Type Q wrote: It seems to me the answer is it depends. Many variables are going to affect the stresses the parts are under whether NA or forced indcution. I am sure Eric (Alfadriver) and people like him who do OEM powertrain development, spend untold hours measuring and modeling these forces to achieve.
I don't, but I did stay at a Holiday Inn last night. Well, know people who do.
And yes, it depends. Always does.
Apologies for the tone of my previous post; Not seeking an argument, just a passionate discussion.
I wonder how many buckled rods are from detonation, not increased BMEP? How does a buckled rod illustrate 'recoverability' from excess compression loads? And if buckled rods are the most common failure in boosted street cars (the result of excess compression forces), than doen't that support the assertion that rods are stronger in tension than compression?
Carter
bigbens6 wrote: KB:58 Yea the theory is that the boost is a cushion near TDC, how ever the net effect is questionable when you look at piston speeds and momentum vs 14 psi on a few square inches...
14 psi on a 3.5" piston is 140 lbs that's working in the right direction, but yes I agree that the forces are on the order of 1000s of lbs regardless.
erohslc wrote: Apologies for the tone of my previous post; Not seeking an argument, just a passionate discussion. I wonder how many buckled rods are from detonation, not increased BMEP? How does a buckled rod illustrate 'recoverability' from excess compression loads?
Detonation does cause an absurd amount of compression and is cause for alot of compressive failure. But realize that when gas is burnt it expands on the order of 10+ times is current volume, typically this is what pushes the piston down so combustion chamber pressure stays controllable, with detonation the volume of gas is increasing while the volume of available space is decreasing, in the scenario of detonation your again talking about going above the rated load by an extreme factor...
erohslc wrote: And if buckled rods *are* the most common failure in boosted street cars (the result of excess compression forces), than doen't that support the assertion that rods are stronger in tension than compression? Carter
This is where you have to differentiate correlation from causation, it is a correlation that more failures are from compression failure, that does not mean it is weaker in compression, it just means we aren't ADDING any tensile strain cause we haven't raised revs or max piston speed at all. (in most cases lets say)
The causation is because it is cheaper to add compressive forces to a motor than tensile. Like I said, a super charger is 4-6K. Con rods, fasteners, pistons, valves, retainers, valve springs, wrist pins, cranks, etc that can handle another 1000-2000 rpms are friggin expensive, alot more time, alost more money, alot more to go wrong... so not many people do it.
But ask yourself this if it is still not making sense, take any modern motor in mass prduction, lets take BMW's twin turbo straight 6 out of the 335i, the N54, stock it runs a 7K redline and 8.8psi and 300 HP
Now ask this, would this motor be more likely to handle 25% more boost or a total of 11psi, or 25% more redline which is 8700 rpm.... i know i would MUCH rather up the boost than the revs, shoot 7500 rpm would make me nervous....
because you can pretty much guarante instant, catastrophic, and spectacular death at 8700 rpms because you will have add absurd amounts of tensile stress, while adding boost will not HELP longevity it will prolly run for many thousands of miles (as evidenced by the current after market) happily and die a few, maybe 20-30K miles shorter than if left stock.
You get more power to the wheels as a result of greater cylinder pressures. I don't buy it.
That said, I will go along with the idea that if you're boosting you can get the same or more power at a lower RPM, and thus don't have to deal with high RPM stresses.
DILYSI Dave wrote: You get more power to the wheels as a result of greater cylinder pressures. I don't buy it. That said, I will go along with the idea that if you're boosting you can get the same or more power at a lower RPM, and thus don't have to deal with high RPM stresses.
You get more torque to the wheels as a result of increased cylinder pressure, you can make more power on the same CP by adding revs, 300 Lbft at 2k is not alot of power = 114HP, 300lbft at 10K rpms is a TON of power = 571HP
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