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Dynomometer Tuning

Dyno Tuning, what is so magical about it, or more to the point, what is the difference between a normal tune-up and a Dyno tune? In a standard type tune-up, the ignition and fuel systems are checked and adjusted at idle and part throttle, but can't be checked under load. Enter the chassis dynomometer (Dyno for short). A dyno is basically two sets of rollers, mounted in the floor, on which the driving wheels of the car are placed and driven. The operator has full control of how much load is applied to the rollers to either slow, accelerate or hold the car at any speed he wishes.

The amount of force required by the car to overcome the load on the rollers is registered on two indicators, one indicates torque and the other Power (HP). An exhaust gas analyser is connected to measure the exhaust content and an oscilliscope is connected to the engine to monitor ignition patterns, so the performance of both ignition and fuel systems can be checked throughout the entire rev range at varying loads, otherwise impossible by any other means.

The chassis Dyno I had was a Dyno Dynamics ATC300. ATC stands for automatic temperature control and the 300 represents the amount of power the dyno was able to absorb, 300 Kilowatts or 402 horsepower. This Dyno was the state of the art at the time. The ATC feature was what sold us on this model, since we did a bit of high performance work. It is a fact that if you took horsepower and torque readings in the morning and then again in the middle of the day, you would get two different readings. This was just not acceptable. This presents a major problem with accuracy, especially if you are experimenting with different distributor advance curves and carburettor settings throughout the day or night, but this was no problem for the ATC300, because it compensated for the difference in air temperature and adjusted the readings to true torque and power, to within 1% accuracy.  

So what else can we do using a Dyno and how do we do it?

With an exhaust gas analyser, vacuum gauge and oscilliscope connected the basic procedure for most standard vehicles was to run the vehicle up to 80 KPH and perform fuel mixture checks at various loads. This would confirm correct operation of the carburettor main jets and power valve operation and the operation of the fuel injection system if fitted. The load was determined by the amount of vacuum in the engine. Cruising vacuum is generally around 15" of mercury, down to about 3" of mercury for full load. The cars speed was maintained automatically by the Dyno in constant speed mode. As we start to apply more load to the engine, most cars would start to develop an engine miss-fire, This is usually the ignition system breaking down - the spark plugs or the fuel system running too lean. If it is missing badly or major problems show up, then we stop & rectify the problems. What we are simulating here in the workshop at 80 KPH is equivalent to you taking the car up a very steep hill under full throttle. Once we have completed the diagnostic phase, we carry out the repairs required. Once this is done the engine is tuned and then back on the dyno to record torque and horsepower improvements if we actually got through the initial run.

When it comes to high performance tuning, we use the same technique as before but add addition tests. The demands on a high performance engine is far greater than on a stock motor so a more rigorous torture test is applied. The dyno is switched to Ramp mode for performance graphing. What the dyno does is apply a load to the rollers that lets the car accelerate at a constant rate. This acceleration rate is adjustable and the slower the rate the more accurate the results are.  So I generally would start at about 80KPH. Activate ramp mode and then tramp the throttle. The car will accelerate slowly untill I decide at what RPM to back off. Generally I would take most high performance cars to 6000 RPM and sometimes more, depending on the camshaft fitted. This test only takes about 15 seconds. Now I have a graph of the torque and horsepower specs for this vehicle before I actually start to tune it. During that 15 seconds of screaming fury I am also observing carby mixtures and ignition patterns. If the A/F ratio drops to a dangerously lean condition during the run or the ignition pattern indicates a serious problem, I abandon the run, correct the problems and have another attempt. The slower the up ramp the more accurate the graph is. On my dyno unlike todays computer generated graphs, my graph did not produce a nice smooth graph like the one you see in my Dyno graph article. The computer version will use software smoothing correction to get a nice clean line by averaging the high and low peaks. In reality with an up ramp of 12KPH/sec the line will be a continous zig zag pattern as the dyno tries to hold the acceleration to the up ramp setting.

Carby jetting was fairly straight forward. We would run the vehicle up to anywhere from 60 - 80 KPH and apply a small load on the rollers to simulate flat road driving conditions. I would monitor the A/F ratio with the objective of getting it as lean as possible without the vehicle surging on the rollers. Once the main jet was set it was time to set up the power valve. To choose the power valve setting I would simply apply more load to the engine and observe the vacuum reading as I applied more load. As soon as the vehicle began to surge, I would note the engine vacuum. For instance if the engine vacuum was 8" of mercury when it began to surge then I would fit an 8.5" or 9" power valve. This ensures that the engine receives the extra fuel it needs to start developing power at just the precise time. If we have a progressive 2 or 4 barrel carb it is now time to wind it out and get the second barrels jetted. This now requires the engine to be revved somewhat higher. All that is required here is to take the engine to about 3/4 revs limit, hold it at full throttle and fit secondary main jets that will get maximum power.

The tune is done and the customer can take his car and thrash the living daylights out of it with confidence that the ignition and fuel system are set-up for max performance. However what inevitably happens is that the car is used for town driving more often than it should. This causes the spark plugs to foul and is very difficult to clean them up on public roads with safety. You would need a long stretch of road that goes steeply uphill to accomplish a good clean out. On the dyno this can be done easily and without revving the guts out of the engine. All I do is run the car up to 100 KPH, set the speed and then open the throttle up fully. The car stays at this speed and quickly the combustion chamber heats up to the self cleaning temperature of the plugs. The car starts to buck and jump as the plugs start to miss there little brains out. Carbon starts to pour out the tailpipes and all the businesses around me think the engine is going to blow up, but the smoke quickly clears up and the engine stops missing. This method of cleaning an engine out of any un-wanted carbon build-up is called De-coking and the best part is it can be done in complete safety on a dyno at much lower speeds and with much better results than on a public road.

Modification of fuel injection components is easily accomplished on a dyno. One of my favourites was the XF falcon with the LE Jetronic system. Modification of the air flow meter was required on quite a lot of this model because most had a problem with lack of power and very poor economy. With the dyno I was able to discover this fault and rectify it by modifying the air flow meter so that a richer A/F ratio was achieved at high speed and also the best A/F ratio was achieved at cruising speed.

The outcome from a Dyno tune was always better than a normal tune-up. In summary you can see that it is just not possible to perform some tune-up adjustments and check ignition and fuel systems under load without one. I did do quite well without a Dyno for almost nine years, but I will admit that I could do a much better tune and in less time with the aid of one.

I have another article in the Tuning sub menu of an actual Dyno Chart so you can see what a typical report looks like. Check it out.