6.2L Best Spark Timing for Fuel Economy

[asquinton]

Folks,

I've spent quite a bit of time looking here with no luck. Does anyone have a best spark timing from cruise conditions? I've tried to work it without a dyno but have too much noise in the data. Does anyone have any input on best cruise spark timing or does GM have it from the factory?

Thanks!

[5FDP]

Around 35-45 degrees, it's hard to nail down an exact number. In my 5.3 truck I can get low 20's for fuel mileage if I am trying to get good mileage on longer highway trips. Going 65-70mph is the key to my mileage.

[Alvin]

Add timing until you start running into knock. You should be on the freeway for best test of this as they will likely gladly take timing at a cruise for back roads country type driving but will easily get into knock as you try to gently pass other cars/climb hills etc while locked up on the freeway.

[BigTuner]

Add timing until you start running into knock. You should be on the freeway for best test of this as they will likely gladly take timing at a cruise for back roads country type driving but will easily get into knock as you try to gently pass other cars/climb hills etc while locked up on the freeway.

Would MBT be knock limited at cruising conditions, I would think MBT would be best for fuel economy as it is where you are most efficient?

[asquinton]

Thanks for the feedback guys. It's currently tuned to knock limit at cruise. I was worried I'd went too far on timing.

[Alvin]

Would MBT be knock limited at cruising conditions, I would think MBT would be best for fuel economy as it is where you are most efficient?

Its possible slightly less than where the knock limit is would be better but not typically and really that is the best anyone can do outside a lab setting.

People will spout "load bearing dyno" this and that.. I have one.. There is no way to keep a truck/car from not melting down and no way to reliably measure exact power output differences holding it at steady state. People often won't realize the insane volume of air passing over the trans oil pan radiator etc.. at a 70MPH cruise and how reproducing that on a dyno is not realistic.

[CaudleDynamicsLLC]

@asquinton do you have a good instantaneous MPG maths for your VCM scanner? If you create one using trans output shaft rpm instead of VSS it will give you MUCH more resolution.
If you can get a good clean, flat, uninterrupted stretch of road that you can make about a 1 mile pass on cruise control down, turn around and do the same in the opposite direction (to negate any wind), then repeat for a total of 4 passes that should give you a pretty solid baseline number if you filter the data well for only the steady state cruise control zones.

Between the precise MPG maths parameter and that drive cycle I've found to be very repeatable in tenths to hundredths of an MPG. Then going down or up in timing should show any gains or losses pretty clearly. All engines (even two stock engines side by side) have slight differences so optimal economy is going to come from data.

[asquinton]

@asquinton do you have a good instantaneous MPG maths for your VCM scanner? If you create one using trans output shaft rpm instead of VSS it will give you MUCH more resolution.
If you can get a good clean, flat, uninterrupted stretch of road that you can make about a 1 mile pass on cruise control down, turn around and do the same in the opposite direction (to negate any wind), then repeat for a total of 4 passes that should give you a pretty solid baseline number if you filter the data well for only the steady state cruise control zones.

Between the precise MPG maths parameter and that drive cycle I've found to be very repeatable in tenths to hundredths of an MPG. Then going down or up in timing should show any gains or losses pretty clearly. All engines (even two stock engines side by side) have slight differences so optimal economy is going to come from data.

I have not. I'll have a look and see if I can get it set up. Thanks very much for your help.

[Fast4.7]

Its possible slightly less than where the knock limit is would be better but not typically and really that is the best anyone can do outside a lab setting.

People will spout "load bearing dyno" this and that.. I have one.. There is no way to keep a truck/car from not melting down and no way to reliably measure exact power output differences holding it at steady state. People often won't realize the insane volume of air passing over the trans oil pan radiator etc.. at a 70MPH cruise and how reproducing that on a dyno is not realistic.

Used one dialing in my Express van. Hardest part was not overheating the dyno itself. With its massive engine driven fan the van never hinted at running hot.

[Alvin]

Used one dialing in my Express van. Hardest part was not overheating the dyno itself. With its massive engine driven fan the van never hinted at running hot.

Headers, cats, trans, rear end, air intake track, etc all remained cool? Congrats.

[Fast4.7]

Headers, cats, trans, rear end, air intake track, etc all remained cool? Congrats.

My air intake heat soaks to 170F at times when the hood is closed around town. With the hood open an dyno fans pointed at the front end and underneath it stays much cooler. Headers did not get hot because of the massive amount of air the fan draws across it. Converter is force locked during tuning so trans stays cool. Hard to heat up a GM full floating 10.5" 14 bolt. We picked about 20 load points then dialed in the spark advance real time. The rest of the spark map between those points merely got interpolated. Then we ran an IM240 schedule which test the engine and drivetrain like normal driving would. Then we made some power pulls from 1,000-5,500 rpm. It was 100F in the shop when we were doing this and the ac ran for all the driveability tuning. The only time the ac was disabled is when the PCM disabled it just like it would be on a hot Texas summer day. We stopped about 6 times to let the dyno cool down. I will say it would be nice to have the chassis dyno setup cell that IIRC Hypertech has. It has a wind tunnel so that they can test under load nearly indefinitely.

[Fast4.7]

Headers, cats, trans, rear end, air intake track, etc all remained cool? Congrats.

My air intake heat soaks to 170F at times when the hood is closed around town. With the hood open an dyno fans pointed at the front end and underneath it stays much cooler. Headers did not get hot because of the massive amount of air the fan draws across it. Converter is force locked during tuning so trans stays cool. Hard to heat up a GM full floating 10.5" 14 bolt. We picked about 20 load points then dialed in the spark advance real time. The rest of the spark map between those points merely got interpolated. Then we ran an IM240 schedule which test the engine and drivetrain like normal driving would. Then we made some power pulls from 1,000-5,500 rpm. It was 100F in the shop when we were doing this and the ac ran for all the driveability tuning. The only time the ac was disabled is when the PCM disabled it just like it would be on a hot Texas summer day. We stopped about 6 times to let the dyno cool down. I will say it would be nice to have the chassis dyno setup cell that IIRC Hypertech has. It has a wind tunnel so that they can test under load nearly indefinitely. Hypertech also uses a superflow that more realistically loads the powertrain because it spins the front wheels as well.