Author Topic: Valve Lash and RPM  (Read 663 times)

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Offline fabr

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Valve Lash and RPM
« on: June 19, 2019, 07:24:14 PM »
I found this short info article damn interesting. Note the bold red number. WOW!!! DAMN!!!! I had no idea anything ran that much lift. Inferring from the article that motor must run close to zero lash. This old nascar engine had almost no lash either.  I'm having a custom cam ground for it to make it a bit more suitable to fast duning/desert running. I'll know tomorrow what the lash spec will be for it. Got me really curious. Anyway,here's the article.


Competition valve spring design and its relationship to lash.

 

A chief misunderstanding of valve lash is how it correlates to rocker ratio. When you compare a 1.5:1 rocker functioning with lash of 0.015in to a 2.0:1 rocker ratio also with 0.015in lash, they are not the same. Given a common lash of 0.015in., the 2.0:1 rocker begins to move the lifter at 0.0075in compared to the 1.5:1 rocker where valve movement begins at 0.010in. Also, the speed of the ramp is changing. A camshaft designed for 0.015in lash but mistakenly adjusted it to 0.025in, would probably increase the speed of the moving parts by 20 times.

As engine speed increases, the valve train components continue to hit harder and harder, which causes them to distort more and more. They all bend: the rocker arms bend, the pushrods bend, the fork on the lifter bends, the camshaft winds up. As a result, valve train components are designed to operate at the engine’s highest rpm—this is the point where they cannot sustain further abuse.

Another misnomer is the relationship between the crankshaft and the camshaft. Though connected by a chain, the camshaft responds only to the clock on the wall. As engine speeds increase, mechanical activity happens faster in relation to the clock. So, this is why an engine spinning at 8,000rpm can operate with valve lift of 1.200in, whereas an engine turning 10,000rpm is likely limited to around 1 inch of lift. This is because there is only a certain amount of time available to move the valve, and that’s a lot faster in relation to the crank.

Joe Hornick has designed successful racing camshafts and valve train for both 903ci Pro Mod engines and NHRA Pro Stock engines. One of his most successful Pro Mod engines operated with 1.440in lift compared to a front-running Pro Stock engine with 1.190in lift. Because the Pro Stock engine revs so much higher, it operates with less lift—although valve speed remains the same.

Competition valve spring designers are obliged to move the valve train at its maximum velocity, and the best way to accomplish this is to integrate the spring’s natural frequency with the correct spring rate. Extreme spring pressure provokes excessive deflection in the valve train and when combined with a disregard of the spring’s natural frequency, the valve train doesn’t react well.

Source

Joe Hornick Enterprises
Mooresville, NC
(704) 664-7322
jheinc.com
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Offline fabr

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Re: Valve Lash and RPM
« Reply #1 on: July 05, 2019, 07:13:37 PM »
I thought someone would have said WTF to 1.440 valve lift. I was stunned.
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Offline dsrace

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Re: Valve Lash and RPM
« Reply #2 on: July 06, 2019, 07:26:17 AM »
that is amazing, 1.440! i assumed that had to by a higher ratio rocker but i cannot imagine the unsprung height of the valve spring for that kind of stroke let alone the beating the valve seat is taking! very impressive indeed!
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Offline fabr

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Re: Valve Lash and RPM
« Reply #3 on: July 06, 2019, 04:18:45 PM »
The cams are almost 3" in diameter for those lifts . The cam in the sb2.2 is approx 2 1/4" diameter with some at 2.5". That allows a much gentler opening and closing ramp. The velocity the valve opens and closes at is just amazing though considering the valve closes rather gently.
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Offline dsrace

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Re: Valve Lash and RPM
« Reply #4 on: July 07, 2019, 06:10:48 AM »
The cams are almost 3" in diameter for those lifts . The cam in the sb2.2 is approx 2 1/4" diameter with some at 2.5". That allows a much gentler opening and closing ramp. The velocity the valve opens and closes at is just amazing though considering the valve closes rather gently.

have never seen that before.....wow! i can only imagine the velocity of the ramp and closing speed and the abuse the poor valve seats take lol impressive for sure!
Don't never argue with an Idiot!
Because he will drag you down to his level and beat you with experience

Offline fabr

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Re: Valve Lash and RPM
« Reply #5 on: July 07, 2019, 08:52:08 AM »
I asked Comp about that. Again, the closing ramp is rather gentle. Remember valves bounce and HP is lost if the valve bounces on closing OR launches off the lobe nose. Those guys aren't losing any HP. With such large lobes and raised cam locations to accommodate the large diameter cams and still have con rod clearance,engineers have the ability to design valve closing rates that are as slow or even slower than small lobes like on small base circle cams that are needed for big lift and rod clearance with cams in OEM or even slightly raised locations.
« Last Edit: July 07, 2019, 08:58:51 AM by fabr »
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)

Offline dsrace

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Re: Valve Lash and RPM
« Reply #6 on: July 07, 2019, 11:12:29 AM »
interesting
Don't never argue with an Idiot!
Because he will drag you down to his level and beat you with experience

Offline fabr

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Re: Valve Lash and RPM
« Reply #7 on: July 07, 2019, 12:02:27 PM »
It is to me. I haven't stayed up on the high end tech like I should have the last 10-15 years. I am doing a lot of catch up .
"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
"I fear the day that technology will surpass our human interaction. The world will have a generation of idiots" Albert Einstein  (IT'S OFFICIAL THAT DAY IS HERE NOW!)