Re: using automotive oil for a 4.3 stern drive
Originally posted by Buttanic:<br /><br />Fuel grade recommendations are primarly based on compression ratio.
And timing.<br /><br />
Both Mercruiser and Volvo's 5.7 engines have 9.4:1 compression ratio.<br /><br />But different ignition modules and fuel mixture calibrations. <br /><br />Timing effects peak cylinder pressure during operation. If two identical engines had the exact same compression ratio, and they were timed identical for optimum performance, the engine running premium fuel would generate less heat, less power, and combustion would occur later. Premium fuels eliminate "pinging" in engines, not because they change the compression, but because they change the combustion timing (later). <br /><br />
Ignition timing will only affect combustion heat rejection to the cooling system if it is out of the normal operating range which is not likely on a computer mapped timing curve whether the engine is operating on regular or preiminum fuel.<br /><br />Any change in ignition timing will effect combustion temperature. The significance of the change is dependent on many factors.<br /><br />
Fuel type itself does not affect combustion temperatures. Fuel/air ratios do.<br /><br />That's why I used the term
"metering".<br /><br />Different fuels burn at different rates under the same temperatures, pressures, and air/fuel ratios. Higher octane or premium fuels burn slower, and cooler.<br /><br />
Leaner ratios raise temperatures and richer ones lower it. Air/fuel ratios change constantly during engine operation depending on load and engine speed.<br /><br />Yes.<br /><br />
As long as they remain within normal ranges heat rejection changes to the cooling system is insignificant.<br /><br />Heat rejection to the cooling system may be insignificant, but parts, like the ring lands, may see much higher and more direct temperatures with leaner metering and more advanced timing. Oil plays the role here not coolant.<br /><br />
Fuel delivery systems can effect oil fuel dilution. Carburators being the worse offender. Electronic fuel injection systems have vastly reduced the amount of fuel dilution<br /><br />Yes. Mercruiser and Volvo use different fuel delivery systems.<br /><br />
In a normally operating engine, engine coolant temperature has little to no effect oil temperature. Coolant temperatures can be normal and oil temperatures can be dangerously high at the same time. High loads WOT operation produces increased oil temperatures with little to no changes in coolant temperature.<br /><br />High flowing, high volume, large heat exchanging cooling systems will have a great effect on an oil's ability to maintain its shear properties. If a low volume and a large volume cooling system were operating at the same temperature, the large volume cooling system would be transferring more heat away from the engine. Closed cooling systems will generally be more prone to oil breakdown due to less heat exchange, and changes in coolant temperatures due to changes in oil temperature will be more sensitive.<br /><br />
It has been proven that 250 to 300 horsepower is required just to overcome internal friction losses in a V8 engine running at at 6000 RPM.<br /><br />You might want to double check that.<br /><br />Yes, synthetics have some advantages. <br /><br />The point was to show that the Mercruiser and Volvo are not "exactly" the same engine. Their recommendations for different oils and fuels may be justified.<br /><br />All good points Buttanic.<br /><br />This topic appears to have run its full course.
).<br /><br />I also learned about the new FC-W specification and companies will stop putting API certs on marine oils.<br /><br />Also, that the FC-W cert is for both outboard 4-strokes and sterndrives, even though Merc will have different oils for each.<br /><br />
