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PostPosted: October 7, 2018, 9:56 am 
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Location: Spencer WV
So my opening post is here: viewtopic.php?f=18&t=19580

Subject to change, but my current plan is to use a basically stock Cadillac 4.9 with basically stock tune (other than programmed to remove the automatic transmission). I'd like to put an AR5 trans behind it out of a Colorado. I'll be running much taller tires than most of you.

So let's say I go with my current choice, a 235/75r15. The trans has ratios of:
1st: 3.75
2nd: 2.26
3rd: 1.37
4th: 1.00
5th: 0.73

The Cadillac peaks at about 4500rpm but has a large amount of it's overall torque by 2000rpm. Supposed metric hp/torque curve attached.

I've popped this info into ratio calculators but essentially, I don't know what to make of the answers. Guys running Fieros say it can cruise at 1500-2000rpm on the freeway so I've picked a FD (I think I settled on 3.55) that puts me into that range at 75-80mph (our Interstates are 70mph in WV), but then I get a bunch of speeds for the lower gears and I don't know what's "good" or "bad". Are there rules of thumb to consult? Do you just keep swapping FD ratios until you get one that works?

What's the process for choosing the FD and matching it to the trans ratios? Besides gearsets not being cheap, not all differentials support all ratios and some require carrier changes across a ratio threshold, so it seems pretty important to have a reasonable idea early on.


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PostPosted: October 7, 2018, 5:10 pm 
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I wouldn't change gearing from whatever you get in a donor until you get the car driving. Use that as a baseline and then you can make changes that better suit your needs.
Any ideas what rear diff or axle you'll be using?


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PostPosted: October 7, 2018, 6:16 pm 
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Looking at a Ford 8.8 IRS for the rear. Plenty of ratios available.

This will be AWD/4WD so the front will most likely have less availability, especially if a reverse cut gearset is required.


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PostPosted: October 7, 2018, 6:21 pm 
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Good place to start shopping is Randy's Ring & Pinion. https://www.randysworldwide.com/product-category/ring-pinion-gear-sets/

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PostPosted: October 7, 2018, 11:54 pm 
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Why that particular 5 speed?

And if you ever think you might give an autocross a try then make sure 2nd goes above 60 mph as that's what speed most autocross courses are planned around. You'll go faster than that, but it will minimize shifting.

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PostPosted: October 8, 2018, 12:41 am 
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For an exclusively street car, I think picking final driver based on your transmission top gear, tire size, and expected highest cruising speed is generally a fine way to go about it.

My calcs show a 235/75-15 with .73 5th gear and a 3.55 final drive running nearly 2200rpm at 70 and nearly 2500rpm at 80. That's probably higher than this engine really needs to be.

With a 3.07/3.08 you'll see around 1900 and 2150 rpm instead, and a 2.73 will drop you to 1700 and 1900. Those both sound a bit more reasonable for such a low revving V8. I'd probably lean towards starting out with a 2.73 myself, but it also depends on whether you are more focused on the lower or higher speed portions of your driving. If you ever wanted to take it to any type of performance driving event with swapping out to smaller/stickier tires, then the revs topping out in that 4750rpm range will leave you with less than desirable gearing either way.

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PostPosted: October 8, 2018, 10:14 am 
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carguy123 wrote:
Why that particular 5 speed?

And if you ever think you might give an autocross a try then make sure 2nd goes above 60 mph as that's what speed most autocross courses are planned around. You'll go faster than that, but it will minimize shifting.


Originally was going to use a T5. The AR5 is considered to have more torque capacity. The 4.9 has a GM 60-degree V6 bellhousing face, so there are bellhousings to mount a Ford-pattern T5 (S-10) or to mount an MA5/AR5/R154/AX-15 (Dakota).

No interest in any sort of competition, just a reasonably competent street vehicle. But that's a good bit of knowledge to have.


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PostPosted: October 8, 2018, 6:16 pm 
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Attached is a gear calc I have made over the years to aide in helping me with choices.
Fill in the yellow boxes and the rest is calculated for you.
I would shift what you have around 2500. Make comparisons to cars you have driven and see what is different.
(Google drive link since xlsx is not supported)
https://drive.google.com/open?id=1-6aNx ... dSDvOrU3hv

What do you want it to drive like?

For comparison a 94-02 Camaro
Rear: 3.42
1st: 2.66
2nd: 1.78
3rd: 1.30
4th: 1
5th: .74
6th: .5

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PostPosted: October 9, 2018, 9:33 am 
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chrisser wrote:
Looking at a Ford 8.8 IRS for the rear. Plenty of ratios available.

This will be AWD/4WD so the front will most likely have less availability, especially if a reverse cut gearset is required.


For ratio, to maximize the torque available in each gear and still reach triple digits near the hp peak (4100rpm), 3.73-4.10 would be better. It makes a big difference in how responsive the car is in any gear.

As for awd, it sounds like you want to rebody a cj or tracker. Why not just build a cj?

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PostPosted: October 9, 2018, 11:05 am 
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Miatav8,MstrASE,A&P,F wrote:
chrisser wrote:
Looking at a Ford 8.8 IRS for the rear. Plenty of ratios available.

This will be AWD/4WD so the front will most likely have less availability, especially if a reverse cut gearset is required.


For ratio, to maximize the torque available in each gear and still reach triple digits near the hp peak (4100rpm), 3.73-4.10 would be better. It makes a big difference in how responsive the car is in any gear.

As for awd, it sounds like you want to rebody a cj or tracker. Why not just build a cj?


Well, not to go too far off topic, but weight distribution, CG and independent suspension primarily. Most of my drive is fun, curvy roads that are 55mph. About 25%, including the road we live on, are one-lane dirt/gravel with steep hills, ruts, rocks, mud, ice and little maintenance. Maybe 10% is Interstate driving at 70mph speed limits. I have a Cherokee, and while it's great on the dirt, it rides hard with the straight axles on pavement and doesn't lend itself to spirited driving. The engine is completely above and partially forward of the axle centerline and is a heavy cast iron lump. Also has a pretty short wheelbase.

I also have a FS 4wd Silverado. It has a long wheelbase and is more stable than the Jeep most of the time, but it actually gets stuck easier than the Jeep. I think the weight is the differentiating factor. That and the weight bias so heavily forward.

My wife drives an AWD Escape (2013), which one would think has the potential to be a better vehicle for our drive, but it's actually the worst. Ground clearance too low, AWD too managed and tires too wide. I got it stuck in a mud puddle once on flat ground because the computer refused to put power to the two tires that had traction when it detected slip on the other two. I will say the Escape handles admirably for what it is. Most of the winter, its stuck in the garage because we can't get it down the road safely either because it doesn't have the ground clearance or doesn't have the traction.

So, taking in my observations from our current livery and seeing what works and what doesn't in what others around me are driving, I'm trying to build something that handles reasonably well on pavement, at least better than the Cherokee which is a pretty low bar, but I can still get to/from home 90%+ of the time, with the Cherokee in reserve for the worst weather or when I need to tow/haul something. Seven-ish inspired vehicle has the engine and passengers low in the chassis, it's very lightweight, has a fairly long wheelbase and independent suspension. With taller wheels/tires I think I can get the ground clearance to where I need it. I only need the 4wd for low speed traction.

Mine will likely not be the pure sports car others have built, but I think it will be an interesting project that might have broader appeal than people outside this part of the country realize.

A lot of people in WV, KY, NC, southern OH and eastern TN are in the same boat I am. Either have a long, rough driveway or live off the pavement on some combination hilly, narrow, rough rural roads. We end up driving 4x4 pickups or SUVs just so we can get to pavement, but then most of the time we're driving a larger, heavier, and more ill-handling vehicle than we need because nothing more nimble can get from the pavement to home. Subarus are probably the closest solution, but even they aren't always viable and seem to be designed with less ground clearance every year.

We also don't have much in the way of entertainment out here in the sticks, so this will keep me out of trouble for awhile. Kind of trying to mash up a sports car with a sporting trials car I guess and add a little hot rod inspiration.


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PostPosted: October 9, 2018, 12:07 pm 
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Miatav8,MstrASE,A&P,F wrote:
For ratio, to maximize the torque available in each gear and still reach triple digits near the hp peak (4100rpm), 3.73-4.10 would be better. It makes a big difference in how responsive the car is in any gear.
For the stated application, 'better' how?

From my perspective, the low redline severely restricts max speed in each gear and won't lend itself to being comfortable at higher (2500+) rpm cruising like a modern 4cyl can. Responsiveness at any given speed wouldn't be substantially improved by a shorter final drive, as that's more about simply being in the right gear at the time, and the engine would be left howling along at an extra 600-700 rpm on the highway over a 2.73-3.08. A first gear that redlines at 25mph may be responsive, and would allow for idle speed crawling along at 5mph rather than 7mph, but isn't something I would find particularly useful otherwise. If picking arbitrary targets outside the primary scope of use, I'd be more interested in what gear it breaks triple digits in, than what rpm it does so without regard to whether it happens at that rpm in 3rd gear or 4th gear. Heck, a 4.1 would realistically require shifting into 5th just to break that last little bit into triple digits.

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PostPosted: October 9, 2018, 6:14 pm 
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Justin, my perspective comes from many gear swaps over the years on many different vehicles for the purpose of improved acceleration. As for how, it is called torque multiplication. The “application” could use the ability to crawl in first and would seldom if ever see anything greater than 80mph. For pampered surfaces, start in second. Yes, make use of all the gears within the operating envelope of the engine and vehicle speed. You’ve obviously have a difference philosophy and that is fine.

Chrisser, not judging, just curious. It should be interesting whatever you come up with.

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PostPosted: October 10, 2018, 11:18 am 
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Miatav8,MstrASE,A&P,F wrote:

Chrisser, not judging, just curious. It should be interesting whatever you come up with.


Not a problem. Most people I've talked to about this project look at me like I'm crazy. After I've explained my thought process, half understand and half are convinced they were right the first time. Thought it'd be easier to just lay out my line of thinking first rather than going back and forth with questions and answers and ending up with the same end result.


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PostPosted: October 10, 2018, 11:32 am 
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I should mention that the transfer case I plan on using has a lo range, so I don't necessarily need to crawl in 1st. I'm basically looking at this as a 2wd manual transmission question, which is why I didn't mention 4wd at the start of the thread.

I will do some analysis of other M/T vehicles and their shift points. During my reading, a lot of sites recommended picking a FD ratio that put 5th in a comfortable RPM range for your desired cruising speed. That makes sense, but then my question was how do you know your 1st gear won't be useless?

I don't mind shifting per se, but I'd rather set it up so 1st through 4th is usable for most driving (which for me is 0 to 60/65mph) and have 5th be used only on the interstate, or even be rarely used at all. That's a better solution, IMHO, than choosing the FD to make 5th usable and ending up with 1st being never used.

To throw a wrench into the works, I was under the mistaken impression that Ford stopped using T5s in the early 00s. Found out they continued to be used until around 2010 on the 4.0 V6 Mustangs. Going to a junkyard this weekend that has six or seven mustangs with V6s so if one has a T5, I may grab it. They don't have any Colorados to look at. The primary reason I'm going is for some parts for my Cherokee, and if there's an NP242 there, I may grab it as well. They have quite a few Explorers, so I may be poking around to see whats out there in terms of differentials.

I figure I can always liquidate the T5 if I decide not to go with it. The later v6 models are rated at 265 ft-lbs for 100k miles. I think it's usable for my purposes given the tires I'll be running and the foreseen vehicle weight even though the engine has a bit more torque available than the trans rating. It's also a bit easier to package than the AR5.


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PostPosted: October 10, 2018, 12:12 pm 
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chrisser wrote:
During my reading, a lot of sites recommended picking a FD ratio that put 5th in a comfortable RPM range for your desired cruising speed. That makes sense, but then my question was how do you know your 1st gear won't be useless?
You can always do the math on 1st gear rpm vs speed, the same as with top gear. But generally speaking, I think if you base your decision on a comfortable RPM range for the engine at your desired cruising speed, most transmissions are geared such that you'll probably have a reasonably usable 1st gear.


chrisser wrote:
To throw a wrench into the works, I was under the mistaken impression that Ford stopped using T5s in the early 00s. Found out they continued to be used until around 2010 on the 4.0 V6 Mustangs.
Be careful with this, as the 05-10 V6 T5's are a bit different than earlier ones, including a change in input shaft length, and as such are not directly interchangeable.

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