I was on the highway in 2021 Cayman S 6 SPD Manual in 6th gear doing 76mph and came up on a long downhill stretch where it said trucks us lower gear. I happen to glance at the boost gauge and it was showing 3 PSI. Curious why it is showing 3PSI if my foot off the accelerator pedal?
Turbo building boost coasting long downhill
- Thread starter raaizin
- Start date
Pushing enough air through to spin the turbo and build boost against a closed throttle. I bet the turbo comes in at quite low rpm
It's a 4 cylinder turbo it makes 309 ft/lbs at 1900 rpm which is max torque. You begin to feel turbo a 2k and at 3k you really feel it. I think RPM were around 3k @76mph. What I don't understand is why build boost seems to be wasting fuel when not needed
Engine braking would be creating excess exhaust energy. Your question should be why Porsche doesn’t have the blowoff valve open whenever the throttle is closed; sounds like the answer is they know how most Porsches are driven (performance-minded) and they do this for the least amount of lag.
Not many, if any, people who have fuel economy concerns buy a Porsche because they’re hoping for miserly consumption.
Just a wild guess, but could it be for emissions?
Is it though? What is the fuel injector duty cycle telling you? If the injectors are not firing, it won't be using any fuel.
No only on longer down hill grades. This makes sense as when you hit the gas it pulls like a train.
I am not overly concerned about fuel economy though I I am impressed with the cars performance while pulling down 30+mpg on the highway. By the same token I like the new Dark horse Mustang but I wouldn't put up with its mileage. I guess old habits die hard.
Just because the manifold pressure is at 3psi, doesn't mean the injectors are not closed when coasting. Boost without fuel is just air pressure and it probably helps some with compression braking and scrubbing off the kinetic energy when going down a hill.
So you downshifted to a much lower gear sufficient to "brake"? It takes sufficient amount of exhaust pressure to build boost.
It’s impossible for a turbo to create boost without burning fuel & using the resultant exhaust energy to spin the exducer fast enough to allow the inducer to compress intake air.
You can bet your $ that anything making 30psi is going to consume plenty of fuel.
Maybe it’s a poor man’s Jake brake like on semis.
Yea I just found it on R&T:
In a Porsche with Dynamic Boost, lifting off the throttle pedal shuts off the fuel injectors, but leaves the throttle partially open. This allows more air to pass through the engine, keeping exhaust flowing and spinning the turbos even during deceleration. Dynamic Boost doesn't get rid of turbo lag entirely, but it reduces the time it takes to get the turbochargers spooled up significantly.
In a Porsche with Dynamic Boost, lifting off the throttle pedal shuts off the fuel injectors, but leaves the throttle partially open. This allows more air to pass through the engine, keeping exhaust flowing and spinning the turbos even during deceleration. Dynamic Boost doesn't get rid of turbo lag entirely, but it reduces the time it takes to get the turbochargers spooled up significantly.
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Indeed! They've especially come a really long way in the past 10 years. I remember back in 2013/4, when Porsche announced the 718 was going to a turbo 4, was how Porsche was going to get around the turbo lag. This seems like an easy and economical way compared to "traditional" anto-lag systems. I believe Merc has something similar with the AMG cars (or it was a Weistech tune, I don't recall.)
Going down hill mine gets as high as - (minus) 11psi
