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Old 04-09-2012, 06:49 AM
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Silver you somewhat threw me off when you mention sticking a 400 shaft in a 350 and having a better rod to stroke ratio. What to you is better?
Lets start out with the basic 350 in stock form
5.703 / 3.480 = 1.63:1

Now a 400:
5.565 / 3.750 = 1.48:1

Budget 383 with stock 400 rods.
5.565 / 3.750 = 1.48:1

383 with 5.7 rods
5.703 / 3.750 = 1.52:1

383 with 6 inch rods
6.000 / 3.750 = 1.6:1

None of these are very good. It's said the optimum rod to stroke for a acceleration engine is 1.8:1 to 1.85:1
Being the most popular 383 combo uses 350 rods, it takes us back to a 1.52:1 ratio. You are right on a longer rod is better, but there are factors in there. longer rods help increase piston dwell time at tdc which helps resist detonation issues in theory by the piston sitting at tdc longer the combusting air/fuel mixture can expand more before the piston moves.
The longer rod also reduces side loading and piston speed.
A example of dwell time is a stock 350 engine, stick a degree wheel on it and a tdc indicator. The dwell time at tdc to 2 to 3 degrees.
Try the experiment again on a 350 with a 6 inch rod, dwell time increases to 8 to 9 degrees. a 6 inch rod in a 350 moves the rod to stroke ratio up to 1.72:1. The lower rod ratio also increases piston side loading into the bore which increases friction and wear. Another issue short rods cause is due to the really low dwell time of the piston at top dead center in the mid range RPMs the piston starts to outrun the flame front. This is especially true in the stock 400 and the 400 rod 383 build. The increased piston speed is also going to put a cap on rpm limit for cast and hyper pistons as they can only take so much before the piston breaks due to the high speed side loading.

Last edited by DoubleVision; 04-09-2012 at 06:57 AM.
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