4 mil Long Rod Crank

[RZBansheeMan]

What is the optimal rod to stroke ratio for two stroke engines ?. Mainly the Banshee I'm in question of, because that's mostly all I care about in 2 strokes.

 

I've done the math several times and this is what I have came up with:

 

110 mm rod = stock length

54 mm stroke = stock stroke

2.037 : 1 = rod ratio

 

so when you have long rod stock stroke crank obviously the rod ratio increases :

 

115 mm "long" rod

54 mm stroke

2.129 : 1 = new rod ratio

 

Seeing that when a crank is replaced, it is always reccomended that a "long rod" crank be used for the added rod ratio, for the reduced piston skirt loading. Well now I'm kinda wondering what gave the 4 mil stroker 115 mm "long rod" crank the term "long rod" when all actuality it has less rod to stroke ratio than a stock stroke / stock rod crank ?

 

115 mm rod = "long rod"

58 mm stroke = 4 mil stroker

1.982 : 1 = less than stock rod ratio

 

120 mm rod = 10 mm longer rod than stock

58 mm stroke

2.068 : 1 = slightly more rod ratio than stock

 

125 mm rod = 15 mm longer rod than stock

58 mm stroke

2.155 : 1 = slightly more rod ratio than "long rod" stock stroke crank

 

I know that this isn't the most feesible way of building a engine, but I do know that there are 125mm rods for a Banshee. I also know that this wouldn't be possible to build without using a spacer plate.

 

What's everyone's thoughts ?

 

RZ

[odaen]

For banshee's, I'd imagine alot of it comes down to piston availability, and working within the confines of the cylinder height. I've got a pretty good spreadsheet of wiseco's pistons that Calvin gave me, and they don't make too many models with a given wrist pin diameter, bore, and pin offset. Adding 15mm of spacer plate, or even 10mm would yield a ton of crankcase volume too I'd imagine.

 

In general, for 2 strokes, I'd guess that it comes down to tradeoffs...

Longer rods give you less rod angle and sideload on the pistons and longer dwell times. But they also give you more mass and higher piston velocities.

 

For target ratios, Dan Wade @ Patriot, or Calvin Pollet would be good people to ask about this.

Edited September 7, 2007 by odaen

[dajogejr]

Very, very interesting....

I would ask it on PS...I'm sure Calvin would pipe in...as well as others....

[RZBansheeMan]

I think you could get away with between 8 mm for 120 mm rod, and 13 mm spacer plate for a 125 mm rod. I say this because on a 4 mill stroker the piston does travel 2 mm above the deck height. I would imagine that a "795" piston still could be used.

 

I have also thought about this adding too much volume to the crank case, but just out of curiosity couldn't the transfers be left a little narrow to keep the transfer velocity up ?

 

I really don't have any reason or want to go over to PS, nothing against anybody over there, but I get all my Banshee info here and all my TRX info over at E2S.

 

I'm not really into Dragging either, I just like to think up some out of the norm engine combinations, more or less to see if it's possible or an improvement.

Edited September 7, 2007 by RZBansheeMan

[odaen]

I think you could get away with between 8 mm for 120 mm rod, and 13 mm spacer plate for a 125 mm rod. I say this because on a 4 mill stroker the piston does travel 2 mm above the deck height. I would imagine that a "795" piston still could be used.

 

I have also thought about this adding too much volume to the crank case, but just out of curiosity couldn't the transfers be left a little narrow to keep the transfer velocity up ?

 

I really don't have any reason or want to go over to PS, nothing against anybody over there, but I get all my Banshee info here and all my TRX info over at E2S.

 

I'm not really into Dragging either, I just like to think up some out of the norm engine combinations, more or less to see if it's possible or an improvement.

 

 

I don't think that leaving the transfers narrow to increase the air/fuel velocity would make up for oversized crankcase volume.

[blowit]

I think you could get away with between 8 mm for 120 mm rod, and 13 mm spacer plate for a 125 mm rod. I say this because on a 4 mill stroker the piston does travel 2 mm above the deck height. I would imagine that a "795" piston still could be used.

 

I have also thought about this adding too much volume to the crank case, but just out of curiosity couldn't the transfers be left a little narrow to keep the transfer velocity up ?

 

I really don't have any reason or want to go over to PS, nothing against anybody over there, but I get all my Banshee info here and all my TRX info over at E2S.

 

I'm not really into Dragging either, I just like to think up some out of the norm engine combinations, more or less to see if it's possible or an improvement.

 

There are several factors that play into conrod length. There is not a set ratio that I am aware of because wrist pin locations vary so much. It really depends on what the OEM is trying to achieve with the engine. It also depends on rod *angle* which is directly related to where the power will be. What is more relevant is the stroke compared to the bore. It has been taught for years that the ideal motor is near "square" or the stroke equals the bore but for the last couple decades, most are going larger bore and shorter stroke.

 

We absolutely love the 4mm long rod setups for the banshee. The numbers simply dictate that all critical areas are still well within tolerance referring to loads while still increasing both top end and bottom end. The motor will last just as long as the 350 stocker but with more punch.

 

Brandon

[RZBansheeMan]

There are several factors that play into conrod length. There is not a set ratio that I am aware of because wrist pin locations vary so much. It really depends on what the OEM is trying to achieve with the engine. It also depends on rod *angle* which is directly related to where the power will be. What is more relevant is the stroke compared to the bore. It has been taught for years that the ideal motor is near "square" or the stroke equals the bore but for the last couple decades, most are going larger bore and shorter stroke.

 

We absolutely love the 4mm long rod setups for the banshee. The numbers simply dictate that all critical areas are still well within tolerance referring to loads while still increasing both top end and bottom end. The motor will last just as long as the 350 stocker but with more punch.

 

Brandon

 

 

I appreciate your wealth of knowledge and you sharing it, but it also seems that if the 4 mil long rod setup is well within tolerence of piston loads, then there wouldn't be any nescessity to use a stock stroke crank with the "long rods", considering when the 4 mil "long rod" has greater rod angularity and less rod / stroke ratio than a stock stroke crank with stock length rods.

 

 

On another note I'm kinda interested in the old RD 400 bore x stroke specs, but that would require 8 mm stroker crank for the Banshee, which I am guessing is an oddball vs. the more common 7 mm stroker, which in my opinion is an oddball number.

 

Mike

[blowit]

I appreciate your wealth of knowledge and you sharing it, but it also seems that if the 4 mil long rod setup is well within tolerence of piston loads, then there wouldn't be any nescessity to use a stock stroke crank with the "long rods", considering when the 4 mil "long rod" has greater rod angularity and less rod / stroke ratio than a stock stroke crank with stock length rods.

On another note I'm kinda interested in the old RD 400 bore x stroke specs, but that would require 8 mm stroker crank for the Banshee, which I am guessing is an oddball vs. the more common 7 mm stroker, which in my opinion is an oddball number.

 

Mike

 

 

 

The long rod motor was popular 10 years ago when you had no option but to rebuild them. A simple swap with the RD rod. You also had to rn the blaster piston and was commonly called the 370 long rod motor because you had to bore the motor to max which we were not a fan of. The reduction of rod angle brought plenty of rpms to the party. You are reducing rod angle with that setup and the introduction of the 4mm stroker brings the rod angle back to near OEM specs. The reason it is so popular now is because it is easier than ever to get a 4mm crank cheap.

 

 

Brandon