Does compression change in different modes?

dmaxdmax · 07-02-2021, 10:21 AM

I have a manual, and maybe I'm imagining this, but when I lift off the gas in Sport, it seems to hold the car back more than it does in comfort.

Neither has any gas going into it, so am I experiencing timing changes?

I've got some steep hills around me and in comfort, just seem to roll freely, without anything holding me back in 2nd, 1st makes my car cry at the bottom.

weehe126 · 07-02-2021, 10:30 AM

Sport would be at a higher rpm, which would have more engine braking than comfort at a lower rpm.

dmaxdmax · 07-02-2021, 01:49 PM

Quote:

Originally Posted by weehe126

Sport would be at a higher rpm, which would have more engine braking than comfort at a lower rpm.

I'm not sure I quite understand that. I have a manual with fixed gear ratios...so in either sport or comfort, if I'm going the same speed in the same gear, aren't the rpms going to be the same?

thejeremyman9 · 07-02-2021, 02:08 PM

It might be in your head. The primary different in mapping between sport/comfort that would be relevant under decel (0% pedal position) is WGDC. In sport WGDC starts at 100% and it opens as you reach boost target but in comfort it starts lower and increases as needed to reach boost target (to make it more responsive in sport). That could in theory increase back pressure and therefore increase engine braking but seems like it would be negligible.

WDE82 · 07-04-2021, 04:54 PM

It's conceivable that the valvetronic does something with valvelift to reduce or increase engine braking on overrun.

OneLastBMW · 07-07-2021, 05:35 PM

How exactly did you thing compression could change? It takes an adjustable connecting rod that I believe only Nissan has tried in the Infinity QX50.

Wires · 07-07-2021, 06:49 PM

In an auto, they lock/unlock the torque converter and it goes into "sail mode". Not sure what that entails, but it might be with the valvetronic to reduce backpressure.

No clue if the 6MT does the same, but perhaps (valvetronic wise).

dmaxdmax · 07-07-2021, 07:04 PM

Quote:

Originally Posted by OneLastBMW

How exactly did you thing compression could change? It takes an adjustable connecting rod that I believe only Nissan has tried in the Infinity QX50.

I don't know much about the insides of engines, but thought that maybe cam timing might be adjusted so it engine brakes better. It knows I backed off the gas, even though in a manual, it also revmatches first, so that's delaying the response I want too.

I feel less engine braking in this car than I did with my e46...probably because it also wants to go much faster than the e46.

thejeremyman9 · 07-08-2021, 05:53 PM

Quote:

Originally Posted by dmaxdmax

I don't know much about the insides of engines, but thought that maybe cam timing might be adjusted so it engine brakes better. It knows I backed off the gas, even though in a manual, it also revmatches first, so that's delaying the response I want too.

I feel less engine braking in this car than I did with my e46...probably because it also wants to go much faster than the e46.

E46 is N/A and a much higher compression ratio engine, which is the main driver of engine braking resistance. Gear ratio is the other major factor.

WDE82 · 07-09-2021, 10:28 PM

Quote:

Originally Posted by thejeremyman9

Quote:

Originally Posted by dmaxdmax

I don't know much about the insides of engines, but thought that maybe cam timing might be adjusted so it engine brakes better. It knows I backed off the gas, even though in a manual, it also revmatches first, so that's delaying the response I want too.

I feel less engine braking in this car than I did with my e46...probably because it also wants to go much faster than the e46.

E46 is N/A and a much higher compression ratio engine, which is the main driver of engine braking resistance. Gear ratio is the other major factor.

Um that's incorrect. The B58's compression ratio is higher than the M54. The N55 is the same.

thejeremyman9 · 07-10-2021, 02:48 PM

Quote:

Originally Posted by WDE82

Um that's incorrect. The B58's compression ratio is higher than the M54. The N55 is the same.

I meant E46 M3 S54 engine, which is 11.5:1.

But you are right that N55 and M54 are both 10.2:1, which is lower than B58 11:1.

07-02-2021, 10:21 AM	#1
dmaxdmax Captain 417 Rep 919 Posts Drives: 2016 M235i 6-spd Join Date: Jan 2020 Location: New Jersey iTrader: (0)	Does compression change in different modes? I have a manual, and maybe I'm imagining this, but when I lift off the gas in Sport, it seems to hold the car back more than it does in comfort. Neither has any gas going into it, so am I experiencing timing changes? I've got some steep hills around me and in comfort, just seem to roll freely, without anything holding me back in 2nd, 1st makes my car cry at the bottom. __________________ '92 325i, '99 328i, '16 340i, '16 M235i all manuals dmax on FB My last pre-covid standup set
	Appreciate 0 Tweet Quote

07-02-2021, 10:30 AM	#2
weehe126 Brigadier General 1161 Rep 3,190 Posts Drives: 2017 340i Join Date: Apr 2014 Location: San Antonio iTrader: (2)	Sport would be at a higher rpm, which would have more engine braking than comfort at a lower rpm.
	Appreciate 1 dmaxdmax417.00 Quote

07-02-2021, 02:08 PM	#4
thejeremyman9 Major General 4368 Rep 7,620 Posts Drives: 2015 BMW 335i Join Date: May 2019 Location: Cali iTrader: (0)	It might be in your head. The primary different in mapping between sport/comfort that would be relevant under decel (0% pedal position) is WGDC. In sport WGDC starts at 100% and it opens as you reach boost target but in comfort it starts lower and increases as needed to reach boost target (to make it more responsive in sport). That could in theory increase back pressure and therefore increase engine braking but seems like it would be negligible.
	Appreciate 1 dmaxdmax417.00 Quote

07-04-2021, 04:54 PM	#5
WDE82 Major 816 Rep 1,199 Posts Drives: 2018 440iGC F36 Join Date: Aug 2015 Location: Torrance, CA iTrader: (0)	It's conceivable that the valvetronic does something with valvelift to reduce or increase engine braking on overrun. __________________
	Appreciate 0 Quote

07-07-2021, 05:35 PM	#6
OneLastBMW Sarcasm while you wait. 149 Rep 118 Posts Drives: 2017 F33 B58 xDrive Join Date: Dec 2016 Location: The Fringe of the Motor City Suburbs iTrader: (0)	How exactly did you thing compression could change? It takes an adjustable connecting rod that I believe only Nissan has tried in the Infinity QX50. Last edited by OneLastBMW; 07-07-2021 at 05:42 PM..
	Appreciate 1 tex26702421.50 Quote

07-07-2021, 06:49 PM	#7
Wires Brigadier General 1660 Rep 4,903 Posts Drives: 2016 340i xDrive Join Date: Apr 2008 Location: Calgary iTrader: (0)	In an auto, they lock/unlock the torque converter and it goes into "sail mode". Not sure what that entails, but it might be with the valvetronic to reduce backpressure. No clue if the 6MT does the same, but perhaps (valvetronic wise).
	Appreciate 0 Quote