Sludge Monster, Easy on Oil, Dirty Engine, Clean Engine
- Thread starter Shel_B
- Start date
Assuming we're talking about gasoline engines here?
1) Blowby: It varies. We've had engines which took forever to darken their oil and others which got dirty real fast.
2) PCV system specifics: An engine can heave a lot of dirty oil vapor into the top end where it can cook and become sludge.
3) General operating temperature: The hotter, the more sludge prone. Under-hood air circulation can effect this.
4) Specifics of heat-soak upon shutdown: Effects/compounds #2+3.
One thing I firmly believe is that my monthly (at least) trips which include 8 hours of steady, highway driving have contributed mightily to the health of my cars over the decades.
1) Blowby: It varies. We've had engines which took forever to darken their oil and others which got dirty real fast.
2) PCV system specifics: An engine can heave a lot of dirty oil vapor into the top end where it can cook and become sludge.
3) General operating temperature: The hotter, the more sludge prone. Under-hood air circulation can effect this.
4) Specifics of heat-soak upon shutdown: Effects/compounds #2+3.
One thing I firmly believe is that my monthly (at least) trips which include 8 hours of steady, highway driving have contributed mightily to the health of my cars over the decades.
dnewton3
Staff member
I'll expand on the post above ...
EASY on oil:
- good oil sump capacity relative to the displacement
- good oil sump capacity relative to the power density levels
- good oil return (doesn't sit in the heads in pockets)
- high enough volumetric oil flow rates to encourage good thermal exchange in hot spots (such as turbos, etc)
- good cooling system design so as to not have "hot spots"
- good combustion chamber/cylinder design such that soot and insoluble values are low
- good system designs such that coolant, fuel and silica don't ingress to the lube system
How are these engines confirmed as "easy" on oil?
* Good wear-metal rates UOAs
* low contamination values in UOAs
* PCs with low contamination counts
* tear-down inspections show excellent physical measurements ("in spec")
* visual inspections show no clues presnt (loss of coolant; no sludge under valve covers; etc)
Hard on oil:
Just the opposite of the above
EASY on oil:
- good oil sump capacity relative to the displacement
- good oil sump capacity relative to the power density levels
- good oil return (doesn't sit in the heads in pockets)
- high enough volumetric oil flow rates to encourage good thermal exchange in hot spots (such as turbos, etc)
- good cooling system design so as to not have "hot spots"
- good combustion chamber/cylinder design such that soot and insoluble values are low
- good system designs such that coolant, fuel and silica don't ingress to the lube system
How are these engines confirmed as "easy" on oil?
* Good wear-metal rates UOAs
* low contamination values in UOAs
* PCs with low contamination counts
* tear-down inspections show excellent physical measurements ("in spec")
* visual inspections show no clues presnt (loss of coolant; no sludge under valve covers; etc)
Hard on oil:
Just the opposite of the above
Mostly feelings based from what I have seen on BITOG over the years. Most Japanese makes will get the “easy on oil” bitog stamp of approval.
I remember the pushback Trav got when he started reporting on Honda Odyssey engine sludge, misfiring and other issues because of the VSM and too long OCI, even with synthetic. People’s brains almost melted from the thought that synthetic could sludge. It is still somewhat inconceivable to many on here.
I remember the pushback Trav got when he started reporting on Honda Odyssey engine sludge, misfiring and other issues because of the VSM and too long OCI, even with synthetic. People’s brains almost melted from the thought that synthetic could sludge. It is still somewhat inconceivable to many on here.
My wild guess would be oil capacity, presence of aux oil cooling, presence of piston coolers (aka piston jets, piston squirters), ring tension
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Toyota, Mazda and Honda do often make engines that show low wear metals via UOA's. Toyota in particular seems to be the most consistent in this area.
Dnewton covered most all of it.
Dynamic compression ratio plays a factor as well. The higher the DCR, the higher the piston temperature and top ring temperature, and thus higher the chances of ring coking.
Dynamic compression ratio plays a factor as well. The higher the DCR, the higher the piston temperature and top ring temperature, and thus higher the chances of ring coking.
This! The Acura J37 comes to mind, low tension rings with high compression ratio.
Yes, and Toyota in particular had a sludge related lawsuit and their high mileage engines show a lot of varnish.
No one is saying every single engine they made was like that, but it's rare and you're cherry picking.
My point is that it’s hard to know if an engine will be clean or dirty just by looking at its specs. We usually find that out years later by sharing pictures and stories.
All the other stuff mentioned in this thread is pure guess work.
Shel_B
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What is dynamic compression ratio?
dnewton3
Staff member
I wouldn't call it "guess work".
There are contributing conditions we know to exist in problem engines.
This comes down to using the English language in proper terms like:
- unlikely
- possible
- probable
- assured
There are some engines which historically have shown themselves to be excellent performers with very few problems, such as the Ford 4.6L 2v in CV/GM/TC applications, as well as many Triton truck engines in 2v configuration. The Mod motor series (4.6L, 5.4L, 6.8L) 2v engines are stout, long lived and have very few lube related problems. They also have very low wear rates with just good basic maintenance. Hence, it's accurate to say these engines are "probably" some of the best in terms of lubrication.
There are some engines such as the old Saturn SL2 engine that don't have any oil drain back holes in the piston rings; these engines WILL coke up the rings if you run ANY oil too long. I've seen data in UOAs that correlate to 5k mile OCIs being too long, even with some syn lubes. These engines run OK, but they burn a lot of oil because of a design decision. This is an "assured" problem simply because of the piston rings.
A lot of other engines fall somewhere in between. Of the factors I listed previously, and those some have also included, you can develop a sense of possible or probable concerns; the more factors present the more "likely" a problem may develop.
I do agree that time is the great equalizer; the longer the engine series is around, the more information we gather and can make swags become educated guesses, and then finally reasonable predictions.
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Dynamic compression ratio (DCR) takes into account the intake valve closing event on the compression stroke. The typical compression ratio we see advertised is the static compression ratio (SCR) which is simply the volume with the piston at bottom dead center (BDC) divided by the volume with the piston at top dead center (TDC). In operation, the intake valve is still open for 30-70 degrees after BDC so the cylinder is not building pressure until that valve(s) closes. At least, that's the case just slowly moving the piston at cranking rpm. At wide open throttle and >3500 rpm, there's wave harmonics in the intake and exhaust that skew that DCR due to short ramming. I won't muddy the water with that here.
Let's take the engine I have in my shop currently as an example. It's a rebuild/upgrade of a 350ci SBC V8 out of a '74 Nova.
Bore = 4.030"
Stroke = 3.480"
Displacement = 355ci
Rod length = 5.700"
Chamber volume = 76cc
Head gasket volume = 4.32cc
Piston volume = +4cc dome
Piston to deck volume @ TDC = 4.18cc
Piston to deck volume @ BDC = 732cc
To find the static compression ratio, we calculate...
(Chamber volume + gasket volume + piston volume (minus for dome) + piston BDC volume) / (Chamber volume + gasket volume + piston volume (minus for dome) + piston TDC volume) = static compression ratio
In the case of this engine...
(76cc + 4.32cc + (-4cc) + 732cc) / (76cc + 4.32cc + (-4cc) + 4.18cc)
(808.32cc) / (80.5cc) = 10.04 : 1 static compression ratio
To calculate dynamic compression ratio, we do the same thing except instead of using the volume at BDC, we use the volume from the point the intake valve(s) closes. For this engine, the intake valve closes at 59° after BDC. In order to calculate this, we need the rod length to know the piston position at that point in the compression stroke. I'll spare that math and just give the value which is 596cc.
(76cc + 4.32cc + (-4cc) + 596cc) / (76cc + 4.32cc + (-4cc) + 4.18cc)
(672.32cc) / (80.5cc) = 8.35 : 1 dynamic compression ratio
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Actually I wasn't going to chime in but it occurred to me some engines have "dead ends" where oil doesn't circulate. I seem to recall the Car Care Nut speaking to that on some of Toyota's variable valve timing components.
I'm not saying it makes an engine "dirty"....but much like stagnant water it can't be great.
I'm not saying it makes an engine "dirty"....but much like stagnant water it can't be great.
Shel_B
Site Donor 2023
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Thanks for that very thorough and understandable explanation. Very much appreciated!
I've heard that timing chains (as opposed to belts) shear oil, shortening oil life.
It depends on the engine. Some MB engines need to have the timing chain/s replaced every 100k miles. I've driven bimmers since 1974 and never had a timing chain problem. However, my 1985 BMW 325e had a timing belt which was religiously replaced every 4 years as it was an interference engine. My BIL's Subie had 200k miles on the clock before the timing belt broken. There was no damage as the engine wasn't an interference engine.
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