Push Rod Length: Shorter vs Longer

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skinny z
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Push Rod Length: Shorter vs Longer

Post by skinny z »

When your calculated push rod length falls between two standard measurements, do you go longer or shorter?
Say I find 7.385" is ideal. So is it 7.375 or 7.40" ?
This is a hydraulic roller with .575" lift via 1.6 rockers.
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Re: Push Rod Length: Shorter vs Longer

Post by F-BIRD'88 »

The rocker nut setting of the hyd lifter preload (amount) changes it a bit anyway.. Better to mock up and test measure with an adjustable push rod than to "calculate" before ordering pushrods.
Use the same rocker arms for mockup as some rockers have a modified pushrod seat height. This ± changes the lift (rate) geometry and pushrod length too.
Not all "1.60:1 roller rockers" are the same.
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Re: Push Rod Length: Shorter vs Longer

Post by ClassAct »

skinny z wrote: Sat Nov 27, 2021 8:32 pm When your calculated push rod length falls between two standard measurements, do you go longer or shorter?
Say I find 7.385" is ideal. So is it 7.375 or 7.40" ?
This is a hydraulic roller with .575" lift via 1.6 rockers.
With a hydraulic lifter it won’t matter. Buy the one you can get and send it.
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Re: Push Rod Length: Shorter vs Longer

Post by skinny z »

F-BIRD'88 wrote: Sat Nov 27, 2021 9:26 pm The rocker nut setting of the hyd lifter preload (amount) changes it a bit anyway.. Better to mock up and test measure with an adjustable push rod than to "calculate" before ordering pushrods.
Use the same rocker arms for mockup as some rockers have a modified pushrod seat height. This ± changes the lift (rate) geometry and pushrod length too.
Not all "1.60:1 roller rockers" are the same.
Yeah. All measured. That's where 7.385" comes from.
Been through the rocker arm differences before too.
There's certainly some tuning via the preload as well.
But what's your take on having choose? Longer or shorter?
Last edited by skinny z on Sat Nov 27, 2021 11:16 pm, edited 1 time in total.
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Re: Push Rod Length: Shorter vs Longer

Post by skinny z »

ClassAct wrote: Sat Nov 27, 2021 10:53 pm
skinny z wrote: Sat Nov 27, 2021 8:32 pm When your calculated push rod length falls between two standard measurements, do you go longer or shorter?
Say I find 7.385" is ideal. So is it 7.375 or 7.40" ?
This is a hydraulic roller with .575" lift via 1.6 rockers.
With a hydraulic lifter it won’t matter. Buy the one you can get and send it.
That was the thinking too. But the question came up anyway.
He went long as it seemed to make the most sense.
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Re: Push Rod Length: Shorter vs Longer

Post by F-BIRD'88 »

Off the top of my head a shorter pushrod should pull the tip of the rocker arm towards to stud. A longer PR should move tbe contact point away from the stud, on the top of the valve.. The hyd lifter preload setting adjustment ± will account for a small pushrod length difference.
Its still a "girlie cam" .....so......
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Re: Push Rod Length: Shorter vs Longer

Post by BOOT »

With hydraulic I prefer to go on the long side when that happens.
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Re: Push Rod Length: Shorter vs Longer

Post by rebelrouser »

I have Manton push rods send me a push rod kit, they leave the one end out, and I cut to exact length and press the tip in myself. I do this mainly on Mopar engines as with the fixed shaft they may be a little more fussy on the exact length. On SBC and BBC seems like there is so much available for them that I can get the right length already made, at least for the few engines I build a year.
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Re: Push Rod Length: Shorter vs Longer

Post by skinny z »

This SBC with AFR 195's was mocked up and measured going for the rocker at 90° to the valve stem at midlift.
The measuring was done several times, different cylinders, intake and exhaust. Also using checking springs and then to confirm some tech article suspicions, also checked with the running springs.
The running springs tightened things up as was thought to be the case. This is what we went with.
End result was 7.385". This takes into account the .070" for the LS7 lifter preload. (which, not being an LS guy, kind of surprised me with how much of it there is. I've been working with .020" with the short travel lifters in the Gen 1)
Thus, the question.

From the replies, both here and elsewhere, the lifter preload and how it can be manipulated, (I've read anywhere from .060" to .100" is workable) can be used to "tune" the overall length which in reality is the distance from the seat of the lifter to the rocker pocket. So the purchased length isn't a 15 thousandths critical decision as we might have made it out to be.
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Re: Push Rod Length: Shorter vs Longer

Post by i82much »

Have you asked your girlfriend this question?
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Re: Push Rod Length: Shorter vs Longer

Post by skinny z »

Cute.
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Re: Push Rod Length: Shorter vs Longer

Post by af2 »

Are you trying to approach 0" on preload?
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Re: Push Rod Length: Shorter vs Longer

Post by skinny z »

No.
.070" is the target preload with that amount added to the adjustable pushrod after measuring.
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Re: Push Rod Length: Shorter vs Longer

Post by racear2865 »

In limited camshaft engines, it can become advantageous when you start and or finish the end of the lift cycle. This also, in part, how much and where the area under the circle is. For a lesson, sit down and sketch out area under the curve at high lift theory and then go .100 under on length and .100 over. See how it affects the area under the curve.
reed
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Re: Push Rod Length: Shorter vs Longer

Post by bigmike »

When your calculated push rod length falls between two standard measurements, do you go longer or shorter?
Longer will in most cases make more power from what I know.

Years ago one test I seen on a gen 1 SBC they dyno tested a optimum length pushrod for geometry than ran a .050 shorter pushrod on the engine, it lost around 7 - 8 horsepower. Than they tested with a .050 longer from optimum length pushrod. Horsepower went up about 7 horsepower from optimum length pushrod.
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