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=brow...rtType=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

 

[SPANISHVETTS]

I have always been taught that a longer rod to stroke ratio is better but when you stroke a SBC with its 9¨deck height it is just not possible. On SBC´s I always go for the longest rod possible.

 

[maxrevs85]

the Longer rods help with rod angle and lessen the strain on the rotating parts...when ever you stroke a 350 for instance, to 383 it is better to use the longer rods(6")....so I was told by pro engine builders.

 

[jenavet]

ive go 2 motors on opposite ends of that spectrum.......ive got a 400 which is .30=406, with a 6 inch eagle rod....and my spare is a 350 with a destroked 3.300 crank with the short aluminum rods with 4.060 bore which makes it a 342.....the 406 uses a flat top piston,60cc head with just the bowls blended,about 12.2-1 compression,a brodix hv1000 intake and 820cfm carb...i played around with camshafts and talked about that in an earlier post how the first cam i had in made more horsepower but less torque than the 2nd cam i put in...but the car went almost 3 tenths faster with the second cam....that really amazed me how a motor would go faster in a drag race with less horsepower but more torque...the cam i settled on is .655 in and .631 ex seat to seat....271 in and .278 ex at .50.....in opens at 29,closes at 61......exhaust opens at 69 and closes at 29.......at a 108.0 centerline which i had them advance 4 degrees.........i think that 6 inch rod 400 is a real nice combination,makes alot of torque......the 342 i have is real high rpm which means high maintenance.......and get this.....the 342 makes 60 more horse than my 406 but 30 less foot pounds of torque.

 

[grumpyvette]

"peak horsepower numbers may be impressive"
but its the average horspower available to you, that the useful torque curve within the rpm band your gearing allows thats important to moving the car!
youll get the best results by building the max average torque at the highest average possiable rpm that the gearing in your car can use effectively it makes zero sence to build an engine that produces 600hp at 6000rpm if your shifting at 4700rpm and your average rpm range is restricted in the 2500-5000rpm range

heres an old post that fits here

we have all heard it, " you need massive low rpm tq" "you need a screaming high rpm hp peak" well heres some info,
More in-depth description:
http://www.revsearch.com/dynamometer/torque_vs_horsepower.html
http://www.dynacam.com/Product/Torque_vs__Horsepower/torque_vs__horsepower.html
http://vettenet.org/torquehp.html
http://auto.howstuffworks.com/horsepower4.htm
http://homepage.mac.com/dgiessel/engine/hpvstq.html
first thing to keep in mind is that theres no such "thing" as horsepower, horsepower is a mathmatical formula for the RATE at which TORQUE can be applied the formula for hp is (tq x rpm/5252=hp
example
450 ft lbs of torque at 3000rpm=257hp
450 ft lbs of torque at 6000rpm=514hp
because the torque at the higher rpm useing gearing can be applied faster
here read this
http://www.69mustang.com/hp_torque.htm
http://www.ubermensch.org/Cars/Technical/hp-tq/
http://vette.ohioracing.com/hp.html
where most guys go wrong is in not correctly matching the cars stall speed and gearing to the cars tq curve, if you mod the engine for increased high rpm performance but fail to also match the stall speed and gearing to that higher rpm tq curve much of the potential improvement is wasted.
example

in the close to stock engine above, the engine should be geared to stay in the 3500rpm-5000rpm range for max acceleration (lower in the rpm range if mileage is a big factor)

in the moded engine above the rpm range moved to 4000rpm-6500rpm requireing differant rear gears and slightly higher stall speeds to gain max acceleration in the same car,
you should readily see that a trans that shifts at 5000rpm will work in the first example but would waste most of the power curve in the second example,where shifting at 6500rpm under full power acelleration would make more sence.
a 3.08 rear gear and 700r4 trans matches the first example well but it would take a swap to a 3.73-4.11 gear to allow the engine in the second example to keep its most effective power band matching that second power curve well.
links youll need to figure out correct rear gear ratios
http://www.miata.net/garage/tirecalc.html
http://www.wallaceracing.com/reargear.htm
http://users.erols.com/srweiss/calcmph.htm
http://users.erols.com/srweiss/calcrpm.htm
http://users.erols.com/srweiss/calcrgr.htm
http://www.prestage.com/Car+Math/Ge...io/default.aspx
http://www.geocities.com/z28esser/speed.html
http://server3003.freeyellow.com/gparts/speedo.htm
http://www.pontiacracing.net/trannyratios.htm
http://www.tciauto.com/tech_info/gear_ratios.htm