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Discussion Starter · #1 ·
Morning crew. Long time lurker, first post. Searched for the below info but didn't find the answer I'm looking for...

On PDK models, if engine start-stop is enabled, the vehicle will disengage the clutches and coast at speed while the rpms drop to idle.

This is great for efficiency, but obviously you lose the benefit of engine breaking.

Question:

If start-stop is disabled, and the clutches stay engaged as you let off the trottle, is fuel cut from the engine even though the rpms remain high?

This is how the DSG in my Audi S4 worked, and I could verify by watching the mpg monitor shoot up to infinity as I engine braked at high rpms.

Thoughts?
 

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Discussion Starter · #3 ·
Well approx 153 kmh, 7th gear, low rpm, fuel cut or not ?


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Thanks, but I know the fuel is cut when coasting at low rpm.

My question is whether fuel is cut when engine braking at higher rpms.

As mentioned, in my Audi I could be engine braking at 5k rpm and fuel consumption would read 0. That meant it was an efficient way to brake.

Wondering if the same holds true with PDK. Does the cayman cut fuel injection when you let off the gas to engine brake?
 

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I'd have to dig to find the old thread, but when the car is using coasting function or when you pull both shift paddles at once (manual way to coast) the car is actually put in neutral, not just disengaging the clutches.
 

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Thanks, but I know the fuel is cut when coasting at low rpm.

My question is whether fuel is cut when engine braking at higher rpms.

As mentioned, in my Audi I could be engine braking at 5k rpm and fuel consumption would read 0. That meant it was an efficient way to brake.

Wondering if the same holds true with PDK. Does the cayman cut fuel injection when you let off the gas to engine brake?
well, cutting fuel is relative, 800 rpm needs fuel too. Never looked up the consumpsion doing this


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Discussion Starter · #6 ·
Had a chance to play around a bit over lunch.

While there is no way to monitor live mpg on my CS, there was an average mpg ("Consumption") field that could be added to the Car display.

So, I ran a comparison, over the same stretch of highway. It's a half mile gentle decline, with long sweeping exit ramp at the bottom. Just prior to each test, I reset the values in the car menu, zeroing out the mpg value.

Run One: Stop/Start enabled, foot off the gas at 75 mph, car switched to neutral and rpms dropped to idle...

- After about 5 seconds, Consumption went read around 155 mpg and stayed there as I coasted for the half mile, to a stop at the bottom of the exit ramp

Run Two: Stop/Start disabled, foot off the gas at 75 mph, manually downshifting as necessary to engine brake....

- No numbers at all as I "broke" in 7th for about 15 seconds, then, as I manually downshifted through the gears, the mpg read 800, 350, 600, 250, 500, etc...

Conclusions:

- With Stop/Start enabled, the transmission goes to neutral and the engine consumes fuel at idle rate. No engine braking.
- With Stop/Start disabled, the transmission stays in gear, but the engine consumes no fuel at all, until you downshift, at which point fuel is used to rev match, then cut again, until the next downshift.


So in the end, I plan to leave Stop/Start disabled, enjoy the benefits of engine braking (i.e. less wear on the brakes and cool downshift sounds) and not worry about fuel consumption, as it seem to be about a rub, overall.
 
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when the car is using coasting function, or when you pull both shift paddles at once (manual way to coast) the car is actually put in neutral.
According to a very knowledgeable Porsche dealership owner, in both cases, the clutches are disengaged. But it might actually be both;). When rolling, if you use paddle-neutral to coast, rev the engine, or whatever, as soon as you hit a paddle (no need to touch the brake, like when stopped), you're instantly in gear again. Can't do that as quickly when in N. BUT if you're stopped, it might be neutral, since if you rapidly click a paddle with the brake pedal depressed and go to the gas immediately, it takes longer for the tranny to engage 1st, and the car jerks forward as a result. Remember it's illegal in some states to ever be in N when rolling (clutch disengaged is permitted), so that alone tells me it can't be N when rolling. There's one experiment you (or somebody else) can do to test if it's N when stopped: rapidly release brake and apply throttle (not too much; just to launch quickly but not too fast, like when you need to change a lane after a light) in both '1' and paddle-neutral, and if the car takes off quicker in '1', then paddle-neutral when stopped is real neutral, like I suspect is the case. At any rate, coasting is not ideal since you're using idle fuel (probably the same as feathering the throttle in high gear), plus clutches are kept disengaged. Just like start/stop, are stupid features on a sports car IMO, especially one as fuel efficient as our 981s, but to each his own. However, it's nice to have paddle-neutral since it's good for other purposes:).

So in the end, I plan to leave Stop/Start disabled, enjoy the benefits of engine braking (i.e. less wear on the brakes and cool downshift sounds)
And if you want to coast, put tranny in N at stoplights, or just rev your engine momentarily for the fun of it, use paddle-neutral, and you can have the best of both worlds;).
 

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You understand owner's manuals are never technical, right?;) They need to be understood by everyone, so the effect is tranny is in N, and that's what an owner's manual will say, regardless how that is mechanically achieved. By the way, I had that discussion with the owner recently. He made the comment about putting a car in neutral while rolling being illegal in some states (I remembered reading that a while ago), and you very well know nothing of that sort would get by manufacturers' lawyers, so it makes perfect sense to me. Besides, it also makes practical sense, since when you move your lever from N to D, it's not an instantaneous engagement, just like when you click a paddle while in paddle-neutral while stationary. But when you're rolling in paddle-neutral, gear engagement is immediate. Draw your own conclusions, but I've seen a ton of errors on owner's manuals (especially on foreign translations, like ours), and we're not even talking technical stuff, so not a bible to me;).
 
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