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I was wondering how much Rod Length really affects an engine.
What is better for what?(torque/horsepower)
What is better on longer track or shorter track?
I am talking about 5.7, 6.0, 6.125 Rods for a SBC motor I know a lot of other factors go into motor too. I know what I think and what I have been told, Just wondering everones elses opinion, and if anyone has any data to back it up. Trying to Figure what I want to build
Thanks in Advance
Longer rods allow for more cyl. fill time at higher rpm, this helps the H.P. curve hold on longer past it's peak. This would be better for restricted carb classes, or where your geared to turn well past the peak h.p. such as on a short track, where you usually turn more rpm then you would on a big track. 6.0" is the most common, and is very versatile.
Rod length analysis NASCAR 358cid
4500 rpm
%
Bore in 4.185 4.185 1.0000
Stroke n 3.250 3.250 1.0000
Rod length in 5.700 6.250 0.9120
mean Piston speed m/s 12.260 12.260 1.0000
max Piston speed m/s 20.250 20.120 0.9936
max flow cfm 367 355 0.9673
reversion flow cfm -61 -80 1.3115
cumulative flow cfm 16726 16727 1.0001
Piston side force int N 1066 947 0.8884
Piston side force combined N 4541 4147 0.9132
9000 rpm
%
Bore in 4.185 4.185 1.0000
Stroke n 3.250 3.250 1.0000
Rod length in 5.700 6.250 0.9120
mean Piston speed m/s 24.510 24.510 1.0000
max Piston speed m/s 40.500 40.240 0.9936
max flow cfm 715 709 0.9916
reversion flow cfm -123 -159 1.2927
cumulative flow cfm 33704 34012 1.0091
Piston side force int N 4264 3788 0.8884
Piston side force combined N 5745 5090 0.8860
Look at the CFM #'s
i dont know about all those numbers and i dont think they mean much to most people what i do know is if you build a 6in rod motor you will never want a 5.7 motor again very big differance especialy in a limited class where you have flat top pistons and low lift cams i have never ran anything longer would like to see if makes as much differance as going from 5.7 to 6in
seems like guys that swear by shorter rods for better torque also are running cams to big for their application. makes you wonder what the real problem is.
do you have more specifics on low lift .we must run 410 lift limited 10.5 compression
the low lift i was refering to is 480 lift we have a couple classes that have that rule around here the other low lift classes are 425-450 lift but they have rules that say 5.7 rods so cant try it unless your cheating so i cant help with that, i do know it free's up the engine some, easyer on pistons because it changes angle at which the rod pushes the piston and they are lighter pistons also thats a plus also
if you built two 400s one 5.7 and one 6" with all the same parts other than rods and pistons, i bet you would never be able to tell the difference, hell i bet a dyno couldn't tell the difference
Right on Bush.
K.I.S.S.
Meaning, stay with a worked out combo, unless you have a lot of time/money to kill.
1. Pick the biggest bore size your rules allow.
2. Get the stroke to make the CI's you want.
3. Get a piston made with a good CH (around 1.200). Too long (1.550), makes a heavy piston, too short (1.000), and the piston is "pivoting" off the ring pack, hurting ring seal and ring life.
4. Buy a rod that will fit the parts above.
Or........buy the rods first, because they were a deal on E-bay.
Wind up with some too long, which the affects are explained above, or too short, and spend major bucks adding heavy metal to the crank because the counterweights had to be shortended to clear the pistons. TB
Oh yea, rods that are too long make the cam selection difficult. Matter of fact, call 5 cam guys, and you'll get 5 different answers on the cam for a long rod engine.TB
Very seldom any reason to run any sbc with a rod over 6.0. No power is gained. 5.7 versus 6.0 the difference is minimal at best. Cam selection plays the biggest part in the equation.
i would never opt for a long rod because i thought it made more hp. for me the benefits are a lighter piston and less side load on bores.
http://www.stahlheaders.com/Lit_Rod%20Length.htm
build the engine to suit your needs.
pick a piston, pick a bore, pick a crank..
then pick the rod that connects them..
i've found just about every "untouchable" motor around here is 6.200"
This is just my opinion and from the experience I have had over the last 23 years......
I've built engines for others and driven hobbies to late models with everything from 5.585 to 6.3 length rods.....and I have found the 6" rod works about the best. Never really thought there was any HP difference with any combination. More of a driver choice really. Seems like the 6" rod was much more drivable and is easier on the engine because it changes the angle of how everything reacts in the cylinder. Too long of a rod makes the engine lazy and too short tends to shove them right out the side of the block.
However once I found something that worked, customers liked it, durability was good, and guys were winning races/getting top 3's....I never ventured away from it. The only thing that changes is the camshaft because every driver has a different feel for his car and where he wants the power at on the track.