I know for E85 there is quite a bit of MPG to be gained by adding timing all over spark advance map. Anyone willing to advise what kind of advance numbers they are using and how they arrived at the numbers? I know the correct way would be to put the car on a dyno that can hold the engine at each RPM/load point and adjust to MBT & then back it off a couple degrees for safety - but that is not in the cards for this vehicle. Driveability is just fine left with the part throttle spark map for 92 octane, but I know I can get a material increase in MPG with more aggressive numbers.

Please share.

Thank you.

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84 NB, 3800SC, E-85, VS Cam, 3.0 Pulley, 4T65E-HD, HP Tuners, AEM Wideband, Regal GS Gauges, S-10 Brake Booster. 1/4 mile -12.15 at 111mph

[This message has been edited by DimeMachine (edited 05-14-2013).]

HP Tuners VCM Scanner should give you the ability to command an absolute timing advance amount (anything you want) in real-time while the engine is running (while the car is driving down the road or on a dyno). You'll find it under the VCM Controls heading -> Fuel/Spark -> Spark Control. What you can do with this is try different values while watching your manifold vacuum levels and throttle position levels. Your goal is to find the amount of timing that requires the least amount of throttle position and gives you the most vacuum while maintaining a certain speed/load.

I doubt you are going to get a one-size-fits-all number from anyone here that is going to match up perfectly with your build. Use HPT's scanner to find the best timing advance number your engine likes at that load and RPM and then input that value into your spark tables in the approprite area. Should be no need to back it off any for "margin of safety" since I presume this is a high-octane / low compression application and you are going to be working in the part throttle areas of the spark table for this.

-ryan

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Custom GM OBD1 & OBD2 Tuning | Engine Conversions & more | www.gmtuners.com

[This message has been edited by Darth Fiero (edited 05-15-2013).]

My experience.. bump it up till it feels right, then bump it more. Not uncommon to run 50 degrees, although the advantages are unknown to me.

Thanks DF & DH - I appreciate how you guys are always willing to share what you have learned. I was not aware of the ability to command timing with HP Tuners and will be trying that shortly. Watching the Vacuum, and TPS is a good idea as DF stated (Perhaps IPW too? & obviously any KR) as it would definately be indicative of identifying ideal timing advance. I was reading an article I saved from Car Craft last night where they were talking about tuning for fuel mileage and like DH mentioned, they too said don't be afraid if some of the cells are 50 degrees of total timing - which seems very high to me at first glance - but when you think about the small amount of intake charge in the cylinder at part throttle & light load it makes more sense. Fun Stuff !

quote Originally posted by DimeMachine: Thanks DF & DH - I appreciate how you guys are always willing to share what you have learned. I was not aware of the ability to command timing with HP Tuners and will be trying that shortly. Watching the Vacuum, and TPS is a good idea as DF stated (Perhaps IPW too? & obviously any KR) as it would definately be indicative of identifying ideal timing advance. I was reading an article I saved from Car Craft last night where they were talking about tuning for fuel mileage and like DH mentioned, they too said don't be afraid if some of the cells are 50 degrees of total timing - which seems very high to me at first glance - but when you think about the small amount of intake charge in the cylinder at part throttle & light load it makes more sense. Fun Stuff !

I don't think you'll ever see any KR at part throttle while using E85 on stock SC compression ratios. I will say that if you give the engine too much timing advance at part throttle, power will start to drop off (vacuum will go down / throttle position will need to be increased to maintain speed) and you may feel it trying to buck / misfire.

On 93 octane fuel, I'm running about 37 deg of total timing advance while cruising down the highway on my 8.5:1 compression Series 2. 50 degrees doesn't sound super excessive for E85 octane levels.

[This message has been edited by Darth Fiero (edited 05-16-2013).]