Quote:
Originally Posted by DavidBoren
Also, I am getting conflicting information regarding possible knock issues.
Everything I read, up to this point, says that the long rods inherent to a long stroke will slow the piston near top dead center. This will provide the increased dwell time at TDC, as you stated.
But I read that decreases the risk of detonation, so I would appreciate some clarification, please.
Does the piston slowing down near TDC simply increase the amount of time it is possible for detonation to occur? Or does something about the piston slowing down actually cause or invite detonation to occur?
My understanding of it, from what I have read, and how it was explained to me, is that the reduced rate of compression decreases the likelihood of detonation. Am I wrong?
I can see detonation issues being inherent to the small bore size, as we discussed earlier... Less surface area to dissipate heat into the cooling system. More latent heat in the engine raises the risk of detonation.
It could be argued that I will see no benefits of what long conrods can provide because my 4" stroke gives me a 1.53:1 rod ratio, which isn't great. So my rod angles are still relatively severe, so I probably won't see any decrease in piston speed near top dead center. If anything, it may act like a short rod motor and increase in speed at TDC.
If it has more to do with the rod ratio than the actual length of the rod, then I probably won't see any increase in dwell at top dead center. And therefore, I probably won't have the detonation issues you warn of.
However, I am still in need of some clarification on this.
Does increasing dwell CAUSE detonation, or simply increase the amount of time it may occur?
When discussing piston speed, does the actual length of the rod matter? Or is it all about the ratio in comparison to the stroke? The change in rod angle determines the relative change in speed, and the ratio is constant in the sense that any length of rod with the same ratio will change the same relative angle, right? So any length of rod with a 1.53:1 ratio will give you the same relative change in speed near top dead center... Correct?
If that's the case, then my stroker is actually more of a short rod motor, because 1.53:1 is kind of low.
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There's trade off between lower thermal efficiency/increased quench and piston speed decrease. Less heat dissipation leaves room for detonation. Piston speed decrease theoretically dispells detonation. And there's theorectically power loss from increased quench (remember our cast iron head bit?)
I don't really think it's a big deal. If you stay Fuel Injected, that's very very tunable. I have timing control for Cylinder Mass every .02 g/cyl or something crazy like that.
Length of Rod and Length of Crank matter for piston speed. But keep in mind, your highest piston velocity is going to be with the crank and rod are at a 90* angle.
You are correct in saying rod ratios correspond to piston speed.
None of it is terribly important. Good tuning can get rid of pretty much all detonation in Fuel Injection. Knock sensors pretty much take care of pitting pistons/breaking ring lands nowadays.
some good reading for you
http://www.contactmagazine.com/Issue...ineBasics.html