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So earlier today CPMorey (a 997 GTS owner) posted an excellent review of the 992 4S he's enviably driving through Germany, Switzerland and Austria. In that review he makes a number of very fair and solid observations about the 992 (these are 3.0 liter engines with turbos) and differences betweenthe 992 and the 997 as well as pros and cons of the 992.
One of the cons was "There is a lag. I don't care what they say. The lizard part of my brain notices the delay between stomping the gas pedal and sinking into the seat." I don't question whether there is lag or not with the 992, I'm certain there is and he is right.
But I did start to question my own understanding of lag and CP's comment got me out in my car today to test my theory that at least the 997.2 TTS has what I might call (as an ignorant person this AM) lag by choice or lag by ineptitude. Turns out there is a formal term for this that I learned later in the day.
My theory before the drive - turbo lag is either 'rpm lag' or 'displacement lag' and that a well designed turbo (as Porsche is absolutely among the best) has no discernible lag when driven properly. (and again not questioning CP's driving)
I went to test this theory by letting the car really warm up - a good amount of driving below 3000 RPM and running the AC - something I rarely do but wanted to get the op temps up to be efficient as possible.
After 30 minutes I got to some open road, turned off the AC and stereo and really listened to the motor.
Cruising (3000 rpm) in normal mode low RPMs and the accelerating - definitely lag.
Cruising in sport mode low RPMs and then accelerating - some lag.
Driving in sport plus mode, which forces high RPMs, and then accelerating - no lag.
Coasting and then accelerating in the lower two modes - definite lag.
Coasting in sport plus mode and then accelerating - no lag. (because the engine remains at high RPMs and PDK shifts almost as soon as you hit the gas)
So basically I found that if the 997.2 3.8 TTS engine is loaded (in the right high RPM range) there is no turbo lag or lag of any sort for that matter.
Whether this was caused by RPM, displacement or engine/turbo design that answer would be found above my pay grade and engineering capabilities.
It turns out what most people call "turbo lag" (and not saying this is what CP did or did not experience) is actually being below and transitioning across the cars "Boost Threshold" and the difference between "Boost Threshold" and "Turbo Lag" explains and validates my informal experiment this AM and my belief that at least the 997.2 TTS does not have any "turbo lag" when above boost threshold. AKA - driven like a Porsche.
So here's a good explanation from a person even more annoying than me.
I'd be interested to hear what other turbo owners of .1s and .2s think about this and what their experience is in their car. (lastly, my apologies if this subject (boost threshold) is already common knowledge. It's new to me and I really enjoy learning something new. It's half the fun of having one of these cars. Happy 4th of July.
Jason Cammisa (in his typically goofy style) did a good video on the effect on throttle response when turbos were added to the 991.2. Worth watching all the way through.
Between idle and boost threshold, there is turbo lag. I've ridden and driven some turbo cycles and cars. I kinda enjoy the lag and hit of acceleration in the right type of vehicle.
Binary on/off drivers do not feel turbo lag, as for them it is all or nothing. For analog, smooth drivers, turbo lag is very much real and intrusive. Just ask Walter Rohrl.
They seem to be saying that if the power is delivered in a linear way then lag isn't apparent and / or if you're accustomed to some measure of lag then you know how to use it and or ignore it or adapt to it.
Here is an explanation for the possible illusion I am experiencing; an appx 160 hp difference between the C4S and the Turbo S at any RPM above ~ 3500. (perhaps an apples to apples ( a 530 hp NA vs a 530 hp turbo ) is a better comparison. And in all fairness maybe CPs #s between the his GTS and the 992 are apples to apples)
I definitely agree with C4SDayton's comment "Between idle and boost threshold, there is turbo lag"
A lot of truth to OP's statements. Another thing to consider is a turbine is response is highly influenced by heat so boost threshold is not directly equivalent to zero lag at the same measured rpm (as to test boost threshold you are building maximum heat at wide open throttle transitioning through that rpm range...) rather than cruising at a lower rpm and downshifting to a higher rpm while going WOT.
Even holding it at a certain rpm before WOT doesn't guarantee full boost vs pulling into that range from a lower rpm.
This partly because, all parts of the system directing exhaust flow to the turbine absorb heat (which doesn't reach the turbine) until they are at full temp.
(So, headers and turbine housing absorb heat until they are up to full temp at which point the turbine receives max heat/energy)
So yes, driven correctly it can certainly be minimized but will always at times be felt in comparison to a naturally aspirated engine.
Jason Cammisa (in his typically goofy style) did a good video on the effect on throttle response when turbos were added to the 991.2. Worth watching all the way through.
In reference to OP- there is always turbo lag, as has been stated by a few already. The torque rush characteristic turbos provide is something that cannot be altered. Personally, I love it and I am also partial to the sound of spool. I do believe you are right that the Porsche pairing in this case is well suited and the rush is more of a threshold than true lag. However, in the video above, it is worth noting the tuning witchcraft that Porsche has implemented with the 991.2s. Those turbo motors have a vastly different feel where; even when throttle is closed, there is air coming into the engine via a slightly opened throttle body. Pretty cool stuff. I think this is what Randy feels when hes on track just because he is always in the performance powerband, even at second going around a turn at "low" speed.
Great explanation of what they call "Dynamic Boost" here:
Well, regarding turbo, my wife has an Audi Q5 with the turbo engine and I feel no lag when cruising and accelerating.
In heavy traffic however, when hardly moving, if there's an opprortunity to swtich to a faster lane you have to calculate your stuff cause as you floor it to get up quick, it's like the vehicle has to think about it, therefore it's nothing short of dangerous.
It will turn 3 years next year and we usually change her car every 3 years, and I guaranty you the next one won't be a turbo. In fact, she wants a Macan and it better be available without a turbo.
That experience with her turbo made me chose the pentastar V6 in my new Jeep wrangler JLU, rather than the 4cyl turbo they came up with.
The new Mercedes AMG engines have repositioned the turbochargers directly over the cylinders to minimize time and distance effects. Turbo lag is virtually nonexistent.
Well, regarding turbo, my wife has an Audi Q5 with the turbo engine and I feel no lag when cruising and accelerating.
In heavy traffic however, when hardly moving, if there's an opprortunity to swtich to a faster lane you have to calculate your stuff cause as you floor it to get up quick, it's like the vehicle has to think about it, therefore it's nothing short of dangerous.
It will turn 3 years next year and we usually change her car every 3 years, and I guaranty you the next one won't be a turbo. In fact, she wants a Macan and it better be available without a turbo.
That experience with her turbo made me chose the pentastar V6 in my new Jeep wrangler JLU, rather than the 4cyl turbo they came up with.
I think most people include boost threshold as part of what they feel as lag. What has not been mentioned is impeller design, intercooler design and location, blow-off valve configuration and length and size of the pipes between turbos, intercooler, and intake port. For example, An S impeller takes a lot longer to spool than an H impeller, but an S impeller has more capacity. Also, as has been mentioned, AMG uses very short runners, so the distance air travels is minimized. Horizontally opposed engines have very long runners.
I’ve owned two Porsche 911 turbos. One was a 997.1, and one is a 991.1. They both lag. The 997.1 had tremendous lag, but that’s part of the turbo experience: lag, followed by the feeling that you’ve been shot out of a cannon. The 991.1 turbo s has less lag, but it’s still there. It’s still part of the Porsche 911 turbo experience. You can obviously minimize lag by keeping the revs up, but in everyday driving, lag is real.
I have instructed many different 911s (GT3, C2S, turbo, etc.) and when you are teaching a novice driver how to drive on a race track the difference in power delivery is very noticeable. I actually had a lady with a 997 turbo this past weekend and it took a bit beforeI could feel the car through her right foot. One thing I found to actually smooth the power delivery was having sport mode on? Not sure if it was because the car was an auto or what, but it really smoothed things out.
The way a turbo works, there's no way to avoid lag. It has two impellers connected by a shaft. One impeller is in the exhaust gas flow and the other in the intake air flow. The speed of the exhaust gas going by the exhaust impeller is what spools it up to cause it to compress the intake air, so you essentially have a flow obstruction in your exhaust system until the exhaust impeller is moving at the same speed as the exhaust gasses. When you push the throttle, it increases the exhaust output, which increases the compression factor on the intake side.
In my DFI Cayenne Turbo S, the best way to avoid the feeling of lag is to manually downshift one gear when trying to accelerate hard. This quickly raises the RPMs, which increases the exhaust pulses going by the turbos, and causes them to spool faster. MexicoBlueTurboS's experiment found similar results when in Sport mode as the PDK downshifts automatically and quickly when in Sport and Sport Plus modes.
Superchargers are different in that their impellers are spinning at a fixed rate relative to crankshaft speed based on the pulley size differences between the crank pulley and the supercharger pulley so a supercharger is always creating boost and that boost rises with RPMs so a supercharged car (like my 997 and my 928) just feels like a larger normally aspirated motor without the slingshot effect felt with a turbo. A turbo is more efficient since it uses the normal combustion process (exhaust gas flow) to compress the intake air and when cruising on the highway with light throttle is essentially idle for better fuel economy, whereas a supercharger places a parasitic horsepower drain on the motor all the time since it's directly driven by the crank all the time.
Each has their pros & cons and both setups drive differently.
07-04-2019, 07:08 PM
Questioning the concept of "turbo lag"
