Why would timing be removed for E85?

[Gh0st]

Not sure if it is just GMT900 tunes that the E85 spark adder table is like this, but I'm wondering why the spark adder table removes spark in the normal driving cells for E85. Maybe I'm missing something...

[brandonleg]

Curious about this too.

I'll throw this out there to get some conversation going.

Though E85 is roughly 104 octane and tolerant of more advance, due to it's burn rate less advance is required.

That's just one school of thought I've found while browsing around. Gotta love the internet, you can find lots of crap that's says the opposite too.

[eficalibrator]

All else being equal, the laminar flame speed of ethanol combustion is faster than that of gasoline. Faster flame = less spark advance required to deliver peak pressure at the right point of the engine cycle.

[BBA]

All else being equal, the laminar flame speed of ethanol combustion is faster than that of gasoline. Faster flame = less spark advance required to deliver peak pressure at the right point of the engine cycle.

To get this right, it has a slower burn rate but faster flame propagation?

[Gh0st]

Makes sense. Thanks as always guys for helping me understand. Learn something new every day!

[GregRexAdams]

To add to what Greg said...where you hear about advancing timing with E85 in aftermarket (mostly boosted) applications is where the timing is limited by knock before optimum power is found. On a stock truck engine with lower compression, that's less likely to be the case, so you may find the timing retarded.

[eficalibrator]

To get this right, it has a slower burn rate but faster flame propagation?

No, ethanol has a faster burn rate than gasoline. It just has a much higher AKI (knock index) so you are rarely limited by knock at WOT which allows you to run more timing at high load.

[LSX378]

All else being equal, the laminar flame speed of ethanol combustion is faster than that of gasoline. Faster flame = less spark advance required to deliver peak pressure at the right point of the engine cycle.

Is there pinging if the timing is not pulled at the lower airflows/part throttle?

And more spark required at higher airflows? Or simply that more spark is tolerated due to octane for better power?

[eficalibrator]

You will not likely experience knock at light load, even if you're running timing greater than MBT. You'll just lose torque and efficiency.

At high load, we're typically limited by gasoline's relatively low knock threshold. Since the knock threshold of alcohols are higher, you can run closer to MBT timing without getting into knock at high load.

[Xjas]

Is it possible to determine MBT at light loads without the use of a dyno? ie. road tuning.
Or at heavy loads when using E85 when knock is not a limiting factor?

[eficalibrator]

The only accurate way to find MBT is by either:

1) use a load bearing dyno that can show instantaneous torque delivery during steady state testings, or
2) use in-cylinder pressure measurement combined with a crank angle encoder and plot cylinder pressure vs crank angle so you can identify location of peak pressure or the CA50 number. This is expensive, but very precise. It's also the way it's done at OEM engine dyno development facilities.

[wyochimneysweep]

That's a bummer for us without a dyno anywhere close and running e85

[Gh0st]

The only accurate way to find MBT is by either:

1) use a load bearing dyno that can show instantaneous torque delivery during steady state testings, or
2) use in-cylinder pressure measurement combined with a crank angle encoder and plot cylinder pressure vs crank angle so you can identify location of peak pressure or the CA50 number. This is expensive, but very precise. It's also the way it's done at OEM engine dyno development facilities.

#2 always makes me wonder if OEMs are giving us MBT timing from the factory or not for "part throttle" areas. I understand why MBT wouldn't be possible for higher loads, but part throttle why wouldn't they design the spark tables to be correct right out of the box?

[LSxpwrdZ]

Loaded dyno is def the trick. I use a Dyno Dynamics which shows instant "Tactive Effort" in lbs and also shows HP. The Tactive Effort is a bit more high resolution because most LSX based engines will produce about 2500lbs of tactive effort or force physically on the roller. Monitoring that tactive effort while adjusting timing up or down at various load/rpm points really makes for accurate timing tables.

Total MBT of an engine cannot be achieved usually on gasoline, so the MBT of the engine on that particular fuel is probably what is used by the OEM. Maybe Greg will chime in but I believe OEM tests normal 93 octane for the Hi Octane tables and then 87 octane for the low octane tables. Then you have your failsafe in the knock detection and learning so that each engine that's in various weather conditions/altitudes etc will still run optimal.