Pushrod stiffness
#2568556
10/23/18 08:48 PM
10/23/18 08:48 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
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It appears that most questions are concerned with "do I need .080 wall or is .060" enough?" when the most important data item is (drum roll): "what's the largest diameter tube that will clear my block and head with that lobe height and specific rocker arm". No, the ratio isn't relevant here, it's the length of the short lever (which positions the upper end of the pushrod).
Dealing with just that variable, the tube sizes commonly available include 5/16", 3/8", 7/16", and 1/2".
To make comparisons between sizes easier, here is the relative stiffness (bending resistance in compression) of these sizes expressed in dimensionless units of resistance, all with the same .080" wall thickness. "Ω": symbol for resistance. 5/16" 13,248Ω 3/8" 25,975Ω 7/16" 45,219Ω 1/2" 72,360Ω
To compare them, divide new size by old size. Duh. Going up from 5/16" to 3/8" (no other changes): 25,975 ÷ 13,248 = 196.1% (almost doubled).
Obviously, any good machinist (including the better pushrod mfgs.) could make any OD and wall thickness you want.
Boffin Emeritus
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Re: Pushrod stiffness
[Re: polyspheric]
#2568561
10/23/18 08:53 PM
10/23/18 08:53 PM
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Joined: Sep 2014
Posts: 4,457 Washington
madscientist
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It appears that most questions are concerned with "do I need .080 wall or is .060" enough?" when the most important data item is (drum roll): "what's the largest diameter tube that will clear my block and head with that lobe height and specific rocker arm". No, the ratio isn't relevant here, it's the length of the short lever (which positions the upper end of the pushrod).
Dealing with just that variable, the tube sizes commonly available include 5/16", 3/8", 7/16", and 1/2".
To make comparisons between sizes easier, here is the relative stiffness (bending resistance in compression) of these sizes expressed in dimensionless units of resistance, all with the same .080" wall thickness. "Ω": symbol for resistance. 5/16" 13,248Ω 3/8" 25,975Ω 7/16" 45,219Ω 1/2" 72,360Ω
To compare them, divide new size by old size. Duh. Going up from 5/16" to 3/8" (no other changes): 25,975 ÷ 13,248 = 196.1% (almost doubled).
Obviously, any good machinist (including the better pushrod mfgs.) could make any OD and wall thickness you want. What about diameter AND taper? I'm all for the biggest diameter pushrod you can fit in there, and when you start offsetting the rockers I found adding taper helped. A bunch.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: Pushrod stiffness
[Re: polyspheric]
#2568578
10/23/18 09:33 PM
10/23/18 09:33 PM
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crabman173
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crabman173
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Smith Brothers can tell you what will work in your application They have the experience Dyno operators can tell you-- stiff is good
One of the most informative things I ever saw was a Spintron at the PRI show--running and we could watch .080 wall push rods look like spaghetti at RPM--shocking!! What the valve train was doing was stunning--Total chaos
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Re: Pushrod stiffness
[Re: ]
#2568695
10/24/18 01:29 AM
10/24/18 01:29 AM
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Joined: Aug 2003
Posts: 43,719 Bend,OR USA
Cab_Burge
I Win
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I Win
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I've told this story on here more than once, I had a set of Smith Bros. custom made 3/8 x.120, was what I ordered and paid for. This was for my old 526 C.I. race gas dyno test mule motor with a set of Indy SR heads with M.W. size intake ports and a custom ground solid roller cam, lifters, proper valve springs, 10 degree retainers and keepers. One pushrod broke into two pieces in the middle where there was hidden defect in the tube during a pull around 5200 RPM I took it and the one that it bent next to it back to Smith Bros. and I was told then that those pushrods where ,083 wall, not .120 wall by one of the shop supervisors I hate being lied to They replaced the two bad ones but did not offer to help pay for the camshaft lobe damage from 1/2 of the broken pushrod falling into the lifter bushing from the lifter getting toss out and the top of the Crower solid roller lifter getting broken off from the other broken piece of the pushrod hitting it I ordered a set of Manton series 3 3/8x.120 wall and ended up retesting that motor with the Smith Bros. first and then after several pull replacing them with the Mantons, there was 8 HP gain and a increase of 300 RPM to peak HP with no other changes Pushrods can, do and will make a difference on any race motor My new bracket motor has a set of Manton 7/16 x.143 wall single tapered pushrods on the intakes and straight 7/16x.143 wall on the exhausts I have friends who race in a heads up no breakout high dollar high RPM motor class and they use 9/16 diameter Mantons
Last edited by Cab_Burge; 10/24/18 01:33 AM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Pushrod stiffness
[Re: polyspheric]
#2568809
10/24/18 12:29 PM
10/24/18 12:29 PM
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Joined: Oct 2007
Posts: 4,219 New York
polyspheric
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diameter AND taper
Definitely, but if the max OD is the same at the taper's major diameter and the straight tube's OD, all it does is save weight. But nice to have, especially w/o penalty.
I would guess that the head's relief holes are the worst spot for many engines, so the longest pushrods (BBM, BBF, etc.) may get away with single taper with the big end at the tappet. A dual-taper minimum OD may be more controlled by what ends you're using (how much ID is needed for insertion), otherwise 1/4" at both ends with 7/16" center will work. The bending moment is far more violent as you approach the unsupported span center, and really low nearest the ends.
Typically, high ratio rockers (conventional shaft or stud, not "individual" like high end Jesel) will have another problem - the ratio is made by shortening the pushrod-side lever, which moves the entire lift range closer to the fulcrum and tighter to the inside of the relief hole.
Boffin Emeritus
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Re: Pushrod stiffness
[Re: ]
#2568884
10/24/18 02:13 PM
10/24/18 02:13 PM
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Joined: Mar 2007
Posts: 3,298 West Coast, USA
jbc426
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Smith Brothers can tell you what will work in your application They have the experience Dyno operators can tell you-- stiff is good
One of the most informative things I ever saw was a Spintron at the PRI show--running and we could watch .080 wall push rods look like spaghetti at RPM--shocking!! What the valve train was doing was stunning--Total chaos
This. I had too low of spring pressure in my 493" hydraulic rollered RB. The 3/8's Manton Stage 5 pushrods were flexing above 5500rpm or so as the springs lost control of the valve train. The intake pushrods flexed until they hit the side of the intake ports and that dampened them, but the exhaust pushrods had all the room they needed to flex like wet spaghetti. It destroyed all the roller bearings in my exhaust rocker arms. They were decent quality T&D's too. I was lucky to catch it when I did, as the motor still was running strong. Since that issue, I learned from some guys like Fastplymoth68, AndyF, Cab, Mike at B3 and Manton's Tech that the harmonics can be dampened somewhat with thicker, stiffer pushrods, but having each and every component of the valvetrain optimized and working smoothly together is just as important. Even 9/16's pushrods will look like spaghetti given enough bad harmonic input. Some degree of bad valvetrain harmonics are probably present in more motors than we know. Now that I've rebuilt my valve train, I think I've felt those same high-mid frequency harmonics in most if not all of my other RB motors. It's extremely damaging to components, and if bad enough, you can feel it. I ended up with a solid roller, Isky Red Zone bushed rollers, 7/16's Stage 5 Manton pushrods, a geometry correction kit & custom T&D's rockers spec'd by Mike at B3 Racing. My engine runs and revs much smoother than I've ever felt an RB run in 4 decades. It's a night and day difference. I believe it is all due to the valvetrain harmonics being eliminated from the motor. Because the bottom end balance was corrected on all my previous motors, I don't believe that is an issue.
1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's 1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)
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Re: Pushrod stiffness
[Re: BobR]
#2569042
10/24/18 07:08 PM
10/24/18 07:08 PM
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Joined: Aug 2003
Posts: 43,719 Bend,OR USA
Cab_Burge
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"I have friends who race in a heads up no breakout high dollar high RPM motor class and they use 9/16 diameter Mantons"
Solid tool steel. Bob, is that on the BAE motor now? If so they Sound like good snake killer tools to me
Last edited by Cab_Burge; 10/24/18 07:09 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Pushrod stiffness
[Re: polyspheric]
#2569082
10/24/18 08:05 PM
10/24/18 08:05 PM
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Joined: Jan 2003
Posts: 15,439 Val-haul-ass... eventually
BradH
Taking time off to work on my car
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Taking time off to work on my car
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Val-haul-ass... eventually
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To make comparisons between sizes easier, here is the relative stiffness (bending resistance in compression) of these sizes expressed in dimensionless units of resistance, all with the same .080" wall thickness. "Ω": symbol for resistance. 5/16" 13,248Ω 3/8" 25,975Ω 7/16" 45,219Ω 1/2" 72,360Ω
Where can data be found that compares something like 3/8" x .120"- or .135"-wall vs other diameters of varying wall thicknesses?
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Re: Pushrod stiffness
[Re: BradH]
#2569084
10/24/18 08:08 PM
10/24/18 08:08 PM
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Joined: Feb 2005
Posts: 20,272 PA.
pittsburghracer
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I know I went with 7/16 pushrods on my engine with B1 heads and 1.7 rockers and boy did I have lots of pushrod clearance grinding.
1970 Duster Edelbrock headed 408 5.984@112.52 422 Indy headed small block 5.982@112.56 mph 9.38@138.67
Livin and lovin life one day at a time
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Re: Pushrod stiffness
[Re: polyspheric]
#2569086
10/24/18 08:09 PM
10/24/18 08:09 PM
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Joined: Oct 2007
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polyspheric
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Solid is better, but nothing like an OD change. Going up in 7/16" from .120" wall to solid improves by (wait for it)... 4.4%.
Boffin Emeritus
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Re: Pushrod stiffness
[Re: polyspheric]
#2569090
10/24/18 08:18 PM
10/24/18 08:18 PM
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Joined: Oct 2007
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polyspheric
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Where can data be found that compares something like 3/8" x .120"- or .135"-wall vs other diameters of varying wall thicknesses?
DIY: use the dimensionless value for the OD above, times the wall thickness for each size. Example: 3/8" × .150" wall = 25,975 × .150 = 3,896.25 To get the same stiffness, a 7/16" OD only needs .086", a 1/2" needs .054".
Boffin Emeritus
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Re: Pushrod stiffness
[Re: pittsburghracer]
#2569103
10/24/18 08:49 PM
10/24/18 08:49 PM
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Joined: Sep 2014
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madscientist
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I know I went with 7/16 pushrods on my engine with B1 heads and 1.7 rockers and boy did I have lots of pushrod clearance grinding. That would have been a good place you use a single taper and put the 3/8 end up.
Just because you think it won't make it true. Horsepower is KING. To dispute this is stupid. C. Alston
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Re: Pushrod stiffness
[Re: pittsburghracer]
#2569187
10/25/18 12:23 AM
10/25/18 12:23 AM
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Joined: Mar 2003
Posts: 2,540 PA
moparacer
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I know I went with 7/16 pushrods on my engine with B1 heads and 1.7 rockers and boy did I have lots of pushrod clearance grinding. I have 7/16 Manton's and 440-1's and had a lot of fun grinding too. But it is worth it.
67 Barracuda street/bracket car 11.27-119 68 Dart 502 BB 8.70s-152 414 cid SB Dragster 7.65-174
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Re: Pushrod stiffness
[Re: polyspheric]
#2569299
10/25/18 11:55 AM
10/25/18 11:55 AM
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Joined: Oct 2007
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polyspheric
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Trying to open against high residual cylinder pressure?
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Re: Pushrod stiffness
[Re: polyspheric]
#2569658
10/26/18 09:58 AM
10/26/18 09:58 AM
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Joined: Oct 2007
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To clarify my lever length remark: If a 1.50:1 (std.) rocker's pushrod lever is 1.000" (adjuster to rocker shaft CL) and you replace it with a 1.60:1 ratio, this is almost always done by keeping the valve side lever at 1.500" but moving the adjuster closer to the shaft by .0625" making it .9375" long: 1.500" ÷ .9375" = 1.60:1. This makes the clearance hole 1/16" tighter on the shaft side, similar to increasing the pushrod OD by 1/8".
To summarize: if you have very little clearance now, installing a high ratio rocker makes it worse.
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