Question about your calculations

N

nosparks

Ox
Joined
Jun 14, 2009
Messages
76
Hamilton Cams said:
boost pressure and spring pressure are a very small part of why a better pushrod is needed. Here is a study I did a while back. No opinions just numbers.

Zach


The following equations are for average cylinder pressure not peak which would be much higher for short moments. At the points in which the pushrods will be affected average is more precise such as exhaust opening. Also for horsepower the equation calls for BHP which is at the flywheel. For this I assume any reading obtained at the rear wheels will have a 16% loss in the driveline. For the equation to see how your cylinder pressure lines up look at the second number whiich is a derivative of the BHP. Also a lot of the trucks I have seen peak at around 2600rpm
792,000xBHP/C.I.x rpm=avg cyl pressure

325hp at crank = 273 at the wheels= 275psi
600 hp at crank=504 at wheels= 509psi.
750=630hp at the wheels=636psi
1000=840hpat the wheels=848psi
1200=1008hp at the wheels=1018 psi

now lets figure load on the pushrod factoring 2.65 sq. in. of valve face surface area

275 hp=273psi x2.65sq in=728lbs +220lbs spring press. X1.34 exh. rocker ratio is 1271lbs.

504hp= 2102 lbs force on pushrod

630= 2553lbs on pushrod

1000=3306 lbs on pushrod

1200= 3909 lbs on pushrod

Now here is where the rubber meets the road. Factory pushods are rated at 1794 lbs, motorsport supply 1982 lbs. Hamilton 3/8" 2064 and Hamilton 7/16" 4035. over 500rearwheel horsepower you at least need the 3/8 .095 wall over 630Rwhp you really need to jump to the 7/16" .120" wall. If you don't the pushrod bends first then opens the valve. It delays the valve opening which reduces spool and power. If you are a low budget street racer you will not notice much difference in spoolup or major power but if you are racing or sled pulling and want to make sure you don't snuff that big charger down the line, use a better pushrod to open the exhaust on time at full power. It will help the chargers stay lit by not retarding the exh opening.
Click to expand...

Zach, I was pondering this and had a few questions to satisfy my curiosity.

Is the PSI that's being calculated the pressure in the cyl at the bottom of the power stroke or is it just after ignition occurs? (I know you said average)
If the piston is at bottom dead center after the power stroke wont the psi be at it lowest compared to just after ignition?

I would assume that the exhaust valve opens just before the exhaust stroke occurs so will the psi really be 509 (for example). I just like to understand the assumptions and nuances of the calculations.
 
nosparks said:
Zach, I was pondering this and had a few questions to satisfy my curiosity.

Is the PSI that's being calculated the pressure in the cyl at the bottom of the power stroke or is it just after ignition occurs? (I know you said average)
If the piston is at bottom dead center after the power stroke wont the psi be at it lowest compared to just after ignition?

I would assume that the exhaust valve opens just before the exhaust stroke occurs so will the psi really be 509 (for example). I just like to understand the assumptions and nuances of the calculations.
Click to expand...


Average means just that average. I assume the pressure would be pretty close for a few reasons. First on performance camshafts in the diesel world the exhaust opens nowhere close to bottom dead center. Most street/strip camshafts open 55-65 degrees before Bdc where there is more pressure. You are right in thinking there would be much less pressure if it was opened at BDCE.

These are basic formulas to have an idea of average cylinder pressure at X rpm if you know at what RPM your truck makes X power. They are also nowhere close to maximum pressure which would be highest at TDCC or within a few degrees of TDCC. I have to lean on formulas others have come up with due to the fact that I know of no way to graph cylinder pressure as a function of crank angle and more important linear piston travel.

As far as when does the valve open in correlation to the exhaust event, as soon as the exhaust valves moves .001", the exhaust event for that cylinder has begun because the pressure is starting to move out of the cylinder instead of act on the surface area of the piston. Like I said previously this varies camshaft to camshaft with differing schools of thought on how an engine's airflow should be managed. The earlier you open the exh. valve the more pressure the face of the valve and the valvetrain as a whole is placed under.



Zach
 
Thanks for the insight on that. I wasnt aware that the exhaust valve opens so early and I can see why the pressure on the port is so high. It seems like you lose energy by opening it so soon but I guess there is a trade off from the early release of heat and the gain from powering the turbocharger with the hotter higher pressure exhaust.
Thanks, I see I still have more to learn.
 
Interesting stuff. I had never given much thought to the benefit of aftermarket push rods. Zach, does any modification need to be made in order to run the 7/16" push rod?
 
When do you need to worry about the intake pushrod. Say on a 12v running your stage 2 springs with a 1.900 intake valve???????
 
Great question. That is relative to the setup, the rpm, amount of boost and the amount of back pressure. There are two main forces acting on the intake valve as it opens. Since the piston is approaching TDCE, the pressure in the cylinder is starting to normalize in comparison to drive pressure. If your chargers are setup correctly you have within 10-20 psi excess drive pressure over boost pressure. Although there are pulses in the intake which makes the pressure vary, it is safe to use boost pressure to factor the force on the plenum side of the valve face and drive pressure on the combustion chamber side. Other factors which would create an imbalance would be the O.D. of the valve face where the exhaust is acting and the smaller diameter where the boost pressure is acting, which is the diameter of the valve face. For arguments sake they almost cancel each other out. The next factor is spring pressure on the seat our springs have about 165lbs of pressure. Multiply this times the rocker ratio of approx. 1.69 and you get 278lbs. Factoring pressure over the nose of the cam and you get around 400lbs depending on lift which translates to 676lbs. Figuring the imbalance of drive to boost pressure and friction due to side loading of the valve could create an additional 100lbs of force which would rise in a non-linear fashion as rpm increases and you come in just under 900lbs of force applied to the pushrod. One last thing to think about is the inertia of moving parts in the valvetrain which adds more stress although I don't have a formula to factor that. I hope that answer satisfied your question.

Zach
 
So I need a push rod that will take 1000lbs of force on the intake side???

So lets say a .090 3/8 rod on the intake and a .120 7/16 on the exhaust will be good enough????
 
Yes the 3/8 will work for all but the people running 600+ lbs of spring pressure in the largest pullers. Our extreme duty kits include 3/8", .095" and a 7/16", .120" with an option for .168" upon request.

Zach
 
You must log in or register to reply here.
Back
Top