anyone living with detonation ?
how many cars were sold with anti-'run-on' valves ?
20 million ? ......... 50 million ?
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It is called 'lufting' and it is common in NASCAR. I am having trouble finding the article on my computer. It is probably in one of my hard copies of Race Engine Magazine.strad wrote:Launching the lifters of the lobe of the cam??? Sounds crazy.
Tommy Cookers wrote:anyone living with detonation ?
how many cars were sold with anti-'run-on' valves ?
20 million ? ......... 50 million ?
That is it. I chose the wrong spelling. Searching under 'loft' works great.gato azul wrote:some call it "loft", if this is what you mean
.Valve loft is intentionally using a controlled valve float to increase lift and duration of the valve open cycle. In some motorsports there are rules that limit camshaft lift; therefore, provoking this type of exploitation. Properly optimizing the system avoids undue stresses to the camshaft lobes and tappets.
.In Winston Cup racing today, a key to improving engine performance is to improve performance of the valve train. A significant portion of Cup team engine development effort is spent working with valve train spin fixtures. These devices allow gross valve motion to be measured at different simulated engine speeds.
What quickly becomes obvious is valve timing and lift effectively change with engine speed (Note Chart 4). Durations measured at 0.050 and 0.100 inch regularly vary 10-15 degrees from low to high rpm. Loft (additional lift from lobe-follower separation at maximum lift) can exceed 0.080 inch, on some roller profiles.
While most racers dont have access to a spin fixture, it is clear that comparing camshafts by looking at 0.050 inch duration numbers and lift is like comparing cylinder heads by valve size. What the Saturday-night racer should do is compare cams at multiple durations like 0.004, 0.020, 0.050, 0.100, 0.150, 0.200, and 0.250 inch. This type comparison will produce a better understanding of how the lobe is likely to perform.
What should you look for? Modern cam profiles have significantly higher low-lift acceleration rates. This means the duration on the seat and at 0.050 inch will actually be smaller, but the lobe will have the same duration at 0.250 inch. This type profile allows you to use a smaller cam for torque but will still make power where expected.
Talk to the cam manufacturer and ask about the dynamic performance of the lobe. You want to know its limit speed. The limit speed is the speed beyond which the valvetrain cannot be turned any faster. This should be much greater than the maximum intended engine speed. Ask at what engine speed does significant bounce occur. This speed should just exceed maximum anticipated engine speed. It is important to know exactly what components were used during the manufacturers spin fixture analysis. If your valvetrain varies from what was tested, the data may not relate to your particular engine.
How do components affect dynamic operation of the valvetrain? In general, you want to remove as much mass as possible from any part on the valve side of the rocker fulcrum (valve, retainer, and locks). This allows the spring to do a better job, helps reduce bounce, and increases limit speed. On the lifter side of the fulcrum, stiffness becomes more important than mass (weight). A stiffer pushrod will follow the lobe better. Reducing the mass of the lifter and/or pushrod will reduce loft, increasing mass will increase loft.
Read more: http://www.circletrack.com/techarticles ... z24UCqSKYQ
By their motion, valvesprings generate heat. It is therefore important that they receive adequate oil for cooling. Also, in the process of being compressed and extended, they never come to rest. Even when valves are seated, residual harmonics or compression and extension of spring coils within a given stack continue between times valves are being opened or closed. These movements are not always along the axis of a given spring, resulting in lateral distortion that accompanies axial motion. This produces components of spring motion that are counter to what might be called pure spring compression and extension.
Despite the efforts of camshaft designers to provide stable valve motion during low-lift opening and closing points, there remains a system of flexible parts between the valve tip and cam lobe (pushrods, springs, and rockers). These components contribute to valve motion that is not true to the cams lobe profiles, thereby producing lift patterns less than what is designed into a given camshaft. During valve seating, reducing the elastic collision between valve heads and seats is an ongoing problem. Engine builders can help the situation by making certain manufacturer-recommended spring installation and pressures are applied throughout the life of a given spring.
Springs should be sufficiently stiff to control valve motion approaching maximum lift as well as during its acceleration to closing. As engine speed is increased, the more important spring forces required to control valve motion are during maximum valve acceleration. Boiled down to its essentials, this points to the importance of maintaining sufficient spring pressure just prior to maximum valve lift in order to maintain lobe/follower contact at and just beyond peak lift.
Read more: http://www.circletrack.com/techarticles ... z24UE1V1Tn
Beehives are OE part numbers now I believe. What I was musing about is if the improvements in springs etc. are such that F-1 engines could now run the somewhat reduced legal RPMs without pneumatic assist –maybe they are since valve trains are old hat topics and aren’t much reported on any more.strad wrote:Got beehives in the Stang..
gato azul Was kind enough to forward a paper wherein they show that there certainly is separation between the tappet and the cam lobe. It's well researched, or seems to be so I have no reason to doubt it, but I'm still not sure,,,haven't read the whole paper,,, that it is predictable or useful as a performance enhancer.
Thanks to the blue cat for some interesting reading
Materials (fatigue) and harmonics are pretty much different considerations. The leaders didn’t include me in their conversations, but back then the answer for valve float tended to be stronger springs. Vacuum cast material was a great improvement for fatigue but didn’t do anything for float. Duel springs with differing natural frequencies were the best answer for the latter. Now there’s effectively a broad spectrum of natural frequencies in a single spring.Tommy Cookers wrote:surely these 'improvements' in springs have been known and some used eg in F1 (or GP motorcycles more likely) for 60+ years ?
(the best US valve spring material was as good as the world best, lots of spring sets were sold (Sparks & Withams etc), with super material less other trick features were needed eg British hairpin springs replaced with S&W or Swedish coils )
(ok they have only recently been introduced to NASCAR, smallblock V8s etc)
BTW this is where high strength Copper Beryllium (2.5% Be) would work better than anything else in all-out performance
(designed for reasonable fatigue life its benefits would be less)