Rear end gear selection?? - Ford Muscle Forums : Ford Muscle Cars Tech Forum

Sifu-Sitaigung · 01-01-2013, 10:35 AM

Thank you everyone for all of your help.

Here's what I want and what I'll be working with - eventually:

390 stroked to 431 (Scat kit) with dual quads, Comp roller, Harland Sharpes, PROBABLY hand made headers (thinking btwn 1 1/2 - 2" down to 3" collectors) Built C-6 (original trans.) with shift kit for now, with 2800-3000 stall..depends on cam..

All that being said, I've been talking with a couple local engine guys and they say it is feasible, depending on cam and heads, to pull around 500hp + at the crank..

NOW- I am debating on what rear end to pick up in my travels should I find a deal:

3:90 or 4:11?? or something else?

What kind of locker/Limited slip should I be looking for? Since you all have experience with brands and drivability.

I have A/C and WILL be re-installing it. I will not be drag racing this car, may track it though ,just to see times.I am not too worried about loss through the chassis and drivetrain. The car will still weigh about stock specs.

I still want to be able to go above 55 - 65 mph for an extended period of time without excessive detriment to the engine; should I choose to drive to a meet or show..

The Beast WILL have reliability to be an everyday driver IF I should so choose. (my goal anyway :-))

Stiffer springs and larger OD sway bar will be installed and adjustable shocks, (urethane bushings if they can be found or made) and overall, from the interior to exterior The Beast will still APPEAR stock. She won't run like stock though.

So what are the pros and cons of the various gear/locker set-ups (that fit my "build") that you have experienced?

(FWIW- GAS mileage is NOT that much of a consideration) - I am thinking btwn 6-11.5 mpg with my foot in it, most of the time. Maybe 9-12 cruising and being easy)

Thanks for reading and providing your experience and knowledge base!!

Tommy

FEandGoingBroke · 01-01-2013, 10:58 AM

OK, check out MY opinion.

Get 2.75:1 or 3.00:1 rear gears.

You tell all those haters to shut the hell up, If you want to ENJOY driving your car, don't gear the silly thing down to 4.11:'s or 3:83's or 3:90's, that totally makes your car a schitt bag on the highway running 3500 rpm's just trying to keep up with traffic.

I am a highway driving fool, and for city driving, well any goof bag knows that there are 3 OTHER gears that you can use

My opinion is not very popular, but it is my opinion.

PSIG · 01-01-2013, 12:39 PM

Your rear gear selection should be based on your camshaft timing and torque converter characteristics. RPMs too low will be a dog and cause excessive converter heat in street driving along with poor mileage. RPMs too high will be more efficient, but with unnecessary fuel consumption from RPMs and more road noise. You need to balance these factors and throw-in your own preferences. It should be a matched package. The whole car is a package and every part should compliment each other.

RPMs too low cause more problems than too high. Take a case in-point - the good old FE powered U-Haul trucks. Those things would spin 4000+ RPMs all day long, back and forth across America, for years on end and hundreds of thousands of miles. Nobody thought badly of that, though the mileage sucked. Also look at boats churning 3500-4500 constantly with no ill effects. However, a lugging engine below it's efficient RPM range and excessive heat at low RPMs can be trouble in the making.

So, decide for yourself, but strive to make a custom package that works together and does what you want it to do. Set the goals, match the parts, tune them in harmony and drive happy.

David

GT300TD · 01-01-2013, 12:49 PM

Sifi-Sit,
are the "local guys" Ford guys? Seems to me 500+ horsepower is a stretch for a stroked 390. You had better have a pretty decent sized cam and a lot of head work to get those numbers. Good Luck

.

Sifu-Sitaigung · 01-01-2013, 01:35 PM

These are the two that I am looking at. Both suggest low rear gears, ones actually specifies. Just seeing what the more experienced guys and gals have to say.

There is a possibility of having one custom ground as well.

33-782-9 - Magnum

33-781-9 - Magnum

Actual BHP of AROUND 500 would be nice, but not necessary. In that area would be fine with me.

I would like for it to out run my '06 mustang GT from the line though.

bartlettracing · 01-01-2013, 02:56 PM

Seems to me 500+ horsepower is a stretch for a stroked 390

Mine got there with no trouble .... 512 to be exsact ... 294s cam ... runs pretty smooth and pulls to 6,500rpm like the wind .....edddy heads with very little work.

FEandGoingBroke · 01-01-2013, 03:03 PM

Quote:

Originally Posted by PSIG

Your rear gear selection should be based on your camshaft timing and torque converter characteristics. RPMs too low will be a dog and cause excessive converter heat in street driving along with poor mileage. RPMs too high will be more efficient, but with unnecessary fuel consumption from RPMs and more road noise. You need to balance these factors and throw-in your own preferences. It should be a matched package. The whole car is a package and every part should compliment each other.

Don't make the mistake some idiot did with his Chevelle that I ran into with my 85 Ford Escort Wagon last night! He had me off the line but couldn't keep up at the 95 MPH mark as I passed him at about 2300 rpm, his engine was screaming out at about 5500 rpm, any ricer could have eaten him for lunch. Which is why my Inline 6 FALCON IS GEARED TO RUN 100 TO 105 mph ALL DAY LONG on the highway and get me 25 MPG while doing it... Your car may look cool and sound good on the street from light to light, but on the road a 91 Civic 2 dr hatch back can embarrass you.

Quote:

RPMs too low cause more problems than too high. Take a case in-point - the good old FE powered U-Haul trucks. Those things would spin 4000+ RPMs all day long, back and forth across America, for years on end and hundreds of thousands of miles. Nobody thought badly of that, though the mileage sucked. Also look at boats churning 3500-4500 constantly with no ill effects. However, a lugging engine below it's efficient RPM range and excessive heat at low RPMs can be trouble in the making.

I thought you were going 4 speed toploader with this? You're not going to run into RPM too low at highway speeds with 3.00:1 gears in it with that tranny............

Quote:

So, decide for yourself, but strive to make a custom package that works together and does what you want it to do. Set the goals, match the parts, tune them in harmony and drive happy.

David

Custom fit all the way. Make the motor, get car on the road, THEN make the decision... THAT is why you got FOUR gears bro

Sifu-Sitaigung · 01-01-2013, 04:09 PM

I am HIGHLY considering a Toploader for it. It depends on if I can find the XL console parts to go to a 4 speed and the clutch/brake pedal assembly and such. I am weighing my options while I make up my mind, as I AM STILL deciding on everything I want to do. I have the general idea and certain goals..

I am locating parts while I do the chassis and body repair work.

This is going to be long term.

ScottURnot · 01-01-2013, 04:22 PM

We put 3.70's in my boys car and really wish we would have went with 3.25's maybe 3.50's. The 3.70's are great in town but for the highway they suck IMO. the motor is stock currently and my boy feels like he is going to blow it up to run with traffic on the interstate. 55-60 is not so bad but who drives that anymore?

I have a 3.70 Posi third member I would trade for a 3.25 Posi staight up.

DeepRoots · 01-01-2013, 04:24 PM

MOTOR - A Rear Axle Calculator
nifty lil rear gear calculator.
You have to be honest with yourself tho... 4.10s are a ton of fun, but if you intend a modern highway speed of at least 75mph, find the rpms your willing to run at that speed.

for me 3.25:1 are a very nice gear ratio. The awesome part of a ford 9inch tho is that in an hour or less you can swap out a third member.

So why not make up a set of 3.00:1 and a third with 4.10's? Use one for cruising, and if you feel the itch to head to the track or whatever, toss in the other third.

Sifu-Sitaigung · 01-01-2013, 04:37 PM

Keep in mind..My stock rear is in place.

It is a 2.75:1 Open diff.

I have (in the past) buried the speedo...1st gear to 50 mph, 2nd to 90 and 3rd off the speedo...120 plus... Nice to be able to...

Just took a while to do it :-)

I would like to shorten that up a bit.

frdnut · 01-01-2013, 06:02 PM

PSIG is correct of course..The gear selection will depend on the cam and convertor..Even if you have a 4 speed and highway gears it will still fall on its face every time you switch to a higher gear unless you have it floored..NOT great for city driving...I would go with a bit of a compromise gear that will allow you to highway drive at the speed you like but be as low (high numerically) as possible...Probably something in the 3.50-3.70 range...Check out an online rpm calculator and see what you can live with.
P.S those are some huge camshafts!.Footnotes recommend 4.10 gears and a 3500+ stall....I think you may have to rethink what it is you want...Unless your building a salt flats racer big cams and highway gears just don't go together.

frdnut · 01-01-2013, 06:14 PM

Quote:

Originally Posted by FEandGoingBroke

Don't make the mistake some idiot did with his Chevelle that I ran into with my 85 Ford Escort Wagon last night! He had me off the line but couldn't keep up at the 95 MPH mark as I passed him at about 2300 rpm, his engine was screaming out at about 5500 rpm, any ricer could have eaten him for lunch. Which is why my Inline 6 FALCON IS GEARED TO RUN 100 TO 105 mph ALL DAY LONG on the highway and get me 25 MPG while doing it... Your car may look cool and sound good on the street from light to light, but on the road a 91 Civic 2 dr hatch back can embarrass you.I thought you were going 4 speed toploader with this? You're not going to run into RPM too low at highway speeds with 3.00:1 gears in it with that tranny............

Custom fit all the way. Make the motor, get car on the road, THEN make the decision... THAT is why you got FOUR gears bro

Must have been a weezing 307 2 barrell in that Chevelle.

ScottURnot · 01-01-2013, 07:22 PM

I'm not trying to say the 3.70's are a huge problem, just dont like spinning the Rpm's on the highway. We currently have 28" tall tires, 3100 RPM yeilds 70MPH, 5000mph is about 110mph. Dont let tires fool you, they are just a important, we originally had 295/50/15's(26.7"tall) and it took 3300 RPM to go 70mph. We are going to a set of 29" tall tires soon that will get us down to 3000rpm for 70mph.

It really depends on a lot of things but I guess it depends on what RPM you want to wined, in hind sight I wish we would have went 3.25's with 29" tall tires. This would give us 2650rpm for 70mph and 132mph at 5000, that probally where we are headed.

Sifu-Sitaigung · 01-01-2013, 07:58 PM

Quote:

Originally Posted by frdnut

PSIG is correct of course..The gear selection will depend on the cam and convertor..Even if you have a 4 speed and highway gears it will still fall on its face every time you switch to a higher gear unless you have it floored..NOT great for city driving...I would go with a bit of a compromise gear that will allow you to highway drive at the speed you like but be as low (high numerically) as possible...Probably something in the 3.50-3.70 range...Check out an online rpm calculator and see what you can live with.
P.S those are some huge camshafts!.Footnotes recommend 4.10 gears and a 3500+ stall....I think you may have to rethink what it is you want...Unless your building a salt flats racer big cams and highway gears just don't go together.

Those are the mechanical lifter cams. I am also looking at the hydraulic roller from crane :HR- 234/354- 2S- 12 camshaft - Fd. FE 352- 428 V8 63- 76 .623 and .651 intake and exhaust respectively @ the valve of course. Still maintaining the 1.76 ratio.

Better cruising speed and with an rpm calc. it puts me around 3-4000 at or around 60mph depending on tires. Of course that is also dependant on whether I use the 3.7 or 3.5.

Comps recomends the 4.11. Not too sure I want to go that steep.