Maximizing a PMR motor....long winded - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 21 (permalink) Old 03-26-2008, 09:27 AM Thread Starter
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Maximizing a PMR motor....long winded

Oh how it sucks to be cursed with PMR's. Ok, we know by now there isn't anything "safe" about modding one. Some people are nervous with stage 1's, some feel great with hybrids and big oil. Of course, there are huge differences in CP's between these trucks, as there are so many factors that contribute to failure, or the lack thereof. There has to be some quantitative way to measure what actually breaks them instead of saying ~401+hp, since the peak in the torque band could be anywhere from 1800-2800 (or someday 3400) rpm. Black widow's dyno chart of a TNBB with mini-me's shows crazy torque down low (a rod-crushing 1070 @ 2100rpm), I'm sure due to excellent spoolup at low rpm; but at a not too-pmr-unfriendly 433 HP. Is this the rig that has been running hybrids for a few years on pmr's? I know "its all in the tuning", but geeze, 1070 @2100 is still 1070 @ 2100. So, when I hear ~401hp is red zone (and since most 7.3's peak [email protected]~2700rpm), I assume ~802 tq. Why the HUGE variance?
I'm kindof a layman in mechanical engineering, but I do know destructive harmonics are the death of most machines. Most reports of PMR failure have been someone going to the grocery store, or leaving the movie theater, not boosted launches and 3000 rpm doughnuts, though of course failures happen then too. Long stroke, high CP, low rpm, heavy truck all = crazy stress on rods. I think of it like standing on the pedals of your 18 spd mountain bike in 12th gear going up hill at about 5 mph; thats when your going to loose a shoe or bend a pedal. So, why didnt BWD's rods break? Are they randomly 30% stronger from the factory then joe schmoe's rod that broke at 700 ft lbs.? I would guess not, though im sure there is some random variance; 15% would seem like alot to me.
Harmonics seems like the culprit to me, which would involve temperature, rpm, CP, Timing, and actual down force on the rod, among others. Torsional vibrations feeding back from the crankshaft and tensile vibrations on the rod/wrist pin itself at just the wrong temp and BOOM! I guess what I'm theorizing here, is what is ultimately leading to PMR death is not a purely a function of HP (since that is based on rpm) or direct torque (to a point), but the vibration levels on the rods and the components connected to the rods causing destructive harmonic resonance. I bet a fluid Damper would help greatly! How bout some other dampening devices? Anyone out there with precision instruments to measure micro vibrations? Cheaper to buy forged or billet, I know. Higher rpm torque peak seems like the band aid, but I dont tow at 2900 rpm, I'd rather be at 1900. If higher rpm helps, this must be a function of harmonics evening out at higher rpms, caused by the upward stresses increased with the rising centrifugal force, aka dampening! Your thoughts......'Leeter

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post #2 of 21 (permalink) Old 03-26-2008, 01:17 PM
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Re: Maximizing a PMR motor....long winded

It's all in cylinder pressure. Take a truck with stock injectors and PMR's, feed it enough fuel and timing, and it will pop. You don't have to reach or go over 400hp to destroy PMR's.

However, you can go over 400hp if you keep pressures in check. That takes good fuel management and timing to remain safe. Heck even Jody Tipton's truck is still running PMR's, and I think he's close to 500hp.

Curtis
2002 F-250 PSD
Gambling with 250/200's on PMR's.
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post #3 of 21 (permalink) Old 03-26-2008, 01:31 PM
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Re: Maximizing a PMR motor....long winded

and a pmr trucked popped with just an 80econo tune...
haulin horses seems to have a pretty good pmr setup.

I would defiantly add a fluid damper to a pmr truck, the less shaking the better. while your there might as well do a melling lpop. unfortunately, balancing and such items are out due to cost..(..if you were going that route then the pmr's would be gone forged rods can be had cheap enough , billets are crazy high..and then there is the whole install fee.)
so really all the mods have to be external. gain power from better fueling
utilizing a reg return/ airdog or a cpr system. I would look at things like keeping the turbo drive pressures lower,bigger oil, but not with high ipr numbers. slightly better injectors but really even stage 2's can push it over the edge.maybe better a better IC, and some headers, to help with the egt's and get the gases out the fastest. did you get pinnicles headers when you got your exhaust, or price them?

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post #4 of 21 (permalink) Old 03-26-2008, 01:43 PM
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Re: Maximizing a PMR motor....long winded

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Originally Posted by southmike View Post
bigger oil, but not with high ipr numbers.
Huh? Can you explain?

To the OP, if you want to remain safe, keep your stock injectors.
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post #5 of 21 (permalink) Old 03-26-2008, 02:19 PM
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Re: Maximizing a PMR motor....long winded

I am certainly no authority on this, but I have heard that constant theme of "keep the stock injectors" many times, there must be some truth to it. I am not in the market for one at this time, but big oil is something that I am still curious about with a pmr motor.

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post #6 of 21 (permalink) Old 03-26-2008, 02:50 PM
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Re: Maximizing a PMR motor....long winded

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I am certainly no authority on this, but I have heard that constant theme of "keep the stock injectors" many times, there must be some truth to it. I am not in the market for one at this time, but big oil is something that I am still curious about with a pmr motor.
My problem is that if I go with bigger injectors, I would have to go all out. I'm not ready to yank out the motor and build it up as a beast yet, and I don't think PMR's would like B codes much.

However, with the big oil on stock injectors, I'm really glad I went that route and would recommend anyone with PMR's to do it. It's funny how I gained more power, but with the big oil tuning I have a smoother running engine and lower EGT's.

Curtis
2002 F-250 PSD
Gambling with 250/200's on PMR's.
478hp/851tq on Haller's dyno - 7/28/12
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post #7 of 21 (permalink) Old 03-26-2008, 04:39 PM
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Re: Maximizing a PMR motor....long winded

Well what I have done hasn't seemed to hurt nothing and I know I am 430+HP and well over year half with countless runs to the drags and a few truck pulls.


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post #8 of 21 (permalink) Old 03-26-2008, 05:09 PM
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Re: Maximizing a PMR motor....long winded

It's a big gamble, but there are lots of folks who have PMR's and addtional mods such as injectors and such. Several of these motors make good power where there setup at. I tow heavy with my setup, go to the strip, take on any challengers at the stoplight, etc... But I do have a spare motor sitting waiting to be built after I finish some other projects though.

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post #9 of 21 (permalink) Old 03-26-2008, 05:21 PM
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Re: Maximizing a PMR motor....long winded

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Originally Posted by kainers View Post
Well what I have done hasn't seemed to hurt nothing and I know I am 430+HP and well over year half with countless runs to the drags and a few truck pulls.
Yeah but your truck is just a freak


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post #10 of 21 (permalink) Old 03-26-2008, 06:14 PM
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Re: Maximizing a PMR motor....long winded

LMFAO


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