I have posted this several times now, so those who have seen it before can skip over it:

It's not "higher boost pressures" that cause rod failures, it is detonation. Even mild detonation causes forces an order of magnitude greater than even huge boost -- so long as that boost ignites normally.

"Knock is virtually always the cause of failure in a supercharged engine." "The pressure spike caused by the [detonation] explosion can reach several thousand PSI, and pressure rise is rapid enough to be considered an impact load. These temperatures and pressures are almost ten times higher than those accompanying controlled combustion. . . No metals in existence today, no forged pistons, no special head gaskets can withstand sustained detonation." Corky Bell, "Supercharged!: The Design, Testing, and Installation of Supercharger Systems," pp. 27, 178.

Further, the load on a rod is a mixture of tensile loads at the top of the compression and exhaust strokes and compressive loads during the power stroke. Yet on the power stroke, the compressive load of combustion helps to mitigate the tensile loads, which are unchecked during the exhaust stroke. Thus, the tensile loads on a con rod are greatest when at TDC on the exhaust stroke, where it is purely a function of piston weight, RPM, and stroke -- not boost.

Think of it this way: Under normal conditions, the explosion just shoves the piston in the butt -- it is already on the way down. But with detonation, the explosion hits the piston in the face with a sledgehammer while it is still on the way up.

Bottom line: At 14 PSIG, peak rod pressure is only up about 20%. See Bell, pp. 21-24. With detonation, peak rod pressure is up about 1,000%.

And detonation does not have to be audible to be damaging. Superchargers mask the sounds of detonation. Detonation strong enough to cause forces many times greater than normal combustion may not be audible -- even on an NA engine.

The Ford engineers use Kisler probes to watch cylinder pressures in real time. Detonation has a distinct pressure spike. This spike can be 100% greater than normal and still not be heard. Under these inaudible detonation conditions, if a rod is marginal, it will break. You now have a broken rod and no audible detonation.

 

food for thought:

Cooling and exhaust!

Arent the rear cylinders the ones that seem to break first (those closest to the firewall? (don't know cylinder number off hand)

If so, consider this.

1. by time the coolant reaches those rear cylinders it has already absorbed a large amount of heat from the front 3 cylinders.

2. Check out the water jacket openeing size, near the waterpump

3. Stock manifolds take all 3 front cylinders exhaust and pass it right by the rear one. Therefore the rear cylinder is Extra hot becuase of increased backpressure and Extensive exhaust passing by it. (one reason I switched over to equal length headers, last week)

4. Stock cats are the bottle neck of the exhaust. By replacing them with HI flows (like I did last week), the exhaust will flow much better, resulting in cooler combustion chamber temps.


NO i don't have scientific evidence, but if you think about it for 2 seconds, common sense will prevail! and it makes sense!


common sense is by lowering the combustion chamber temps will reduce the chance for detonation. (premature ignition of the a/f mixture in the combustion chamber)


5. Using a cooler plug will also help lower temps.

6. water wetter will absorb the more heat than water alone or anti-freeze and water.

7. water absorbs more heat than antifreeze/water mixture.

8. add cooling fans on the heat exchanger to help lower temps.



ANYTHING you can do to lower the combustion chamber temps will HELP prevent detonation. NOTHING will completely prevent it.


things I've done:
1. equal length headers
2. Hi Flow cats
3. Water wetter in I'C fluid
4. br-7 plugs (2 steps cooler than stock)
5. I'm debating on fans on the heat exchanger (have to find good price)
6. E-Fan that flows more than stock fan

 

pssssssssssst TIM, please use the search function.

https://www.f150online.com/forums/sh...ses+detonation

J/K

 

the chevy guys say they have cintered rods and they call the new small block (bullet proof) so how come ours are junk

 

Junk? I would consider an engine that can handle 450-500 rwhp and 550-600 rwtrq JUNK.

Figure 14% drivetrain loss to the rear wheels..

You do the math and see if the stock internals are junk.

 

opps sorry I didnt mean it that way I agree with you , justb seems to be lots of complaining about rods,and im real glad i dont own a bullet proof cheby anymore

 

Now the spark plug spitting heads are another story....

 

So again guys as long as i dont run the truck hard at all i should be fine til i get the btm end built ?To be on the safe side may be add a couple gal.of 100oct. to a tank of gas?Thanks for all the info.im taking it all to heart ,I will be showing my truck this year im sure the polished Kennebell will give me some browie points at the shows.
Thanks all Bob

 

I would add more than a "couple." If you add 2 gal's of 100 to 23 gal's of 93 , you will only get 93.5oct. If you add 5 gal's you will get 94.4oct. 7 gal's is 94.9 oct. All you have to do to calculate this is, take the gallons of gas of 93, and multiply it by 93. Then take the gallons of gas of 100, and multiply it by 100. Then divide by 25 gallons, which is a full tank. 1/2 tank is 12.5, and 1/4 is 6.25.
23 gallonsX93 octane=2139
+2 gallonsx100 octane=200
2339

2339/25gallons=93.56

Hope this helps some.

 

hey l-menace,

have you considered the "cooling mod" that the mustang guys are doing to increse coolant flow the the rear cylinders?

 

opening up the inlet? If, it's not that then what is it?

 

read about it here:

http://forums.modulardepot.com/showt...d&pagenumber=1

 

Not intending to offend but I am chewing on your food for thought

a few things you've done dont in themselves do anything to lower combustion chamber temps.
#1, #2, #3
A big obstacle for increased performance in IC engines is the need for the exhaust temperature to be high enough for the catalytic converter to operate (thanks CA) so your new cats are the same temp as your old cats...unless they are made of a low-temperature catalysts that are under design I read about a while ago. The only info I have read is that the cost of the catalyst goes up exponentially when the heat requirement is lowered...their is no measureable difference in how much heat is absorbed by the exhaust manifold and engine block with cats that have a different flow rate if the catalyist operates at the same temp.....

 

something I just learned about 15 minutes ago.
(Thanks Wydopnthrtl)

The Lightning has the same Exhaust Manifold as the regular 5.4L F150.

And we are pumping a lot more exhaust through it. An engine is an air pump, the more air you pump through it the more power you make. The S/C helps air get into the engine, but the stock manifolds hinder it's exit. resulting in heat.

 

huh... is this your final answer (hinder it's exit. resulting in heat.)