2340 long rod or 2540 long rod?

[Doug_Guilds]

Ok so I have been running a 2340 stock rod engine at the tracks I have around me. All are 1/4 mile or tight 1/3 mile. Gearing and tire size are the same at all 3 tracks for my car. I run about 7600-8000 and the car feels good there. I grenaded my engine last weekend and have another I am going to build for next season but am trying to decide wether a 5.7 2500 is worth the trouble of building since I have a large journal block right now. Or would I just be as good running a 5.7 rod 2300.

[84Dave]

Doug..... Regarding rod-length to stroke ratio. A lot depends on the operating rpm band @ the tracks you run. It's a fact that for any given stroke, a longer rod slightly narrows the rpm band between peak torque and peak HP. There is also a very slight mechanical advantage at the rod journal, due to a slightly decreased rod angle, with a longer rod. Additionally, 'dwell time' of the longer rod in approaching/departing TDC is increased slightly, allowing some extended time for torque application and a faster burn rate from increased effective mixture density. HOWEVER..... if your race tracks are fairly narrow in the straights and/or corners regarding the ability to pass, whereby you're forced into a fairly wide operating rpm band, a shorter rod might serve you better. PLUS..... dyno your completed engine and find the peak torque rpm. Then gear the race car so that peak is reached just as you exit the corner under a 'free run' on the track. 'Bout all I know and have experienced on the Subject! -Dave-

[Doug_Guilds]

Thanks for the info Dave. I will definitely consider the advise about the rod length vs the operating rpm of the engine.

[englertracing]

displacement is a much more important operating parameter than rod to stroke ratio
r/s does make a difference
but there are prostock engine builders that will tell you the rod just connects the piston to the crank
they use shorter r/s rations in the pro stocks because the lower deck height allows better optimization of the intake manifold,
so r/s should be way way way down at the bottom of the list

i would take a 2.5l any day of the week
ive dealt with 2.3 vs 2.5 from stock motors in rangers to pro 4, a 2.5 all else the same always wallops a 2.3