively creating a smaller, lower dynamic compression ratio engine with a high expansion ratio for improved thermal efficiency. That's also why the Prius can run 13:1 static compression ratio on 87 octane gas.

I will say though, that the cam ramp angles are limited by the weight of the valves and stiffness of the springs. At some point, the improved efficiency from reduced pumping losses will be outweighed by the increased friction caused by the need for stiffer valve springs.

[serialk11r]

What I'm saying is that first of all it probably costs some extra money upfront to create specifically weakened springs for the Prius, and also that valves that open and close quickly would reduce variation in charge density over the rpm range.

Since the Prius operates at extremely low rpms by typical car standards and has a very high static compression ratio, it's important that at higher rpms the VE does not increase more than it has to. If the valves are restrictive, the piston doesn't push air back out as well when it's travelling faster, and while this can be offset by a further increase of intake duration, the effective compression ratio at low speed goes down which is bad.

I think it only makes sense that the Prius has a relatively "standard" valvetrain aside from the abnormal camshaft.

[old greg]

Originally Posted by serialk11r

What I'm saying is that first of all it probably costs some extra money upfront to create specifically weakened springs for the Prius...

I think it only makes sense that the Prius has a relatively "standard" valvetrain aside from the abnormal camshaft.

It doesn't cost any more to design a valve spring from thinner wire.

I never said it didn't have a normal valvetrain. I was responding to your assertion that valves should open/close as fast as possible. In an economy-oriented engine this isn't necessarily true.

[serialk11r]

Well yea, but I was responding to Dimman's speculation that the valve acceleration would be lower than normal.
"fast as possible" is variable in interpretation

[Dimman]

Originally Posted by serialk11r

Well yea, but I was responding to Dimman's speculation that the valve acceleration would be lower than normal.
"fast as possible" is variable in interpretation

The lower acceleration would be so they don't have to use the harder higher friction springs. Fast-opening valves are hard on valvetrain and need to be controlled with the harder springs. And then the harder springs have more valvetrain friction.

[Ryephile]

Originally Posted by fito68

This is going to be a noobish question but does anyone know if the engine is going to have to be taken out to install the headers??

Before any flaming occurs the only reason i ask is because i had to do that to install on my Z and it was an expensive pain in the ass.

Should be a piece of cake. Since it's a boxer, all the exhaust access will be on the very bottom of the engine. Remove the aero pan and BAM, there's everything.

[OrbitalEllipses]

Originally Posted by Ryephile

Should be a piece of cake. Since it's a boxer, all the exhaust access will be on the very bottom of the engine. Remove the aero pan and BAM, there's everything.

As Rye pointed out, it's quite easy. Taking the headers off my WRX is painless, especially since I cut-out a slot for the A/F sensor in the heat shield.

People talking cams and headwork though...that's not so fun. That will require dropping the engine. For instance: valve adjustment on EJ powered Subarus typically runs ~$1000 due the requirement of dropping the engine. Another example: spark plugs are pretty much in the fenders, requiring dexterity and small hands (otherwise enjoy your cuts/bruises). First time I did plugs I think it took me 2-3 hours.

[serialk11r]

Originally Posted by Dimman

The lower acceleration would be so they don't have to use the harder higher friction springs. Fast-opening valves are hard on valvetrain and need to be controlled with the harder springs. And then the harder springs have more valvetrain friction.

As I said, the valves have a huge effect on the combustion cycle, so there are very good reasons to not want to downgrade them.

I know that valvetrain friction is somewhat significant, but a Prius is almost always operating at full load, so I'd think that this is less important than how closely they can control the VE, as the higher end of the rpm spectrum is what limits the compression ratio. When you only have like 8:1 effective compression at low speeds or less, that matters a lot when you're trying to build the most efficient engine possible for a low cost.

If valvetrain friction were that important, I'd think that more engines would be rev limited to their hp peak and fitted with the weakest springs possible. Maybe that's an overstatement, but whatever.

[Tradewind]

Originally Posted by OrbitalEllipses

As Rye pointed out, it's quite easy. Taking the headers off my WRX is painless, especially since I cut-out a slot for the A/F sensor in the heat shield.

People talking cams and headwork though...that's not so fun. That will require dropping the engine. For instance: valve adjustment on EJ powered Subarus typically runs ~$1000 due the requirement of dropping the engine. Another example: spark plugs are pretty much in the fenders, requiring dexterity and small hands (otherwise enjoy your cuts/bruises). First time I did plugs I think it took me 2-3 hours.

Certainly true enough

The Subaru flat 6 is absolutely terrible to get plugs in an out of. The 4s in Impreza NA are easy tho

The 86 could be ok tho, however valve adjusting and cam changing is a cruel exercise, pass go on that idea of cam changing for a few measly extra hp


Historically Subaru flat four valve springs have been good for max rpm of 7500rpm since 1998

The Subaru 6 flat (not in this discussion really) is only good for safe 7000rpm - weak exhaust springs

[Ryephile]

The FA20 has a 7450 RPM fuel cut, and you can bet safely the OEM rev limit isn't anywhere near the RPM when valve float becomes a concern.

[arghx7]

Originally Posted by old greg

I'm assuming that the specs you're listing for the Prius are for the intake cam and not for the exhaust cam. The long duration is needed because it is an Atkinson cycle engine. The intake valves stay open during part of the compression stroke, effectively creating a smaller, lower dynamic compression ratio engine with a high expansion ratio for improved thermal efficiency. That's also why the Prius can run 13:1 static compression ratio on 87 octane gas.

The Prius is on its third generation and is using a different engine and engine family now, the 2ZR-FXE (1.8) vs the old 1NZ-FXE (1.5) engine. The valve timing range is different between the two, in part because the 2ZR has an EGR cooler.

Originally Posted by serialk11r

What I'm saying is that first of all it probably costs some extra money upfront to create specifically weakened springs for the Prius, and also that valves that open and close quickly would reduce variation in charge density over the rpm range.

Since the Prius operates at extremely low rpms by typical car standards and has a very high static compression ratio, it's important that at higher rpms the VE does not increase more than it has to. If the valves are restrictive, the piston doesn't push air back out as well when it's travelling faster, and while this can be offset by a further increase of intake duration, the effective compression ratio at low speed goes down which is bad.

I think it only makes sense that the Prius has a relatively "standard" valvetrain aside from the abnormal camshaft.

On the 2ZR, the key is the balance between EGR rates and cam phasing:



neither engine lugs at low rpm for extended periods... here are the brake specific fuel consumption maps:

Originally Posted by Ryephile

Should be a piece of cake. Since it's a boxer, all the exhaust access will be on the very bottom of the engine. Remove the aero pan and BAM, there's everything.

That's assuming they don't have some kind of weird subframe or mounting system.

Originally Posted by OrbitalEllipses

People talking cams and headwork though...that's not so fun. That will require dropping the engine. For instance: valve adjustment on EJ powered Subarus typically runs ~$1000 due the requirement of dropping the engine.

For all we know this new engine has hydraulic lash adjusters.

[serialk11r]

Oh, did not know about EGR cooler. Thanks for the info. Regarding low rpm operation though, that "basic operating line" that they give, I am fairly sure it typically runs near the lowest rpm part of it because if you calculate the approximate cruising power required at say 60mph, it's around 80 Nm at 1000rpm (I don't like explicit hp calculations, I just multiplied through by a lot of ratios comparing with other cars). I believe the Prius is supposed to cruise at max efficiency on the highway.

[old greg]

Originally Posted by Ryephile

Should be a piece of cake. Since it's a boxer, all the exhaust access will be on the very bottom of the engine. Remove the aero pan and BAM, there's everything.

Yeah, but on the AS1 the engine seems to be sitting right on top of the front subframe, so it may not be all that easy.