I was sure this topic was discussed before, but couldn't turn it up in a search.
Anyone who's ever worked on a head for the first time may have been surprised at the amount of pressure valve springs exert on the cam. Try turning a lubricated cam in a head - there's an unexpected amount of resistance!
The idea of going with valve springs with less resistance was suggested - as something that would likely return a very small improvement in efficiency.
I happened across a reference that suggests using lighter springs is actually a tactic automakers have used to reduce fuel consumption:
Quote:
Pulling the engine from the Altima meant adopting some of that car's bias toward fuel economy, smoothness and torque. That's the excuse for a rev limiter that smacks you in the face on every shift. Super-light valve springs designed to get that last fraction of a mile from every gallon of gas mean no more revs until you buy 16 more springs and an ECU.
Source is a discussion of putting an Altima engine in a Sentra (and then building/tuning it for more power).
I suppose "too light" would be when the springs don't exert enough force to shut the valve completely before the next stroke. If for instance the intake valve isn't closed when the compression stroke starts, it'd shove some of the fuel/air back out the intake...
On old, very low-speed engines, you may find no valve springs at all. Air opens and closes the valve. The main job of the valve springs is to avoid "float" -they overcome the inertia of the valve and lifter and keep them in contact with the cam as they start to close. That force goes up as the square of speed, so it pays to avoid high revs. If you are going to lower the red-line, a cam with less dwell and overlap is better, as there is less inertia in the manifold gasses.
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On old, very low-speed engines, you may find no valve springs at all. Air opens and closes the valve. The main job of the valve springs is to avoid "float" -they overcome the inertia of the valve and lifter and keep them in contact with the cam as they start to close. That force goes up as the square of speed, so it pays to avoid high revs. If you are going to lower the red-line, a cam with less dwell and overlap is better, as there is less inertia in the manifold gasses.
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Mercedes sports-racing cars from the 50s had them, and perhaps a few other exotics. It would be nice to see that combined with variable valve timing. Perhaps hydraulic lifters plus hydraulic closers would cure the clearance woes.
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most springs are just steel. Depending on the strength of the steel and the amount of flex they're under will determine the life before unacceptable margin of stress makes them un-useful. That is a very spring specific thing. BUT! The springs will most likely die long after piston rings or bearings fail. Diesels last what, 1-2-4 million miles? I don't work on industrial diesels, but I'm betting the springs in OTR trucks aren't ever replaced unless they are stretched or compressed out of spec.
A friend of mine does local deliveries, home every night; anyways, his truck just had its first serious engine service at 1 million miles. Sure it's had the weekly and monthly oil, fluids, leakdown etc tests, but never the engine cracked open. At the 1 mil mark they put new crank bearings in, new piston rings, new turbo seals, and put it back together (he's a car nut, so he asked the mech what was done). He says it runs a little stronger now, probably has slightly less friction and higher compression. Netted him about .3mpg more too.
Placibo? maybe, but the .3mpg is proof its running better. When you average 6.85mpg over 800 miles in a day, .3 is a huge savings.
Bottom line I'm saying: Steel springs last forever. Titanium lasts even longer (less pron to fatigue). Go with lighter springs if you want slightly better FE, but keep the rpms down to prevent valve float. Don't want any head crashes...
Steel springs are run below their fatigue limit, but only became really reliable after shot-peening became common. That was the secret of Toledo swords, too, BTW. Anyway, if you were to have your springs etched down, you could get any strength you want, and the loss of the surface treatment would probably be compensated by the reduction in strain, if the change is substantial. Springs, valve train weight, RPM and lobe shape all interact. The reduced duration on an XFI cam gives it a peakier lobe, even with less lift, so the need for spring strength is not reduced along with the power band. If you are willing to be religious about a low red-line, there are definite gains available, though.
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