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 LOSSES IN V-8s
 By
 Robert "Jocko" Johnson


  

 

Since I've recently studied engine designs I've been given a new piece of information regarding drag race engines. Western Wrist Pins' owner, Manuel Maldonado sent me a listing of his products along with the bending pressures for each product.

  The wrist pins used by the nitro racers in their V -8s are bending a little. To bend those pins in a press it required 110,000 pounds of force! I figured that if 110,000 Ibs. of force are generated inside the combustion chamber and only 10,000 Ibs. are required to accelerate a 2,000 lb. dragster at 5 Gs then there must be a loss of 100,000 Ibs. of force somewhere between the wrist pin and the tire patch pushing the car. To pinpoint the exact point/location of that loss I've done some math on V -8 nitro burners.

  The pistons each get to do their work only 400.6 times per 5-second blast at 7,800 RPMs. Nowadays the Ets have gotten down to 4.5 seconds so each piston achieves that max pressure less than 40 times in a quarter mile, yet that is enough to do the large damage to parts.

  The car is pushed by residual pressure leftover from the decompression of that peak at TDC. The pressure at TDC can't push the car because it is busy pushing the crank down and compressing con rods. Plus stretching the main webs to the rip and tear point. AtTDC there is no leverage to do useful work.

  Huge cylinders full of exploding nitro causes 110,000 lbs. of force at TDC but since the rod is straight up there is no place to go. The piston tops feel heat and pressure; the pins feel pressure from above and resistance down below so they yield.

  Taking a look at the rod. It is standing straight up at TDC but the forces it feels are extremely high pressure from the piston and pin pushing down and a sideways travel down below but not much downward travel until the throw passes 24° ATDC. At that time the leverage is less than 1" (but is increasing to 2" at 90° on a 4" stroke) the rod is leaning over and . allowing piston down and relieving some of that peak pressure of 8,360 PSI.

Downward travel of the piston quickly doubles the space inside the combustion chamber. Each doubling of that space reduces the pressure by 50%. So before the crank throw reaches 90° A TDC where leverage is 2 1;2" (on a 5" stroke the pressure is down to about1,000 lbs. per square inch.

All that nitro pressure gone to waste! The power is in the fuel; the engine is merely the mechanical handle on the explosions. It puts a spin on them that we can use. Torque is the weight mover. Revs are how fast the torque is doing its work. Horsepower is measured by doing work and measuring how much and how quick. The formula: