502rwhp/437rwtq 347 c.i. AFR 205/Fast92/TSP 1x7/8"
07-22-2014, 04:12 PM
#32
It is nice to use a 11/32 or even 3/8 as IMO you cannot put too large of a push rod in any engine.
When the power level starts going up so does cylinder pressure. The exhaust push rod and rocker takes a beating(as does the lifter) trying to open against that cylinder pressure.
Over 600rwhp with stock rockers and a hydraulic roller cam I like to mandate a 11/32" push rod. That is good IMO to up to 800-900rwhp. Over 900rwhp I like to mandate a 3/8". Of course there have been numerous times where people have used 5/16" at well over those power levels. My opinion is that when you're building a motor of that caliber you don't skimp on anything.
In a 500rwhp N/A deal deflection is not happening, but may account for a 2-4hp difference, possibly 5hp going from 5/16 to 3/8.
I have seen 10-12hp gains going from 5/16 push rods to 3/8 in combos that have roller rockers making 500-550rwhp already. In those cases the moment of inertia is much higher from the roller rockers mass and is just beating the poop out of the 5/16 push rod turning it into a guitar string.
On a cam only deal, or head cam LS1 it's not going down like that with a stock rocker arm though.
07-22-2014, 05:05 PM
#35
Got a link? With stock rocker arms at this power level 5/16" is sufficient.
It is nice to use a 11/32 or even 3/8 as IMO you cannot put too large of a push rod in any engine.
When the power level starts going up so does cylinder pressure. The exhaust push rod and rocker takes a beating(as does the lifter) trying to open against that cylinder pressure.
Over 600rwhp with stock rockers and a hydraulic roller cam I like to mandate a 11/32" push rod. That is good IMO to up to 800-900rwhp. Over 900rwhp I like to mandate a 3/8". Of course there have been numerous times where people have used 5/16" at well over those power levels. My opinion is that when you're building a motor of that caliber you don't skimp on anything.
In a 500rwhp N/A deal deflection is not happening, but may account for a 2-4hp difference, possibly 5hp going from 5/16 to 3/8.
I have seen 10-12hp gains going from 5/16 push rods to 3/8 in combos that have roller rockers making 500-550rwhp already. In those cases the moment of inertia is much higher from the roller rockers mass and is just beating the poop out of the 5/16 push rod turning it into a guitar string.
On a cam only deal, or head cam LS1 it's not going down like that with a stock rocker arm though.
It is nice to use a 11/32 or even 3/8 as IMO you cannot put too large of a push rod in any engine.
When the power level starts going up so does cylinder pressure. The exhaust push rod and rocker takes a beating(as does the lifter) trying to open against that cylinder pressure.
Over 600rwhp with stock rockers and a hydraulic roller cam I like to mandate a 11/32" push rod. That is good IMO to up to 800-900rwhp. Over 900rwhp I like to mandate a 3/8". Of course there have been numerous times where people have used 5/16" at well over those power levels. My opinion is that when you're building a motor of that caliber you don't skimp on anything.
In a 500rwhp N/A deal deflection is not happening, but may account for a 2-4hp difference, possibly 5hp going from 5/16 to 3/8.
I have seen 10-12hp gains going from 5/16 push rods to 3/8 in combos that have roller rockers making 500-550rwhp already. In those cases the moment of inertia is much higher from the roller rockers mass and is just beating the poop out of the 5/16 push rod turning it into a guitar string.
On a cam only deal, or head cam LS1 it's not going down like that with a stock rocker arm though.
I always related lobe characteristics to pushrod requirements more so than cylinder pressures. Like an LSK lobe operating steel shaft mount rockers and 2.080" aftermarket solid stem valves would see huge gains going to a 3/8" over a 5/16" pushrod, especially as RPM increases. Cylinder pressures relating to horsepower figures can be misleading as a 600whp 440ci engine peaking at 6500RPM could easily have less cylinder pressure than a 346ci engine making 475whp at 6500 RPM.
07-22-2014, 05:38 PM
#36
No, I don't remember specifically where I had seen it, but I remember several instances when a set of baller *** heads didn't make the power the owner expected and they suspected the pushrods were to blame. I don't think 10-15hp would have made much difference, I think they were expecting 20-50hp more.
I always related lobe characteristics to pushrod requirements more so than cylinder pressures. Like an LSK lobe operating steel shaft mount rockers and 2.080" aftermarket solid stem valves would see huge gains going to a 3/8" over a 5/16" pushrod, especially as RPM increases. Cylinder pressures relating to horsepower figures can be misleading as a 600whp 440ci engine peaking at 6500RPM could easily have less cylinder pressure than a 346ci engine making 475whp at 6500 RPM.
I always related lobe characteristics to pushrod requirements more so than cylinder pressures. Like an LSK lobe operating steel shaft mount rockers and 2.080" aftermarket solid stem valves would see huge gains going to a 3/8" over a 5/16" pushrod, especially as RPM increases. Cylinder pressures relating to horsepower figures can be misleading as a 600whp 440ci engine peaking at 6500RPM could easily have less cylinder pressure than a 346ci engine making 475whp at 6500 RPM.
When you get into really high horsepower forced induction and nitrous applications, cylinder pressure pushing against the exhaust valve becomes an issue. Deflection at the teeny 8mm bolt also becomes an issue too and can cause an issue with valve stem tips.
Darrin Morgan cringed when I mentioned making the power we do with stock rockers and 8mm fasteners!
