Dynamometer Tuning

By Daniel Stapleton, England

Having spent many pounds on building and preparing your engine, the next vital step once the engine is run in, is to have the engine tuned properly. The only proper way to tune the engine is on a rolling road dynamometer (dyno). A dyno will tell you how much Brake Horsepower (BHP) is transmitted to the wheels. This BHP figure is not to be confused with the manufacturer’s BHP quoted figure, which is always taken from the flywheel and is consequently higher than the figure achieved at the wheels. However, with a modified engine the manufacturer figures become irrelevant.

When the car is driven on the rolling road dyno, there is constant read-out on how much power is being produced at the wheels. Consequently, when any adjustments are made to the engine, it can be seen whether or not there is a resultant increase or decrease in power. I recently had my car, with its Oselli built engine, tuned on a rolling road dyno and the best way to describe what happens at a dyno tune, is to relate my own experience.

To choose a dyno and operator, seek the advice of your own engine builder. In my case Oselli recommended Mike Baldwin of Marshalls of Cambridge. On the day of the tune, before starting any engine runs, I had a chat with Mike about the engine specification. At this time I suggested possible carburation changes, and gave details of how the engine was currently set up. Once ready, I drove the car onto the rollers and the front wheels were chocked. Mike then connected up an electronic engine analyzer to the ignition system and placed an exhaust gas analyzer up the exhaust tail pipe.

The first run was a static run, to check out the entire ignition system. It confirmed my ignition system was in good order. If for instance, I had a bad lead or other ignition fault it would have shown up straight away. The second run was done driving the car in top gear at 3,500 rpm. This was to show up any weakness in carburetor mixture before full power tests, this again was O.K.

On the third run the ignition timing was set. The engine rpm used for the setting is dependent on the advance curve of the distributor, and anticipated power peak of the engine as dictated by the cam. My distributor produces its maximum advance at 5,000 rpm and we used this rpm to find the optimum ignition timing. As I drove the car on the rolling road, Mike swung the distributor to advance and retard. Eventually, the ignition timing was set to TDC, not what I had expected, and I doubt if I would have ever tried this setting had it not been for the rolling road tune. Mike also did a static check to tell me the total advance the distributor produced.

Before commencing the fourth run we changed the idle jets. We used a richer jet with emulsion properties, which would give a much later lean out. On this run I could feel the engine felt much smoother. I drove the car as before, but at 6,000 revs Mike restricted the engine’s ignition so it would not exceed 6,000 rpm, with the throttle floored. The BHP readings looked very promising, and on completion of this run a check was made on engine temperature. Although there is a large fan to cool cars on the dyno, most cars will run on the hot side. My car has an excellent cooling system and there were no problems, though the oil temperature read hotter than normal. Another run was completed, this time at 6,500 rpm.

As well as keeping a close eye on the dyno readings, Mike had also kept a close eye on the exhaust gas analyzer. This told him how rich, or lean, the engine was burning. From this information he decided to go on to a larger main jet size. Although my main jet was sufficient for full power, it was desirable to try a larger size and I had expected this. After the change another run was undertaken. The dyno showed an increase of 10 BHP, which was both surprising and pleasing. It is worth noting that the previous mixture was in no respect too lean and the car had always felt good at full power. The air fuel ratio used to set the carburetor was 12.5 to 1 at full power, full throttle. On the last run the engine was taken to its full rpm limit. The dyno showed the peak power to hold for a full 500 rpm.

Now that we had achieved the full power tune, a check was made to see if the engine remained flexible. What I like to call a ‘slam" check was undertaken. This is where the throttle is slammed to the floor at low revs, to see if the engine would miss or hesitate. This would determine whether or not the engine was running the right accelerator pump. My engine was fine in this respect, and showed no-signs of detonation or engine miss. The car was also driven at a low rpm to see how it felt. I was most impressed, not only was the car much smoother, it had better throttle response and more power across the rev range. All this from an engine, which had given pleasing, results before the tune.

The final test was a short drive on the road. As experienced on the dyno, the car was most impressive. I now had not only an improved car, but also a set of figures for engine power output from which I could obtain further information, such as power to weight ratio. I am now also able to verify Oselli’s projected power output for my specification engine build. Furthermore, their claims are realistic to the point of positive modesty, unlike some engine builders. It can also be seen how important a rolling road tune is. I doubt if any amount of road or track testing could have given me the ideal set up, let alone in such a short time or minimal expense.