Norton rod/stroke ratio

Here's a little (edited) info I just got from Jim Cossins who did some number crunching with JS longer rods/shorter lighter pistons. For you tech heads out there.

When a Norton crank is at 30 BTDC the distance from piston crown to TDC with the JS longer rod is 6.8mm compared to 9.2 mm with standard rods. So both sets of pistons are going from 6.8 and 9.2 mm BTDC to TDC in the same amount time. You can see that the piston with the longer rod spends more time closer to TDC (more efficient burning & power) and is moving slower when in reverses direction (less vibration).

Jim

Very informative to me Jim. 6.4" rod / 89 mm = 1.8265168539325842 ratio.
http://www.stahlheaders.com/Lit_Rod%20Length.htm

The main advantage for me is torque side of it which also tends towards detonation.
The lower G force jerk down of longer rods helps piston rpm tolerance, if that is even a factor on your piston kit on jump rope crankshaft. The main down side to longer rods is said to slow down the intake pressure drop in some applications. Nortons rev so slowly there's enough fill time - its a non issue. I make exception to Jim's twin showing tach needle heading beyond the red zone over 9,000 rpm!!! More mean hearted than anyone to Nortons you are.

Hi Jim, does it means that we could/ should, retard ignition timing ?

marinatlas said:

Hi Jim, does it means that we could/ should, retard ignition timing ?

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NO

Jim Cossins was concerned that the ignition timing should be changed for longer rods but decided not to because at a given RPM it takes the exact same amount of time for a crank to get from 30 degrees BTDC to TDC no matter what rods are used (or no rods at all).

Jim

Good to know!! thanks for the post Jim. Cj

Obviously fireing with the Piston in a differant Position though, olde in. before tdc types would be flumoxed . :?

Rotational speed of crank per revolution , or two. is irregular , fastest during late combustion ( the Torque Spike )
Do these suckers even it out a way more ? thus less stressing on the poor old crankshaft ? ? :mrgreen:

[quote="jNO Jim Cossins was concerned that the ignition timing should be changed for longer rods but decided not to because at a given RPM it takes the exact same amount of time for a crank to get from 30 degrees BTDC to TDC no matter what rods are used (or no rods at all). Jim[/quote]

You may not have to, but there could be gains in power by retarding the ignition timing. With the longer rod the the piston stays at tdc longer, the combustion space stays smaller longer, so the fuel charge should burn faster in the smaller space-time. Any time you can retard the timing and still get all the fuel charge burned it cuts down the time that combustion pressure is building against the piston when it is trying to travel upwards on compression.

If you could build an engine that burned the charge so quickly that it could do the job with zero degrees ignition advance then the compression stroke would do the least fighting against combustion pressure and it would release even more power.

When one engine parameter is changed, then the optimum settings for all other engine parameters must change also, it is a different engine.

My two cents.

More little tid bids on rod length and cam breathing.

Another factor that influences DCR is rod length. It's length determines the piston location at intake closing, different rod lengths change the DCR. Longer rods position the piston slightly higher in the cylinder at intake closing. This decreases the DCR, possibility necessitating a different cam profile than a shorter rod would require. However, the effect is slight and might only be a major factor if the rod is substantially different than stock. Still it needs to be taken into account when calculating the DCR.

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