Optimising Ignition advance with a home made Detonation Detector

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Charlie54043
Posts: 14
Joined: Tue Jun 24, 2014 5:57 am
Location: Lincolnshire Coast

Optimising Ignition advance with a home made Detonation Detector

Post by Charlie54043 »

This post is part of a series about building a Fuel Injection system for a Hillman Imp engine in a Mk 5 Davrian, previous posts were:

Inlet manifold length vs Torque and power: https://davrian.net/forum/viewtopic.php?t=716
Fuel Injection Build: https://davrian.net/forum/viewtopic.php?t=1026
Determining actual WOT of TB's and its application: https://davrian.net/forum/viewtopic.php?t=1176
Notes on optimising Fuel Injection maps “on the Road”: https://davrian.net/forum/viewtopic.php?t=30476

These notes are a practical way of optimising the Ignition timing on the road with Programmable Engine management and although a Megasquirt ECU was used, they are applicable for most ECU's as they mainly concern the Maps and optimising process.

Optimising the Ignition Timing to engine requirements will have the single largest effect on how it performs and while Air Fuel ratio is important (my engine will run with AFR's from 11 to 15:1) making small changes to the timing has an immediate effect on “drivability” and when optimised, you'll be surprised how well a 50 year old Imp engine runs.

The ignition timing is controlled by the Ignition Map and put simply, the ECU measures the angle of the throttle and the revs, then looks in the Ignition Map to see how much advance is required. With a Mechanical distributor the advance can only stay the same or increase as the revs rise, whereas with programmable ignition you can pretty much “dial in” whatever the engine needs. An example of this is, at peak torque the advance needs to be reduced as a result of efficient cylinder filling, then as the revs rise, the advance needs to be increased again.

Generally, Ignition advance is limited by detonation, not only at peak torque but all through the rev range and if allowed to become severe, it will damage the engine. So how do we know when the engine is detonating? Well, we can hear it! but with all the rest of the racket in the car (plus wearing ear plugs! ) it will probably be quite severe by the time you hear it! It would be much better if could hear it at a very low level, and this is the subject of the first part of this post.

The Detonation detector consists of a Sensor head attached to a cylinder head bolt (an Electret Microphone in a housing), a simple Audio Amplifier and ear buds.

The Sensor head mounted on the engine:
Sensor head (2).jpg
Sensor head (2).jpg (1.59 MiB) Viewed 177 times
As can be seen above, the Sensor head is pressed onto the head bolt between No 2 & 3 cylinder

Construction of the Sensor head:
Sensor head.jpg
Sensor head.jpg (105.5 KiB) Viewed 177 times
The sensor is made from 25 x 10mm flat bar Aluminium.

Sensor Amplifier Circuit:

Below is the Amplifier circuit used with the Sensor head, it's a standard low power audio amp circuit using an LM386 chip (ready assembled boards can be found on Ebay) The Sensor head (Electret) is connected to the Amplifier on the left and earphones connected on the right of the diagram. The 10k variable resistor is the Volume control.
Det Amp circuit.jpg
Det Amp circuit.jpg (95.69 KiB) Viewed 177 times
The first time you switch the Amp on with the engine running will be really off putting!! you can hear all sorts of HORRORS...timing chains rattling, tappets tapping, gearboxes grinding but hopefully no pistons knocking!

However, Detonation sounds just like a bunch of keys being rattled, it's quite distinctive.

Setting up Ignition advance:

To start, you will need some figures for the Ign Map and a good place to start is standard “distributor” timing, (taken from the manual)

Centrifugal advance + static: Degrees 4 @ 800, 21 @2000, 29 @ 4000, 37 @ 6000

Enter these into the “bins” of the Ignition Map:
Dist Adv Map.jpg
Dist Adv Map.jpg (236.8 KiB) Viewed 177 times
This map will get you going, but is only Adv/Revs just like a mechanical distributor, the throttle opening is configured with subsequent drives.

Using the Detonation Detector to set up the Ignition Map:

Megasquirt has the invaluable facility to record most engine conditions while you are driving, such as Ignition advance, Revs and Throttle position (TPS) so before you go for your first drive, familiarise yourself with how to record these conditions.
Now, with the engine running, the ECU software recording a minimum of throttle position, Ign advance and revs.......go for a drive with the Detonation Detector switched on and earbuds inserted. If you don't hear any Detonation, advance the whole map above 1500 revs by 3 degrees and repeat, keep doing that until you hear detonation, then reduce the TPS/RPM “bin” with detonation by 3 degrees.

But, there is a practical problem! While driving, it's difficult to read off the TPS and Revs when you hear detonation and even if you could, trying remembering multiple Detonation Revs and TPS's at the end of the drive is near impossible (especially at my age!) so a method of “marking” the TPS/Revs at Detonation while driving had to be found.

In my Megasquirt set up, only the Throttle Position Sensor is used to tell the ECU the engine load, but the software can also record the Manifold Air Pressure sensor. So adding the MAP to the recording conditions and attaching a “squeezy” bottle with a long tube to the MAP sensor, you will be able to “mark” when detonation is heard by a quick squeeze of the bottle (increasing the MAP) which will be seen in the recording. A note can be made of the TPS/revs and the advance can be reduced for that “bin”

All ready for Ultimate Detonation Detecting!
IMG_20220418_113208618_HDR.jpg
IMG_20220418_113208618_HDR.jpg (1.7 MiB) Viewed 177 times
The Squeezy bottle can be seen next to the Gear Stick with it's tube going back to the MAP sensor.
Behind it is the Detonation Sensor Amp with earbuds and the wire going back to the Det Sensor on the Cylinder head.

So, you've been out for a drive, heard Detonation, marked it and now it's time to review the recording.

Below is part of a recording to show when Detonation was heard and the bottle squeezed!
Log view9-4-22.jpg
Log view9-4-22.jpg (411.52 KiB) Viewed 177 times
The green trace show the Revs = 6538 rpm, TPS in White = Wide Open Throttle , O2 in Red = 1.922v ( Afr = 13.0 : 1), Ign advance in Pink = 34.4 degrees, the MAP is in blue and the 2 peaks are when Detonation was heard. So now we go to the Ignition map, look for the wide open throttle/6500revs bin and reduce the advance by 3 degrees, it was 34 so was reduced to 31 degs.

But look carefully at the Ignition Advance trace in pink, you will see the advance increasing with revs, but around 3400 rpm the Advance reduces to 28 degs where there had been detonation in the past, then the advance increases as the revs rise without Detonation. This reduction in advance, then increase, can't be done with a mechanical Distributor, so the Max advance at peak torque effectively limits the advance for the whole Rev range and in all probability the engine would be able to run with more advance at other Revs.

Probably the best place to start mapping is on the Wide Open Throttle line of the Ign Map where if there is detonation it can very quickly damage the engine. Choose 3rd gear and slow the revs to as low as the engine will take WOT without “baulking”, now whack the throttle wide open and let the engine rev to 6000 while listening closely for detonation. If none is heard, increase all the bins on that line (WOT) by 3 degs, then go out for another drive repeating the WOT to 6000 revs routine until Detonation is heard, mark it by a squeeze of the bottle, review the recording and reduce the advance at that point by 3 Degs.

When you have the the Ign Advance at WOT to a point where increasing any “bins” in this TPS “line” by 3 degs causes detonation, move down a couple of TPS lines on the map (say to 105 in my map) and go for a run, but this time hold the throttle at the 105TPS opening while reving the engine from 1500 to 6000 revs. If you here detonation, mark it with a squeeze of the bottle, review the log and reduce the advance in that “bin” by 3 degs. If no detonation is heard, increase the advance by 3 degs. for the whole “line” until detonation is heard. Carry on doing this for every other TPS “line” and interpolate the advance between the lines

For drive-ability, the bottom left corner of the map is most important and for engine safety the top right corner of the Map.

My Current Ign Map:
Final Ign Map 2.jpg
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Points to note in the above Ignition Map.

Advance reduces as the throttle is opened because of better cylinder filling.

Advance reduces at max torque...... max cylinder filling.

The Advance at idle is arranged to increase as revs reduce:
800rpm = 4degs btdc, 700rpm = 8 degs btdc......this is idle stabilisation. As the engine begins to stall, the advance increases and the engine speeds up.

Do not reduce the advance too much at 800 revs as the engine will loose all power when crawling along in traffic and feel as if its about to stall, however too much advance means less air flow through the throttle bodies and the engine will surge.

Low advance on closed throttle overrun:
This increases engine braking. Don't over do this as the engine will start popping and banging down the exhaust...sounds very “Fast and Furious” but expensive on manifolds and silencers!


Programmable Ignition is the most cost effective way of making the Imp engine run like a modern engine and requires little extra work to the engine. I first used a Luminition programmable ignition system, a locked distributor with optical sensor and a Throttle position sensor on Twin 1 1/2” Strombergs and the improvement convinced me to go to Fuel Injection and full engine management.


The next and final post in this series will be a Rolling Road report to see if it has all been worthwhile!
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