octane and timing pull

[cjpp78]

I searched but didnt find exactly what I was looking for so... Anyway, over on SF there was a thread on a guy logging his timing on stock ecu with different fuels. pump 93 , a toluene mix and 105 race gas. He was seeing how much the ecu was pulling timing with each fuel at different boost levels.

It was mentioned in this thread that toluene, when mixed with pump only has a mon octane of 94. well basicly he concluded from testing that no more than 15psi should be run on pump, no more than 17psi on pump/toluene and around 19-20 psi for the 105 pon race gas.

The testing above was done on a mkiv , does anyone here have any data on timing on stock ecu for the mk3 with various fuels? I used to run 17psi on 93 pump in cold weather but after reading that I keep it 15psi and below on pure 93 octane. I dont have a way to monitor timing or reliably knock so I follow guidlines like this.

 

[starscream5000]

It was mentioned in this thread that toluene, when mixed with pump only has a mon octane of 94. well basicly he concluded from testing that no more than 15psi should be run on pump, no more than 17psi on pump/toluene and around 19-20 psi for the 105 pon race gas.

I should be noted that roughly 1 gallon toluene mixed with a full tank of gas will net you roughly 94 octane using a full tank of 91-92 octane .

 

[jdub]

Not familiar with the Mk IV ECU, but the Mk III ECU is pretty simple when it comes to timing control. What it does (assuming the motor is at operating temp and above idle) is use a table stored in memory. The ECU senses intake air volume from the AFM (Vs), engine speed (Ne), throttle position above idle (IDL - off), coolant temp (THW), and the knock sensors (KNK). The ECU "looks" at the table and selects the value closest required for the combination of Vs and Ne per 1 revolution of the engine. It then does a proportional calculation to determine optimal timing for the conditions present.

This is subject to certain corrections when the engine is above idle. If the ECU senses THW is high, it will retard timing to prevent knock due to an over temp. If the ECU senses the KNK signal, it will retard timing based on knock intensity (strong, medium, or weak)...the max the ECU will retard timing for knock is 10 deg. When the knock stops, the ECU stops retarding timing and then begins to advance at set angles a little at a time. This is a feedback loop...if knock re-occurs, the ECU retards until the knock stops and begins to advance again.

The most appropriate function of the Mk III ECU regarding your question on timing is related to knock (or detonation) and that can be a result of several things...octane, dynamic compression, A/R ratio, and spark advance. As you alluded to, boost (as well as cam timing, piston shape, and head combustion chamber size) will impact dynamic compression making knock more likely as it increases. A/F ratio determines if the mixture is more "explosive". Octane has an impact on how fast the flame front moves across the cylinder as the mixture is compressed. Spark advance is when the mixture is ignited (controlled by the ECU).

Why am I telling you all this?

Because there's too much variation between 7M motors to give you a valid answer that fits YOUR motor. Head gasket thickness and the pistons used (Ross, Wisco, JE) will affect dynamic compression. Injector size and duration affect the A/F ratio. To determine the "safe limit" of boost you can run, the data will have to come from your motor. The Mk IV data you mentioned is invalid...too many differences.

BTW - the formula for determining octane when mixing in Toluene or Xylene:

(A x gas octane) + (B x 114 or 117) / (A + B) = Octane Value

A = gallons of gas
B = gallons of Toluene or Xylene
114 = octane value of Toluene
117 = octane value of Xylene

Also - you're smart for thinking about this...most guys push till the motor blows

 

[cjpp78]

jdub, thanks for posting the info on how the mk3 ecu goes about dealing with timing control.
I already had a good understanding of the rest though and I fully understand that the max boost limit would be different depending on your own engine set up. I just wanted to see if any mk3 owners had data logged any similar information with their set ups. To see if anyone had tested the limits of a 30% toluene mix on their 7m to see how much boost before any timiing pull.

As far as the octane of toluene/pump, the thread was stateing that the motor method octane of toluene/pump is 94. The guy with the mkiv stated that the 94 motor octane of toluene/pump made sense based on how it performed in comparison to the 105 pon octane race gas. The toluene mix at 17psi acted about like the 105 pon octane race gas at 19psi as far as timing went.

The formula above gives you pon octane right, not mon?

 

[jdub]

The values for both Toluene and Xylene is (R+M)/2 the same as ordinary fuels are rated in the US.
There's really no way he would know the octane unless he mixed according to the formula above.

 

[Doward]

jdub, I remember talking about the Knock routing with JJ over @ mkiiitech... did we ever reach a conclusion on the active timing control over a narrow band (part throttle cruise?)

Maybe I should go get some timing logs done this weekend, on the 7M, so we can see

I just want to point out that saying xx psi is the max is completely pointless. Too much is dependant on intake temps. 15psi off a 14b (tiny turbo) on the 7M is a hell of a lot different than 15psi off a JT58 (a much better sized turbo).

 

[jdub]

I'm fairly sure that it does...the ECU uses the table to control above idle timing based on Vs and Ne. Any time the knock sensor "hears" a 7 kHz frequency is when the ECU pulls timing. But, on the other hand, knock should not happen until the upper power band.

After start timing advance begins with the basic ignition advance angle and then goes to corrective ignition advance angle based on the parameters above (plus a couple others...mostly concerned w/ idle). The ECU does have a Max/Min advance angle control:

Max Advance Angle 35 deg ~ 45 deg
Min Advance Angle -10 deg ~ 0 deg

This is to protect against abnormal ignition timing: The Advance Angle = Basic Advance Angle + Corrective Advance/Retard Angle is subject to these limits.

I'm quite sure JJ has a lot more insight on this

It would be interesting to see the logs as well...the MAFT Pro has the ability iirc

 

[Doward]

Yep, it sure does. I'll be setting mine up tomorrow to start logging. I'm interested in what timing values I've got going on, under boost.

 

[jetjock]

You're doing fine Dub. Although Toyota uses two different types of knock sensors along with two different algorithms (wide and narrow band on both) I don't recall which is used in our cars. I believe it's the narrow band setup but I could be wrong. Doward's data would be enlightening and would keep me from digging out and setting up the simulator.

 

[Nick M]

As I was reading my own link from another thread, the one with old technology, it indicated the timing retard was scaled amounts depending on the knock frequency. If the knock persists, it goes back another amount, until it stops. If it does not stop, the MIL is illuminated and 53 is stored.

This is different from the new stuff, that is fully operation closed loop knock control.