Is 35 to much
09-16-2002, 08:45 PM
#21
Guest
Posts: n/a
1.88:1 rod ratio
I realize that a long rod ratio makes the piston dwell longer at TDC and this might help (theoretically) with air flow, it reduces side loading on the piston skirt/cylinder wall and it might have some effect on ignition timing but the more I read and talk to really good engine people the more I am convinced that the connecting rod simply connects the crank to the piston, I really think it is about as simple as that.
I also believe that moving the piston pin really high on the piston (crowding the rings) to use long rods and using really short pistons to clear a long stroke crankshafts counterweight hurts performance more than helps. Forged aluminum pistons have very large cylinder wall clearances in the first place and if you add to that short piston skirts the piston becomes very unstable in the cylinder walls. The sloppy piston fit is constantly unloading the rings from the cylinder wall upsetting ring seal. This unstable piston will allow oil to pass by the rings and enter into the combustion chamber causing detonation, an unstable piston can also crack a piston skirt very easily.
If I had my choice of combinations I would go with a longer ratio but I would do that with a standard length stroke crankshaft and I wouldn't spend a great deal more time or money arriving there!
I hope this doesn't start a big discussion about rod to stroke ratios because I don't think it would ever be answerd.
About the ignition timing. The only thing you can do is try different timing figures and use what is right for your boat/engine/drive combination. All setups have different loads placed on the engine and all engines are a little different. Generally we can use 32 - 34 degrees advance but I think you really want to zero in on the timing (if I am reading you correctly by the fact that you mentioned rod to stroke ratio). Experimentation is all you can do!
I hope you are not disappointed with my answer but I think that stroker motors are on everyones mind these days and I thought that I would address some of their questions as well.
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE
I also believe that moving the piston pin really high on the piston (crowding the rings) to use long rods and using really short pistons to clear a long stroke crankshafts counterweight hurts performance more than helps. Forged aluminum pistons have very large cylinder wall clearances in the first place and if you add to that short piston skirts the piston becomes very unstable in the cylinder walls. The sloppy piston fit is constantly unloading the rings from the cylinder wall upsetting ring seal. This unstable piston will allow oil to pass by the rings and enter into the combustion chamber causing detonation, an unstable piston can also crack a piston skirt very easily.
If I had my choice of combinations I would go with a longer ratio but I would do that with a standard length stroke crankshaft and I wouldn't spend a great deal more time or money arriving there!
I hope this doesn't start a big discussion about rod to stroke ratios because I don't think it would ever be answerd.
About the ignition timing. The only thing you can do is try different timing figures and use what is right for your boat/engine/drive combination. All setups have different loads placed on the engine and all engines are a little different. Generally we can use 32 - 34 degrees advance but I think you really want to zero in on the timing (if I am reading you correctly by the fact that you mentioned rod to stroke ratio). Experimentation is all you can do!
I hope you are not disappointed with my answer but I think that stroker motors are on everyones mind these days and I thought that I would address some of their questions as well.
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE
09-16-2002, 11:00 PM
#22
Guest
Posts: n/a
I spoke to my engine builder today and he insists that 35 is the way to go. He said that was the point that the motor started to loose power on the dyno. This is the second time I have rebuilt this motor this year. Both times it whent out it was because of detonation. I dont want to go through this whole process again it costs to much and I have only been able to use my boat a few times this year. Should I try it at the 35 and just keep a eye on it or shoould I insist that he retard the timing?
09-17-2002, 05:41 PM
#23
Charter Member #601
Charter Member
26 CHECKMATE,
I would start at 30 degees.. If the motor just wont run, then add a little at a time. I think it is less likely you can hurt it at 30, but I think you may at 35. What kind of gas was he running on the dyno?? If it was race gas, then I can see his point, but you need to know about the real world gas..
Dennis,
Not disappointed at all . I appreciate the thoughts.
I guess the question I am asking is, if you increase rod ratio, would the motor tend to like less advance? I have talked to a few people and they say it should like less. I have not done any experimenting to see what it likes. But hope to in the coming months or next spring. Short of running it on the dyno, which isnt a feasable option at this point, I was just curious if any one has put any theory to the long rod issue.
I have a very conservative pin height.. 1.544", so the rings arent crowded and only a 4.25" crank. So no need to cut the skirts short. In the beginning I was looking for an octane insensitive motor. But after several failures,(machine work, broken heads, and roller lifters) I gave up on the experiment and just put it together and it has run for about 50hrs without problems.. Not sure if I will be able to make any serious power with this combination, but I cant stop thinking about not doing it!!!
Thanks for the input!
Dick
I would start at 30 degees.. If the motor just wont run, then add a little at a time. I think it is less likely you can hurt it at 30, but I think you may at 35. What kind of gas was he running on the dyno?? If it was race gas, then I can see his point, but you need to know about the real world gas..
Dennis,
Not disappointed at all . I appreciate the thoughts.
I guess the question I am asking is, if you increase rod ratio, would the motor tend to like less advance? I have talked to a few people and they say it should like less. I have not done any experimenting to see what it likes. But hope to in the coming months or next spring. Short of running it on the dyno, which isnt a feasable option at this point, I was just curious if any one has put any theory to the long rod issue.
I have a very conservative pin height.. 1.544", so the rings arent crowded and only a 4.25" crank. So no need to cut the skirts short. In the beginning I was looking for an octane insensitive motor. But after several failures,(machine work, broken heads, and roller lifters) I gave up on the experiment and just put it together and it has run for about 50hrs without problems.. Not sure if I will be able to make any serious power with this combination, but I cant stop thinking about not doing it!!!
Thanks for the input!
Dick
09-17-2002, 07:39 PM
#25
Guest
Posts: n/a
35 ?
Dear 26 Checkmate,
I have a feeling that the engine builder was saying that the engine power was beginning to drop off with anything MORE than 35 degrees advance. This would seem probable because most engine builders try to run as much advance as possible without the engine detonating. They believe that the engine will put out max power at the very edge of engine detonation.
What you should be asking is if the engine power drops off with anything LESS than 35 degrees.
It is a common misconception that you should ADVANCE the timing until power starts falling off and then retard it a little.
The reality is that you should RETARD the timing until power starts falling off and then advance it a little.
I hope I haven't insulted everyones intelligence with my simple logic!
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE
I have a feeling that the engine builder was saying that the engine power was beginning to drop off with anything MORE than 35 degrees advance. This would seem probable because most engine builders try to run as much advance as possible without the engine detonating. They believe that the engine will put out max power at the very edge of engine detonation.
What you should be asking is if the engine power drops off with anything LESS than 35 degrees.
It is a common misconception that you should ADVANCE the timing until power starts falling off and then retard it a little.
The reality is that you should RETARD the timing until power starts falling off and then advance it a little.
I hope I haven't insulted everyones intelligence with my simple logic!
Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE
Last edited by Dennis Moore; 09-17-2002 at 07:42 PM.
