I was looking at intake runner calculations and considering the following equation.

L = ((ECD × 0.25 × V × 2) ÷ (rpm × RV)) - ½D

ECD = Effective Cam Duration (720 - (Adv. Duration - 20*))
V = Speed Of Sound
RV = Reflective Value (going with 2)
D = Diamter of Runner
RPM = Desired RPM to peak torque

So I noticed that as air temperature cools down the speed of sound drops. At 104*F your at 1167 ft/s and at 158*F your 1227 ft/s, I'm assuming this is at sea level, but the relationship should stay consistant.


So as an example I did

(465 x 0.25 x 1227 x 2) ÷ (8000 x 2) - 0.5(2)

This gives me 16.82" total runner length.

If all else stays equal, but we lower the speed of sound to 1167 ft/s, then that same runner length is now equal to peak at 7600 RPM. So am I understanding that right? An engine will create peak torque lower on a colder day? Not necassarily lower overall numbers because the cooler denser air and thus can make more torque, but just peak effeciency will happen at a lower RPM?

 

What you are talking about is called accoustic offset and yes it is a real phenomena.

 

whoosh (right over my head )

well the only thing i dont get is the relfective value