Spark Timing, Rod Ratio, Crank/Piston Offset

[Whittey]

While laying awake staring at the ceiling last night I was thinking of rod ratios and offsets and whatnot, and how optimum timing would change with differing offsets and rod ratios....

As I understand it, when you offset the cylinder from the crank you change the piston TDC to be different from crank TDC. This sounds good and bad. 1) It seems to give good initial leverage on the crank but seems like it would make for a shorter usable down-stroke. 2) Increased load on the bearings at piston BDC.

How does a long-rod combo work with more offset? It would seem that a short-rod would work perfectly fine on a straightup (no offset) engine while a long-rod would help out on an engine with an offset.

Anything I said wrong? Anything to add?


-=Whittey=-

 

[buzz41]

I'm waiting for an expert, too. Maybe, a very short course on the why's and wherefore's of offsets.

I've worked the geometries out a few times in the past, but never used it. I have an app soon that might benefit from a little pin offset, but I don't know why or which way.

 

[evelrod]

I am in a bit of a rush this morning, guys. I'll try to look up the threads later but, somewhere in these automotive sections are several discussions of pin offset, ad nauseum. Isaac (Ivymike) has some real insight to the physical properties, stroke change and, math on the subject (he converted me). In my own experience with several race engines, I have found pin offset to be a curiosity and not much help in making more power. Long rods on the other hand have proven themselves to me time and again. Look up the other threads, lots of info on the subject.


Rod

 

[TonyG714]

There's alot of talk on the discussion board at

http://www.ford-trucks.com

about the subject especially as it relates to 460 motors. The consensus is that cam retarding was done for emission purposes. This was recently stated and confirmed by a ford mechanic with 27 yrs experience. Many guys are going with pre '71 timing chain when they rebuild their 460's. They say it gives a great deal more torque down in the lower rpms.
My confusion or question would be when it comes to choosing a cam. Are the aftermarket cans designed to work with the retarded timing chain setup or are the designed to counteract the emissions retard. My guess is that they cams are set to a standard 5 degrees or so and that the timing chain retard is still in effect. ........ So I shoulda put a pre '71 chain on the new motor....

Tony

 

[ivymike]

TonyG, I must have missed something - is your post related to the initial post?

Whittey, here is a link to a pdf I put together showing slider eqns with and without offset:

http://www.geocities.com/compact_enthusiast1/slider_eqns.html

In regard to your specific questions,
1) that depends on the direction of offset relative to the direction of crank rotation. You can have a longer or shorter downstroke (in degrees) depending on which way the crank spins. If you make a 2-d slider drawing with the piston shown above the crank, and offset the crank center to the right (vs cylinder axis), and then make the crankshaft rotate clockwise, you'll have a shorter downstroke. With the piston up and the crank to the right and CCW rotation, you'll have a longer downstroke. I'm tempted to describe it as "rotating towards cylinder axis = longer, rotating away from cylinder axis = shorter" but I know that's not clear.

-more later-

 

[ivymike]

whittey-
2) the increased BDC load that you mention - was that due to increased acceleration at BDC? If so, it seems that the effect will be (realistic max) only a few percent, but would be magnified by having a shorter rod.

buzz41-
Usually offset is used to allow a more controlled transfer of the piston from the thrust side of the cylinder to the anti-thrust side. This is usually done to reduce piston slap, and thus the pin offset is usually in the direction of the thrust side (so the skirt jumps first). Offset is often in the close neighborhood of 1mm (0.5mm - 1.2mm seem to be common). I suspect that you are talking about offset in the opposite direction (perhaps for some performance gain)? I'm not versed in the benefits of this other reason for offsetting the pin, so I probably shouldn't say too much about it. It doesn't seem to me like a 1mm offset is going to make a noticeable power (or torque) difference in an engine with a 85mm or greater stroke, but I dunno...

 

[franzh]

Tony:
The pre-71 Ford 429 and 460 timing sets used a reindexed crank gear (about 3 degrees crank retarded), the chain and cam gear were the same. I used to order the pre-71 set for all of them. Claims were about 15 bhp and about 25 ft/lbs improvement. The changes were for emissions only, it reduced cylinder pressure.

Franz

 

[Whittey]

ivymike: I pulled that whole initial post out of my head, so if you think i'm wrong please let me know. I am assuming more load on the bearings because the vertical loads should be the same while you're adding horizontal loads as well. I haven't read the linked PDF yet as this is the first time i've been back to the board since the initial post (drive controller died on my computer).


-=Whittey=-

 

[ivymike]

Well, the vertical loads are actually increased slightly at BDC (slightly greater acceleration), and the conrod is at an angle. The real deciding factor will probably be how big of an offset you're looking at. Now you're starting to get me more curious about it... I hadn't put too much thought into the behavior w/ very large offsets. I'll have to look more carefully at the situation and get back to you on that.