Chevy V8 performance and wear vs. coolant/oil temperature.

[CentralCoaster]

I'm surprised I couldn't find info on this in the archives, it's an ongoing debate over engine performance and oil & coolant operating temps. Can any of you in the automotive industry clarify some of these questions?

The old wives claims are:
Cooler is always better for horsepower
Engines are only run hot to reduce emissions

This specifically refers to the gen-1 Chevy small block V8, and the gen-2 Chevy LT1 (with reverse flow cooling), or any other relevant applications. The newer gen-1 motors use a 195F thermostat, regulates coolant flow leaving the engine (2-way valve). The LT1 uses a 180F thermostat, regulating coolant temp entering the engine (similar to a 3-way mixing valve).

No one that I have seen yet has come in here with any evidence that your motor will suffer any undue harm by running it in the 180 range, by the same token that no one has shown that running at high temperatures will do any harm.

I agree, I'm having trouble finding enough info on this, short of purchasing SAE papers. I hate relying on second hand information. There's advantages and disadvantages, the question is, where is the best compromise?

Facts...

Hotter heads promote detonation.
Hotter heads rob less heat energy from engine combustion, increasing combustion efficiency.
Hotter heads give higher cylinder pressures.
Hotter intake reduces density of intake air, reducing VE.
Hotter blocks reduce oil contamination.
Hotter/thinner oil reduces friction losses.
Piston/wall clearances are optimal at a certain temperature.
GM engineers care more about emissions effects than we do.

The questions are...
Which factors outweigh the others?
Is thicker oil viscosity the primary reason for claims of lost horsepower and increase wear on cooler engines? (easily fixed by running thinner oil), or is it due to temperature effects on piston-cylinder wall clearance?

 

[reidh]

"Hotter/thinner oil reduces friction losses"

Thinner Oil does reduce friction losses, but multi-grade oils contain a viscosity improving additive, which actually allows the oil to increase viscosity with higher temperatures.

That being said, oil viscosity is not the primary reason that an engine is more powerful the hotter it gets. The main factor is simple thermodynamics.

Fuel contains a certain amount of energy. In the cylinder, that stored energy is dissipated several ways, some exits through the exhaust, some is used to overcome friction, some is transferred to the crankshaft, and some is rejected to the engine coolant as heat. The more energy (heat) rejected to the coolant, the less available there is to transmit to the crankshaft.

However, there are two factors working against a hot engine. One is the induction air temperature, the other is detonation (as you mentioned). All else being equal, running an engine on the brink of detonation is going to extract the most power from the fuel. Of course that is a fine line between maximum power and ruining your engine, so engineers will design to stay a good margin below the detonation limit. If you keep a close eye on detonation, you can easily extract more power out of a stock engine.

Induction air also plays a role, as shown by the SAE J1349 formula for correcting horsepower at a given ambient (assuming constant pressure):

cf=1.18*{[(T+273)/298]^.5}-.18-----------T is in Celsius

So assuming your induction air is the same as your water temperature, your correction factor would be 1.10 for the 180F thermostat and 1.122 for the 195 F thermostat. This is less than a 2% difference. The thermodynamic gain will more than offset the loss due to the increased induction air temperature.

Sorry about the rambling, but I have always found this an interesting discussion. Hope this answers your question.

-Reidh

 

[GregLocock]

Title: Some effects of internal coolants on knock-limited and temperature-limited power as determined in a single-cylinder aircraft test engine
Author(s): Slough, James W
Abstract: No Abstract Available
NASA Center: Glenn Research Center
Publication Year: 1944
Document Source: CASI
Download Document: View PDF File
Document ID: 19930093209
Accession ID: 93R22499
Report Number: NACA-WR-E-24
Price Code: A03
Related Information: NACA Wartime Report E-24.
Keywords: DETONATION-EFFECT OF COOLANTS ENGINES-TEMPERATURES FUEL-AIR MIXTURES FUEL CONSUMPTION CRANKCASES-WAUKESHA CUE ENGINE CYLINDERS-WRIGHT C9GC COOLANTS ENGINES-PERFORMANCE EFFECT OF INTERNAL COOLANTS ENGINES-COOLING, INTERNAL
Accessibility: Unclassified; No Copyright; Unlimited; Publicly available;
Updated/Added to NTRS: 2006-11-06

Is probably relevant

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[romke]

there are two distinct things two consider here: internal temperatures in the engine and the related coolant and lubricant temperature and on the other hand the temperature of the air sucked in and the fuel.

oil temperature: oil temperature has a relation to viscosity and therefore to the ability to lubricate the engine. however, at higher temperatures the oilviscosity is already very low indeed and 10 degrees more or less do not make a significant difference. the frictional losses will not vary very much and therefore will not have a large influence on net poweroutput - bear in mind that socalled fuel efficient oils will maximally save you about 4% percent of fuel - as long as the coolant temperature stays really low. at normal driving speeds and temperatures the differences between a standard oil and a fuel efficient oil can hardly be measured.

another thing to bear in mind is that the oil temperature in the engine varies quite a lot from location to location: you may measure 110 degrees C in the sump whereas the oil leaving the crankshaft bearings may be 160 C, the oil in the ringbeltzone around 180 C and the oil trying to lubricate the valvemechanism may be 190 C occasionally. luckily the oil used to lubricate the valvetrain is put under extreme pressure and thereby temporarily nearly solidifies - so there is no need to worry about actual viscosity.

as for the coolant temperature: the higher the better. firstly because the heat transer capacity depends on the coolant temperature entering the radiator minus the ambient air temperature and secondly engines work better at elevated temperatures from a thermodynamic point of view. you should therefore choose a coolant temperature that is as high as possible, without introducing detonation. normally that is quite a bit higher then the standard temperatures because the cooling system is usually overdimensioned to cater for situations of decreased efficiency (clogging) and higher ambient temperatures then usual. be aware though that maximum coolant temperature may be restricted by the pressure cap setting: changing the pressure cap for a higher set type may help.

you should not worry too much about the engine overheating: remember that the parts to be cooled are always quite a bit higher in temperature then your coolant - otherwise no heat transfer would be possible.

the factor most likely to have influence on poweroutput is the intake air temperature. you should try to keep that as low as possible or better as close to the ambient air temperature as possible.

keeping it low has two advantages. first the lower the air temperature the more mass of air you can actually get into the engine and therefore the more fuel you can introduce - and power take out. the other advantage is that introducing cooler air into the cilinder will lower the octanerequirement of the engine, and thus reduce the chance of detonation. detonation might ruin the engine due to local overheating and also reduces poweroutput.

usually production engines can be improved quite a bit - of course the manufacturer has his cost constraints and therefore utilizes a larger range of things acceptable then you would accept when setting up a race engine, where cost is less a constraint. that way poweroutput can be increased quite considerably without hardly changing the engine - a standard spread in poweroutput in production engines might be in the order of 15 to 20 %.

the temperature of the fuel is less important. to get the maximum poweroutput keeping it cool is advised but the effect is far less prominent then with the intake air. remember: the mass of air that you are able to get into the engine determines the maximum amount of fuel that can be utilized and hence the poweroutput.

 

[reidh]

Good points Romke.

I had completely neglected the fact that a cooler intake air charge will raise the detonation limit.

-Reidh

 

[patdaly]

CentralCoaster:

I dont know which LT1 you refer to, but at least in the Iron head Caprice version, stock thermostats are 195 F., and the first cooling fan turns on at 225 F. The second fan comes on with either the AC or when the temp hits 323 F.

It could be different in the Camaro or Corvette versions, I am not familiar with the specifics on those.

 

[patdaly]

Whoops, second fan on at 232 F.

 

[CentralCoaster]

The common thing I find when researching this, is running oil temps below 180F decreases engine life.

But there's nothing magical about 180F. If warmer is better, why not 190F? Why not 220F? Below 180F is too cold, okay, what's too hot?

It seems like these old tests didn't look past 180F.

Obviously the coolant and oil and trans temps are all related if you have heat exchange between them, so they have to be in the same ballpark. The trans fluid, for example, can be cooled on the outlet side of the radiator to maintain a substantially lower temp than the coolant in the heads.

 

[JimCasey]

I can't let this one go.

>> but multi-grade oils contain a viscosity improving additive, which actually allows the oil to increase viscosity with higher temperatures.<<

Widely believed but incorrect.
Take an SAE 5W40 oil for example.

The viscosity at -25 F is within the SAE 5 range, 3500 cp.
At 100C it has thinned to the viscosity of an SAE 40 oil: between 16.3 and 12.5 cSt.

Meanwhile a straight SAE5 would be at 3.89 cSt at 100C.

So the Oil definitely gets thinner as it gets hotter, but the multigrade oil thins out on a flatter curve.
Ref: SAE J300 Dec95

More wear at temps less than 180 has to do with quenching of the F/A ratio and washing of the oil film from the cylinder wall, also dilution of the oil with both fuel and with condensation from moisture from combustion byproducts blown by the rings. This leads to production of corrosives and corrosive erosion which a micrometer would measure as material loss/wear.

 

[patprimmer]

Viscosity improver's are added at a rate that decreases the rate of thinning rather than actually increasing viscosity. This is indicated by change of oil pressure with change of temperature.

Oil temperature and water temperature will be indicators of block and piston temperatures.

Minimal safe piston to bore clearance will give least wear. temperatures of both block and piston substantially effects running clearance.

Water collects in engine oil and is normally lost by evaporation. close to water boiling point or slightly over drives of the water, but the hotter the oil the quicker it oxidises. Maybe 180 is a good compromise, however I doubt it is that sensitive and consistent. Once warmed up, running as low as 160 and occasionally as hot as 220 will be a good practical real world range for very good engine life I would think.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[patdaly]

CentralCoaster:

I believe the old tests didnt go above 180 because of thermal degradation of Dino oils. Now that more of us are running Synthetics, we can run hotter oil temps. I'm not 100 percent sure, but I believe they run Synthetics from the factory in new Corvettes because coolant temps are now in the 240 range?

 

[Knap]

CentralCoaster
This is old

http://www.carnut.com/ramblin/_cool3.html

I believe Vizard recommended lower temp for more power but again many years ago.
From Internal Combustion Engine Handbook
The important factor regarding operational life and reliability is the component temperature. The piston head (in SI engines) has a combustion side heated by hot gases and high pressures and the bottom side cooled by oil and crankcase air. The combustion (hot side) of this plate gets up to 570 F. The underside, central (hottest) portion gets up to 500 F. The upper ring temperature, bottom edge is at 400 F. The total surface area compared to other things is small but it gets this hot so oil must be able to withstand this environment.

For F1/performance engines, esters based oils are used without viscosity improvers for the highest fluidity, lowest friction resistance, high wear protection, maximum shear stability and heat tolerance.
Viscosity is decreased to achieve power gains

http://www.silkoleneoil.com/techtip6.htm

 

[lowtechgrad]

CentralCoaster posted <<< The common thing I find when researching this, is running oil temps below 180F decreases engine life. >>>

Why is that?

I'm just an occasional shadetree mechanic,
never torn into one- so go easy please<g>

I understand if oil gets too hot it does not lube as well ("breaks down"?),
but why would 'cool oil' be detremental ?




Rich