Rod/stroke ratio effects explained

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worntorn

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A very thorough examination of rod ratios and their effect on engines.

Stock Norton Commandos have a 1.68 rod ratio. It would appear to be a reasonable choice for the RPM and performance levels involved ( lower output per cc than the Hyundai commuter!)

 
Oh dear. I barely understood anything. I’ll watch it again sometime and see if it sinks in.
 
That's a big rabbit hole to go down. :)

I used a PW3 cam in my old 500 Dommie race bike. A Dommie has a 2.1 ratio and I spent some time convincing myself I should retard the cam to allow for the difference. Comparing the piston location in the bore for the different rod ratios of a Commando versus the Dommie found a different percentage of stroke had been covered for a given cam timing. I therefore adjusted cam timing on the Dommie to match the Commando piston location in percentage terms. Dyno testing did show a measurable increase in bhp in the 3000 to 5000 part of the rev range but no difference above 5750 to 7000.
 
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worntorn said:
A very thorough examination of rod ratios and their effect on engines.

Stock Norton Commandos have a 1.68 rod ratio. It would appear to be a reasonable choice for the RPM and performance levels involved ( lower output per cc than the Hyundai commuter!)

Click to expand...

Excellent video.

I think Jim Schmidts long rods take the ratio up to 1.83:1
 
Some good info in there re the benefits of a longer rod reducing vibration. And let’s not forget that video assumes no change in the piston… whereas in a Norton, the main reason for using longer rods is to allow a much lighter piston.
 
Fast Eddie said:
Some good info in there re the benefits of a longer rod reducing vibration. And let’s not forget that video assumes no change in the piston… whereas in a Norton, the main reason for using longer rods is to allow a much lighter piston.
Click to expand...
I think he talked about longer rods allowing a shorter skirt, and different wrist pin heights, but the assumption was same bore and deck height.
 
Fast Eddie said:
Some good info in there re the benefits of a longer rod reducing vibration. And let’s not forget that video assumes no change in the piston… whereas in a Norton, the main reason for using longer rods is to allow a much lighter piston.
Click to expand...
I also think the slightly longer dwell at top of stroke is beneficial, especially since ours aren't the most efficient engines.

As an aside, I use the additional 9 degrees timing advance that Triumphs have (38 degrees vs our 29) as proof positive of the comparatively poor combustion chamber shape of the Triumph vs Norton.
Gets a few "rises" anyway!;)
 
My short stroke uses JSMs longest rods (6.57") to give 2:1 rod ratio.

There is a suggestion made elsewhere that if you increase the rod ratio with shorter dwell you should also reduce the max advance. This guy does not mention ignition advance (about the only thing he doesn't).

I have tried retarding the ignition a little, I know others who say it works and use as little as 26 degrees, (JimS incuded), but I am not convinced and currently plan to go back to 29 to 30 degrees, if not the full 31 degrees.

I am also interested in head temperature, which seems to be higher with less advance, which is surely counter intuitive. (and part of the reason why I am fitting some temp gauges to get a better look).

Anybody else have experience here?
 
No wonder my 301 SB Chevy (283 bored .120" over) revved so gleefully and made so much power. With a 5.7" C-C rod and a 3" stroke, it had a rod ratio of 1.9:1
 
Fast Eddie said:
Some good info in there re the benefits of a longer rod reducing vibration. And let’s not forget that video assumes no change in the piston… whereas in a Norton, the main reason for using longer rods is to allow a much lighter piston.
Click to expand...
I believe he mentions that, although he puts it slightly differently.
He suggests that the extra weight of the long rods can be offset by use of a lighter piston. The short rod set up needs a longer heavier piston to deal with those side forces.
The stock Norton set up uses an extremely light aluminum rod with no top bushing, moderately heavy fairly long skirt piston and a middle of the pack rod ratio.

Glen
 
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Dan1950 said:
No wonder my 301 SB Chevy (283 bored .120" over) revved so gleefully and made so much power. With a 5.7" C-C rod and a 3" stroke, it had a rod ratio of 1.9:1
Click to expand...
He also explains that dramatic rod ratio changes have only tiny effects on power. The low rod ratio pulls a bit harder down low and in the middle and gives better throttle response in low rpm engines like our 6500 rpm Nortons or a low revving American v8. A higher rod ratio is needed for very high rpm engines such as modern Sportbike engines or F1 engines. (14-16,000 rpm)
Our Nortons would be scattered in many pieces long before those rpm levels!

The Norton clones built from aftermarket parts could probably hang together to 8000 rpm or better, but I wouldn't expect them to make it much past ten k, not for very long at least!

Glen
 
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worntorn said:
He also explains that dramatic rod ratio power changes have only tiny effects on power. The short rod pulls harder down low and in the middle and had better throttle response in low rpm engines like our 6500 rpm Nortons or a low revving American v8. The long rods ratio is needed for very high rpm engines such as modern Sportbike engines or F1 engines. (14-16,000 rpm)
Our Nortons would be scattered in many pieces long before those rpm levels!

The Norton clones built from aftermarket parts could probably hang together to 8000 rpm or better, but I wouldn't expect them to make it much past ten k, not for very long at least!

Glen
Click to expand...
The 301 didn't make much low end torque but as the revs climbed it would pull harder and harder. I shifted at 6200 but it could peg the 8000 RPM tach in the lower gears.
 
worntorn said:
I believe he mentions that, although he puts it slightly differently.
He suggests that the extra weight of the long rods can be offset by use of a lighter piston. The short rod set up needs a longer heavier piston to deal with those side forces.
The stock Norton set up uses an extremely light aluminum rod with no top bushing, moderately heavy fairly long skirt piston and a middle of the pack rod ratio.

Glen
Click to expand...
...and it's mainly the small end of the rod that contributes to reciprocating vibration. The big end is pretty much the same anyway.
 
If the longer rod slows the piston, that would mean more dwell time at TDC?!?! I know a long stroke creates a longer dwell time at TDC. Would that call for more or less timing advance?
 
mean gene said:
If the longer rod slows the piston, that would mean more dwell time at TDC?!?! I know a long stroke creates a longer dwell time at TDC. Would that call for more or less timing advance?
Click to expand...
Definitely not more - potentially less (BTW on a Commando you're talking longer rod, not longer stroke - right?)
I think the change in advance would be fairly insignificant.
 
" Commando versus the Dommie found a different percentage of stroke had been covered for a given cam timing. "

Somewhere along the way , didnt they throw the cylinder back ( desaxe ) so the rods dont go through the camshaft .
( This gets things all unequal & out of kilter, if fractionally , on a two smoke it unsymetricalises the cylinder port timing ,
in regard to crankshaft rotation at least . Obviously Hight linear isnt altered . seabelow .


" As an aside, I use the additional 9 degrees timing advance that Triumphs have (38 degrees vs our 29) as proof positive of the comparatively poor combustion chamber shape of the Triumph vs Norton. "

RISES . OH DOES IT NOW ! .

30 on the Norton is something like 3/8 B.T.D.C. , like wot a Triumphs Got @ 38 . So ignition point , linearly , is pretty much THE SAME . So There !

A thousand years ago , what the Norton Twin was renowned for , was its DEEP BREATHING , which gave it the volumetric efficiancy , and Tourque .
Even if they did blow up .

Actually , Id think the ' blowing Up ' had more to do with wear / excess clearances , & 18 yr old budding agostini's . Tho doubling the oil pump speed cant have hurt , there .
 
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