Engine building question about tdc
- Thread starter paulkish
- Start date
Yes
Offset wrist pin in piston, or offset bore centerline to crank centerline.
Chevrolet did this in early sbc days to combat piston slap noise. Smokey Yunick turned the same pistons 180 in the bore to increase dwell at tdc.
Most tech is based on tdc, not necessarily at tdc only.
Ie, ignition timing. cam timing, etc.
Offset wrist pin in piston, or offset bore centerline to crank centerline.
Chevrolet did this in early sbc days to combat piston slap noise. Smokey Yunick turned the same pistons 180 in the bore to increase dwell at tdc.
Most tech is based on tdc, not necessarily at tdc only.
Ie, ignition timing. cam timing, etc.
Offset in rods can change TDC which effectively changes cam timing. But TDC is simply a matter of piston location. I am thinking you are asking about piston location relative to when the crank pin is centered in the bore? In tech that would be somewhat irrelevant as TDC is properly determined by piston location alone.
steve baker
Moderator
As chipg56 eluded to, in the tech they measure the highest position the piston reaches, so the crank pin location is irrelevant. So in other words TDC in tech is determined when piston is at the highest point above the block deck surface.
alvin l nunley
Site Supporter
Help me understand, you want to know if the piston can go higher once it's gone as high as it can go??
If you had a device that could measure to .000000", you would find that between .000" BTDC and .000" ATDC (as close as most instruments can measure to) there is a small amount of piston movement. That's what TDC (top dead center) stands for . The the position of the piston between those 2 numbers.
I'm sure you know, others may not, when the crankshaft is exactly mid--way between 90° positions, if the piston has a piston pin offset, it will not be at TDC.
You can Calculate that movement, but at 3 decimal places (most instruments) it's hard to see.
alvin l nunley
Site Supporter
To show just exactly how far a piston does travel, with .1° of crankshaft rotation, I've expanded some cells to 6 decimal places.
For a real kick, offset wrist pin .050 to the side opposite rotation and look at piston dwell in degrees between .050 before and .050 after tdc vs straight up.
http://blog.wiseco.com/what-is-wrist-pin-offsetA little info from piston manufacturer.
http://blog.wiseco.com/what-is-wrist-pin-offsetA little info from piston manufacturer.
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steve baker
Moderator
Good stuff, as piston location, dwell and piston velocity changes the torque and camshaft design design changes. Back in my Chevy SB race engine building days ( late 70's ) we studied the stroke vs rod length ratio and how this affected the piston velocity and crank angle when max velocity occurred.
Steve
paulkish
old fart
was offset something a sign of when box stocks rattled when at idle on the grid?
was told often the ones that rattled were the good ones
edit: But we ran a house motor which didn't rattle.
I figure house motors were those far and few inbetween motors when done and put on the dyno just happened to give special high hp numbers.
They were then kept and the customer got something else which was good but not special.
was told often the ones that rattled were the good ones
edit: But we ran a house motor which didn't rattle.
I figure house motors were those far and few inbetween motors when done and put on the dyno just happened to give special high hp numbers.
They were then kept and the customer got something else which was good but not special.
A study
Amazing how a simple change can throw all else out the window.
Every advance offers multiple opportunities for different camshafts.
The flathead is a great example of possible combinations.
Couple guys on here believe clone cam design has progressed to the ultimate already.
Amazing how a simple change can throw all else out the window.
Every advance offers multiple opportunities for different camshafts.
The flathead is a great example of possible combinations.