Some motorists are afraid to use full throttle for long periods in case they damage the engine, whereas others employ full throttle far too often for their own good. This raises the very interesting question: How long can an engine run at full throttle without wearing out at a very fast rate or even self-destructing?
By Jake Venter
Without a load, most engines should last about three or four minutes, if that long. With a load that restricts the engine to its designed rev range, the engine should last at least 500 hours, if not more.
How do I know? I’ve been there, and I’ve got the T-shirt. My first entry into the automotive manufacturing environment was as a dynamometer technician at the old Chrysler SA assembly plant in Cape Town.
The dyno was a hydraulic unit, connected to the engine’s flywheel by means of a short shaft. Initially my weekly routine consisted of selecting a Chrysler Valiant engine from the assembly line, performing a running-in procedure on the dyno, and then measuring the maximum power output.
The running-in procedure consisted of starting at low revs with a small load on the dyno, and then gradually increasing both until after nine-and-a-half hours the engine was running at full throttle with the dyno load adjusted to keep the revs at a maximum of 4000 r/min. The maximum power output curve was then obtained at full throttle by taking torque readings every 500 r/min, from 1500 r/min to 4 000 r/min.
These “quality control” engines had to be test-run in the conditions in which they were received from the engine line. This sometimes meant that the ignition timing was too far retarded. All would appear to be well until the full-throttle run at the end, but then the complete, full-length car exhaust system we were using would turn red hot – a graphic illustration of the evils of such a condition.
Afterwards, I had to strip the engine completely and award demerits for any assembly mistakes. The most commonly occurring fault was dirt in the bearings.
After the test, I reassembled the engines, marked the engine blocks with a spot of white paint, and sent them back to the engine line.
Was that fair? Yes, because these engines were run-in scientifically. I can say this because we connected an airflow meter to the engine to measure the amount of blow-by (the gas that passes the rings) and drew a graph from the readings. It invariably showed that after about eight hours the high initial readings would settle to a steady low reading, meaning that the engine was run-in. The blow-by will stay at this level for years until the gaps between the rings, pistons and bores start to grow larger.
This dyno was also used for endurance testing. Most engine components were expected to last 480 hours at full throttle, and I completed four such runs with local components. The schedule called for 10 hours at each 400 r/min interval, starting at 2000 r/min, and ending at 4000 r/min. This gives 60 hours, after which time the head was torqued down, spark plugs changed, tappets and ignition timing adjusted, and the oil and oil filter changed. Eight such cycles constituted a full endurance test.
I remember the first time I had an engine in the dyno at full load. The mechanical noise of the engine going full blast next to me was such that I could not imagine the engine lasting more than an hour. But it did, and 480 hours later when I stripped it there was very little bore wear, but some of the big-end bearings were in need of replacement.
When sitting close to an engine screaming away under load, it is very easy to start imagining abnormal noises. On a number of occasions I imagined that the engine had started to knock. Of course, the power of suggestion is such that the moment I mentioned it to my assistant, he could also hear it. On two occasions we stopped the engine, dropped the sump and inspected all the bearings, but found no cause for the knocking. When we started the engine again, the knocking was gone.
We discussed the situation and formulated a possible solution for this. A flat surface will vibrate in various modes, depending on the amount of energy causing the vibration. Any of these patterns will result in areas of more intense vibration, flanked by areas of less intense vibration.
Our dyno cell was on the second floor of a building, and the floor vibrated quite readily while an engine was on test. We therefore took off our shoes and walked around the engine from time to time to familiarise ourselves with the level of vibration. We found we could clearly identify lines of greater or lesser vibration. The next time the ghost vibration started we were able to establish that the floor did not amplify any knocking, allowing us to ignore the sound. It went away as soon as we started to think of something else…