wrist pin offset

[bigtomwcp]

anyone have experience with offset toward the direction of rotation. most OEMs are running about 1mm against the directioin of rotation because of noise from rocking pistons. i am starting off with .5mm in the other direction, then try 1mm. any info on piston life and rod ratio used with what offset would be great and save me some development time hopefully.

 

[SMOKEY44211]

I have run .030" offset in the direction of rotation on a small block Chevrolet 350. Gained about 5 hp@ 5,200 rpms BSFC was reduced about .1lb/hp/hr. Most of my customers complained of the noise. "Sounds like a cement mixer" was the general assessment. Durability problems never arouse running this combo.-------Phil

 

[bigtomwcp]

what kind of piston-wall clearance were you running? i am thinking .002 should keep the noise down with the idea that there is no place to rock.

 

[Andy330hp]

You reduced BSFC by 0.10??? or 0.01?

 

[Tranger]

I have no experience with turning around pistons, or playing with offset. What I might suggest is very limited clearance. If you run cast pistons and have a reliable cooling system, you can get the clearance to .001 or less. Cadillac ran .0005 on 307 Olds used in 80's RWD sedans and Chevy also ran .0005 on a 350 used in a 3/4 ton truck in the late 80's. These are 2 engines I've rebuilt personally and can speak with knowledge. Just food for thought.

Mark

 

[Tranger]

When I suggest this limited clearance, I'm saying it with the thought in mind you will be running the offset in reverse, thus the limited clearance will help eliminate rocking noise. Sorry if I wasn't clear in my first post.

 

[SMOKEY44211]

Initial cold piston skirt clearance is dictated by material expansion characteristics and the shape of the elipse ground into the skirt profile. Independant of which one you are using they all should end up with zero clearance at operating temps. I believe the trend to hypereutectic pistons with thier fairly tight recomended cold clearance is dictated by OEMs need to pass emissions test at cold start conditions. Tight clearance keeps the rings perpendicular to the bore under those conditions. That being said the only gain by reducing cleance with the offset you're cosidering would be in noise reduction during cold start conditions. My reasoning for running reverse off set and/or longer rods is that the cylinder wall side loading is decreased= lower internal friction.------Phil

 

[bigtomwcp]

thanks phil

i will let you know how it goes with the .020 offset i guess. im not worried about the noise too much as long as its reliable for a reasonable time. and yes the tight cold clearance is for cold start conditions.

 

[patprimmer]

If this is a race engine, the normal solution is to warm the engine up before any power runs.

Smokey44211 is spot on with his comments, so the cold clearance must be large enough so as not to scuff, seize or gaul at the maximum expected piston temperature. Sorry smokey if I restate some of your comments.

To overcome piston slap:-
Piston materials with minimal expansion should be used (so long as other properties are not unduely compromised).
A steel expansion control strut is often cast into the piston in the skirt around the height of the gudgeon pin. As the steel expands a lot less than aluminium, and is stronger, it restrains the expansion to some degree.
Piston shape has a bit to do with it (especially if steel inserts are used) as the clearance can be tighter in the restrained portion of the piston, and expanding the piston with some areas restrained will cause some distortion.
Skirt design is the main area to control slap, as the longer the skirt, the less the rock at the same clearance.
Flexibility in the skirt(normally by slots behind the oil control ring) also alows a tighter clearance at the skirt.
One from left field would be to use a higher expansion material for the bores, so as to reduce differential expansion, but this still does not correct for temperature differential.
Higher coolant temp and lower oil temp will help, as will extra oil flow aimed at the piston.
I dissagree with Smokey to a very minor degree on one point. That is that ideal running clearance is not zero, but zero plus oil film thickness, which is so close to zero as to be hardly worth mentioning






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Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[bigtomwcp]

thanks but i recall asking about pin offset and experience with the durability and the noise related to moving it. thanks for the not so necessary words about skirt design.

 

[tbuelna]

Hey SMOKEY44211-

Speaking of wrist pin offset, here's some text from Henry (Smokey) Yunick's magic motor patent:

"In order to achieve the earlier discussed "piston dwell" of 0.001" piston movement at TDC for 13 degrees of crankshaft rotation, the wrist pin position is offset approximately 0.060" in the direction in which thrust is applied to the piston from the diametric center of the piston to accommodate a rod length of 6.5". Additionally, a piston radial clearance of 0.006" was selected to provide a small amount of piston "rock" which adds to the piston dwell. The combined effects of the offset and clearance permit thrust forces to offset the piston as the rod connected crank journal passes over dead center resulting in a closer spacing of piston top to journal axis than in the case with conventional construction. After the journal passes dead center the thrust forces are relieved and the piston centers itself. This centering action has a movement vector away from the journal and therefore it assists in maintaining the piston near top-dead-center."

Boy, talk about an engine with piston slap!!!

Regards,
Terry

 

[patprimmer]

OK

To answer your exact question directly, It will sound like a cement mixer full of marbles, but as far as I know, it won't break.

I have used zero offset in race and High Performance road and boat engines nemerous times without to much noise and with excellent durability.

I have used wrong ofset once, with considerable noise and no durability problems, but lack of courage prevented extensive testing. This was in a side valve Dodge in a ski boat were performance was important, but not paramount. As we were "Barefooting" it often sustained 5000 rpm.

It had a 4.25" stroke, and we put about 30 hours on it before we chickened out and pulled it down and turned them around. There might have been a slight but measurable lossin power, but it was not noticable when driving nor skiing.

I have been told that it is regular practice for formular "V' motors, but they are VERY lightly stressed for a race motor, and 1/2 hp is the difference between winning and looseing.

In a race engine, I would certainly try installing the pistons backwards in any class that required dead stock high volume production pistons.

eng-tips, by professional engineers foe professional engineers

Regards
pat

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Tmoose]

"experience with the durability and the noise related to moving it."
A friend chased a light rap in a freshly done Blazer Chevy V-8 motor.
It took a few teardowns to notice half the V-8 pistons were assembled to the rods backwards, creating the "wrong" offset for quietness.

The shop that had put them together was experienced and reliable, but simply thought (incorrectly) that the notches in the piston tops should all go toward the cam.

The offset of the bearing inserts in the rods prevented simply installing the 4 assemblies rotated 180 degrees.

 

[SMOKEY44211]

Hello to all that read this: ANDY330hp you are correct. Please forgive my misplaced decimal point. To TBUELNA: I've read everything I could get my eyes on that Smokey Yunick wrote. (and yes I borrowed my screen name from him. Most of my friends call that)As much as I admired the man and his work I have to dissagree with some of his reasoning. If you take a given bore, stroke, rod legnth combination example 4" bore 3.5" stroke 5.7" rod legnth and introduce say 100psi (closed system) and add all the instant torque values from tdc for 180deg. crank rotation. Now we do the same routine with a 6" rod. Results are the same. Do the same drill with piston pin offset in either direction. Still come up with the same results. But in actual practice I have found long rods always make more power at every rpm level. Same results with reverse offset pin location. Even if you air cycle the engine. Hence my conclusion decreased piston side loading = decreased internal friction. Smokeys theory of the longer dwell at tdc I don't think is the correct conclusion. I can't see any advantage to running additional clearance then what a piston was designed for. For what its worth I'm headed down to Florida because Smokey's family is having an open house at " The Best Damn Garage in Town" saturday night Feb.14 between 5pm and 9pm. They are going to be selling the real estate soon. One last look at some of his handiwork and get away from this miserable New England winter. If anyone else is going be sure to say Hi.--------Phil