[Olasek]

Hi, this is my first post here. I am completely new to the Porsche brand though will be picking up a pre-ordered 2020 Macan GTS next month (my mountain-ski car) and was considering getting a base Boxster (or S) for my beach/wine country trips.

1. What about this boost pressure, I have been reading the base 2.0 engine has boot pressure of up to (20 psi) 1.4 bar versus the S version is at (14.5 psi) 1.0 bar. This confuses me, doesn't it mean that at sea level the turbocharger is not pumping any extra air into the cylinders in the 2.5 l engine? Then there is no benefit from turbocharging or am I missing something? Never owned a turbocharged car.

2. Rev matching with manual transmission - the Porsche configurator says that with chrono package you get rev matching in Sport mode when downshifting. But wouldn't such rev matching be also beneficial for upshifting, say you stay in a lower gear for too long before upshifting? Is it really limited to downshifting only? Since my Macan comes with PDK I am considering the manual transmission for contrast and extra fun, I enjoy driving my current 2001 BMW Z3 roadster stick.

 

[CaymanMatt]

Hi, this is my first post here. I am completely new to the Porsche brand though will be picking up a pre-ordered 2020 Macan GTS next month (my mountain-ski car) and was considering getting a base Boxster (or S) for my beach/wine country trips.

1. What about this boost pressure, I have been reading the base 2.0 engine has boot pressure of up to (20 psi) 1.4 bar versus the S version is at (14.5 psi) 1.0 bar. This confuses me, doesn't it mean that at sea level the turbocharger is not pumping any extra air into the cylinders in the 2.5 l engine? Then there is no benefit from turbocharging or am I missing something? Never owned a turbocharged car.

2. Rev matching with manual transmission - the Porsche configurator says that with chrono package you get rev matching in Sport mode when downshifting. But wouldn't such rev matching be also beneficial for upshifting, say you stay in a lower gear for too long before upshifting? Is it really limited to downshifting only? Since my Macan comes with PDK I am considering the manual transmission for contrast and extra fun, I enjoy driving my current 2001 BMW Z3 roadster stick.

Rev match is on downshift only. Engine revs fall when you lift off the throttle so you don't need matching on upshift. BTW the only up shift that can be rough is 1->2. I find that if you're not driving hard it still helps if you carry 1st gear through 4500 to 5000 rpms. The shift into 2nd is smoother as a result. If you're really cranking your revs will be high enough anyway and that up shift will be smooth.
MOO

 

[Olasek]

Engine revs fall when you lift off the throttle so you don't need matching on upshift.

Say for 1-->2 transition your engine revs fall to idle but your car speed is too fast for engine idle speed in 2-nd gear so in fact you would need to rev-up engine for smoother clutch operation. It happens to me occasionally in my Z3 stick when I stay too long in lower gear.

 

[CaymanMatt]

Say for 1-->2 transition your engine revs fall to idle but your car speed is too fast for engine idle speed in 2-nd gear so in fact you would need to rev-up engine for smoother clutch operation. It happens to me occasionally in my Z3 stick when I stay too long in lower gear.

If your engine speed falls to idle, you're not shifting fast enough! BTW, this is the reason I carry 1st gear up to 4-5K rpms. Yes, it's a large drop from 1st to 2nd and it's the hardest shift so it takes a bit longer. Starting at the higher revs means the engine speed doesn't drop as low, helping to make the shift to 2nd cleaner. Give it a try.

 

[JazzCatGab]

1. What about this boost pressure, I have been reading the base 2.0 engine has boot pressure of up to (20 psi) 1.4 bar versus the S version is at (14.5 psi) 1.0 bar. This confuses me, doesn't it mean that at sea level the turbocharger is not pumping any extra air into the cylinders in the 2.5 l engine? Then there is no benefit from turbocharging or am I missing something? Never owned a turbocharged car.

I think this is a much more complex question than it seems. For whatever reason, you need a bigger displacement, heavier engine to get 14.7 psi in naturally aspirated cylinders than you would get in turbocharged 2.5 engine. Also, you could only get that at sea level in a naturally aspirated engine (I would assume). It also has to do with air flow and how quickly you can fill a vacuum in a cylinder. I haven't found a good explanation either. But we do know that turbocharged engines do better at elevation than naturally aspirated engines. A pilot may be able to explain this better. I think it took jet engines to exceed the sound barrier (or was it a rocket engine?) and to allow higher altitude flight. Someone, please, set me straight on this.

 

[vosadrian]

The boost pressure question is simple. A boost of 0psi (or less with pressure drops in intake/throttle) is a naturally aspirated car with no turbo (or a turbo running no boost). A boost of 14.5psi is 14.5 psi above atmospheric pressure... so at sea level, the absolute pressure is ~29psi. So the base engine runs an absolute pressure of ~34.5psi at sea level.

The benefit with turbos is that (if boost control is programmed in this way) they can maintain the absolute pressure. So as you go up a mountain, the boost pressure (pressure above atmospheric) can go up to maintain the same absolute pressure in the manifold which will then maintain similar power to sea level. A naturally aspirated engine does not have this option so will lose absolute pressure and will therefore lose power at higher altitude. Note that this depends on the boost control algorithms in the ECU. The ECU can do what it wants with boost, but the above mentioned approach is commonly used.

Now given the base engine takes 20psi boost, it appears the engine internals are probably good for that in the 2.5L.... A 2.5L at 20psi sounds pretty good to me!! It sounds like thermal management becomes the limiting factor for sustained power... but for occasional power bursts on the street?!?!

 

[Olasek]

But we do know that turbocharged engines do better at elevation than naturally aspirated engines. A pilot may be able to explain this better

I am a pilot , I fly a turbocharged 4-seater Cirrus SR22T with the Continental IO-550 engine.
Most of my confusion is probably rooted in the fact that in aviation we deal differently with turbocharging, there is no such thing "boost pressure", we don't use PSI. We use manifold pressure (MP) and units are inches of mercury (29.92" HG for sea level). I am not allowed to run this Continental engine at anything above 37" Hg. We also have a 'critical altitude' - this is the altitude above which the turbocharging can no longer compensate for thinner air, in case of this engine it is 20,000 ft.

Thanks to all for the input.

 

[vosadrian]

The limit with turbos at altitude is related to over revving the turbo. To make the same Manifold pressure at altitude compared to sea level requires the turbo to spin faster as the air is less dense. As altitude goes higher, turbo speed goes higher to maintain the same manifold pressure. Eventually one of two things happens. Either the turbo spins too fast and risks failure... or the turbo becomes inefficient (falls off compressor maps) and needs too much energy out of the exhaust to maintain pressure which results in high exhaust back pressure which kills the power it is trying to make by shoving more air in. Either way, you are either power limited or risking damage to parts, so no point. I imagine aviation would leave some headroom to ensure an engine failure does not happen.

 

[GTSTALKER]

Boost is irrelevant with different sized motors and different sized turbos.

say we have two identical 2.5l engines, one with a tiny turbo and one with Bigger turbo. Say we are running 20psi on both. The bigger turbo motor will make more power as at 20psi,even Though same boost pressures, it’s moving more volume of air or CFM. In a nutshell

 

[GTS18BGTS]

Regarding 1st-2nd shifting, you should not have to have the RPMs above 2000 to 2500 to shift as long as you're on relatively flat ground and drop the shifter into 2nd quickly. You just need to get the car rolling. If you have issues, throttle blips help to keep the car from a stall. There's no magic here.

 

[Bob V]

The OP is wondering about a boost pressure figure that appears to be very close to atmospheric pressure (BAR being a figure of metric measurement versus imperial PSI, it's very close but is not technically "barometric" or atmospheric pressure per se), and it's a good question I never saw it that way. If you were trying to force air into something, you'd usually want higher than atmospheric pressure or a balloon would never inflate. Luckily, engines aren't balloons.

Wouldn't a normally aspirated engine have vacuum at the intake manifold? Some NA cars have a vacuum meter labeled as "fuel economy +/-" (I first noticed that on a 1987 mercedes, but a recent vintage Subaru has it too). My understanding - and please educate me if if this is just a wild assed guess - a forced induction engine is bringing that pressure back up so the cylinders aren't sucking air in anymore it's being fed; even though you wind up near atmospheric pressure it's still feeding more air than there would be without the turbo.

 

[vosadrian]

You are correct in some of what you say.... but the boost in bar is guage pressure... meaning that 1.0 bar is above atmospheric pressure... so at sea level, the absolute pressure is 2.0 bar in the manifold.