Standard rod vs +10

[alvin l nunley]

Way to come in here and call everyone wrong... That helps this thread.
And for the record I never once claimed which rod was better short vs long.
Nor did I mention anything about stroke.
All i was trying to explain is piston acceleration at tdc and bdc are not the same.[/QUOTE
Ssrperformance
Ssrperformance is online now
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Join Date:Nov 2013Posts:17

Quote Originally Posted by Kart43 View Post

Rod length directly affects engine performance, you are correct displacement stays the same.
-- Long rod will improve top end and also reduce piston and cylinder wear.
--Short rod can improve bottom end performance but causes an increase in piston and cylinder wear

--The long rod will cause the piston to dwell at TDC longer and move away from TDC more slowly, same at BDC.

The relationship between connecting rod length and crankshaft stroke is crucial in engine development.

The stroke controls the piston velocity -- if the crankshaft travels from TDC to BDC in one sec and the stroke is 1 inch the piston must travel 1" per sec. now if the crank travels from TDC to BDC in one sec. and the stroke is 2" the piston must travel at 2" per sec. The time does not change the piston velocity changes.


That's not correct. The longer rod will dwell more at TDC, but less at BDC. Opposite is true for short rod. A short rod will also have a faster peak piston speed than a longer rod (not to be confused with mean piston speed). Shorter rod will have more demand at peak piston.
So who's calling everybody wrong?
__________________[/QUOTE
You don’t say what the difference is you’re talking about. Long rod, short rod, hard to calculate. What difference in length are you talking about?
I haven’t done the calculations on a “really” long rod, but I could. I have done them with a -.040” change in rod length and say no difference in dwell degrees at BDC. Measured to 3 places of precision on the piston travel and .1 degree of crankshaft rotation. Now if I go to 6 place precision, I might see some difference, but nobody has the tools to measure that so it really is academic.
Comments, compliments, criticisms and questions always welcome.

 

[flattop1]

it seems per the jere stahl article . That: Time is measured in Crankshaft Degree's.
Three Cheers ; AL
Good info here though gone of topic a bit.

 

[C&L_MOTORSPORTS]

nice work Al, thanks!!!

 

[supermod]

The longer dwell period is most beneficial at TDC just after a properly timed ignition event, the piston is at TDC longer allowing greater pressure to build while the chamber is at it's smallest size.

Before the crash a poster claimed he used a screwdriver in the spark plug hole to find TDC, you will find you can roll these engines about 3-4 degrees either side of TDC on your degree wheel without the dial indicator indicating piston movement.

I raced supermodifieds and saw some differences in rod ratio in alky V8s. THe 6" rod with a 4" stroke was the best saturday nite combo with 23 deg. heads. When I first started we could run a 430, but that needed bigger heads. Had a friend who had a hi-buck 430 w/6" rods(1.41 to 1) and a 450 small block. You could hear that motor above others and it was a rocket, torque + but needed freshened often( hellacious rod angle 1.2 to 1) . Yes, the rod will dwell ATDC longer with a long rod. And in a limited motor, it may mean that yuou can get more timing and if its cammed right better induction(keeps the air flowing longer) sometimes likes a short exhaust.