Sponsored

Any 18+ GT's dailying the 4.09 rear?

OP
OP

Schwerin

Well-Known Member
Joined
Jan 31, 2018
Threads
179
Messages
3,993
Reaction score
2,496
Location
Home
Vehicle(s)
2019 Mustang
It may not pose a real problem but it is always something to consider when you are spinning the driveshaft faster and faster. At some point you will hit the critical speed limit of the shaft and it will catastrophically fail. Most quality aftermarket shafts are rated to 9,000 shaft RPM which should be more than enough, but some aren't balanced to those speeds(certainly not the stock shaft) so it is questionable how long the shaft will tolerate high speeds.
The Shafts max RPM won't change with rear gearing though. It's going to match engine RPM as that is what is driving it. The shafts speed when reving the engine to 7500 with a 3.27 is the same speed it will spin when reving the engine to 7500rpm with a 4.56.
Sponsored

 

Grimace427

Well-Known Member
Joined
Dec 6, 2013
Threads
14
Messages
6,470
Reaction score
1,699
Location
NoVA
Vehicle(s)
2011 Mustang 5.0
The Shafts max RPM won't change with rear gearing though. It's going to match engine RPM as that is what is driving it.
Only in 1:1 trans gear ratio will the driveshaft match the engine RPM.
 

Blue Horse

Banned
Banned
Banned
Joined
Jun 13, 2014
Threads
8
Messages
626
Reaction score
73
Location
USA
Vehicle(s)
Mustang GT
I loved how the 4.10 helped my Mach1 and the 18+ gearing is very close to the Mach1's (+/- 0.1) I was wondering who had the 4.09 and how they like it.
Although I don't drive my Mustang daily,every time I do get in and drive it, I put the CD in and listen to--- SHE'S REAL FINE MY 409,I have to say I am real happy with that CD, and makes my driving experience a pleasure.I have not noticed any difference in performance or MPG while that CD was playing,but I will check for sure, and report back as to my findings.
 

Grimace427

Well-Known Member
Joined
Dec 6, 2013
Threads
14
Messages
6,470
Reaction score
1,699
Location
NoVA
Vehicle(s)
2011 Mustang 5.0
Yes but the transmission ratio is not changing, thats my point.

The transmission ratio is not relevant so I'm not sure what your point is? At a given road speed the car with 4.56 gears will be spinning its driveshaft faster than a car with 3.73 gears.

I'm trying to find a calculator so as to show the actual difference based on speed and tire height so that you guys can see the specifics for your vehicle. My fuzzy math shows a 1,000rpm increase going from 3.73 to 4.56.



edit: this is the best calculator I've found so far, feel free to post a better on if you find one.

(speed ______x 336 (a constant) x rear ratio (like 4.10), then divide by the tire height (28” tall tire))
 

Sponsored

OP
OP

Schwerin

Well-Known Member
Joined
Jan 31, 2018
Threads
179
Messages
3,993
Reaction score
2,496
Location
Home
Vehicle(s)
2019 Mustang
The transmission ratio is not relevant so I'm not sure what your point is? At a given road speed the car with 4.56 gears will be spinning its driveshaft faster than a car with 3.73 gears.

I'm trying to find a calculator so as to show the actual difference based on speed and tire height so that you guys can see the specifics for your vehicle. My fuzzy math shows a 1,000rpm increase going from 3.73 to 4.56.
Yes, we all know that, but your talking about limitations for some reason over just a couple hundred RPM. Which is a pointless thing to worry about If you're no buying gears just because you have to spin your engine faster to do the same MPH due to worrying about durability and balance then you literally sweating the pointlessly small stuff. If your shaft cant do 45MPH with any of the gears then your DS already had problems. Who cares if the DS is limited to 9k when the engine would blow up over 8? It will never rev that high.

If you're shifting at 8k down the track with a 3.73, you're going to shift at 8k with a 4.56, 430, or 4.09. and in none of those cases will your DS be spinning any faster than the other. The ONLY time your DS will need to spin faster is because you need more RPM to keep highway speeds. If you cant do HW driving at 3000 RPM vs 2000 RPM due to a DS imbalance then it should not be on the road no matter what gearing youre doing.


If you are trying to explain something, you are doing a HORRIBLE job at it.
 

Grimace427

Well-Known Member
Joined
Dec 6, 2013
Threads
14
Messages
6,470
Reaction score
1,699
Location
NoVA
Vehicle(s)
2011 Mustang 5.0
Yes, we all know that, but your talking about limitations for some reason over just a couple hundred RPM. Which is a pointless thing to worry about If you're no buying gears just because you have to spin your engine faster to do the same MPH due to worrying about durability and balance then you literally sweating the pointlessly small stuff. If your shaft cant do 45MPH with any of the gears then your DS already had problems. Who cares if the DS is limited to 9k when the engine would blow up over 8? It will never rev that high.

I'm assuming people aren't only going to drive 45mph though. If you intend to race the car your driveshaft will be spinning faster. Like I said earlier it may not be a problem but driveshaft safety loops exist for good reasons.
 
OP
OP

Schwerin

Well-Known Member
Joined
Jan 31, 2018
Threads
179
Messages
3,993
Reaction score
2,496
Location
Home
Vehicle(s)
2019 Mustang
I'm assuming people aren't only going to drive 45mph though. If you intend to race the car your driveshaft will be spinning faster. Like I said earlier it may not be a problem but driveshaft safety loops exist for good reasons.
It will only be spinning faster if you raise your rev limiter. Rear Gearing has NOTHING to do with that.

You are still limited by the max RPM you can do in 6th gear. Which is the same speed for the DS no matter the rear gear. It's rarely RPM that breaks a DS it's TQ.
 

Grimace427

Well-Known Member
Joined
Dec 6, 2013
Threads
14
Messages
6,470
Reaction score
1,699
Location
NoVA
Vehicle(s)
2011 Mustang 5.0
It will only be spinning faster if you raise your rev limiter. Rear Gearing has NOTHING to do with that.

You are still limited by the max RPM you can do in 6th gear. Which is the same speed for the DS no matter the rear gear.

Again transmission gears are irrelevant. Only vehicle speed, tire height, and rear gear ratios matter. If you are racing your car, at whatever speeds you get your driveshaft will be spinning faster with the shorter gears.
 

Cobra Jet

Well-Known Member
Joined
Feb 12, 2015
Threads
705
Messages
16,230
Reaction score
17,942
Location
NJ
Vehicle(s)
2018 EB Prem. w/PP and 94 Mustang Cobra
The transmission ratio is not relevant so I'm not sure what your point is? At a given road speed the car with 4.56 gears will be spinning its driveshaft faster than a car with 3.73 gears.

I'm trying to find a calculator so as to show the actual difference based on speed and tire height so that you guys can see the specifics for your vehicle. My fuzzy math shows a 1,000rpm increase going from 3.73 to 4.56.



edit: this is the best calculator I've found so far, feel free to post a better on if you find one.

(speed ______x 336 (a constant) x rear ratio (like 4.10), then divide by the tire height (28” tall tire))

Here ya go this online Calc (or the other link below) may help:
https://spicerparts.com/calculators/driveshaft-safe-operating-rpm-calculator


https://www.differentials.com/technical-help-2/differential-gear-ratio-calculator/
 

Sponsored
OP
OP

Schwerin

Well-Known Member
Joined
Jan 31, 2018
Threads
179
Messages
3,993
Reaction score
2,496
Location
Home
Vehicle(s)
2019 Mustang
Again transmission gears are irrelevant. Only vehicle speed, tire height, and rear gear ratios matter. If you are racing your car, at whatever speeds you get your driveshaft will be spinning faster with the shorter gears.
Yeah.. like we all said we already knew...like 3 times. Your point?

None of that has to do with your DS being unbalanced. If its unbalanced with a 3.27 its unbalanced with a 4.56. Either way that is a separate issue NOT related to your rear gears. Did you know that if you have a small hole in a tire and change those tires to new wheels you will probably also still have a small tire leak?
 

Mustang_Lou

Well-Known Member
Joined
May 1, 2016
Threads
13
Messages
1,817
Reaction score
773
Location
Toronto, Canada
Vehicle(s)
2020 Mustang Bullitt
So trying to simplify the discussion about driveshaft speed being affected, or not, by the axle ratio that's in the rear pumpkin so I can understand.

In simple terms, the front of the driveshaft is connected to the engine (via the tranny) and the back end to the axle-gears.

The determining factor of how much the driveshaft spins is the engine. It cannot spin any faster than the engine tells it to.

The gears on the end of the driveshaft will THEN determine how fast the wheels spin.

Am I right??
 

Tone Everready

Banned
Banned
Banned
Joined
Mar 9, 2019
Threads
0
Messages
210
Reaction score
124
Location
Albuquerque, NM, USA
First Name
Mark
Vehicle(s)
2016 EB and 2019 GT
This is giving me a hard on. 20 years ago, I had a full bolt-on DOHC Cobra. She came alive with 4.56 gears. Us DOHC Cobra guys used to say “don’t fear the gear”.

These gen 3’s rev to the moon. If you can put it down effectively, why the hell not? Go nuts!
 

Silver Bullitt

Well-Known Member
Joined
Feb 26, 2018
Threads
17
Messages
2,355
Reaction score
2,163
Location
Parkville, MO
Vehicle(s)
2018 GT Coupe PP2
So trying to simplify the discussion about driveshaft speed being affected, or not, by the axle ratio that's in the rear pumpkin so I can understand.

In simple terms, the front of the driveshaft is connected to the engine (via the tranny) and the back end to the axle-gears.

The determining factor of how much the driveshaft spins is the engine. It cannot spin any faster than the engine tells it to.

The gears on the end of the driveshaft will THEN determine how fast the wheels spin.

Am I right??
Yes, this is about as simple as it can be explained. Changing the rear end gears only will not change the rpm of the driveshaft, it will only change how fast the wheels spin for any given engine rpm. You can change the rpm of the driveshaft based upon engine rpm via the gears in the transmission which will also effect how fast the wheels spin (i.e., by shifting to another gear or changing the actual gear ratios in the transmission altogether), but that's a totally different discussion than what is being discussed here.
 

Ghost50

Well-Known Member
Joined
Sep 17, 2018
Threads
26
Messages
972
Reaction score
586
Location
Houston by way of Philadelphia
Vehicle(s)
2019 Magnetic Metallic GT PP1
I daily 4.09 in my 18 pp1 manual. First is a little short in traffic but u can start off in second without too much clutch feather. As for mileage, 21.4 average. 70 mph is under 2500 rpm in 6th gear. Stomp it above 3500 rpm in first and it's sudden, u will spin 275 ps4's thru first and into second. MT street ss will hook good on that roll.
I have long tubes, pmas, and lund tune. Also have pedal commander set at sport minus 3.
A blast to drive. Going procharger soon.
Nice.

I’ve been batting around the idea since I got mine. I want more lower end feel. Where did you get the gears? Did you buy a whole pumpkin and just swap out?

I had 4:10’s in my 94 Supercharged GT and loved them.
Sponsored

 
 




Top