Diagnosing misfires can be difficult

Much as we admired the long-stroke vintage sportscar engines, there is no denying that they were incredibly inefficient.
Dec. 6, 2011 - PRLog -- Much as we admired the long-stroke vintage sportscar engines, there is no denying that they were incredibly inefficient. For example, in the 1920s even the 4.5-litre Bentley, at one time king of British sportscars, took about 15 seconds to reach 100km/h from a standing start. Today a small-capacity family runabout will have no trouble achieving or bettering that. OK, I admit the Bentley’s weight didn’t help acceleration times.

While long-stroke engines had their admirers, the stresses in the long connecting rods were partly responsible for the low practical engine speeds. This was one of the reasons why later cars had a more square bore/stroke configuration and became higher revving “buzzers” if you like. Added to this, there is no doubt that a small stroke/bore ratio more easily lends itself to higher volumetric efficiency. It’s a matter of space too; the bore size dictates the maximum size of the inlet valve.

Whereas at one time not so long ago, in the quest for power we would fit a sports camshaft, high-compression pistons and a special exhaust, along came exhaust emission controls.

Moves to reduce harmful emissions included reduced compression ratios and changes in combustion chamber and piston-head design.

Close clearance spaces in the combustion chamber, which tend to quench the flame before the fuel/air mixture has been fully burned, have been eliminated.

This quench height reduction has brought a worthwhile reduction in harmful emissions.

Valve overlap (when inlet and exhaust valves are open at the same time) has been increased to bring about some dilution of the air-intake mixture, thereby lowering peak combustion temperatures and reducing emissions.

The introduction of lead-free fuel initially brought rapid wear on valves and valve seats. To overcome this, during engine production exhaust valve seats were heated by induction coils to about 930 deg C and hardened to a depth of 1.27mm to 2.032mm. Other hassles on some cars included (for example, on one of the Volvo models) the fitting of two head gaskets, resetting the ignition timing to seven degrees BTDC at 750rpm with vacuum pipe disconnected. Possible alternatives were the use of higher octane petrol or octane-boosting additives.

All history now, of course; but it does go to show to what lengths we were prepared to go to enable us to breathe easier for a bit longer.

As I have said many times, an intermittent misfire can be difficult to diagnose because unless you get the vehicle when the misfire is present, it is largely a matter of guesswork. For example, a car was left with us with a note asking if a misfire could be cured.

On a test drive, no misfire was found. A phone call to the owner gave us a bit more information; the misfire occurred only after a long run. The plugs, HT leads and other normal suspects proved to be okay, so I suggested that he take the car away and return it when the misfire recurred.

The following afternoon he brought the car back and it was misfiring badly. I reckoned it was something amiss with the ignition amplifier and fitted a replacement. The job took only a couple of minutes and the misfire disappeared.


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Stephens Gaskets Ltd based in Oldbury, Birmingham manufactures Gaskets, Exhaust Gaskets, Cylinder Head Gaskets, Ring Shims, Precision Washers, Shims in Brass, Steel, CS4, Stainless Steel, Plastic, Copper and many others.
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