Any 18+ GT's dailying the 4.09 rear?

[Grimace427]

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.

Driveshafts can vibrate based on resonant frequencies from specific RPM's of the shaft itself. The shaft can be perfectly balanced and still cause a vibration at a specific RPM based on its length, shaft wall thickness, and construction(AL, steel, CF). So with lower(numerical) gears you may have a perfectly smooth driveline but when you install the shorter gears(thus spinning the shaft faster for the same road speed) you may hit the resonant frequency of the driveshaft causing a vibration. You may have seen people install an aftermarket driveshaft and get a vibration at a specific speed and have to 'clock' the driveshaft in order to mitigate that frequency.


My overall point is that because installing shorter gears will cause the shaft to spin faster you have a higher chance of hitting the critical frequency of the driveshaft which will cause a vibration and even reduce the life of the shaft or cause sudden failure.

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??

No.

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.

This is also wrong.

You can completely remove the engine and transmission from the car and push the car down a hill as far as this discussion is concerned. The rear gear ratio is the relationship between the rotation of the driveshaft and the rotation of the driven wheels. With 3.73 gears for example, the driveshaft spins 3.73 times for every rotation of the rear wheels. Thus with 4.56 gears the shaft spins 4.56 times for every rotation of the wheels.

Going from 3.73 gears to 4.56 gears means you are spinning your driveshaft about 19% faster for any given road speed.

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[Grimace427]

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

So based on this formula(and assuming you have 27.7" tires) you would only need to go 127mph with 4.56 gears to exceed 7,000rpm shaft RPM's which is higher than most shafts are rated(certainly the stock driveshaft).

With a high quality driveshaft rated to 9,000rpm you can go 163mph with 4.56 gears and 27.7" tires before you exceed the max shaft RPM rating.

 

[tw557]

I'm following this thread because I feel this would be my best mod when I get a GT to give me some of the acceleration/ torque I feel the GT is lacking.

But seriously though, the driveshaft can not spin any faster then it did before because no gears have changed in between the shaft and the motor. At 7000 rpm the drive shaft is the exact same rpm with what ever gears are run.

 

[Mustang_Lou]

I think I may be catching on to what Grimace is saying and both viewpoints are correct but I think it's perception of what EXACTLY is being discussed that is not clear.

The gears change the RATE at which the engine spins. In other words, a 4.09 gear will make the engine spool up at a faster rate than say a 3.15 and that makes the driveshaft spin faster. I think we can all agree with that.

However, the INITIATOR of the driveshaft's spin rate can only be the engine. However, the gears will alter the rate at which the driveshaft spins once the engine gets past 0 rpm.

So we're both right but we need to each put ourselves in the others perception to understand the others viewpoint.

Am I getting warm Grimace?

 

[Silver Bullitt]

I'm following this thread because I feel this would be my best mod when I get a GT to give me some of the acceleration/ torque I feel the GT is lacking.

But seriously though, the driveshaft can not spin any faster then it did before because no gears have changed in between the shaft and the motor. At 7000 rpm the drive shaft is the exact same rpm with what ever gears are run.

You might as well give up. He does not understand the relationship between the engine, transmission, driveshaft, and rear end gearing. He's now even removed the engine to try and prove his point, which while making his argument plausible from a reverse engineering perspective, has no foundational basis whatsoever assuming a rear wheel drive vehicle being propelled by a front mounted combustion engine.

 

[tw557]

Yeah, Once the engine was removed from the equation (which is literally the driving and limiting factor), I figured the reverse engineering aspect just wasn't valid considering we do actually have a motor attached and don't just have a glider rolling down hills.

But I really would love to drive or have a ride in a GT sometime that has the gearing in the 4:09 range. I really don't think I would mind the 6th gear or 10th a bit higher and not lugging around. Current gearing means I will only ever hit redline in 2 gears but close to redline is where the fun is. I soon (year or 2)plan to get a GT as the prices are dropping and I won't feel bad about modding one that is out of warranty.

 

[DuRsT]

Grimace, drag cars have driveshaft safety loops because they are running a great deal of power into a suspension built to take a large tire and plant that torque directly into the ground. Driveshafts can and have broken under this load. Can someone spin their stock Mustang GT to a speed where the driveshaft will implode? Possible, but unlikely and says that there was a problem with that driveshaft to begin with.

For your view on how all of this works.....you're just wrong, and there's no other way to say it. For your viewpoint to work/be valid the power would have to be coming from the rear differential. Unfortunately for your argument the power comes from the engine through the transmission. Let's look at a constant that we know. Sixth gear's final drive in the MT-82 trans in my 2019 GT is .62. This does not change regardless of the gears in the rear diff. So, the power of the engine coming through my six-speed MT-82 will transfer .62 of it's rpm's to that driveshaft....REGARDLESS OF WHAT GEAR IS IN THE REAR DIFF.

 

[Smokey613]

On my 2004 Ford Crown Vic I went from a 2.73 to a 3.55 and had driveshaft vibration above 100mph. I went with a Marauder aluminum DS ( shorter), a longer trans output shaft and housing. Problem solved. It was smooth as silk afterwards. It was a known issue in the Panther platform. So yes, the DS does rotate at a higher RPM when changing from say a 3.15 to 4.09 diff ratio and can cause the DS to operate outside of it design limits.

 

[DuRsT]

On my 2004 Ford Crown Vic I went from a 2.73 to a 3.55 and had driveshaft vibration above 100mph. I went with a Marauder aluminum DS ( shorter), a longer trans output shaft and housing. Problem solved. It was smooth as silk afterwards. It was a known issue in the Panther platform. So yes, the DS does rotate at a higher RPM when changing from say a 3.15 to 4.09 diff ratio and can cause the DS to operate outside of it design limits.

lol........just no.

Sounds like you had an issue with your driveshaft. You guys are cracking me up today.

 

[Smokey613]

lol........just no.

Sounds like you had an issue with your driveshaft. You guys are cracking me up today.

Google is your friend. Look it up........ never mind, I will do it for you....

https://adtr.net/kb/crown-vic-driveshaft-swap/

 

[DuRsT]

Google is your friend. Look it up........ never mind, I will do it for you....

https://adtr.net/kb/crown-vic-driveshaft-swap/

A fabulous link on how to spend some money on your grocery getter to install an aluminum driveshaft..fantastic.

Nowhere in that link does it support your assertion that changing the rear gears causes the driveshaft to rotate faster. Your engine will have to turn more RPM's at 100mph after the gear change which causes your driveshaft to turn more...yes...but again that is dictated by required engine revolutions, not the gear itself.

 

[Smokey613]

A fabulous link on how to spend some money on your grocery getter to install an aluminum driveshaft..fantastic.

Nowhere in that link does it support your assertion that changing the rear gears causes the driveshaft to rotate faster. Your engine will have to turn more RPM's at 100mph after the gear change which causes your driveshaft to turn more...yes...but again that is dictated by required engine revolutions, not the gear itself.

A fabulous link on how to spend some money on your grocery getter to install an aluminum driveshaft..fantastic.

Nowhere in that link does it support your assertion that changing the rear gears causes the driveshaft to rotate faster. Your engine will have to turn more RPM's at 100mph after the gear change which causes your driveshaft to turn more...yes...but again that is dictated by required engine revolutions, not the gear itself.

Bottom line, yes, a DS will rotate at a higher rpm relative to mph when axle ratio increases if all other variables are constant.

Just put a car on a lift, drop the DS so it nor the trans is part of the equation. Rotate the tire 1 revolution and count how many times the pinion yoke completes a full revolution. In a 3.15 axle the yoke will turn 3.15 times per 1 revolution of the tire/ output shaft. If a tire/output shaft turns 700 rpms in 1 mile the pinion yoke will make 2205 revolutions in 1 mile. A 4.09 rear axle diff will turn the pinion yoke 2863 revolutions in 1 mile. 2863 RPMs is faster than 2205 RPMs or approximately 23% faster. The DS obviously rotates at the same speed as the pinion gear hence the 4.09 gear set WILL rotate the DS at a higher RPM than a 3.15 gear set.

 

[Silver Bullitt]

 

[Silver Bullitt]

Guys, we've been doing this all wrong. We've been trying to eek out every last horsepower we can get out of our engines to go faster. All we need to do is disconnect the driveshaft from any power source, swap rear end gears, and we can go as damn fast as we want. Hell, we don't even need an engine. Who would have thought. And, I always thought a shorter gear would hurt my gas mileage. I'm such a dumbass!

 

[DuRsT]

Bottom line, yes, a DS will rotate at a higher rpm relative to mph when axle ratio increases if all other variables are constant.

Just put a car on a lift, drop the DS so it nor the trans is part of the equation. Rotate the tire 1 revolution and count how many times the pinion yoke completes a full revolution. In a 3.15 axle the yoke will turn 3.15 times per 1 revolution of the tire/ output shaft. If a tire/output shaft turns 700 rpms in 1 mile the pinion yoke will make 2205 revolutions in 1 mile. A 4.09 rear axle diff will turn the pinion yoke 2863 revolutions in 1 mile. 2863 RPMs is faster than 2205 RPMs or approximately 23% faster. The DS obviously rotates at the same speed as the pinion gear hence the 4.09 gear set WILL rotate the DS at a higher RPM than a 3.15 gear set.

We're not going to agree on this one.

Final drive of the trans decides rotational speed of the driveshaft, diff gears and tire height dictate how many times your tires turn in a mile.

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