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Engine load metric?

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I am reading an article about vacuum advance - in it, there is reference to a "Jet" ignition table with a metric I don't understand nor can I find a description of what it is. Can't find any reference to the brand "Jet" to help clarify the reference.

Along the X-axis is RPM. Along the Y-axis is identified as gm/cyl. The range is 0.08 - 1.20 and higher - I can only see a limited range in the image clip. I know the Y-axis engine load, I just don't understand the term gm/cyl. Can anyone clarify this.

this is a link to the image: http://image.superchevy.com/f/197472323+q80+re0+cr1+ar0/8-jet-efi-screen-timing-map-stock-5-3l-gm-efi-truck-engine.jpg

and this is a link to the article: http://www.superchevy.com/how-to/97438/

grams per cylinder maybe? whatever it is, how would it relate to Kpa or inHg



The required spark lead (Ignition Advance) is dependent upon the density of the mixture. Density is the mass of the molecules in a given volume. So a load parameter of mass (in grams) per volume (of one cylinder) make perfect sense as the load axis.

The GM engine likely has a Mass Air Flow (MAF) sensor which is calibrated to identify how much air is entering the intake, and the ECU adds the necessary fuel mass (based on it's density and the target Air-to-Fuel Ratio) to arrive at how many grams of mass are in one cylinder's worth of volume.

To figure out what the equivalent manifold pressure would be, you would need to know the air temperature and the targeted Air-to-Fuel Ratio.

PS. I think the proper article is found here: http://www.superchevy.com/how-to/engines-drivetrain/1607-tune-your-vacuum-advance-for-better-drivability/

And I think the "JET EFI" mentioned might be one of these wonderful products:


The load units of gram/cyl or gram/rev are both very common in OE ECUs equipped with mass airflow meters. What the ECU really needs to know is the mass of air entering the engine, and the MAF sensor directly measures this, hence the load is expressed in units of mass. In the aftermarket we're much more likely to use an ECU that operates on the speed density principle where the airflow is calculated using the ideal gas law rather than being measured. in this instance manifold pressure is a key input to the calculation and that's why we see the load axis using units of pressure.

Thanks for your help and input. I am wanting to modify the ignition table of my ECU (Performance Electronics) for better economy, as we did not have a chance to do this while on the Dyno. Looking for various ignition tables to see how much and where they added advance rather than just blindly throw it in. Tuning now, at this point, is done on the road and hoping to be able to hear any ping. Difficult to do as it is a large (7 liter) Ford motor with side exhaust making it very difficult to hear the can of marbles. I've run upwards of 50degrees advance when carbureted and manual ignition so I kind of know what the engine likes, just trying to become more familiar with the ECU side of the world.

I see the load side of the table moves in equal increments so for my purposes, it really doesn't matter what the number is, although it would be nice to know (roughly) where 12"Hg is as it would give me a starting point to compare tables.

Us old guys struggle as we transition from old school mechanical stuff to state of the art technology. Thanks for helping us


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