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Dirty Stuff - Part 5

The emissions changes from simple tuning alterations.

by Julian Edgar

Click on pics to view larger images

It's easy to look at the results of emissions testing and not be able to see the wood for the trees. Some gases go up, others go down. To be even more specific, in some points of the drive cycle, some gases increase by larger amounts than others. So what's it all mean? Which gases increase when the air/fuel ratio is made richer? Which gases decrease when the ignition timing is retarded? We decided to make some broad-brush tuning changes, running a car through the full I/M 240 drive cycle dyno test after each change was made.

Click for larger image

AutoSpeed contributor Graham Pring again made available his awesome test vehicle - and as you can see here, drove the car through the test cycle as well. Probably the most tested standard 1989 Suzuki Sierra SJ50 4WD in the world, the car has some significant advantages in this type of exercise. We wanted a car where easy tuning changes were made, and where those changes would give the desired outcomes. (We could have used Graham's Holden V8-powered Cobra replica, which is controlled by lap-top tuneable Kalmaker software. But, for example, when you change the volumetric efficiency charts for fuel, the closed loop function would negate some of the affect. Then when you disable close-loop, the hundreds of other fuel maps will each have their own part to play in the resultant mixtures. We wanted something simple!)

So the Sierra was it.

Click for larger image

Again Stephen Bell made available his I/M 240 emissions test dyno for AutoSpeed's use. As discussed previously in this series, the dyno is identical to those used in the US to make sure that vehicles continue to comply with emissions standards. In order that it can do this accurately, the car is driven through a drive cycle test, with the mass of the emissions measured in grams per distance travelled. In other words, it is far more representative of real world driving that an idle (or even full load) gas analysis. Stephen uses the emissions dyno during his tuning work, finding that the real-life gas emissions give a very good indication of combustion efficiency as well as legalities. For example, emissions substantially improved on a VT Commodore after the injectors were cleaned....

So that the raw emissions could be better seen, the Sierra lost its cat converter for this testing.

Rather than looking at the four gases in each state of tune, we'll examine the gases separately, seeing how their output was affected by the changed tune of the engine. Remember, the emissions are being assessed through a full drive cycle, not just at idle or at full-load.


  Standard Timing
retarded 10 degrees
Timing advanced 10 degrees Mixture richened Mixture
HC 1 0.78 1.14 1.39 0.88

Hydrocarbon (HC) is an organic compound comprising Hydrogen and Carbon. This emission can be thought of as the unburned petrol exiting the exhaust - the greater the amount of incomplete combustion, the higher the HC values will be. As you'd then expect, rich air/fuel ratios result in high HC outputs; however, excessively lean air/fuel ratios will also cause HC to skyrocket. Any misfire - whether through an ignition breakdown, incorrect ignition timing, no compression, etc - will also result in high HC. In addition, combustion chamber shape and injector spray quality will help determine HC levels. According to Aire Kare (makers of the emissions dyno system), the lowest HC emissions will occur at an air/fuel ratio of 16:1.

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As can be seen from the table and graph, in standard (but, remember, cat-less) form, the Suzuki produced a total of 1.00 HC through the drive cycle test (we'll ignore the units to make it all a bit simpler!). With the mixture richened (just a jet change in the Suzuki's carb) the HC rose by 39 per cent to 1.39. Conversely, leaning out the mixtures (another jet change for the long-suffering Graham) reduced HC levels by 12 per cent. So, much as you'd expect - too many hydrocarbons going in equals too many coming out.

However, changing the timing was more interesting. When the ignition timing was advanced by 10 degrees, the HC emissions rose by 14 percent, while with a timing retard of the same amount, HC emissions dropped by 22 per cent. So, is retarding the timing a way to better emissions? Well, maybe - but you'll also be dropping power pretty quickly too!

Carbon Monoxide

  Standard Timing
retarded 10 degrees
Timing advanced 10 degrees Mixture richened Mixture
CO 14.45 11.59 12.96 81.04 9.79
Click for larger image

Carbon monoxide (CO) is formed in the combustion chamber by the partial burning of the fuel mixture in an atmosphere that lacks oxygen - resulting in the combining of carbon atoms with oxygen atoms. A rich fuel mixture will starve the burn of oxygen; in fact, with air/fuel ratios richer than stoichiometric (ie 14.7:1), the increase in CO corresponds very closely the richening of the air/fuel ratio. Leaner mixtures than stoichiometric result in less change to the CO output - from an AFR of 16:1 and leaner, the CO output stays largely constant.

Excessive CO levels can normally be traced to incorrectly rich air/fuel ratios. During the emissions drive cycle, each acceleration event is accompanied by a burst of CO as the acceleration enrichment occurs. However, emissions spikes aren't all that critical to the end emissions result - it's the area under the curve that's much more important.

Click for larger image

As you'd expect from that prelude, the big mover in the CO stakes was when the air/fuel ratio was richened -CO then went out of sight, increasing by over 4.5 times! Compared with that change, carbon monoxide was relatively unchanging in the others state of tune.

Nitric Oxide

  Standard Timing
retarded 10 degrees
Timing advanced 10 degrees Mixture richened Mixture
NO 2.91 1.89 5.18 0.71 3.03

While this emission is most commonly referred to as Oxides of Nitrogen, the Aire Kare emissions test gear actually measures nitric oxide (NO) to get a handle on these emissions. In fact in most emissions testing, analysers generally assess nitric oxide, although the generic term 'NOx' may still be used.

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Most of the air that is breathed by the engine is made up of nitrogen - 78 per cent in fact, versus the 21 per cent that is oxygen. The nitrogen is neutral - it doesn't contribute or detract from the combustion process. However, when exposed to temperatures above about 1100 degrees C, the available oxygen and nitrogen molecules combine to form oxides of nitrogen. Since combustion temps can easily exceed this - especially under load or with lean mixtures - the production of NOX can be a major problem. In fact, it is NOx that is the big hurdle to overcome with current lean-burn, ignition-mapped engines. While a light load ignition advance of 40 degrees and an air/fuel ratio of 16:1 looks great on paper for fuel economy and cruising response, the NOx emissions are likely to be blown out of the water. The formation of NOx per se does not affect engine performance; however, good engine performance may produce excessive NOx and so reducing it may in turn harm engine performance.

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Since hotter combustion leads to NO production, as you'd expect, leaning-out the mixtures increases NO production. In fact, the changing to a smaller jet size on the Suzuki increased the emissions of NO by a little during the drive cycle. However, it was the increase in ignition timing that really inflated NO emissions - by no less than 78 per cent! It is this dramatic change in NO emissions with advanced ignition timing that makes quite questionable the claim by many manufacturers of 'hot' chips (that generally run more advance in low load ignition timing) that emissions standards are still being met. In terms of the fuel side of things, a richer mixture reduces combustion temps, in turn lowering NO emissions.

Increased NOX may be caused by:

  • Exhaust Gas Recirculation not working (5 per cent EGR can reduce NOX emissions by 40 per cent)
  • Valve timing
  • Ignition timing - an increase in advance will result in an increase in NOX emissions;
  • Load - higher loads result in faster, hotter combustion and higher NOX outputs;
  • Revs - higher rpm with rich mixtures equals faster burn times, resulting in less heat loss and so higher NO outputs.
  • Compression - higher compression results in hotter combustion, increasing NOX


It can be seen that simple tuning changes can result in radical changes to emissions. By looking at which emissions are excessive on a modified car - and then making some tuning changes derived from the results covered here - some significant progress towards emissions compliance can be made.

  Standard Timing
retarded 10 degrees
Timing advanced 10 degrees Mixture richened Mixture
HC 1 0.78 1.14 1.39 0.88
CO 14.45 11.59 12.96 81.04 9.79
NO 2.91 1.89 5.18 0.71 3.03
O2 22.29 19.77 23.76 17.4 25.36
CO2 305 307 298 246 310


Bell's Auto Service

+61 8 8231 6211

+61 411 840 167

Bell's Auto Service is happy to do one-off emissions testing and tuning for a quite low charge.

Dirty Stuff - Part 1
Dirty Stuff - Part 2
Dirty Stuff - Part 3
Dirty Stuff - Part 4

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