SBC Shaft Mount Geometry with different int/exh lifts

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by lekid » Sat Jan 13, 2018 2:59 pm

So I checked my geometry/sweep again and tried 0.050, 0.085, 0.100, 0.115 & 0.140 shims and the best one was 0.100" shims that gave me a sweep of 0.065" on the intake and 0.060" on the exhaust. Geometry is almost perfect as well with the mid-lift at 90deg of the valve.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by steve316 » Sat Jan 13, 2018 5:12 pm

.050 more lift will add .005+ to your sweep. Sounds like you have it.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by lekid » Sat Jan 13, 2018 5:35 pm

Thanks!! Took long enough, lol! I have around 4-5 hours to get it right. Do it right the first time I guess, lol!

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by randy331 » Sat Jan 13, 2018 6:05 pm

Below is from Jesel's website.
As you can see they don't go along with mid lift on their shaft set ups.
If you set up one of their shaft systems, with the tool they send, it won't be mid lift.

Randy


ROCKER GEOMETRY

Rocker geometry is a function of the arc generated from the rocker arm and the relationship of the valve tip to rocker shaft height. Using this arc correctly is the difference between a smooth operating valvetrain and a valvetrain of worn out parts. Jesel’s Low Pivot geometry utlizes the portion of the arc that produces a minimal sweep pattern from half to full lift, a point at which spring pressures are exponentionally increasing. These added spring forces transferred against the nose roller have the potential to cause the roller to skid instead of roll across the tip bending the valve stem and wearing the guides. By minimizing the roller travel distance under high spring loads, the potential of roller skidding is reduced and valve guide wear decreased tremendously. As illustrated below, the Jesel Low Pivot geometry yielded almost .020” less roller travel during the critical stages of lift compared to a rocker set up for a symmetrical half-lift geometry.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by MadBill » Sat Jan 13, 2018 7:19 pm

The Jesel method would yield a lower shaft location than the above, as it would result in 90° at ~ 3/4 lift instead of half. Their logic does reduce scrub during the high spring force part of the cycle, but at high RPM much* of that is working to overcome valvetrain inertia as the valve slows approaching max lift, resulting in less roller to stem tip contact force. Also, to nit-pick, for a standard design spring, its load increases linearly not exponentially. *In fact at the point of valve float the spring load on the roller is zero.

Seems to me you would want minimum scrub in the lift region where the combined spring and inertia forces max out, probably centered near the 40% lift point. :-k

(Un-oh, my mind just flashed to a lengthy and acrimonious S/T thread from a few years back on this subject; I may come to regret sharing the above musings... #-o)
Felix, qui potuit rerum cognscere causas.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by cjperformance » Sat Jan 13, 2018 7:46 pm

MadBill wrote:
Sat Jan 13, 2018 7:19 pm
The Jesel method would yield a lower shaft location than the above, as it would result in 90° at ~ 3/4 lift instead of half. Their logic does reduce scrub during the high spring force part of the cycle, but at high RPM much* of that is working to overcome valvetrain inertia as the valve slows approaching max lift, resulting in less roller to stem tip contact force. Also, to nit-pick, for a standard design spring, its load increases linearly not exponentially. *In fact at the point of valve float the spring load on the roller is zero.

Seems to me you would want minimum scrub in the lift region where the combined spring and inertia forces max out, probably centered near the 40% lift point. :-k

(Un-oh, my mind just flashed to a lengthy and acrimonious S/T thread from a few years back on this subject; I may come to regret sharing the above musings... #-o)
:lol: oh no what have you started!

But on that with roller rockers for long life i prefer (on the valve side) mid lift or just shorter on pushrod length where compromise must be made.
I check the pushrod side aswell and sometimes compromise has to be made there too.
Craig.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by panic » Sat Jan 13, 2018 8:17 pm

And, of course, Jesel does not bother to elaborate that the penalty for saving the guide is lower lift during the opening phase of the intake stroke where vacuum is highest. If your heads work well but springs are harsh this may be what you want.
However, it you're starved for air it may not be.

If you can't measure sweep accurately, another metric is sweep direction. Using .800" gross valve lift:
At lift 0% (valve closed) the roller tip should be at its closest point to the rocker shaft.
At .200" (25% lift) is has walked across the stem and is mid-way (but may not be exactly centered) in its path on the stem tip.
Then it crosses the stem tip completely and arrived at the farthest point (away from the shaft) at .400" (50% lift).
It then reverses direction, and repeats those steps:
At the mid point @ .600" (75% lift).
Back at the closest point at .800" (100% full lift).
Rinse, repeat.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by cjperformance » Sat Jan 13, 2018 10:45 pm

panic wrote:
Sat Jan 13, 2018 8:17 pm
And, of course, Jesel does not bother to elaborate that the penalty for saving the guide is lower lift during the opening phase of the intake stroke where vacuum is highest. If your heads work well but springs are harsh this may be what you want.
However, it you're starved for air it may not be
.

If you can't measure sweep accurately, another metric is sweep direction. Using .800" gross valve lift:
At lift 0% (valve closed) the roller tip should be at its closest point to the rocker shaft.
At .200" (25% lift) is has walked across the stem and is mid-way (but may not be exactly centered) in its path on the stem tip.
Then it crosses the stem tip completely and arrived at the farthest point (away from the shaft) at .400" (50% lift).
It then reverses direction, and repeats those steps:
At the mid point @ .600" (75% lift).
Back at the closest point at .800" (100% full lift).
Rinse, repeat.
A very good point indeed!
Craig.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by randy331 » Sat Jan 13, 2018 11:51 pm

There are arguments for the various different geometry set ups, the application would be the biggest deciding factor on what to use.

You can go for mid lift, most total lift, Jesel's method, etc.

How do we decide what is best for our application ?

Randy

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by Geoff2 » Sun Jan 14, 2018 3:33 am

Bill,
Good catch about 'exponential' spring forces. One wonders if Jesel can get something THAT simple wrong, what else could they have got wrong...

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by randy331 » Sun Jan 14, 2018 9:56 am

Geoff2 wrote:
Sun Jan 14, 2018 3:33 am
Bill,
Good catch about 'exponential' spring forces. One wonders if Jesel can get something THAT simple wrong, what else could they have got wrong...
So you think Jesel has it wrong ?

Randy

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by MadBill » Sun Jan 14, 2018 12:09 pm

Yes, unless there are factors I've overlooked. (Or if they are improbably more concerned with low RPM guide wear.)
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Happy is he who can discover the cause of things.

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Re: SBC Shaft Mount Geometry with different int/exh lifts

Post by randy331 » Sun Jan 14, 2018 1:34 pm

I agree with Jesel in some applications.

Randy

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