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Discussion Starter · #1 ·
I dropped my '87 roller 302 block off at the machine shop this morning. We discussed the idea of getting good flowing aluminum heads and/or a stroker kit. For the stroker option he recommended getting a 347 kit with the shorter rods to avoid the piston pin/oil ring issue. I have heard of these kits but not much about what people think about them. Any feedback would be appreciated.
 

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The shorter rod 347 kit is "DOT legal" the long rod versions aren't. Even FMS sells the short rod kit, short block. I doubt if the difference, performance-wise, will even be noticeable by anyone, other than you - and then only because you had it installed.

I really don't see where you could go wrong, either way. If it was me ... I probably get the heads, the short rods and a decent cam; then call it a day. If your buddy gets the long rod kit and maybe 10 ft/lb extra torque, so what? That is nothing more or less than a statistical anomaly anyway. It’s just too close lose any sleep over.
 

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Discussion Starter · #3 ·
I'm not really concerned about the 347 short rod vs long rod in respect to performance. I agree that they should be very similar. It is the reliability and longevity of the pieces that I am more curious about.
 

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On 2006-05-10 16:39, Beoweolf wrote:
The shorter rod 347 kit is "DOT legal" the long rod versions aren't. Even FMS sells the short rod kit, short block. I doubt if the difference, performance-wise, will even be noticeable by anyone, other than you - and then only because you had it installed.

I really don't see where you could go wrong, either way. If it was me ... I probably get the heads, the short rods and a decent cam; then call it a day. If your buddy gets the long rod kit and maybe 10 ft/lb extra torque, so what? That is nothing more or less than a statistical anomaly anyway. It’s just too close lose any sleep over.
"DOT LEGAL"??????????????I have a 5.4 rod 347 and it does not smoke.Any motor will smoke if it is not put together right or if it is abused.
 

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Everything Ive read on the subject is that the 347 'long' rod burns oil no matter what. The shorter rod was introduced to get the pin out of the oil ring groove. The trouble with the shorter 5.315 rod is that the angularity on the rod gets worse and side loads the piston even more than the 5.4 does.
Id like to know how rings can be set up any different to keep the 5.4 rod from burning oil. Unless you run gapless rings?
 

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"DOT LEGAL"??????????????I have a 5.4 rod 347 and it does not smoke.Any motor will smoke if it is not put together right or if it is abused.
Is that a question or a comment? The only point of mentioning "DOT" (dept of transportation) legal is if you live in a "smog enforcement" area, it has passed emissions testing. The 347 with long rods either hasn't been submitted or didn't pass...it doesn't have an exemption.

All engines smoke, to some degree or another. As far as to whether or not the long rod with oil support rings is going to smoke "noticeably" more than one built with short rods or a tighter ring package... has nothing to do with most performance engines. As long as it isn't leaving an "Oily Con-trail" as you run down the freeway or kill mosquito while idling, I would be happy.

Ultimately it still boils down to what the owner is willing to put up with. Remember, there are a lot of people that find a choppy idle, solid lifters, free lowing mufflers are unacceptable...which to many of us is music, sweeter than a symphony.
 

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I have the "short rod" 347. I liked the idea of not dealing with support rings for an imperceivable power gain, but in reality, if the engine is built right you won't notice the difference either in power or oil consumption. It's all about the machine work and assembly.
 

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(5.4 - 5.315)/5.4 = 1.6% difference in length... so if you think 1.6% more piston side load (which it would actually be more like the squareroot of 1.6 which is 1.26) will make a significant difference in your engine's performance and longevity then go with the long rod. Me personally, I think the choice is clear... if you went longrod and it burned oil you'd be kicking yourself in the rear... if you went shortrod and lost 3hp you'd never know and never care.

_________________
'86 Bronco fullsize, 460, E4OD
'85 Ranger, 340hp 289, T5


<font size=-1>[ This Message was edited by: Motorhead on 5/12/06 3:05am ]</font>
 

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Discussion Starter · #12 ·
After talking to machinist some more today, I realized that I misinterperted what he was saying. He was implying that the 5.4 rod kits have been improved with a piston design that solves the oil issue. I feel pretty comfortable with his explanation and plan to go ahead and get the extra cubes.
 

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Hell half the factory engines burn oil....so what if a 347 uses a quart every 5000 miles. Build it the way YOU want it....not the way someone else says to. My Lightning uses oil..so what. Most of them do (PCV design flaw).
 

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I think you have your answere!
 

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short rod 347 rod angle = 18.65 degrees
long rod 347 rod angle = 18.35 degrees

whoop dee doo. Not an issue. Rod angles above may not be exactly correct if the wrist pin is off center on the piston which I believe it is.

Rod length does very little to change piston motion unless the rod is SIGNIFICANTLY longer than the shorter rod. Even 1/2" longer will do very little (literally about .015 or so difference max in piston motion throughout the stroke cycle). Ask me how I know...I purposely designed and built a short rod 331 using 5.155 289 length rods and I wrote a program to compare the piston motion of a 5.4 rod 331 vs a 5.155 rod 331. The difference was .012 max at 90 degree crank angle and even less everywhere else. The rod angle is 0.85 degrees more than a 5.4 rod (yet it's the same as a 5.4 rod 347), but when you do the math to determine the increase in side loading ,it's small (about 5%) and my piston has skirts that are more than 5% bigger so the force is spread over more surface area helping to negate the increase in side loading. Longevity was my concern when I designed my 331. I just went road racing (hi performance driving event) and drove 100 miles each way and I didn't burn any oil. The main thing a longer rod buys you is reduced internal friction because the piston skirts are smaller and the rod angle is smaller, but it costs you in higher potential of piston rocking and ring instability (doesn't seem to be a big factor with well built 347s nowadays).

I built my 331 3 or 4 years ago and your options were less than they are today. They may have indeed improved upon the oil ring wrist pin intersection issue with better piston design.

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Tracy Blackford: Corona, Ca
'65 FB Mustang 331, 282S cam, ported 351W heads. T5z, 3.50 9" posi.
346 [email protected] on a warm spring day (335 RWHP SAE corr.)




<font size=-1>[ This Message was edited by: blkfrd on 5/13/06 1:04am ]</font>
 
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