Originally Posted By: ARCOgraphite
Originally Posted By: Bluestream
Originally Posted By: ARCOgraphite
I dont doubt much, but I doubt that, Mega.
Actually Mega is correct, only they run the test at -20 not -40.
Here is from bbobynski GM engineer (Post 130078)
"In the case of the Mobil 1 in the Corvette I would hazard a guess that the number of dyno engines run with Mobil 1 is in the hundreds....like somewhere between 200 and 300 all total. These are engines that run for 200, 300 , 400 or more hours at full throttle, max RPM, max power and are then torn down and analyzed in a variety of ways for wear and other lubrication performance. This is on top of the countless specific bench and dyno tests run on the specific lubrication system, cold start lube system performance, cold overpressurizaration (start at -20 and go immediately to 5000 RPM), cold start field testing and driveability, engine cooling testing, etc. Fleets or test cars are running with the production intent designs and lubes on accelerated durability, endurance, emissions, track testing, etc... The products are testing far in excess of what any customer can do.
"
So how do they expain away the major piston slap problems with their V8s? All the leaky IMs? I Guess they were not as thorough as they made out. The proff is in the pudding and the pudding aint fully cooked. I do all
my torture testing on a warm engine - I gotta give the 'lil bugger sumptin of a fighting chance
ARCOgraphite, piston slap is very normal for a modern design engine. See post #129898 below from GM Engineer bbobynski
Are you talking about piston slap during the first few minutes of warmup..??
If so...that is what it is. Nothing to fear or nothing really wrong...just the piston noise until the piston warms up to fill the clearance to the bore.
Pistons rock as they cross TDC where the thrust load from the rod angularity shifts from one side to the other. This makes noise. Harmless..but it is the source of piston slap in most all situations.
To prevent this, pistons are designed with long, tapered, flexible skirts so that they can be fitted very tight in the bores when the bore and piston is cold. The taper of the skirt and flexibility of the skirt then prevents scuffing when the piston is hot. Also, the piston pin in OEM production pistons is always offset to one side....it is NOT in the middle of the piston. By offsetting the pin in the piston, artificial thrust load is created to control the piston "rocking" as it crosses over TDC.
Unfortunately, all of the above control techniques, common in past model engines to the extreme, create excess piston mass, cause friction and cost power and fuel economy. With the desire to build in as much power and free-revving capability and to improve fuel economy as much as possible thru friction reduction these design features are pushed in the other direction on modern engines.
Piston pin offset has been reduced over the years to a bare minimum today to reduce the thrust load generated and reduce friction. Pistons have been lightened up considerably by shortening the skirts. This creates less rotating/reciprocating mass which is good for power, free revving capability and fuel economy. Light weight pistons are great but the skirts, by necessity, are short making it hard to make them both strong and flexible and the shorter ckirts make them more prone to rocking.
Unfortunately, when the performance and fuel economy oriented pistons are run cold they are very prone to "slap" until they warm up to operating temperature.
The piston designers and development engineers are always treading the fine line between piston slap cold and friction and power/fuel economy loss when the engine is warm.
It is possible that you are hearing piston noise from an engine that is on the "high limit" for piston clearance so that it makes some noise cold. The good news is that the condition is harmless and that engine is probably a little more powerful (due to less friction) than a "quiet" counterpart. The bad news is that...it makes noise cold.
