Why is a Hellcat rear geared higher than my Grand Marquis?

[ford46guy]

Put power aside for a moment, both being V8s. Maybe the transmission lower gear difference?

I guess for top speed? Wouldn't that hamper acceleration?

A 4R70W is geared:

• First: 2.84 :1.
• Second: 1.55 :1.
• Third: 1.00 :1.
• Overdrive: 0.70 :1.
• Reverse: 2.32 :1.

2.73 Open diff. for the MGM.

Finally, the 6.2-liter Hemi in the SRT Hellcat and limited edition 50th Anniversary Edition Daytona comes mated to the TorqueFlite ZF 8HP90 automatic transmission. The asymmetric limited-slip 2.62 rear gear ratio is standard and unlike the Challenger, the 3.09 is not offered as an option with the muscular sedan. The gear ratios for the Hellcat transmission are as follows.

1st 4.71
2nd 3.14
3rd 2.10
4th 1.67
5th 1.29
6th 1.00
7th 0.84
8th 0.67
Reverse 3.32

The 411 hp 6.2L in my F150 is geared at 3.73. Raptors had 4.10s
6R80 Offers six forward speeds, with gear ratios of 4.17, 2.34, 1.52, 1.14, 0.87, and 0.69:1.

 

[Brian123]

Because it has more Gears!

 

[Astro14]

Your question is answered by your own list - look at the first gear ratio in both transmissions - do the math - much greater torque multiplication with that 4.71 first.

 

[Kira]

I was doing the math whilst you typed, Astro14.
These are the final drive ratios
For the 4R70W. For the ZF 8HP90
1= 7.753......................12.340
2= 4.231........................8.227
3= 2.730........................5.502
4= 1.911........................4.375
5 = NA............................3.379
6= NA.............................2.620
7=NA...............................2.200
8=NA...............................1.755

Get up and go gearing vs cruising down the open highway gearing.
The 8-speed has your motoring needs covered.

 

[eljefino]

Throw tire size into your math. The dodge probably has larger diameter rubber. This will make the gearing seem even taller.

 

[supton]

It's an automatic--put your foot in it and it'll downshift. When it runs out, it upshifts. When not in the throttle, it upshifts and lowers rpm as much as it can. When your MGM was popular, the OEM's couldn't (or wouldn't) justify more than a 4 speed auto, which lead to what it was, a compromise with the rear end ratio to get a reasonable final drive for mpg and first gear takeoff.

I've read that OD was a popular idea in the 80's as OEM's did not have to redesign the rear end. In order to get a taller final drive, the pinion gear would have to be made larger and/or ring gear smaller. Ring gear can only get so small before it compromises the spiders I bet, and a huge pinion is probably costly to make, heavy, or some other drawback. The MGM has its roots back to then. I'm not sure but would not be surprised if the Hellcat had little desire to reuse tech from the 80's, and thus getting a tall FD with high torque handling was just a challenge for the design team to overcome--as opposed to something barred by the bean counters wanting to reuse as much as possible.

 

[Chris142]

any steeper gears likely would just blow the tires off. Plus with that kind of horsepower you dont need em

 

[dgunay]

There is not enough traction, why would you need shorter gears if you can't put the power down - it'd just make lower gears unusable. You also have more number of gears and more torque compared to the other vehicle. So it's really apples to oranges comparison.

 

[supton]

any steeper gears likely would just blow the tires off. Plus with that kind of horsepower you dont need em

Good point. Pickups have had torque management for a while, and that was one of the tricks to make "faster", remove that in a tune. But that could also break things... transmissions and rear ends have max input torque limits. And the tires have only so much traction.

But once past first or second gear, full torque can be used.

 

[wpod]

The Hellcat also has much more torque and HP.

 

[i6pwr]

Put power aside for a moment, both being V8s. Maybe the transmission lower gear difference?

I guess for top speed? Wouldn't that hamper acceleration?

A 4R70W is geared:

• First: 2.84 :1.
• Second: 1.55 :1.
• Third: 1.00 :1.
• Overdrive: 0.70 :1.
• Reverse: 2.32 :1.

2.73 Open diff. for the MGM.

Finally, the 6.2-liter Hemi in the SRT Hellcat and limited edition 50th Anniversary Edition Daytona comes mated to the TorqueFlite ZF 8HP90 automatic transmission. The asymmetric limited-slip 2.62 rear gear ratio is standard and unlike the Challenger, the 3.09 is not offered as an option with the muscular sedan. The gear ratios for the Hellcat transmission are as follows.

1st 4.71
2nd 3.14
3rd 2.10
4th 1.67
5th 1.29
6th 1.00
7th 0.84
8th 0.67
Reverse 3.32

The 411 hp 6.2L in my F150 is geared at 3.73. Raptors had 4.10s
6R80 Offers six forward speeds, with gear ratios of 4.17, 2.34, 1.52, 1.14, 0.87, and 0.69:1.

You listed a large, cruiser family sedan and a very high performance muscle car.

You also have to factor in torque curves, intended usage, as well as the technology avail for the time period, mainly the available transmission.

The Grand Marquis is a simpler design, starts in 1st every time from a start, and goes through every gear. If you start winding that motor out at 55 MPH then your EPA target will likely be a bust, among other factors.

The Hellcat can use different gears based on load and need, also has almost 1000 more RPM to work with.

The Hellcat has newer tech and can benefit from a shorter 1st gear for launches, and still keep a consistent RPM drop through gears.The GM would have a huge deficit trying to make up a large gap in gearing from 1st to 2nd or 2nd to 3rd if given a shorter 1st ratio.

 

[MRC01]

This reminds me ... Years ago I owned a Panoz Roadster, a 2500 lb. car powered by the DOHC Ford Cobra V8. From the factory, it had a 3.27:1 rear diff ratio and redline (7000 RPM) in 2nd gear was around 80 mph. Geared much too tall! Before swapping the ring & pinion I called Panoz to get their advice. They said with the high revving engine and large wheels, the car was made for a 3.73 or 4.1 ratio, take your pick. I asked if so, why ship it with a 3.27 ratio? They said the bigger ratios reduced fuel economy by around 3%, not enough to notice but just enough to fall below the threshold that would incur a gas guzzler tax on sale. And swapping the ring & pinion is cheaper than the gas guzzler tax, so even folks who want the bigger ratio still come out ahead. I went with the 3.73 and that 14% or so boost really woke up that car.

Modern cars with auto trannys are a whole 'nuther thing and diff ratio selection becomes moot, or at least not the big factor that they are with manual tranny cars having only 5 gears.

​

 

[JeffKeryk]

There is little doubt in my mind that the entire package is optimized for 1 thing.

 

[AdmiralYoda]

With 600-700 lb-ft of torque there just isn't the need for steep rear end gears, especially with the low gearing in the transmission. If you swapped 3.73's or 4.10's in it first gear would probably be unusable and you'd probably be even more traction limited.

 

[ARCOgraphite]

Daytona Edition needs Daytona gears, lol.

I just calc'd it would shift out of 1st at 40 mph then go thru the traps at the top of 4th gear (117mph).
(shift point set at 6000 RPM, tire diameter 28.7in. (275/40 R20)

Sounds reasonable. But the Dodge could get by with a 5 speed AT with a 1.00 or slight O.D. TOP gear to cruise 70mph at 2100 rpm.
(Top speed 190+ at 6000rpm if not drag limited)

I know I would valve float my '74 Vega drag car thru the traps with a THM350 and 3:55 rear gear so I had to let off.

 

[Allaneddie]

Put power aside for a moment, both being V8s. Maybe the transmission lower gear difference?

I guess for top speed? Wouldn't that hamper acceleration?

A 4R70W is geared:

• First: 2.84 :1.
• Second: 1.55 :1.
• Third: 1.00 :1.
• Overdrive: 0.70 :1.
• Reverse: 2.32 :1.

2.73 Open diff. for the MGM.

Finally, the 6.2-liter Hemi in the SRT Hellcat and limited edition 50th Anniversary Edition Daytona comes mated to the TorqueFlite ZF 8HP90 automatic transmission. The asymmetric limited-slip 2.62 rear gear ratio is standard and unlike the Challenger, the 3.09 is not offered as an option with the muscular sedan. The gear ratios for the Hellcat transmission are as follows.

1st 4.71
2nd 3.14
3rd 2.10
4th 1.67
5th 1.29
6th 1.00
7th 0.84
8th 0.67
Reverse 3.32

The 411 hp 6.2L in my F150 is geared at 3.73. Raptors had 4.10s
6R80 Offers six forward speeds, with gear ratios of 4.17, 2.34, 1.52, 1.14, 0.87, and 0.69:1.

Put power aside for a moment, both being V8s. Maybe the transmission lower gear difference?

I guess for top speed? Wouldn't that hamper acceleration?

A 4R70W is geared:

• First: 2.84 :1.
• Second: 1.55 :1.
• Third: 1.00 :1.
• Overdrive: 0.70 :1.
• Reverse: 2.32 :1.

2.73 Open diff. for the MGM.

Finally, the 6.2-liter Hemi in the SRT Hellcat and limited edition 50th Anniversary Edition Daytona comes mated to the TorqueFlite ZF 8HP90 automatic transmission. The asymmetric limited-slip 2.62 rear gear ratio is standard and unlike the Challenger, the 3.09 is not offered as an option with the muscular sedan. The gear ratios for the Hellcat transmission are as follows.

1st 4.71
2nd 3.14
3rd 2.10
4th 1.67
5th 1.29
6th 1.00
7th 0.84
8th 0.67
Reverse 3.32

The 411 hp 6.2L in my F150 is geared at 3.73. Raptors had 4.10s
6R80 Offers six forward speeds, with gear ratios of 4.17, 2.34, 1.52, 1.14, 0.87, and 0.69:1.

Because burnouts are bad, unless you want to burn rubber. Plus, who intentionally does burn outs in a Grand Marquis?

 

[SwampSurvivor]

Because the power train engineers at Ford have no concept of BSFC. OHC modular v8s are not low speed engines. They never should have been infront of the stupid 70s/80s fuel economy gearing. I'm 4000 miles into my 3.55 rear swap on my 01 - in addition to the car being able to get out of it's own way now, fuel economy is up! That includes 40 miles of highway commuting at 70 every day.

Like another pointed out - the Hellcat has more gears, closer and starting way lower.

The 4.6 and 4r70w will easily outlive a hellcat but they just can't be performance oriented.

 

[SwampSurvivor]

It's an automatic--put your foot in it and it'll downshift. When it runs out, it upshifts. When not in the throttle, it upshifts and lowers rpm as much as it can. When your MGM was popular, the OEM's couldn't (or wouldn't) justify more than a 4 speed auto, which lead to what it was, a compromise with the rear end ratio to get a reasonable final drive for mpg and first gear takeoff.

I've read that OD was a popular idea in the 80's as OEM's did not have to redesign the rear end. In order to get a taller final drive, the pinion gear would have to be made larger and/or ring gear smaller. Ring gear can only get so small before it compromises the spiders I bet, and a huge pinion is probably costly to make, heavy, or some other drawback. The MGM has its roots back to then. I'm not sure but would not be surprised if the Hellcat had little desire to reuse tech from the 80's, and thus getting a tall FD with high torque handling was just a challenge for the design team to overcome--as opposed to something barred by the bean counters wanting to reuse as much as possible.

I've always wondered why overdrive was so popular. Just run a higher gear ratio with lower , direct transmission gearing. I can see that being a reason.

Dana made the 61 and 71 differentials to accommodate highway gears behind diesels in HD pickups.

 

[supton]

I've always wondered why overdrive was so popular. Just run a higher gear ratio with lower , direct transmission gearing. I can see that being a reason.

Dana made the 61 and 71 differentials to accommodate highway gears behind diesels in HD pickups.

Yeah, technically to step up/down incurs some driveline loss--but in the gears in the transmission, I'm not sure there much loss. [Manual transmissions don't need coolers, an automatic needs one due to the pump and worse, unlocked TC.] So it was a minor loss to speed up the driveshaft, but a major gain to reduce engine RPM.

What I don't know about is the hypoid gear. Seems to me that every driveshaft rotation has to have some amount of loss, and given the long teeth, it's worse than transmission gears. Still... if it was really lossy, there'd be a cooler out back. Which I don't believe is all that common--it can get warm, sure, under heavy load. Anyhow, I'd think deeper gearing would have more driveshaft rotations thus more loss--but perhaps under light loading it just isn't a major issue.

I still find it wild that a 90 degree transfer (hypoid gear) is more lossy than a 180 degree transfer (regular gears). FWD eeks out a bit more mpg due to not having that hypoid gear (among other details). Yeah if you look at it, it's obvious--but if you just ponder that one is a lesser angle, it seems... odd.

 

[Pablo]

For EPA MPG of course!!

I do notice, even from the 1960's factory rear gears are too tall for my liking. Even small trucks. I think the new GMC Canyon 4x4 is 3.42. Heck I regeared my Tacoma to 4.88. Yes apples to oranges, but I need low grunt....hahahaha