Tuning Tip: Detonation "Fingerprints"

[Rich Z]

Tuning Tip: Detonation "Fingerprints"

Here's a log from one of our ST-12 customers. I had never seen this before in this context. The interresting part is that on this engine the knocking cylinders cause a specific AFR signature. I suspect the abnormal combustion during knock causes part of the mixture to not burn at all, causing excess O2 in the exhaust.

This is off a 632 ci Ford drag racing engine with one 1150 CFM Holley 4 barrel. The engine came from the builder with 36 deg. of ignition timing. The engine was designed to run with a 400 HP shot of NOS. Engine was dynoed on a DTS Dyno equipped with an Innovate ST-12. Ignition timing was adjusted anywhere from 28 to 36 degrees. Engine made 850 HP at 36 degrees Ignition timing on VP C16 fuel no NOS.

The engine did not knock audibly, but was not making the expected power. The first session in the log (named "Detonation 36 deg Advance") is with C16 fuel and 36 deg. advance. Notice cylinder number 1 and 2 (purple and black) showing lean spikes, but not the characteristic sharp spikes of ign. misses, but a lean area with overlaying wild excursions. These cylinders are knocking, badly, as was determined later.

The last session (named "Session Normal 28 deg") was on C16 with ign. advance at 28 degrees. NO changes in fueling at all. In that run the engine made 970 hp (no NOS), a 120 hp gain compared to 36 advance. Because the engine owner insisted that he needs more timing to make more power, different things were tried. The second session is with 32 deg timing and cam retarded 4 deg on Sunoco 118. The lean excursions on Cyl1 and 2 look almost the same as in the first session if you overlay the two. The third session was with 32 deg. and Sunoco 118 and cam reset to spec. It looks normal but had less power.

In the end the engine was dialed in with 28 deg. advance. With NOS it made 1450 hp.

Again, the fascinating part is that knock can have such a large effect on AFR data when seen on a cylinder by cylinder basis. When using the WB in the collector the effects will be of course diminished as the lean gas will be diluted by other cylinders normal gas. This dilution would make it harder to see. But when you see sudden lean areas that are not typical spikes from ignition misses and can't be explained by carb/injection events, high speed knock is something to consider. This is especially true for engines with factory knock sensors, as most factory ECUs don't listen to knock sensors above ~4k RPM anyway because engine noise masks the signal too much.

As an additional note, the owner had already blown up two engines at the track. He didn't believe that datalogging and testing on the track is neccessary because the engine was dialed in on the dyno already. After this dyno-tuning session, he proceeded to blow up another one. He now understands that there are vehicle issues (intake dynamics, fuel delivery, etc.) that need to be tuned at the track. So they are setting up a DL-32 system."

-Innovate Motorsports

Source: http://www.innovatemotorsports.com/resources/news14.php