why change rear gear ratios?

[grumpyvette]

I got EMAILED asking how and why swaping gear ratios works and why if your using the same engine does it even matter?

 

well, thats a reasonable question if your not up on the basics of how it works so Ill try and explain it in basic terms

first you need to under stand that the rear gear ratio and the transmission work as a combined team to provide the transfer of rotary force from the engine to the pavement.

 

first you need to understand that horsepower is simply a mathematical way to express the RATE at which TORQUE is available and applied

and torque is basically the force in the cylinders thats trannsfered to the crankshaft thru the connecting rods and the leverage in the cranks throw TIMES the NUMBER of POWER STROKES PER SECOND and that the current design of auto engines make thier best power over a narrow rpm range, usually in the 4000-6000rpm range, below that rpm range theres just not quite as effective of cylinder filling and torque is still building, since theres less power strokes per second theres less total torque,and above the engines power band,efficiency falls off rapidly after the 4000rpm-6000rpm zone,(depends on components used)

so the KEY is both KEEPING the engine IN that effective torque production range and having the most power strokes applied to the pavement thru the drive train.

 

example

a 3.07 rear gear allows the engine to spin 3.07 times in direct drive or top gear in a NON-OVERDRIVE type trans

but in first gear,which could have a 2.52 first gear ratio like a TH 350

 

http://www.oldengine.org/unfaq/leadfoot/trans.htm

 

we see it has a 2.52 too 1 ratio in first gear

with that 3.07 rear gear we effectively have a 7.74 :1 ratio so if the tires turn over once the engine has spun over and applied approximately 7.75 times in low gear,swap to a 4.11 rear and youve just increased the number of power strokes applied to 10.35 timesor 34% more torque avalible at the rear wheels, from the same engine

the down side is that the engine runs above its effective rpm range at a lower top speed and gets less efficient mileage

 

http://users.erols.com/dmapes/GEARCLC.HTM

 

 

http://users.erols.com/srweiss/calcmph.htm

 

below is a closely related post/thread

 

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