just wondering why it is that the more horsepower you are running the more undependable your engine seems to be? is it higher revving creating more stress and wear on things? are some mods worse then others? like i can see how agressive cams are bad as they have a lot of lift and can allow the engine to rev really high but what harm can highflow heads do? do they hurt and engine also? what makes a high horsepower engine so undependable?

 

to create more power, you create more stress.

it is that simple. be it an increase in piston speed or compression ratio, it creates more stress on the bearings, rods, rings, valves, take your pick.

A good example, a Formula 1 engine is a 3 litre V10. Putting out 800+hp at some 17000RPM... and they last exacty 2.5 hours, and then they are torn down...

but a 3.6 litre Audi R8 (ALMS Prototype) puts out 610hp or so, and can last 24 hours.

or a NASCAR 358 V8... 850hp or so... and they can last 600 miles...

or, the clincher, a 500ci Top Fuel or Funny car motor... 7000hp and they last 2 minutes.

pressure. stress. heat. fuel wash. it all adds up.

 

Yup .. and GM spends a LOT of time tuning the engine they make. It's a little hard to tune our engines so perfectly as they do after we've made a gazzillion modifications to them.

 

Race engine longevity is different from street performance engine longevity. Race engines are designed to run the entire race at WOT at peak hp or torque. All engines will fall apart in a matter of hours if they are run constantly at peak hp or torque. Race engines are not a good guide to street engine performance lifespan. Boat engines are closer to race engines in the way they are operated.

Street performance engines are sometimes less dependable than stock engines because of parts used and user setup. A stock engine comes with parts that are designed and tested to run at least 100K miles without any problems. Stock tuning is conservative. A lot of aftermarket parts like cams, valve springs, and some rocker arms are never tested or designed properly to run 100k miles. Aftermarket cam lobe profiles are the biggest souce of reliability problems because of the aggressive lobe profiles that are used. Notice how much more lift aftermarket cams have versus stock cams. These steep profiles are very hard on valve seats and springs.

Engine tuning is another source of reliability problems. Everybody wants max hp so they put the car on a dyno and start looking for hp through air fuel ratios. They start leaning the fuel curve out and get more hp and end up running air fuel ratios above 13:1 which is generally considered the safe limit for WOT operation. The car will appear to run fine at these lean ratios, for a while, and then suddenly a ring fails, a valve gets burned, or pistion gets a hole in it.

Fuel injection has created a bigger chance for problems because the WOT fuel air ratio can be set too lean through out the rpm range. When you tune a carb, you set the jetting so that the leanest part of the power curve is the air fuel ratio you want. The rest of the range is richer. This reduces the amount of time the engine is running at the leaner setting so a too lean setting won't affect the engine as much. If you have a speed density fi system, tuning gets much more involved and can only be done using a wide band O2 sensor.

Add guys trying to lean their Nitrous for max hp, running without fuel pressure safety gear, blowers using cast pistons, injectors being run at 79% duty cycle leaving only a 1.25% margin for error, and solid roller cams and longevity goes down.

Here are the best ways to improve dependability.

Do not go to the limit of the engine's components. Leave some margin of safety. Avoid detonation at WOT. It breaks pistons and fatigues the rotating assy. The best way to do this is to stay conservative on compression and timing, don't run too lean, don't run more NOS than the motor CR safely allows and stay rich. Keep your plug wires well separated so they don't cross fire. Stay away from solid roller cams. Use lobes from cam mfrs catalogs that emphasize stability and long valve spring life.


[Modified by AquaMetallic94LT1, 12:00 AM 7/20/2002]

 

thanks AquaMetallic94LT1, that actually made a lot of since. i htink i understand that a lot better now!