mopops
Well-Known Member
i have 294 rear gears in my mirada and am trying to figure out engine rpms for the different ratios say for example 294 rears at 2500 rpms compaired to 355 rears at 2500rpms just cant seem to find info tks
engine speed per rear gear ratios
- Thread starter mopops
- Start date
MiradaMegacab
Well-Known Member
Last edited:
mopops
Well-Known Member
thank you for the info,now ill figure out what ratio i want to go to
SixBanger
Well-Known Member
Maybe you could play around with this calculators.
Not directly a gearcalculator. But some start point with car characteristics spec to acceleration or max speed simulator.
Acceleration simulator Vehicle aerodynamics. I like the way they show the formulas for the calculators and possibility for some more advance input.
Not directly a gearcalculator. But some start point with car characteristics spec to acceleration or max speed simulator.
Acceleration simulator Vehicle aerodynamics. I like the way they show the formulas for the calculators and possibility for some more advance input.
Last edited:
AJ/FormS
Well-Known Member
- Joined
- Apr 26, 2016
- Messages
- 1,291
- Reaction score
- 305
MiradaMegacab has your numbers. and here's how he got them;
mph={rpm (TC)}/1056xR1xR2 where
TC is tire circumference or properly tire rollout,In your case; 28xPi=88inches; and
R1 is the rear gear, and R2 is the trans final drive ratio; usually 1.0
If you have an automatic without a loc-up convertor, you can factor in some slip; From about 5 to 8% at WOT after the tires quit spinning, to 2 or 3% while cruising.
But in your case you can take a shortcut cuz same tires, same cruise speed, same transmission. And it eliminates the slip-factor.
In this case; New ratio/Old ratio x Old rpm@ chosen speed gets you the new rpm at that chosen speed.
Example 3.55/2.94 times 2500rpm =3019rpm with 3.55s
If 2500rpm was 71 mph, with 2.94s, then it will be about 3019@71with 3.55s. This works for any rpm.
You can apply these numbers to speed just by flipping the gears; example 2.94/3.55times 71 mph=59@2500 with the new 3.55 gears
It also works with any gear; example
4.10/2.94x2500=3486rpm at 71mph, and
2.94/4.1x71@2500=51mphwith 4.10s
and the proof is;
mph={rpm (TC)}/1056xR1xR2 From above
mph=(2500x88)/(1056x4.1x1.0)=50.813mph; 88s being 28" tires
In case yur wondering; 1056 is a constant to convert the measurements to commonality.
Happy calculating.
mph={rpm (TC)}/1056xR1xR2 where
TC is tire circumference or properly tire rollout,In your case; 28xPi=88inches; and
R1 is the rear gear, and R2 is the trans final drive ratio; usually 1.0
If you have an automatic without a loc-up convertor, you can factor in some slip; From about 5 to 8% at WOT after the tires quit spinning, to 2 or 3% while cruising.
But in your case you can take a shortcut cuz same tires, same cruise speed, same transmission. And it eliminates the slip-factor.
In this case; New ratio/Old ratio x Old rpm@ chosen speed gets you the new rpm at that chosen speed.
Example 3.55/2.94 times 2500rpm =3019rpm with 3.55s
If 2500rpm was 71 mph, with 2.94s, then it will be about 3019@71with 3.55s. This works for any rpm.
You can apply these numbers to speed just by flipping the gears; example 2.94/3.55times 71 mph=59@2500 with the new 3.55 gears
It also works with any gear; example
4.10/2.94x2500=3486rpm at 71mph, and
2.94/4.1x71@2500=51mphwith 4.10s
and the proof is;
mph={rpm (TC)}/1056xR1xR2 From above
mph=(2500x88)/(1056x4.1x1.0)=50.813mph; 88s being 28" tires
In case yur wondering; 1056 is a constant to convert the measurements to commonality.
Happy calculating.
Last edited:
mopops
Well-Known Member
thank you all for the input looks like 373s for me
AJ/FormS
Well-Known Member
- Joined
- Apr 26, 2016
- Messages
- 1,291
- Reaction score
- 305
3.73s can be a very fun gear for a city car
To get your numbers of 2500 with 2.94s, in a Mirada with something like 225/70-15s (which are ~27.4 tall and might roll out at 86inches),would get you 69mph with a loc-up TC or 67 at 3% slip.
If you have a regular 727 in there with ratios of 2.45-1.45-1.00, then 2500 in first gear should be about 27 mph @5% slip. So hyup I can see that being a lil sluggish, if she's a slanty or a tired low-C 318. A 360 might only need a lil more stall.
But with 3.73s, 2500@3% slip should get you ~21.6 mph in first gear;a 26.9% improvement. If a slanty, she's still gonna be sluggish, and a 318 will still want more stall.
But the ol' 360 will be a lot less grumpy.
For me. I like to nail it at 30mph, and get tirespin to intimidateold ladies in the K-cars, in the next lane,lol. For you with 3.73s, 30mph should be ~3500 rpm, about peak torque with a 360, quite a bit past peak torque with a 318, and almost time to shift with a slanty.
For most stock low-C Mopar engines of this vintage, when they hit second gear, they go kindof numb on account of the huge drop in rpm at the 1-2 shift, namely to 59%. So the bigger the engine you have, the better to pull second gear with.
3.73s will get you ~72 mph @5000 in second, and 65=2980@zeroslip/3040@2% slip.
all numbers are for an 86" roll-out which I guessed at.
To get your numbers of 2500 with 2.94s, in a Mirada with something like 225/70-15s (which are ~27.4 tall and might roll out at 86inches),would get you 69mph with a loc-up TC or 67 at 3% slip.
If you have a regular 727 in there with ratios of 2.45-1.45-1.00, then 2500 in first gear should be about 27 mph @5% slip. So hyup I can see that being a lil sluggish, if she's a slanty or a tired low-C 318. A 360 might only need a lil more stall.
But with 3.73s, 2500@3% slip should get you ~21.6 mph in first gear;a 26.9% improvement. If a slanty, she's still gonna be sluggish, and a 318 will still want more stall.
But the ol' 360 will be a lot less grumpy.
For me. I like to nail it at 30mph, and get tirespin to intimidateold ladies in the K-cars, in the next lane,lol. For you with 3.73s, 30mph should be ~3500 rpm, about peak torque with a 360, quite a bit past peak torque with a 318, and almost time to shift with a slanty.
For most stock low-C Mopar engines of this vintage, when they hit second gear, they go kindof numb on account of the huge drop in rpm at the 1-2 shift, namely to 59%. So the bigger the engine you have, the better to pull second gear with.
3.73s will get you ~72 mph @5000 in second, and 65=2980@zeroslip/3040@2% slip.
all numbers are for an 86" roll-out which I guessed at.
mopops
Well-Known Member
ok i have an 89 360 with roller performance RV cam 28" tires 904 trans this motor is about 8.5 to 1 stock stall converter so it doggie iam not going back into the motor to make changes( cant do that work anymore) so a new rear n ratio change is were iam going car will be around town. i just don't want to kill the motor with rpms but would like to get a little more out the hole math was not a good subject 4 me tks
225/70R15 is 27.5" tall
Gears for a 1981+ A904 are:
2.74 / 1.54 / 1.00
Gear Ratio Calculator
Plug that info in to the link above and hit the "Calculate Speed" button. Then play with different rear ratios. If your tire is different than above you can plug those numbers in too the lower section to fix that too. (Width/Ratio/wheel diameter)
If you are using an older pre-1981 A904 the gears are like a A727:
2.45 / 1.45/ 1.00
Gears for a 1981+ A904 are:
2.74 / 1.54 / 1.00
Gear Ratio Calculator
Plug that info in to the link above and hit the "Calculate Speed" button. Then play with different rear ratios. If your tire is different than above you can plug those numbers in too the lower section to fix that too. (Width/Ratio/wheel diameter)
If you are using an older pre-1981 A904 the gears are like a A727:
2.45 / 1.45/ 1.00
mopops
Well-Known Member
well i found the build sheet n axel is 2.24-2.26 ratio 7.25 -trans is A999 std duty no wonder its a slug ,now i don't know whether 373 r too low might be better with 355s
mopops
Well-Known Member
28" tire
Well at least you have that going for you. I personally wouldn't go more than 3.55 if it ever had to cruise on a highway that has 80mph big rigs on it. But like you said any of these choices are going to be a huge improvement over stock. Might want to price out a stronger 8.25" rear swap for behind that 360 though if you haven't already.
mopops
Well-Known Member
i have a ford 8.8 i will be using and am now thinking a 331 would be a better choice--big change from slug gears -posi,n still have some out the hole n ability to cruise
Anything in the 3’s is far better than in the 2’s.
A 3.21/3.23 from a 2.2 is a huge jump and it is possible you might be happy there (maybe).
My ’77 wagon has 318 2-bbl with 2.94’s. My ’86 Fifth Ave also has a 318 2-bbl but has 2.21’s. Both cars have about the same HP and I’d guess weight to be close.
From a takeoff, the wagon will circle the Fifth Ave about twice before cars get across the intersection – at WOT. Even with 2.94’s the wagon is a lot more fun to drive – but I prefer 3.2’s
Tossing those 2.2’s will make a HUGE difference without anything else to be done. I would highly encourage the gear change before the engine change – if possible.
On a different matter – the 7¼” won’t last long (maybe one takeoff) behind a built 360 – but it sounds like you are already on that issue. Again, I would do the differential before the engine (if not already done so).
BudW
A 3.21/3.23 from a 2.2 is a huge jump and it is possible you might be happy there (maybe).
My ’77 wagon has 318 2-bbl with 2.94’s. My ’86 Fifth Ave also has a 318 2-bbl but has 2.21’s. Both cars have about the same HP and I’d guess weight to be close.
From a takeoff, the wagon will circle the Fifth Ave about twice before cars get across the intersection – at WOT. Even with 2.94’s the wagon is a lot more fun to drive – but I prefer 3.2’s
Tossing those 2.2’s will make a HUGE difference without anything else to be done. I would highly encourage the gear change before the engine change – if possible.
On a different matter – the 7¼” won’t last long (maybe one takeoff) behind a built 360 – but it sounds like you are already on that issue. Again, I would do the differential before the engine (if not already done so).
BudW
mopops
Well-Known Member
in the process but car will be put up for the winter
AJ/FormS
Well-Known Member
- Joined
- Apr 26, 2016
- Messages
- 1,291
- Reaction score
- 305
Let's
call that RV cam a 262/110 and,
the MAGNUMS were 9.4Scr engines IIRC,
and you have the 2.74-1.54-1.00 gearset,
with a 2200 TC.
then the Wallace spits out
Static compression ratio of 9.4:1.
Ica of 57*, at 500ft elevation.
Effective stroke is 2.95 inches.
Your dynamic compression ratio is 7.92:1 .
Your dynamic cranking pressure is .............................158.75 PSI.
V/P (Volume to Pressure Index) is ............................ 143
That 159psi pressure is borderline for 87/89 gas
That 143 VP says you have the potential to have an awesome street engine. She would be happier with more stall cuz you could run less rear gear............. but you have that 2.74 low in the trans so;
IMO, a starter gear of 10:1 will be just right; and you get that by;
10/2.74=3.65s.. Ok so this is halfway between 3.55s and 3.73s so you can go either way.
Ok but, I bet you want some nice pull at 30mph on the kickdown into first; right?. Ok that will take something like 3500 rpm so; doing the math I get 29mph with 3.55s and 27.5 with 3.73s; no big thing there. However, 3.73s will multiply your torque an extra 5% over 3.55s. So if your combo is capable of say 380ftlbs at 3500. then the 3.73s will give you a 19 ftlb kick in the azz over 3.55s. And that 5% will be there at every rpm; only the number of ftlbs changes. Now; 19 ftlbs doesn't sound like much , but that 5% applys to the power too. So if your engine makes 300hp then the 3.73s will make it feel 5% bigger than if it had 3.55s; therefore the 3.73s will make it feel like the engine is 15hp more powerful........... which is about the same as one camsize.
So, getting back to the kickdown at 30 mph; I vote 3.73s. For two reasons;
1) it's a city car, and
2) the extra gear will help compensate for the lack of stall. So you don't have to pay someone to install one.
Finally;
going from a 2.25 to a 3.73 will multiply your engine torque an additional ; 3.73/2.25=64.8% ......... which is a huge increase , that you can only match with supercharging.
Say your current engine is sitting at .83 hp per cube, which is 300 hp. This is an easy build. Adding 64.8% to that will get you the performance of guessing 425hp into those earlier 2.25s. That's a real nice improvement.
It sure won't be doggie any more.
Remember I guessed at the camsize, to come up with thischit.lol.But even If I am, you can see the Science from the math
Whichever you chose it will vastly outperform those 2.24-2.26s
Last edited:
mopops
Well-Known Member
no i am still trying to put mine out along with a brain fart --thats on fire too
