longer RODS???

[grumpyvette]

"All other things equal (cam, heads, exhaust), would a 383 Chevy motor built with stock 400 rods (5.56"?) versus 6" rods perform poorer? "

 

in that case the shorter rod will tend to build torque earlier and peak earlier in the rpm range, but designing an engine to take advantage of the longer rod ratio will require that cam,headers,compression and parts be changed to match the rod ratio or theres not a great deal to be gained, the longer rod ratio requires the rest of the combo take advantage of the potential the longer rods, lighter piston combos, lower side loads, and better rod angles,and longer effective pressure above the piston per revolution that the longer rods potentially have, combos must be made from parts designed to match the operating perameters if effective results are expected, generally the shorter rods like a tighter LSA and slightly lower scr read below

 

 

 

 

things to look over and read

 

5.7" vs 6" rod piston possition vs crank degree chart

http://www.iskycams.com/ART/techinfo/ncrank1.pdf

read the articles

http://www.hotrod.com/techarticles/82378/

 

http://www.grapeaperacing.com/GrapeApeRacing/tech/rodslength.cfm

 

http://www.geocities.com/MotorCity/Garage/1020/rods.html

 

http://www.victorylibrary.com/mopar/rod-tech-c.htm

 

http://www.stahlheaders.com/Lit_Rod%20Length.htm

 

http://www.airflowresearch.com/

(articles)

(The 350 Engine, Chevrolet Should Have Built)

 

while the gains youll see are small and mostly noticable above about 5000rpm due to the slight mechanical advantages the longer rods, lighter piston combos, lower side loads, and better rod angles, that the longer rods normally have the ADVANTAGES ARE THERE for the longer rods, and are measurable, look at the piston position vs crank angle chart again, the longer rods move away from TDC slower in relation to the crank angle allowing a longet time spent at the higher pressure part of the cylinder pressure curve, at higher rpms thois results in a very slight advantage in useable torque (useful cylinder pressure) if a matching cam timing and exhaust scavaging header is used to take full advantage of that longer pressure peak as the piston moves away from tdc all the factors must be in place before youll see a noticable advantage to the longer rods, especially the exhaust scavaging and cam timing

[grumpyvette]

heres the software to figure dynamic compression equally

 

http://cochise.uia.net/pkelley2/DynamicCR.html (thanks to pat)

 

lets take these two engine and run them in a software dyno

all cam timing figures Ill use are at .020 lift to keep it simple) in very similar 383 engines with the exact same combo EXCEPT FOR ROD LENGTH,AND THE CAMS LSA KEEPING IN MIND THAT the longer rods and wider LSA will permit a very slight increase in STATIC compression to reach the exact same DYNAMIC COMPRESSION RATIO, and a slight change in header tube length to match the cams and dynamic compression change the longer rods allow

 

first combo with first cam

383

8:1 dynamic cpr Requires a 10:1 scr with 5.7 rods and that cam

pistons need a 3.3 valve dish

afr 210cc heads 74cc chamber

.021 thick head gasket

4.06 gasket bore

piston .023 down the bore

super vic intake 850 holley

1 3/4" tube headers 44" long and 16" long 3"collector

int opens 34cls 66 extop 70 cls 38

 

long rod combo. with wider lsa BUT IDENTICAL DCR

383 with the same cam ground on wider lsa not a 106 like the first engine

8:1 dynamic cpr Requires a 10:5 scr with 6" rods due to the longer LSA spread and rod length and that wider lsa cam to produce the IDENTICAL DYNAMIC COMPRESSION of 8:1

flat top pistons need a 1.7cc dome

afr 210cc heads 74cc chamber

.021 thick head gasket

4.06 gasket bore

piston .023 down the bore

super vic intake 850 holley

1 3/4" tube headers 40" long and 14" long 3" collector

 

heres the change in cam timing to compensate (remember to boost the static compression so the dynamic compression stays the same)

int opens 30 cls 70 ex op 66 cls 42

 

below is the standard cam for the first engine

http://www.cranecams.com/?show=browseParts&lvl=3&prt=5&Vehicle_Type=Auto&Cylinders=8&Engine_Make=CHEVROLET&Year=1978&Engine_Size=262-400%20C.I.&partNumber=110921&partType=camshaft

 

and yes before you point it out both the cams wider LSA and the longer rod has an effect in droping the dynamic cpr,to identical 8.05 requireing the second engines SCR to be raised

but after building several of these in real cars and swapping back and forth between the 114681 and 110921 cams in both 5.7 and 6 inch rod combos I think youll see what I have, that the longer rod engines tend to like the wider lsa and detonate less and accept a smidge more compression like 10:1 vs 10.5:1

notice that the entire torque curve moved slightly higher in the rpm range but the combo of longer rod length and wider LSA allowed the DYNAMIC CPR TO STAY THE SAME

 

heres what my custom software makes its wild guess at

first engine

rpm hp/tq

2000 142/372

3000 244/427

4000 352/462

5000 450/473

6000 496/434

7000 491/368

 

second engine

rpm hp/tq

2000 141/370

3000 242/424

4000 351/460

5000 453/476

6000 507/443

7000 505/378

 

now I DON,T TRUST SOFTWARE EXCEPT TO SHOW BASIC TRENDS and I SERIOUSLY DOUBT the results would be that large but I do feel that there is and has been a slight edge in performance available in matching the longer rods to the wider lsa