Downshifting LS3 - Camaro5 Chevy Camaro Forum / Camaro ZL1, SS and V6 Forums

Nate76 · 12-08-2015, 07:49 PM

I was wondering if downshifting burned fuel. Firstly, I don't really care if it does- I didn't buy the SS for fuel efficiency, however I'm curious. So let's say I'm driving and slow down from highway speed. Press the clutch and put it in third, let the clutch out and the rpms go up to the 3500 range. My foot is off the gas, it's only my prior speed that caused the engine to rev that high. Is this burning gas at a higher rate or just at an idle rate? If it's not burning gas what making that beautiful, loud sound coming out of the exhaust as the engine slows??

paul84043 · 12-08-2015, 07:55 PM

Not really. The EFI doesn't demand fuel delivery unless the throttle sensor is asking for it.
It's probably not even burning it at the idle rate, most engines with the instantaneous readout will go to some high value.
Even carbureted engines only deliver the gas the foot pedal or Idle stop is asking for.
Whether or not it shuts off completely probably varies by manufacturer..

Engines with direct injection have even finer control.. Some of the TDI diesels have injectors so fine they can actually release a set number of fuel molecules..

NC_SS · 12-08-2015, 07:56 PM

Fuel and air are probably in the equation, but I'm not an engineer

Easy answer is probably. I don't think revving your engine higher helps MPG's lol

paul84043 · 12-08-2015, 07:59 PM

Quote:

Originally Posted by NC_SS

Fuel and air are probably in the equation, but I'm not an engineer

Easy answer is probably. I don't think revving your engine higher helps MPG's lol

But he's not revving it.. he's downshifting.. Different animal..

NC_SS · 12-08-2015, 08:00 PM

Quote:

Originally Posted by paul84043

But he's not revving it.. he's downshifting.. Different animal..

right, like I said I'm no engineer

idgaf about MPG's in my SS lol

rebelyell22 · 12-08-2015, 08:07 PM

Quote:

Originally Posted by paul84043

But he's not revving it.. he's downshifting.. Different animal..

I think he means rev matching

Spec · 12-08-2015, 08:18 PM

Higher RPM burns more fuel than lower RPM. Your engine goes through more cycles in a minute obviously at higher RPM than at lower RPM... thus burning more fuel measure on a timeline... even if you don't press on the pedal, its still needs to spray more fuel into the cylinders to maintain the higher rpm... I think, lol... I'm no mechanical engineer either...

Irace16 · 12-08-2015, 08:44 PM

I know for sure a carbureted engine pulls fuel at high rpm while not on the throttle. I believe vacuum pulls it. That's why some race cars have a flame out the exhaust in the middle of a corner. That said I have no idea if that happens with fuel injection

Revrider550 · 12-08-2015, 08:46 PM

If your car revved up because of the syncros in the transmission and the fuel delivery didn't pick up accordingly I'd assume it would lean out. So yes it dumps in more fuel when it revs up or there would be a lot more burnt down manual camaros out there.

Sent from my XT1080 using Tapatalk

havelegs · 12-08-2015, 10:13 PM

Still needs fuel to get those rpms up and keep the engine running.

Nor Cal ZL1 · 12-09-2015, 12:51 AM

Downshifting and not touching the throttle will use no more fuel than at idle. The throttle plate is closed so essentially no fuel is needed at all. Back when I raced mechanical injected cars there was a fuel cut off solenoid that dropped all fuel flow to the injectors until 1200 RPM. So from 7500 rpm down to 1200 no fuel was injected at all when decelerating.

two_wheel_mayhem · 12-09-2015, 01:19 AM

They decel pop when you left off the gas, what's that tell you? There is no fuel being put into the engine when you are at 0% throttle position and the vehicle is moving.

The speed sensor tell the ECM when the car is not moving and it's time to idle. Therefore, coasting or engine braking burns less fuel than idling.

Bodywerks · 12-09-2015, 01:50 AM

This is why you don't ask this type of question in the general forum...

FunkedOut · 12-09-2015, 02:48 AM

DFCO = deceleration fuel cut off
You are injecting zero fuel until the rpms drop very low.

12-08-2015, 07:49 PM	#1
Nate76 Drives: 2022 2SS Convertible Join Date: May 2011 Location: Ontario Posts: 96	Downshifting LS3 I was wondering if downshifting burned fuel. Firstly, I don't really care if it does- I didn't buy the SS for fuel efficiency, however I'm curious. So let's say I'm driving and slow down from highway speed. Press the clutch and put it in third, let the clutch out and the rpms go up to the 3500 range. My foot is off the gas, it's only my prior speed that caused the engine to rev that high. Is this burning gas at a higher rate or just at an idle rate? If it's not burning gas what making that beautiful, loud sound coming out of the exhaust as the engine slows??

12-08-2015, 08:44 PM	#8
Irace16 Drives: 2010 1SS RS M6 Join Date: Jan 2015 Location: Appleton, Wisconsin Posts: 521	I know for sure a carbureted engine pulls fuel at high rpm while not on the throttle. I believe vacuum pulls it. That's why some race cars have a flame out the exhaust in the middle of a corner. That said I have no idea if that happens with fuel injection __________________ Vararam intake, gmpp exhaust, borla stainless long tubes, cat less, jannetty tune, bmr lowering springs, heritage grill , smoked lenses

12-08-2015, 10:13 PM	#10
havelegs Drives: 24 LT1 M Join Date: Oct 2015 Location: North/Central FL Posts: 1,121	Still needs fuel to get those rpms up and keep the engine running. __________________ 24 LT1 M6 Sharkskin Metallic 22 Mazda CX-30

12-09-2015, 01:19 AM	#12
two_wheel_mayhem Would rather be riding Drives: No car no more Join Date: Jul 2009 Location: Indiana Posts: 2,750	They decel pop when you left off the gas, what's that tell you? There is no fuel being put into the engine when you are at 0% throttle position and the vehicle is moving. The speed sensor tell the ECM when the car is not moving and it's time to idle. Therefore, coasting or engine braking burns less fuel than idling. __________________ No dog in this fight anymore. 5th Gen owner 2009-2016.

12-09-2015, 01:50 AM	#13
Bodywerks Drives: 2015 2SS/RS/1LE Join Date: Feb 2015 Location: Tucson Posts: 866	This is why you don't ask this type of question in the general forum... __________________ 2015 SS/RS/1LE HSA delete, VMAX throttle body, CAI inc. Intake, PRC255 heads, BTR 660 springs, BTR cryo-treated rockers, PAT G 232/240 .654/.626 110+2, johnson 2110's, Manton 502 pushrods, C5R, Melling high volume/high pressure, Powerbond 25% underdrive pulley, Proform covers, Kooks stepped headers, catless mids, Doug Thorley exhaust. 512rwhp, 446rwtq on a mustang dyno, tuned by Cunningham motorsports.

12-08-2015, 07:55 PM	#2
paul84043 Drives: 2015 Camaro 2SS RS 1LE Join Date: Nov 2011 Location: Lehi, Utah Posts: 1,349 View My Garage	Not really. The EFI doesn't demand fuel delivery unless the throttle sensor is asking for it. It's probably not even burning it at the idle rate, most engines with the instantaneous readout will go to some high value. Even carbureted engines only deliver the gas the foot pedal or Idle stop is asking for. Whether or not it shuts off completely probably varies by manufacturer.. Engines with direct injection have even finer control.. Some of the TDI diesels have injectors so fine they can actually release a set number of fuel molecules..

12-08-2015, 07:56 PM	#3
NC_SS Drives: 2012 M6 Camaro SS (RIP) Join Date: Nov 2014 Location: Virginia Posts: 1,684	Fuel and air are probably in the equation, but I'm not an engineer Easy answer is probably. I don't think revving your engine higher helps MPG's lol

12-08-2015, 08:18 PM	#7
Spec Account Suspended Drives: 2010 Camaro 1SS LS3 Join Date: Jan 2015 Location: Riverside, CA Posts: 4,018	Higher RPM burns more fuel than lower RPM. Your engine goes through more cycles in a minute obviously at higher RPM than at lower RPM... thus burning more fuel measure on a timeline... even if you don't press on the pedal, its still needs to spray more fuel into the cylinders to maintain the higher rpm... I think, lol... I'm no mechanical engineer either...

12-08-2015, 08:46 PM	#9
Revrider550 Drives: 2015 1SS/RS Join Date: Apr 2013 Location: New York Posts: 104	If your car revved up because of the syncros in the transmission and the fuel delivery didn't pick up accordingly I'd assume it would lean out. So yes it dumps in more fuel when it revs up or there would be a lot more burnt down manual camaros out there. Sent from my XT1080 using Tapatalk

12-09-2015, 12:51 AM	#11
Nor Cal ZL1 Drives: 2013 ZL1 M6 Join Date: Oct 2013 Location: Nor Cal Posts: 1,825	Downshifting and not touching the throttle will use no more fuel than at idle. The throttle plate is closed so essentially no fuel is needed at all. Back when I raced mechanical injected cars there was a fuel cut off solenoid that dropped all fuel flow to the injectors until 1200 RPM. So from 7500 rpm down to 1200 no fuel was injected at all when decelerating.

12-09-2015, 02:48 AM	#14
FunkedOut Drives: Black 2010 SS Join Date: Dec 2012 Location: FL Posts: 447 View My Garage	DFCO = deceleration fuel cut off You are injecting zero fuel until the rpms drop very low.