calculate gear ratios, and when to shift calcs



calculate gear ratios, and when to shift calcs

Postby grumpyvette » October 19th, 2008, 4:39 pm

you really should read all the links and sub-links below to maximize your understanding of the concepts, in the thread

if we assume your looking at maximizing a performance cars acceleration while maintaining a reasonable cruise rpm for mileage, and you have a tire diam range of between 26-29" tall as, most muscle cars have, you can just find the ratio that multiplied times your transmissions first gear ratio falls in the 10:1-10.25:1 range and you'll find it works out rather well, especially with the OVER DRIVE TOP GEAR TRANSMISSIONS
if you don,t have an over drive transmission most guys just select the gear ratio that when matched to their tire diam. that will have the engine rpm range fall in the 2400rpm-2600rpm range at 70mph
Gear comparisons for 700R4 vs. other GM Transmissions:
.............................1st.......2nd....3rd....4th
Power Glide..............1.76......1.0
TH350 .....................2.52.....1.52....1.0
TH400......................2.48.....1.48....1.0
700-R4/4L60..............3.06.....1.63....1.0.....70
200-R4.....................2.74.....1.57....1.0.....67
4L80E......................2.48.....1.48....1.0.....75

EXAMPLE
http://www.vibratesoftware.com/

http://www.5speeds.com/ratios.html

http://purplesagetradingpost.com/sumner ... index.html

http://www.jegs.com/p/Richmond-Gear/Ric ... 3/10002/-1

http://tire-size-conversion.com/tire-height-calculator/

http://www.summitracing.com/expertadvic ... calculator

http://www.offroaders.com/info/tech-cor ... ph-rpm.htm

http://www.prosystemsracing.com/calculate.html

http://www.fatboyraceworks.com/gears/

http://www.wallaceracing.com/RPM%20Drop ... ulator.php

http://www.wallaceracing.com/gear-speed.php

http://www.wallaceracing.com/gearcalc.php

http://robrobinette.com/et.htm

http://tire-size-conversion.com/tire-size-chart/
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you should ideally select a cam , tire diam. converter stall speed and differential gearing that put the engine rpm range in your power curve about 90% of the time
if your first gear ratio in your 4l80e transmission were to be a 2.48:1 you divide 10.25/2.48 and you find the closest ratio is a 4.11:1, if your more concerned with mileage than brute acceleration ,you should drop back to a 3.73:1 or a compromise 3.90:1 ratio
you get the advantage of reasonably low gears and highway drive ability if you install the correct over drive transmission, and reasonably low rear gearing, everything's a compromise but its easily possible to get a decent combo, lets say you want want to cruise at 2500-2800rpm and a top speed in excess of 120mph at a reasonable 6000rpm and a good launch with torque, and you multiplication that low gears provide, your running lets assume a 24"-26" tire

you want the first gear ratio times the rear gear ratio to fall in the 10:1-11:1 range

Image
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lets assume your dealing with a th400 and 2.73 rear gears you fall into the 2.48/2.73=6.77:1 range killing fast acceleration
swap to a 5.13 rear gear with that 2.48 th400 trans gear and you fall in the 12.73 range killing cruising speed, as well as top speed, which would be around 80mph in top gear.
Image

but lets swap that 5.13 rear gear into a car with a 4l80e TRANSMISSION with its overdrive gearing, its 0.75 brings you a top speed of near 117mph , and a launch ratio of 12.7:1, not bad with big sticky slicks on a light weight car, but not ideal on the street

now lets try to get both a great top speed and a great launch plus a decent cruise rpm,
with a 26" tire

(obviously youll need to measure and calculate your particular needs)
we start with the 4l80e trans first gear divided into 10:1-11:1 and find a 4.11:1-4.33:1 ratio is a great choice, a 4.11:1-4.33:1 ratio gives you a 140mph-150mph theoretical top speed, and a cruise speed near 65mph, a 3.73-3.90:1 rear gear would probably be better on a car used mostly for cruising as it will drop the cruise rpm several hundred rpm
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trans gearing

http://vexer.com/automotive-tools/speed-rpm-calculator

http://www.vibratesoftware.com/

http://www.stl-vettes.com/65Vette/Trans ... earing.htm

http://purplesagetradingpost.com/sumner ... index.html

http://purplesagetradingpost.com/sumner ... index.html

http://www.drivetrain.com/autotranscrossref.html

http://www.bgsoflex.com/rpmmph.html

http://700r4.com/speedoCalc/rpmgearscalc.shtml

http://www.chevyhiperformance.com/techa ... index.html

http://www.tciauto.com/tc/trans-dim

http://www.tciauto.com/tc/gear-ratios

impalla65ss posted this link below


This is pretty extensive. Everyone will learn a lot from it.
get this excel-file downloaded and play with it <<<<<CLICK ON LINK

youll find what you want here


https://www.google.com/search?q=how%2Bt ... ar%2Bratio

http://www.btc-bci.com/~billben/reargear.htm

http://www.thirdgen.org/calculations

http://www.geocities.com/z_design_studi ... zx_tt.html

http://www.datsuns.com/Tech/whentoshift.htm

FIRST, THANK YOU IFATGMC!!!
THATS VERY USEFUL, IF YOU DON,T MIND ILL MAKE THIS A STICKY :grin:

http://purplesagetradingpost.com/sumner/bvillecar/bville-spreadsheet-index.html

HERES OTHER INFO LINKS

http://www.xracingsports.com/tirecalc.html

http://www.secondstrike.com/Technical/GearCalc.asp

http://www.5speedtransmissions.com/calculators.html

http://www.geocities.com/z_design_studi ... zx_tt.html

http://www.datsuns.com/Tech/whentoshift.htm

http://www.corvetteactioncenter.com/tec ... scalc.html

http://www.wallaceracing.com/reargear.htm

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

http://www.tciauto.com/Products/TechInf ... tors.asp#8

http://www.grimmjeeper.com/gears.html

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

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

http://users.erols.com/srweiss/transc.htm#tabtop

http://users.erols.com/srweiss/transc.htm#Auto

http://www.gmtuners.com/gmtransinfo.htm

if your looking for a 3.73-4.11 rear gear for your corvette,and can,t find a 4.11 gear,the reason is that the highest COMMON gear ratio is 3.73:1 in that differential for that C4 corvette, but heres a link, btw you do realize that 4.09, 4.10, 4.11:1 rear gears are all the same exact SAME ratio and the difference is in how they were calculated

the RATIO IS CALCULATED BY COUNTING THE NUMBER gear teeth on the RING GEAR are divided by the NUMBER OF TEETH ON THE PINION GEAR

Image

btw, for anyone reading thru this thread,
if you've got a transmission without an over drive top gear ratio like a th400, th350, etc I would consider ,STRONGLY CONSIDER something in the 3.07-3.31 rear gear ratio range, if you have an over drive top gear, like a 700r4, 4l80e, 200r4 the same criteria apply but youll find , a 3.54.-3.90:1 rear gear as a good choice
remember this is for effectively gearing a mixed performance/transportation street/strip car, combo and
Id suggest gearing the car so your drive train gearing falls in a range where the transmissions FIRST GEAR x REAR GEAR ratio fall close to being in the 10:1-to-11:1 range and if your engines spinning at 6000rpm in top o.d. gear places the car speed no higher than 135 mph, the closer you can get the gearing to fall in that gearing range the more responsive most cars will be,get the rear gear ratio low enough that the rear gear ratio times first gear is over 11:1 and your first two gears become far less useful without major suspension mods and SLICKS, and truthfully, how many times do you need to exceed 135mph?

http://www.datsuns.com/Tech/whentoshift.htm

http://www.welltall.com/ymc/discovery/car/shiftpt.html

http://www.bgsoflex.com/shifter.html

http://autospeed.com/cms/title_Finding- ... ticle.html

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IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: calculate gear ratios

Postby grumpyvette » October 31st, 2008, 1:11 pm

BLACKWIDOW POSTED THIS INFO, and hes made a few good points
First off there are two things you need to find the optimal shift point.

1. a dyno plot of the torque curve your engine puts out
2. all the gear ratios you will be shifting through
3.(optional) is the differential gear, the reason this is optional is that the gear never changes which makes it a constant that does not effect where you need to shift. But i will show you why it is fun to have.:twisted:

All the math I give you is pretty simple. It is just that the process is lengthy.


Finding true torque.
The equation for this is
Flywheel torque x gear ratio you are in.
for example if you are making 150 lbs of torque at the flywheel and have a first gear of 3.549, and second gear of 2.197.

150 x 3.549=532.35 You are making 532.35 lbs in first.
150 x 2.197=329.55 You are making 329.55 lbs in second.

As you see here as your gear ratio drops you don't make as much torque. Which is why your car doesn't pull as hard in 2nd as it does in 1st, and so on all the way through the gear box.
Now for some real fun. These numbers are to our theoretical drive shaft. Now lets take into account the optional differential gear. And put in 20% drivetrain losses. For ease i'll use the numbers from above with a diff. gear of 3.9:1.

(flywheel torque)x(gear ratio you are in)x(final drive ratio)x(.80)
150 x 3.549 x 3.90 x .80= 1660.93 lbs at the wheels :shock:

Now if you didn't know before this is what you are really seeing at the rear wheels. Now the next time your friends ask what your making at the rear wheels, tell them the real number.

For reference your dyno plot to the wheels already has the drive train loss in so use the same equation but without the drive train loss on the end.


Finding RPM drop when shifting
The equation for finding RPM drop is

gear your shifting into
---------------------- x RPM before shift = RPM after shift
gear your shifting from

If your shifting from 1st(3.54) to second(2.197) at 7000 rpm it would be

2.197
------- x 7000= 4333.33 rpm
3.549

So you would be at 4333.33 RPM after the shift. Pretty easy stuff.
http://www.grumpysperformance.com/gearratiochart.gif

Making the graph and finding the best shift
Now lets take our math we have learned so far and put it to use. Take the formulas i have given you and make a chart so that you can see shifting points and
torque numbers for the corresponding shift(ie. first to second). now i am going to use MONZTER's dyno plot from the N/A engine setups with dynos and the gear
ratio's from a factory 240z 4 speed as my examples. You can find the dyno plot here in this post
http://forums.hybridz.org/showthread.php?t=139485 , and the gear ratios here http://www.geocities.com/z_design_studio/transmission.html

Image

Start at your red line and go down 1200 rpm in 200 rpm intervals, charting your rpm start and drop, and your torque start and drop side by side. Along with the drop in torque from the shift.

First column is 1st gear rpm
Second column is 1st gear torque at rpm
Third column is torque diffrence before the shift
Fourth column is 2nd gear torque at rpm
Fifth column is 2nd gear rpm after the shift
Sixth column is torque loss after the shift
Seventh column is torque diffrence after the shift

7200..... 514.61..... -107... 369.10..... 4456 ....-145.51 .....+157
7000..... 550.10..... -71..... 336.14..... 4333 ....-213.96 .....+89
6800..... 578.49..... -43 .....329.55..... 4209 ....-248.94 .....+54
6600..... 596.23..... -25 .....318.57..... 4085 ....-277.43 .....+25
6400..... 603.33..... -18 .....318.57..... 3962 ....-284.76 .....+18
6200..... 610.43..... -11 .....307.58..... 3838 ....-302.85 .......0
6000..... 621.08........ 0 .....318.57..... 3714 ....-302.43 .......0


So now that we have our first to second shift chart lets find that perfect shift point. The key to finding the best place to shift is to find the point at
which you lose the least amount of torque during the shift without losing to much forward momentum before the shift. I do this by comparing the torque diffrence before the shift( Which goes with your forward momentum) and the torque diffrence after the shift.On this particular setup I think he should shift right around 7000-7100 RPM, if he had the gearing I used here. While there are less torque losses after the shift above this rpm, you have to much torque drop off before the shift, which causes a loss of the forward momentum you were building. Once you find what you think is the optimal shift point vary it up and down about 400 RPM to find what suits you best. Oddly enough most drag racers have a rule of thumb that the best shift point is 10% beyond peak horsepower. Which if you look at his graph is is right around 7100 RPM. But you should take this as a coincidence and do the math yourself.On his setup he said his redline is 8000 RPM but he needs to shift well before that. What this shows is just because you can rev to 8000 doesn't mean you should shift there.

Now take this info and do every gear shift in your setup. Plus don't forget that since the ratios change from gear to gear the best place to shift in one gear might be different than where to shift in the next. But it should be pretty close.

After I did this I found this was went over on someons page but I don't think it elaborated as much, but just for more reading you can find it here http://www.datsuns.com/Tech/whentoshift.htm . I hope it helps some people out.
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
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Re: calculate gear ratios

Postby grumpyvette » November 24th, 2008, 7:28 pm

btw I got asked
"how do you figure out the gear ratio of a ring and pinion set if its not stamped with the ratio"

, WELL,I GUESS THATS A VALID QUESTION,
if youve never dissembled a rear differential,
you find the ratio,by counting the pinion and ring gear teeth and divide the pinion count into the ring gear count?
http://tire-size-conversion.com/tire-height-calculator/

http://www.miata.net/garage/tirecalc.html
EXAMPLE
If the ring gear has 37 teeth and the pinion has 9 thats a 4.11 set...
OR If the ring gear has 41 teeth and the pinion has 11 thats a 3.73 set

beats guess work

Image

SOME GUYS STAY WITH THE STOCK 3.07-2.57 REAR GEARS to minimize the tire spin, and at lower power levels that works to prevent the tire spin that a 3.73-4.11:1 rear gear ratio would produce due to the extra torque multiplication,but its not that effective once youve got the necessary power to blow the traction at will,
OBVIOUSLY KNOWING YOUR TIRE HEIGHT HELPS IN CALCULATIONS
Image
the PROBLEM with that approach is NOT the traction, or lack of it in most cases,but the fact that your engine spends very little time in the engines most effective section of the torque curve , theres not much sense in building a killer combo with a cam and heads that flow impressive numbers and allow impressive hp in the 5000rpm-6500-7000rpm if your spending 70% of your time below 5500rpm and the transmission shifts at 5500rpm, due to the rear gearing and transmission shift point.
Its rather pointless to have a great deal more hp than you can effectively transfer to the pavement, but gearing the car so you can,t get into the most effective part of the power curve much of the time is hardly the best option.

play with the info in the links posted above and youll soon see the distance traveled and mph prevent a car with a 2.57 rear gear from spending the majority of its time in the efficient part of the tq curve.
yes your likely to need slicks and suspension mods but the car will be far more effective if your using the full potential of the power curve.
yes your going to need slicks and suspension mods to get the necessary traction.

quick posi check =put the car up on jack stands then, turn one axle and watch the other axle. If they both spin the same direction, its a posi. if the other axle spins the opposite direction its an open diff.



it may help if you think of it this way
if your engine puts out 400ft lbs of torque
a 2.57:1 rear gear ratio puts about 1000 ft lbs of twisting power to the axles
a 4.11 rear gear ratio puts about 1600 ft lbs of twisting power to the axles

if your car weights 3500lbs
youve got 29 percent of the cars weight applied in rotational force with the 2.57 ratio
youve got 46 percent of the cars weight applied in rotational force with the 4.11 ratio

its about mechanical leverage, or efficiency
assuming a 26" tire height and zero mechanical losses, the tires need to turn about 200 revolutions to cross a 1/4 mile
a 2.57 rear gear lets the engine spin thru about 2060 power strokes
a 4.11 rear gear lets the engine spin thru about 3290 power strokes
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: calculate gear ratios, and when to shift calcs

Postby grumpyvette » October 30th, 2010, 4:57 pm

IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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Re: calculate gear ratios, and when to shift calcs

Postby grumpyvette » September 23rd, 2014, 9:22 am

Crusty66 wrote:I’m looking for a stall speed recommendation for my ’66 Caprice.

Here are the specs
496ci – 454 with 4.25 stroker
9.5:1 compression
Oval port Edelbrock E-Street

Comp 276HR hydraulic roller;
Int 224, Ex 230 @ .050”
Lobe separation 110, centreline 106
Lift .510”

Edelbrock Performer intake with Quadrajet 800cfm
2” full length headers
Rear end ratio is 2.73 with 26” tall tires.
TH400

Maximum speed limit is 60mph in my country, trans guy supplied me with a 2,200 rpm stall speed converter which I think is too high & probably unnecessary with the torque this 496 should have.



I think you have the wrong Idea about how a 2200 rpm stall speed converter WORKS, that 2200 rpm stall speed seems about ideal to me,for the combo you list in the car you list above, and Ive built dozens of similar 454-496 engines.
many guys who have never had a different aftermarket stall speed converter are under the mistaken idea that the car won,t move until the engine rpms reach , the rated stall speed such as the 2200rpm,you mentioned, that ideas incorrect!
the car will drive and move at well under the rated stall speed, the stall speed just indicates what rpm the engine will jump too under wide open throttle if you stand on BOTH the brake and throttle petals so both are fully down under your foot towards the firewall floor, the FUNCTIONAL POWER RANGE of a higher stall speed allows you to jump into your more effective power band under high load and wide open throttle conditions but something like a 2200rpm stall converter on a 496 BBC will have little too no negative effect on daily driving, at part throttle

viewtopic.php?f=71&t=1715

viewtopic.php?f=71&t=741


Crusty66 wrote:Hi Grumpy,

I do understand torque converter operation, my only concern was that the stall speed was very close to cruising rpm where the usual cruising speed is 55-60 mph with the very tall 2.73 ratio.

I know I might be better off with moving to a 3.08, but I'm running short on funds right now.


I know that "SHORT ON FUNDS DEAL ALL TOO WELL"
but I also know most pick & Pull salvage yards frequently sell complete differentials for full size older cars for $50-$150 for older chevys, so I'd at least look into the possibility of finding a similar car in a salvage yard with a different different ratio gears and swapping differentials, ideally youll find a 3.54:1 or 3.54:1 rear gear differential

If you had more funds Id suggest upgrading the differential to a FORD 9", (ideally with disc brakes) from a later full size ford or pick up truck

viewtopic.php?f=71&t=1261&p=10180&hilit=differential+to+a+FORD#p10180

viewtopic.php?f=39&t=7&p=10#p10

http://www.oldride.com/library/1966_che ... price.html

http://www.crankshaftcoalition.com/wiki ... asurements
IF YOU CAN,T SMOKE THE TIRES AT WILL,FROM A 60 MPH ROLLING START YOUR ENGINE NEEDS MORE WORK!!"!
IF YOU CAN , YOU NEED BETTER TIRES AND YOUR SUSPENSION NEEDS MORE WORK!!
grumpyvette

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