strok question
10-28-2011, 11:41 PM
#46
Three Wheelin'
Actually this is how we drew engineering models in school to solve problems like this, its called "Free Body Diagram". However I can represent the forces involved with computer model (ANSYS) a lot better, I may actually do that in the near future. In the real world software like ANSYS are used to accurately calculate the forces involved. Some of our projects required us to compare the calculation results between analytical model and our computer model to see if we got the computer model right. Results were very close. The equation I posted above is a very simple dynamic equation so that others can easily understand. The actual equation that I learned in "Mechanical Design 1" as a senior has a little more to it. I recommend posting this problem on some Mechanical Engineering forum, they can help in understanding what the offset will do in this situation.
10-28-2011, 11:48 PM
#47
Rennlist Member
It seems to me that your example of a fabrication press includes an assumption not relevant to an engine - and that is that the mold is supported on a stable immobile surface that is perpendicular to the force applied?
A piston on the other hand is not a stable surface - it is subject to movement within the clearances, both in the bores, and on the wrist pin and big end bearings, and thus its able to cant and the moment of force is thus not perpendicular to the face of the piston.
Question for you jerry as a fabricator of much experience. How many times have you, when slip-fitting something, had it **** and jam in the hole?
Now, if you were to push a cylinder into a barely-larger cylindrical hole with a flat-ended dowel, and were pushing against the cylinder slightly off-center, what do you think the chances of it sliding into place without cocking would be?
As far as I can tell (having not yet built an engine of any type), the offset issue is pretty straightforward - and that given the huge forces in play at high rpm's in an engine, even a slight offset is going to cause issues.
Sure, with a very small offset, an engine might run okay for a while, but I'd expect the ring wear to be very unevenly distributed to one side of the piston, and the engine would not last as long as one that had no rod offset.
A piston on the other hand is not a stable surface - it is subject to movement within the clearances, both in the bores, and on the wrist pin and big end bearings, and thus its able to cant and the moment of force is thus not perpendicular to the face of the piston.
Question for you jerry as a fabricator of much experience. How many times have you, when slip-fitting something, had it **** and jam in the hole?
Now, if you were to push a cylinder into a barely-larger cylindrical hole with a flat-ended dowel, and were pushing against the cylinder slightly off-center, what do you think the chances of it sliding into place without cocking would be?
As far as I can tell (having not yet built an engine of any type), the offset issue is pretty straightforward - and that given the huge forces in play at high rpm's in an engine, even a slight offset is going to cause issues.
Sure, with a very small offset, an engine might run okay for a while, but I'd expect the ring wear to be very unevenly distributed to one side of the piston, and the engine would not last as long as one that had no rod offset.
Your other assumption is also wrong becaue as I pointed out above, the factory and many other piston designers routinely design their pistons with the wrist pin off-center. That essentially entirely cancels your argument about how terrible any offset must be. The offset issue is merely a myth.
Jerry Feather
10-28-2011, 11:54 PM
#48
Hmm, a controlled explosion in a confined space is not going to react the same way as pressing 1,000,000 lbs on a part with a press.
I'm glad I'm not a lawyer, I'd know way more than I really do.
I'm glad I'm not a lawyer, I'd know way more than I really do.
10-28-2011, 11:56 PM
#49
Rennlist Member
10-29-2011, 12:00 AM
#50
Three Wheelin'
The Piston pin offset your referring to is for the smooth transition of the rocking motion between the two piston skirts to reduce knocking sound when engine is cold. Some car manufactures don't even have this offset built into their piston, piston pin is centered. What I drew is representing the wrong offset that has been discussed on the previous thread and this one, also pictured by the OP on this thread.
10-29-2011, 12:01 AM
#51
Nordschleife Master
All I know is that I would prefer an engine that works in practice to one that works in theory. Build your motors and show us the error in our thinking by going fast on a track.
Information in order of usefulness;
Forum bench racing
Magazine ads
Dynojet
Track.
Information in order of usefulness;
Forum bench racing
Magazine ads
Dynojet
Track.
10-29-2011, 01:39 AM
#52
Former Sponsor
Hi everyone,
I made a very simplified drawing of what Greg could be referring to. Greg, if this is wrong, simply say so. This is a very simple dynamic model.
Note: Force * distance = Torque; in this model.
The green horizontal bar represents the piston pin.
F1 and F2 represent the force transferred from the combustion forces to the two piston pin bosses. Do all of you agree that these two forces are equal? Okay.
Now "R" is the small end of the connecting rod that comes in to contact with the piston pin. As you can see on the drawing below, the rod is slightly offset towards F1, similar to the picture on the first page of this thread. The rocking motion that Greg is referring to will happen when one of the forces exerts more torque to the point of contact R. This will happen when there is a difference between lengths F1R and F2R.
As pictured below F2 will exert more torque at point of rod and pin contact compared to F1 due to the larger distance difference, so a clock wise torque will be applied to that point according to the drawing. Obviously this will severely wear the piston wall and cylinder wall including the rod bearing that Greg has been mentioning on the other thread. This could also possibly bend the rod.
Now if the distance was equal (zero offset). The torque applied by F1 and F2 would be canceled (remember, one would be clock wise and the other counter clock wise), eliminating piston rocking motion from side to side of the pin bosses.
I made a very simplified drawing of what Greg could be referring to. Greg, if this is wrong, simply say so. This is a very simple dynamic model.
Note: Force * distance = Torque; in this model.
The green horizontal bar represents the piston pin.
F1 and F2 represent the force transferred from the combustion forces to the two piston pin bosses. Do all of you agree that these two forces are equal? Okay.
Now "R" is the small end of the connecting rod that comes in to contact with the piston pin. As you can see on the drawing below, the rod is slightly offset towards F1, similar to the picture on the first page of this thread. The rocking motion that Greg is referring to will happen when one of the forces exerts more torque to the point of contact R. This will happen when there is a difference between lengths F1R and F2R.
As pictured below F2 will exert more torque at point of rod and pin contact compared to F1 due to the larger distance difference, so a clock wise torque will be applied to that point according to the drawing. Obviously this will severely wear the piston wall and cylinder wall including the rod bearing that Greg has been mentioning on the other thread. This could also possibly bend the rod.
Now if the distance was equal (zero offset). The torque applied by F1 and F2 would be canceled (remember, one would be clock wise and the other counter clock wise), eliminating piston rocking motion from side to side of the pin bosses.
10-29-2011, 01:42 AM
#53
Former Sponsor
Actually this is how we drew engineering models in school to solve problems like this, its called "Free Body Diagram". However I can represent the forces involved with computer model (ANSYS) a lot better, I may actually do that in the near future. In the real world software like ANSYS are used to accurately calculate the forces involved. Some of our projects required us to compare the calculation results between analytical model and our computer model to see if we got the computer model right. Results were very close. The equation I posted above is a very simple dynamic equation so that others can easily understand. The actual equation that I learned in "Mechanical Design 1" as a senior has a little more to it. I recommend posting this problem on some Mechanical Engineering forum, they can help in understanding what the offset will do in this situation.
The pressures exerted on the rod, the piston, and the bearings are very large, when the connecting rod isn't centered on the wrist pin.
All engine manufacturers, engine builders, and rod manufacturers go to extreme lengths to keep their rods perfectly centered on the piston pin. Jerry Feather apparently thinks they are wasting their time, doing this engineering. He thinks any old rod, with any old offset, sitting anywhere it can on the wrist pin is fine.
Jerry Feather correct.....everyone else in the internal combustion engine business is wrong. Hmmm. Interesting concept.
The "original" builders of 928 stroker engines "picked" pieces they could use off the shelf. They used small block Chevy connecting rods. These rods have an incorrect offset for use in a 928 engine. The picture on the very first post of this thread shows this incorrect offset. Engines assembled using a connecting rod that is not centered with-in .010" of center will have accelerated wear on the piston, the cylinder bores, and the rod bearings. Alusil engines have very little chance of survival.
I've tried very hard to show, tell, warn others of this problem.
At this point, I don't have much reason to care if anyone building a stroker engine pays any attention to my posts, or not.
I certainly don't see any other 928 engine builders telling people their "secrets" to building these engines, on this or any other Forum.
In the past few days, I've had quite a few PMs asking me why I bother telling people any of the things I've found out about these engines. I love these vehicles. I get a lot of phone calls about these cars, every week. It really hurts me when I talk to someone that has invested 30K+ into an engine that ran for very short periods of time, due to mistakes made by the builder. This "hurts" the entire community of 928 owners.
I'm not going to continue to debate the "offset" rod issue with Jerry Feather any farther. I'm convinced that Jerry wouldn't accept the correct answer, if God himself came down and told him.
This entire problem is pretty simple. Follow the "old" methods of building 928 stroker engines and join the ranks of the people that have very expensive scrap metal sitting in their garage or pay attention to all the designers of internal combustion engines, internal combustion parts suppliers, and 99.99% of professional engine builders and center the connecting rod.
Small block Chevy connecting rods don't work in a 928 engine.
End of story.
Last edited by GregBBRD; 10-29-2011 at 02:31 AM.
10-29-2011, 06:37 AM
#55
Hi Guys, Greg have reasson, the standard chevy rods not work well in 928 engine.
Today im investigate of my rods and crank and pistons.
My crank is a moldex, with 968 je pistons and i use iron cast sleeves, not alusil or nikasil.
The chevy rod is assimetrical, one side have more distance into the rod bearings and the counterweight of the piston. This is for the curve that have the crankpin.This rods only can mount in one position, you can not turn the rods 180 for one reasson:
if you turn 180º the rod , when you screw it the bearing is mounted on the curve that have the crankpin and not assembly correctly. only can turn one rod of the same crank pin but the problem persist in the other rod.
im thinking to machining the rod for can turn the 180º, but also is not posible because you increase the clearance and its wrong because the rod move excesively.
The problem with the force in the piston.... also wrong if you mount in the correct position.. but in other position can not mounted...
If i mounted in the correct position, not have the 1.27mm clearance the rod with the piston and when the piston dilate there is not sufficient space and broke it.
JE says that is necesary 1.27 mm minimun clearance the piston with the rods.
The first photo you can see the rod in correct position, the bearing not touch the curve of the crank and turn without problems.
The second photo is the same rod but 180º turned and the bearing mount into the curve and is wrong. dont turn, there fore you can not turn 180º.
the third photo is the rod turned 180º and you can see that the distance is very similar.. but you can not doing this because te problem exist in the other rod of the same crankpin that you can not turn.
i dont knwo if undertand very well...
Today im investigate of my rods and crank and pistons.
My crank is a moldex, with 968 je pistons and i use iron cast sleeves, not alusil or nikasil.
The chevy rod is assimetrical, one side have more distance into the rod bearings and the counterweight of the piston. This is for the curve that have the crankpin.This rods only can mount in one position, you can not turn the rods 180 for one reasson:
if you turn 180º the rod , when you screw it the bearing is mounted on the curve that have the crankpin and not assembly correctly. only can turn one rod of the same crank pin but the problem persist in the other rod.
im thinking to machining the rod for can turn the 180º, but also is not posible because you increase the clearance and its wrong because the rod move excesively.
The problem with the force in the piston.... also wrong if you mount in the correct position.. but in other position can not mounted...
If i mounted in the correct position, not have the 1.27mm clearance the rod with the piston and when the piston dilate there is not sufficient space and broke it.
JE says that is necesary 1.27 mm minimun clearance the piston with the rods.
The first photo you can see the rod in correct position, the bearing not touch the curve of the crank and turn without problems.
The second photo is the same rod but 180º turned and the bearing mount into the curve and is wrong. dont turn, there fore you can not turn 180º.
the third photo is the rod turned 180º and you can see that the distance is very similar.. but you can not doing this because te problem exist in the other rod of the same crankpin that you can not turn.
i dont knwo if undertand very well...
10-29-2011, 03:34 PM
#58
Three Wheelin'
Cool drawing, except it does not come close to representing what is actually taking place, as I have already pointed out in my analysis. The connecting rod is not connected to the wrist pin at such a single point and is in fact connected over a span of about and inch and an eighth which clearly brackets the center point sufficiently to cancel any tendency for the piston to rock, as suggested.
Thanks for ltrying. Please reread my analysis because I think I have already pointed out the flaw in this simple suggestion.
Thanks for ltrying. Please reread my analysis because I think I have already pointed out the flaw in this simple suggestion.
10-29-2011, 04:41 PM
#59
Former Sponsor
Yes, obviously the piston will not be rocking like suggested, due to the tight clearances. However, there will be an unnecessary torque applied to one side of the piston, trying to rotate it side ways. This will create extra friction, which will not only reduce power but also increase wear that eventually causes failure.
Since Carrillo almost always uses perfectly "centered" rods (people are generally not using a rod designed for one engine in another engine), they had never done a study of the forces involved.
They were very surprised at the huge loads on the pieces, as I was.
10-29-2011, 04:52 PM
#60
Former Sponsor
Hi Guys, Greg have reasson, the standard chevy rods not work well in 928 engine.
Today im investigate of my rods and crank and pistons.
My crank is a moldex, with 968 je pistons and i use iron cast sleeves, not alusil or nikasil.
The chevy rod is assimetrical, one side have more distance into the rod bearings and the counterweight of the piston. This is for the curve that have the crankpin.This rods only can mount in one position, you can not turn the rods 180 for one reasson:
if you turn 180º the rod , when you screw it the bearing is mounted on the curve that have the crankpin and not assembly correctly. only can turn one rod of the same crank pin but the problem persist in the other rod.
im thinking to machining the rod for can turn the 180º, but also is not posible because you increase the clearance and its wrong because the rod move excesively.
The problem with the force in the piston.... also wrong if you mount in the correct position.. but in other position can not mounted...
If i mounted in the correct position, not have the 1.27mm clearance the rod with the piston and when the piston dilate there is not sufficient space and broke it.
JE says that is necesary 1.27 mm minimun clearance the piston with the rods.
The first photo you can see the rod in correct position, the bearing not touch the curve of the crank and turn without problems.
The second photo is the same rod but 180º turned and the bearing mount into the curve and is wrong. dont turn, there fore you can not turn 180º.
the third photo is the rod turned 180º and you can see that the distance is very similar.. but you can not doing this because te problem exist in the other rod of the same crankpin that you can not turn.
i dont knwo if undertand very well...
Today im investigate of my rods and crank and pistons.
My crank is a moldex, with 968 je pistons and i use iron cast sleeves, not alusil or nikasil.
The chevy rod is assimetrical, one side have more distance into the rod bearings and the counterweight of the piston. This is for the curve that have the crankpin.This rods only can mount in one position, you can not turn the rods 180 for one reasson:
if you turn 180º the rod , when you screw it the bearing is mounted on the curve that have the crankpin and not assembly correctly. only can turn one rod of the same crank pin but the problem persist in the other rod.
im thinking to machining the rod for can turn the 180º, but also is not posible because you increase the clearance and its wrong because the rod move excesively.
The problem with the force in the piston.... also wrong if you mount in the correct position.. but in other position can not mounted...
If i mounted in the correct position, not have the 1.27mm clearance the rod with the piston and when the piston dilate there is not sufficient space and broke it.
JE says that is necesary 1.27 mm minimun clearance the piston with the rods.
The first photo you can see the rod in correct position, the bearing not touch the curve of the crank and turn without problems.
The second photo is the same rod but 180º turned and the bearing mount into the curve and is wrong. dont turn, there fore you can not turn 180º.
the third photo is the rod turned 180º and you can see that the distance is very similar.. but you can not doing this because te problem exist in the other rod of the same crankpin that you can not turn.
i dont knwo if undertand very well...
If you were to cut another champher on the rod and cut some of the rod bearing material away, to clear the large radius on the crankshaft, the rod would fit onto the crankshaft and probably function fine.
However, you can easily see from your photograph, that the rod, when it is turned 180 degrees, still isn't centered on the piston pin. Although this "improves" the problem, it doesn't cure it. As I said, the study that Carrillo did for me, showed that the rod needs to be within .010" (.25mm) of being centered. More than .020" (.5mm) off and the forces on the piston, rod, and bearings is virtually the same as if the error is .060" (1.5mm).
You need a correct rod for the 928 engine.