red hot valve springs

[Tmoose]

A motorcycle tuner with a good reputation spanning decades hangs around another bulletin board. He reported that a ProStock drag race (car, I think) builder had told him that their valve springs get red hot in operation. I find that kind of hard to believe, and would just ignore the comment coming from a lesser source. To be fair I have never been at a ProStock engine dyno session, but the valve spring metallurgy required to buy durability would seem to require operating temps way less than 1000F. And I'd expect the oil to char way below red heat as well. (Maybe the existence of char is the source of the red hot theory)
Any supporting experience here ?

thanks

Dan T

 

[140Airpower]

I've heard this before in a post describing an Isky research setup. The claim was that SBC springs would get "red hot". On the face of it it's ridiculous for the reasons you list. I can envision Isky fighting oil fires in his test valve trains. However, the springs do get hot when worked hard and stock SBC springs had dampers that would add more heat. I have also heard of oil being skirted on valve springs, especially exhaust springs where there is heat from the valve stem and guide.

 

[izzmus]

I'm not so sure that there is durability involved with any top-tier drag racing valve springs. I haven't paid attention in years, but at the time, valvespring life was measured in numbers of passes, and the number was barely into the double digits.

 

[patprimmer]

some components in Top Fuel have a life of several hundred turns of the engine at WOT.

Valve springs can certainly char mineral oil in some racing applications like sprint cars, and mineral oil tends to be more compatible with alcohol fuels.

I very much doubt actual red heat though as the temper would quickly disappear. I suspect the term red hot was used to describe very hot, then quoted as literal. They do get hot enough for carbon to form on the springs and spring seats.

Regards
Pat
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[tbuelna]

As patprimmer notes, if a steel valve spring becomes "red hot" it will have also lost its temper, and will take an immediate set when compressed. Valve springs do get hot due to friction and conducted heat from the head and valve. Friction is the primary source of heat in the valve spring, and this friction mostly results from rubbing between the coils of double/triple springs or where a spring dampener is used. It is common practice to use lube oil to cool the valve springs in these instances.

"Red hot" steel implies a temperature of 1000 to 1200 degF. Most valve spring steel alloys lose temper strength above about 475 degF. So the notion that steel valve springs can endure even very brief periods of operation at "red hot" temperature is sheer fantasy. As for the presence of carbonized oil deposits on valve springs subjected to severe duty, that is entirely conceivable. Engine oil can produce these type of deposits at temperatures below the temper limit for alloy steel springs.

 

[pontiacjack]

I could only believe this if "in operation" could include dyno testing- with a run-time significantly exceeding the few seconds duration of an on-track run.

 

[patprimmer]

Another point.

These real high output race engines almost always have aluminium heads. If the springs where red hot, they would melt their way down through the head.

Regards
Pat
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[140Airpower]

Pat, absolutely correct. The description "red hot" is a huge exaggeration. But, they do get hot and they do fatigue with heat contributing to it. When you say there is coking of the oil on the spring or spring seat, is it the exhaust or both? If only the exhaust, I would expect that from exhaust heat primarily.

 

[swall]

As a metallurgist, I can't buy the melt down theory. An aluminum head is an excellent heat sink and the end in contact with the head would be cooler than the "red hot" spring.

 

[140Airpower]

swall, now that you say that I am sure you are right. Mere contact with a red hot part might not melt Aluminum that has heat-spreading mass and jacket water on its opposite side. However, as a metallurgist what is your take on the detempering issue? What temperature range would be gradually ruinous to highly stressed steel springs?

 

[swall]

The overtemp I worry about killing the springs starts around 450F or so. That will kill the shot peening and the fatigue life will be degraded.

 

[patprimmer]

The heat sink will keep the head substantially cooler, but aluminium can be somewhat below red heat and still recede under high pressure.

I am certainly not a metalurgist, but the number 600 seems to spring to mind in association with aluminium having a dramatic drop in strength.

Regards
Pat
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[140Airpower]

Pat, I too remember 600 degrees being a critical temperature, one reached in air cooled engines under worse case conditions. You get recession of valve seats and dropped valve seats. At lower temperatures, from perhaps 400F you lose the 356-T6 heat treat. I think water cooled Aluminum heads should be safe from those kinds of temperatures.

 

[franzh]

Although Prostock and straight line drag racing is impressive, I have to shudder on the valve train stresses an F1 or Indy car engine experiences during a 2 hour run. F1 was just completed about an hour from my house and I could hear the shrieking engines if the wind was just right. 18,000 rpm for two hours? Realize that is the top rpm, they run right close to it for a pretty good portion of the time.

Franz

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[140Airpower]

Franz, did you consider going? How much were tickets?
F1 engines have to last between 2 and 3 entire race weekends including qualifying. Ironically, or predictably(?), they are much more reliable under these rules than they were when they only had to do one race. But everything is critical and super expensive. At least they don't have to worry about red-hot valve springs since they use air springs. 18,000 rpm is only about 75% of the rpms they would be doing by now had the rules not limited them (some were already at 21,000 back in 2006).

 

[patprimmer]

Air at really high pressures can get very hot unless allowed time to cool after compression. Of course if cooled after compression it can get very cold when released.

Regards
Pat
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[BigClive]

A well-known Oz cam grinder once told me that he built a motored test rig to test cams and springs etc. He said that without the oil supply turned on the valve springs were smoking and "turned blue" with heat within seconds.

 

[140Airpower]

Pat, apparently the pneumatic systems they use are very efficient, leaking very little air. So, the adiabatic compression/decompression cycle should be virtually a zero sum for the heating and cooling of the air spring. Even the conduction of heat in and out of the spring cylinder walls should be a zero sum. However, I am very sure they adjust the pressure to compensate for heating of the head. In fact, I heard they even adjust the pressure over the rpm range, but this would require some hp.

BigClive, it belatedy dawns on my numb skull that when you drop a valve spring squarely onto a hard floor, it bounces back to only 90-something percent of its starting height. I think the loss is a few percent. A little is, of course, air drag, but what remains is the inefficiency of the spring. That directly results in the heating of the metal. How much power does it take to compress the spring? A few percent per cycle of that is what heats the spring. There is also friction in the rest of the valve train contributing to the total power cost to run the valves. Note that at low rpms, the spring pushes back on the cam, returning power in valve closing, but at float rpm, the spring is not able to deliver power back to the cam. I assume the stored energy in the spring gets progressively dissipated in spring surge and coil collisions as the rpm increases. You can imagine that the heating of the spring increases faster than the rpm..