×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Wrist pin offset or Cylinder bore offset

Wrist pin offset or Cylinder bore offset

Wrist pin offset or Cylinder bore offset

(OP)
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.

Cheers

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!!!!!)


Rod
 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Resources

White Paper - The Evolving Landscape of Commercial Battery-Powered Trucks
What’s driving the evolving landscape of truck electrification? What are the barriers, motivators and strategies for accelerating the electric transition? What insights and resources are available for today’s design engineers working to achieve industry disruption and evolution? For answers to these and other pertinent questions, read this white paper. Download Now
eBook - Rethink Your PLM
A lot has changed since the 90s. You don't surf the Web using dial-up anymore, so why are you still using a legacy PLM solution that's blocking your ability to innovate? To develop and launch products today, you need a flexible, cloud-based PLM, not a solution that's stuck in the past. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close