MembersPage/JorgenKarlsson/TwentyFourX (2011-05-03 12:01:01)

General motors 24X trigger system

Background

This trigger is used on the very popular LS1, LS6 engine and many other V8 engines in this engine family. All these engines are popular to retrofit in cars as it is very light and weigh less then many four cylinder engines. We should try to support it properly.

It is very hard to modify the secondary trigger angle on these engines to get it exactly between the primary trigger pulses.

Problem

It works fine on 48+1 trigger on most cars. The cam sync trigger is however a bit too close to the primary trigger pulses and it sometimes cause a race condition. This seem to have become worse on 1.1.88 compared to the results seen on 1.1.76.

The primary trigger has 24 pulses per revolution where the falling edge of the trigger signal is at 15 deg intervals and the pulse length of the logical '0' is 12 or 3 degrees. As the pulse density is high and the difference is fairly large I think that we can detect this reliably even when cranking. The sync is possible in 90 deg.

Solution

We need to make the engine time the engine from the primary trigger and only use the cam sync sensor for sequential operation. The secondary trigger can possibly be used to simplify starting at low battery voltages.

1.1.91 has a special Ls-1 forgiving option in secondary trigger dialog

Bench tested config at http://vems.hu/vemstune/sharingcenter/reports.php?cmd=view&key=vsMcdX

The 24X trigger pattern look like this:

LS1_crank_timing.jpg

LS1_timing_chart.jpg

Scope captures around #1 TDC:

cam_sync_and_1deg_spark_dont_worry_about_noise_bad_scope_ground.png

The capture above show the spark from #1 together with the cam sync. This was taken with the timing locked at 1deg. Ignore the noisy signals, I used a chassi ground for the scope. I only took this to verify the timing of the cam sync sensor

Primary_and_secondary_trigger_around_cyl1_TDC_idle.png

This how how the primary and secondary trigger signals look around #1 TDC. Bigger version of the same is found here:

Primary_and_secondary_trigger_around_cyl1_TDC_idle_big.png

Trigger log and a short normal log is found here, Primary=falling edge, Secondary=falling edge. Bad sec trig position shows up around 1 minute in the log:

http://www.vems.hu/files/JorgenKarlsson/24X/Peter_Cab-2465.zip

The pattern can be broken down to this:\nÿ1ÿ

If I only specify the length of the logical '0' we get:

0deg to 90deg:

12,3,3,3,3,3

90deg to 180deg:

12,3,3,3,12,12

180deg to 270deg:

3,3,12,12,12,3

270deg to 360deg:

12,3,12,12,12,12

The cam sync has a 360 crank degree duration and will change from 0 to 1 or 1 to 0 around 0 crank degrees. The spec sheet say that it should do it at 0 deg but in reality it occur a few degrees from the previous falling edge of the primary trigger. At least of the engines I have run a trigger log on.


Div-by-N

Auditrigger (traditionally divby3, that would be tooth count=16 in this case) always allowed 1 tooth sloppiness in the sectrig.

We'll run tests with these as well (and with the new solution below also).


Trigger-detection with sectring LS1 forgiving bit - implemented (under testing)

From 1.1.91 secondary_trigger bit2=1 ("LS1 forgiving"=enabled)

LS1 forgiving=yes behavior:

Another solution could be to turn on primtrig rising-edge irq at some point (around campulse ?). But - if otherwise sufficient - just peeking at the polarity is simpler.

It might be useful for 24+1 (c024) type triggers also. But the recommendation remains:


Yet another solution postponed for now:


This engine family also use a trigger type called 58X which is what other manufacturers call a 60-2 trigger wheel. For example the LS2 engine use this trigger.