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stress on pmrs

5900 Views 66 Replies 21 Participants Last post by  PowerstrokeJunkie
hey guys.....verified that i indeed have pmrs....and was thinking about this......it seems generally accepted that 400 hp is a limit on pmrs......my truck has dp 80 tow 4 in exhaust 35s and i tow heavy at 22,000 gross combined......my question...is this....if you build a pmr motor to 350.....then tow heavy like i do pushing up hills deep into it.....can you under those circumstances...exceep the stress levels of pmrs and fold a motor...ok ok you can ....but is it likley.....it seems to me that with all the weight you would in effect....be putting as much or more stress on them.....as it would building the motor to 400 hp.....am i makeing sense........................................................q2...also i dynowed 275hp but they couldnt tell me the torque anybody know what 250 hp is in torque...david
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I think what you said was pretty good. You just need to make sure and replace the word POWER or Horsepower in your description with the word Torque.

What you described was how you make more Torque, not power.

You do NOT have to increase BMEP even one point to double power output. You can double the BMEP and halve power output if you wish. BMEP deals in force. It has no relation to time, or subsequently rate. That is why it is basically proportionate to torque, and at constant engine rpm, that would be fully proportionate to power.

But we don't have to only use the same old engine rpm over and over. We can also increase power output by producing the same BMEP (cylinder pressure, and torque) at a higher operating rpm. The rate has gone up, but not the engines stresses associated with BMEP and cylinder pressure. And since the BMEP/cylinder pressure/Torque have remained the same and rate has gone up.....we can produce more power output.

This is precisely why I plan to make the same cylinder pressure/torque/bmep I make now (1000ish ft/lbs) but at 3400rpm instead of 2700-3000 or so rpm.

That will take this engine from 500ish, to 650ish without increasing anything you just posted about. I might actually lower them some for insurance.

There are two aspects to engine stress (other than thermal), those are Compressive Load, and Inertial Load. Compressive is comprised of Exactly what you described in your post. Cylinder pressure, BMEP, torque, basically the brute force trying to push the crankshaft through the oilpan. Inertial Load is the load from stopping and starting the reciprocating components of the rotating assembly, those being Piston, rings, wristpin, and connecting rod. The failure point here is in the Tensile loads applied at TDC exhaust stroke (on exhaust there is no compression to negate the tensile load with compressive load). It is here that the wristpin can pull out of the piston, the rod bolts can stretch/break, rods can fatigue and break and so on. Of the these two stresses, Compressive and Inertial, the powerstroke is not even remotely in the same galaxy on pushing the inertial load to the same degree as it is pushing the compressive load.

So why do we keep trying to push even harder on the compressive side with more, and more, and more torque? The block itself is ripping in two.

The powerstroke has an excellent rod ratio. And after somewhat extensive testing, the actual forces acting on the conn rod, wristpin and so forth at elevated rpms are not out of the scope of practicality. Usually less than or equal to that of a "stroker" big block running in the 7000ish rpm zone.

I think it's time to spin em up, don't you?
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I dont see what 10-20lbs of drive pressure has to do with it when the cylinder pressure is in the thousands of psi....

and another thing, increase timing and you WILL increase cylinder pressure and stress on the rods.
Well, the 10- 20 pounds of drive pressure dont make ahuge difference , other than parasitic losses. I agree, so what if I have 30 pounds drive pressure?
Does it really make a big difference? Probly not much.
And yes, increasing timing will increrase cyl pressures. But ask your self why.

Because it makes the mixture fire off earlier when ther cyl volume is less, and the pressure rises faster. And the peak is higher.
What does this do? It cuases the cyl pressure to occur before the ideal crankshaft degrees of rotation. Like I posted earlier, it needs to happen sometime AFTER TDC. (peak firing pressure) Around 18-22 gegrees after.
Does this make more power (or tq as was pointed out)??
Maybe. Depends on where the timing was to begin with , and how far you advance it. I believe you can take it to the point that a further increase does not make more tq/hp.
It is a bit more complicated than that in practice. But yes, overly advanced timing does do things with cylinder pressure that engines don't like. It's not necessarily absolute cylinder pressure though.

FWIW, BMEP over the surface area of the piston is basically torque in a nutshell. Your description is 100% in refrence to torque.

And drive pressure is a joke. It has zero to do with this issue other than the effect of elevated drive pressure negating BMEP requiring a higher cylinder pressure per torque produced. In other words, lower drive pressure and for any given torque output you will have subjected the engine to a lower cylinder pressure load. The drive pressure itself does no harm. It is a miniscule value in the grand scheme of things. It is a general way of looking at pumping losses. An efficiency gauge of sorts.


On Edit:

Because it makes the mixture fire off earlier
Intuitively accurate, but actually backward in reality. Usually does the exact opposite when overly advanced. Fires in to a relatively low pressure/cold cylinder (too early, not enough compression yet) and begins to fill it up, dropping temp/pressure even more. Shows up as an actual drop in pressure just before TDC. Then as the fuel finally begins to adequately self-ignite, flame front propogation and cylinder pressures FLASH to full burn/pressure VERY rapidly. Like a damn cliff straight up near TDC. The difference between setting a tandem dumptruck on a connecting rod nice and easy, and dropping it from a foot above. And no, setting a tandem dumptruck on one con rod is not an exadurated load to demonstrate the actual forces in our engines.
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Exactly. Thats the point I was trying to make.
Tuning can mean the difference in having a fairly safe 350 hp, and having a very hard on the engine 300 hp.
Still learning on diesels. I have done cyl. pressure checks on natural gas fired engines for years, not so much on a diesel. They are very similar to a diesel in the way they react. It is neat to see what effect timing plays on the pressure/ time trace. I did alot of testing on one of our engines to find the ideal timing for fuel efficiency. Found out that it gets better up to a point , then starts falling off again. As long as the ambient temp.s will allow it , without detonation this engine ran the best fuel rates while at 14 degrees BTDC. Any less and it lost eff. . Any more and it lost .
Not to mention that it was much easier to get in to detonation with the timing advanced past that point.
We use peak pressures to determine the condition of the cylinders, and to balance the loads between them.
These engines being on the lean side of the curve are different animals than a typical gas engine. Add fuel and the cyl temps/ cyl pressures go up, take fuel out and the temp cool down, cyl pressures go down. Way different than a gasoline. Had to learn everything all over again from what I though I knew.
And then theres diesels. Where the heck are the spark plugs anyway??:rolleyes:
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The part about the dumptruck was cool. And true. Multiply the cyl pressure by the surface area of the piston, and the pressure being applied to the wristpin/ rod is unbelievable!
Throw in some screwed up timing , and parts come out of the block.
I calculated it once before, and it came out to be somewhere around 34,000 pounds of force on the connecting rod (max) for every power stroke
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