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tuning ignition timing

EFI Tuning Fundamentals

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HI all.

i have a question about ignition timing.

what are the mechanical parameters of the engine that may determine the final ignition advance to produce the peack cylinder pressure in the 16 to 18 degrees ATDC?

So for example:

if we have a 4 cilinder NA engine with 9:1 dynamic compression with 81mm bore and 87.2mm stroke with 20 deg of advance (suposed to be MBT) at 5000 and athmosferic pressure (WOT).

Can we interpollate in some way the MBT ignition advance for an NA engine with 9:1 dynamic compression with 83mm bore and 85mm stroke at 5000 rpm and WOT, suposing also the same VE number and Lambda?

Would it be the same in both engines?

if not, could it be due to a diferent piston position relative to TDC at 20 degres BTDC, afecting the actual volume and as a result chamber presure when the mixture is ignited?

i hope you undertand my doubts

thank you in advance

The point in the engine cycle where we want to achieve peak cylinder pressure is based around the engine geometry and in particular the stroke and rod length. It's quite possible that software could simulate the engine operation and find the optimal ignition timing for a variety of operating parameters but in all honesty, this simply doesn't matter. For our requirements as aftermarket EFI tuning, we simply want to test and find MBT for ourselves using a quality load bearing dyno.

Thank you Andre for the relpy.

i thought knowing how to make the conversions would be usefull when you want to tune an ignition map without a dyno, from another map in another engine that has been tuned to MBT, i know that wouldn't be to much accurate, but maybe you can be more or less near MBT.

what do you think about it?

Thanks

When I'm trying to tune spark without a dyno, it's really just a preliminary starting point before going to the dyno. So I'll take spark maps from a similar engine and start from there. Calculating MBT typically needs expensive commercial software that still needs to be correlated to physical measurements.