OCI correlation with fuel mileage

[novadude]

Take 2 engines with a similar wear signature... let's say two Chevy 350s. One has an EFI induction system, OD trans, high gears (2000 rpm @ 60 mph), and gets 25 mpg. The other is carbureted, no OD, and 4.11 gears (3500 rpm @ 60 mph), and gets 12 mpg. It seems to me, these engines would have vastly different OCI needs. Is there anyway to correlate MPG vs. recommended interval for a given engine.

This was a thought I had when driving to work... it seems to me that shoving more a/f through the engine and more rpms per mile would beat up oil more, requiring more frequent changes.

Anyone have any research on this subject?

 

[XS650]

All else being equal, the 2,000 rpm engine is running 1.75 times the average cylinder pressure of the faster turning engine. It's not exactly resting.

It doesn't look like all else is equal though. It looks like the engine attached to the 4.11 is pumping a lot of unburned gas through it, unless it's in a motor home

Some people recommend OCI based on the amount of gas consumed. Not an illogical approach.

 

[JHZR2]

quote:

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Originally posted by XS650:
All else being equal, the 2,000 rpm engine is running 1.75 times the average cylinder pressure of the faster turning engine. It's not exactly resting.

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Please explain

 

[XS650]

quote:

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Originally posted by JHZR2:

quote:

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Originally posted by XS650:
All else being equal, the 2,000 rpm engine is running 1.75 times the average cylinder pressure of the faster turning engine. It's not exactly resting.

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Please explain

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If they are pushing the same load at the same speed, they are putting out the same amount of power.

In ft-lb-s units

Torque = (HP x 5252/rpm

Assuming they are pushing the same load, HP and 5252 are constants, so required Torque is proportional to the inverse of RPM.

Average cylinder pressure on a given engine is directly proportional to developed torque being devloped at the time.

Frictional losses and couple of other things fudge those numbers a bit, but not much in the ranges we are talking about.

 

[Gary Allan]

quote:

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Originally posted by JHZR2:

quote:Originally posted by XS650:
All else being equal, the 2,000 rpm engine is running 1.75 times the average cylinder pressure of the faster turning engine. It's not exactly resting.

Please explain

If they are pushing the same load at the same speed, they are putting out the same amount of power.

In ft-lb-s units

Torque = (HP x 5252/rpm

Assuming they are pushing the same load, HP and 5252 are constants, so required Torque is proportional to the inverse of RPM.

Average cylinder pressure on a given engine is directly proportional to developed torque being devloped at the time.

Frictional losses and couple of other things fudge those numbers a bit, but not much in the ranges we are talking about.

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can we shorten this to "load" or "stress"

It's probably a wash ..although I find the example extreme (probably to make a point). Anything with a designed engine speed of 3500 rpm @ 60 mph is either a rice burner with a sewing machine stroke (low piston speed) ...or a street rod with a 7000 rpm limit. Anything else with a 4.1 rear and a long stroke (relatively) will have tires of substantial size and will probably be around 2000-2500 rpm (or less) at that speed. That is, you would be hard press to see two like engines in similar chassis being this polarized in this driving mode.

 

[dustyjoe1]

Too Slick shared the following formula with me a while back: oci = (constant x sump capacity in quarts x Miles per gallon x Cubic Inches) / Horsepower. The constant is 40 for dino 60 for g3 80 for m1 and 120 for amsoil. Might want to bump up the 40 a bit given the increasing quality of dino base stocks. If you do a little algebra its fairly easy to rewrite the above realtionship in terms of MPG.

Regards,

R.

 

[dustyjoe1]

also regarding beating an engine vs babying it; Just use your horsepower output at the different rpms to simulate the beating. At least that's my take on it....

 

[novadude]

quote:

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Anything with a designed engine speed of 3500 rpm @ 60 mph is either a rice burner with a sewing machine stroke (low piston speed) ...or a street rod with a 7000 rpm limit.

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The example was purely arbitrary, HOWEVER, the example is not all that extreme. My '65 Nova with 3.08 gears and a stock powerglide cruises right around 2800-3000 at 65-70 mph cruise. In fact, this is the norm for most 1960's cars. It is not at all uncommon for 3.55-3.73 geared cars to roll along at 3200-3500 at around 70 mph to keep up with the flow of traffic. These gears were commonplace in the 1950's and '60s, and most of these cars were not big cammed monsters.

For a point of reference, my 3.27 geared Mustang with the same diameter tires (25.6") as the above Nova cruises at 2000 RPM @ 70 mph. OD is a wonderful thing!

 

[Gene K]

Well I would say you are not that far off the mark...

Caterpillar specifically recommends against using
miles and instead using fuel use to determine OCI.

On a CAT 3406E (14.6L) Diesel of 355-550 bhp with a oil capacity of 10.0 Gal (40.0 Qt) they specifically recommend you change the oil every 2500 gal of fuel use + adjust for any add oil up to a maximum of 30,000 mi. They have a chart in the owners manual based on mpg.

Example:
2500 gal x 6.0 mpg x 1.0 (No Add Oil) = 15,000 mi OCI

2500 gal x 8.5 mpg x 1.2 (2.0 Gal Add Oil) = 25,500 mi.

They do require you use the CAT SOS Analysis program for OCI's over 15,000 mi though.

An additional problem with carbed engines is the possibility of excess fuel in the oil. On a carbed engine Amsoil would be the oil of choice because the fuel dilution offsets the oxidation thickening.

Gene