Bearing clearances, oil weight, and more...
 

I'd like to start a thread on this topic. Lately, it seems I've been hearing alot about oil viscosities, and how thinner oil and tighter clearances are the way of the future, and loose clearanced engines with higher viscosities are old school technology. I'd like to hear some more input on this. I'll state why I favor clearances on the looser side on most marine builds, and would love to hear some input on whether or not my school of thought is correct or incorrect.

With todays power adders, such as cylinder head technology, camshaft technology, and forced induction, making lots of horsepower is getting easier and easier. There are alot of guys today, in our hobby, building up their engines. Lots of us are using things like production based engine blocks, with iron main caps, budget style crankshafts, and spinning our engines faster than we were 20 years ago. Back when most of these blocks and parts were mass produced, they left some things to be desired as far as rigidity.

We stress these components further and further, things can start to move around. Added RPM, longer stroke, heavier crankshafts, possibly turning a supercharger, etc. For some of us, we operate in rough water conditions, where the crankshaft is being loaded and unloaded extremely often. I was always under the school of thought, that by allowing a bit more bearing clearance, coupled with a higher viscosity oil, gave you a little more safety factor when pushing the limit on these types of parts, and possibly help keep the bearings running a little cooler. I do not know if that is absolute or not.

I am curious and always into new technology. Can any of the pro engine builders explain the benefits, or negatives, of tightening up the clearances and running a thinner weight oil? Is anyone out there running a tightly clearanced engine and say 5w20 or 5w30 engine oil in offshore engines??

I was lead to believe that the thinner oil drama like 5w-20 revolves around CAFE ratings in the automotive world.There is evidence that thicker oil reduces efficiency.But that is a fleet study that raises average MPG to meet federal regulations.But I'm old school.

I'm not a pro builder, but I'll toss in my line of reasoning that guided my decision. As far as I've been aware, cold tolerances are just that, cold. As parts heat up they change dimensions. Piston to wall clearances as well as ring gaps are suggested to be looser on harder ran engines. I applied this to the crank as well since these engines operate inside a small box under high load and generate a lot of heat. I would rather see the crank grow to a normal tolerance and run it a little loose cold than have it wipe bearings at the worst possible time. My block has new caps because of that exact thing. The bearings got wiped, the crank started flexing, and it beat the caps to death.

From what I've seen and been around as horsepower goes up so does clearances and oil wt. 3600hp .006" main clearance 60wt oil. That's what it needs to live.

Yes.

Many cars that come in here have oil caps that state 0W-20. Some 5W-20, etc, etc, you get the drift.

So two things, not only thinner oil but only synthetics found in that weight category.

Every couple of 10ths per mile make a huge difference for CAFE ratings.

That's why the synthetic drivetrain switchover too.

Up here in Northern north America, all of us can see the difference in fuel mileage between synthetics and non-synthetics in our personal vehicles during th cold months. Especially 4wds.

Good info guys keep it coming

Here's something to ponder also.

Journal or 'bearing' speed.

The larger the journal at X rpm, will have more speed on it's OD than a smaller journal.

Thus why many small block race motors have been switching (machining down to) Honda journal size.

Not something we have to worry about (I don't think ) with our type of rpms, but I'm sure is all a factor on why some clearnaces and the resultant weight of oil seems to work better.

Just winter round table talk...nothing more.

I don't have a ton to add but there was always a saying a loose motor is a fast motor. Works for me. Haha.

In 2009 the federal government mandated auto manufacturers recommend 20wt oil. If the manufacturer did not comply they had to pay a fine for that model. Not all manufacturers have complied with every model. What I have noticed when the switch to 20 wt was made. Oil consumption is very high most cars are running out of oil before there normal change interval. I work on a ton of cars. Probably 30-50 different makes and models a week. This is what I have witnessed as a trend with 20wt oils. You can have them.

The Honda journals are a thing of the past. Everyone I know that was running them has switched back.