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Wrist pin offset or Cylinder bore offset

Wrist pin offset or Cylinder bore offset

Wrist pin offset or Cylinder bore offset

Anyone here has dyno experience or has studied the effects
of wrist pin offset ???
And what about cylinder bore offset in either
with crank rotation or against crank rotation ???
Know the net effects ???

I've offset bored blocks along the crank axis to unshroud
larger intake valves, but never tried the other ways...
its one of those things i often wondered about or thought by know i would have tried !!

I did switch the pin offset (reverse pistons in bores)
on an Olds 455 cid Stocker engine in the late 1970s
..relatively small intake port cross-sectional area
for that size engine (455 cid W30 Olds) ...from that point on the car was faster in MPH trap speeds and a little faster in ET ...but does anyone here have better evidence ??? or data ?

Larry Meaux  (meauxrace2@aol.com)
Meaux Racing Heads
MaxRace Software
ET_Analyst for DragRacers

RE: Wrist pin offset or Cylinder bore offset

Wristpin offset has a big impact on piston secondary motion (slap, crown guidance, etc), as you might expect.  I've never looked at its effects on net power output.  I can't think of a good reason why it would affect power output for better or worse, unless the piston started dragging the crown, or started having trouble generating an oil film.

Never looked at crank offset either, except for a few brief calcs to answer a question here (did you know it changes the # of crank degs between TDC and BDC?).  

RE: Wrist pin offset or Cylinder bore offset

hmm, now that I've spent a couple minutes thinking about it, I guess that offsetting the wristpin can slightly alter the angle of the conrod @ a given crank angle, which could raise or lower the torque & power output by a small amount.

RE: Wrist pin offset or Cylinder bore offset

If you offset the crank enough then you /could/ change the way that the gas expands, by altering the 'dwell' (wrong word but you know what I mean) of the piston at the top of its stroke. Various nut-case engines have proposed all sorts of mechanisms to get that effect.


Greg Locock

RE: Wrist pin offset or Cylinder bore offset

Could offsetting the cylinder bore w.r.t. the crank reduce angularity of the connecting rod during the power stroke and thus reduce the friction of the piston on the cylinder wall?

It seems that offsetting the wrist pin would increase the tendency of the piston to rock in the cylinder bore.

RE: Wrist pin offset or Cylinder bore offset

yeah, it could.

Yes, you can alter the rocking behavior of the piston by offsetting the wristpin.  Picking a good offset allows you to better control the piston rocking motion (to reduce "slap").  This is what I was alluding to in my prev. post about piston secondary motion.  There are programs out there that allow you to simulate piston secondary motion, given sufficient information about the engine geometry.  A search on yahoo for "piston secondary dynamics" should turn up such products (ADAMS, PisDyn, etc).  I've had good experiences with the PisDyn - in the few times I've used it, I've found that the predictions match real-world evidence (measurements, skirt/liner wear patterns, etc) pretty well.

RE: Wrist pin offset or Cylinder bore offset

Larry, In an article Smokey wrote in Hot Rod, I think, back in the '60's or '70's he addresses the pin offset and rod length vs. rod angularity vs. piston dwell @ TDC (yes Greg that is the right term) problem.  If I remember correctly, he reversed the offset as you did for a  performance increase (???).  He then increased the rod length by as much as .25 inch on a BB and slightly more on a SB.  Dwell time increases dramatically and ignition timing and valve clearance becomes a big problem if not attended to. But power was up on all as I recall. It is of interest to note that in the 302 engine I saw apart at tech Riverside when John Timanus was tecking Smokie's Trans-Am attempt appeared to NOT have offset????? (The car failed and never raced, it now belongs to Vic Edelbrock Jr. and is raced in vintage events.  It seems the entire car was 7/8 ths. scale!!!!!)


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