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What rear differential gear do you recommend for the 4.6L?

ModFoxMustangs

Well-Known Member
Staff member
What rear differential gear do you recommend?
2V GT: The GT reacts to gear changes much the same way a 5.0 does although the real problem is that they have less torque than a 302.

For stick cars we recommend 3.73 gears. Because of the lack of low end torque, we don't feel 3.55's are enough. The 3.73 gives an excellent increase in acceleration and top speed isn't decreased because you are able to wind out overdrive further. With a 3.73 your rpm in 5th gear at 60 mph will only be 2005, not at all what you might call "too high". As we said, your top speed will not decrease when putting lower (numerically higher) gears in a car with overdrive. You will simply wind out 5th gear further than you could with the higher gears (numerically lower).

For AODE cars, 3.73's are still a good choice but we recommend 4.10 gears. A 4.10 may sound extremely steep, but because the AODE's 1st gear is higher (numerically low) and 4th gear (overdrive) is steeper than the T-5, it's a very good choice for the driver interested in making their car perform better.

4V Cobra: The Cobra has an entirely different engine than most people are used to. It performs best at high rpms, upwards of 7000. It's power band is relatively high due to the deep breathing nature of the two intake valves. When gearing a Cobra, 3.73's just aren't enough. They don't allow the engine to reach its powerband quickly enough. 4.10 gears are an excellent choice and 4.30's are even better. This may sound very steep if you're used to the average 5.0 which needs to be shifted by 5600 rpm. But since this engine will easily rev to 7000 it needs the extra gear and it likes it. 4.30's are our #1 choice, naturally aspirated or supercharged.At 80 mph in fifth with 4.10's the engine only turns about 2700 rpm, completely tolerable.

AutomotiveAlternative
 

87_notchfox

justr another notch among the rest...
Donator
i got 3.73 on my coupe with a 98 cobra engine and trans...it seems to move pretty good and i do about 70 mph at about 2400 rpms...the irs i have has 4.10
 

87_notchfox

justr another notch among the rest...
Donator
i got 3.73 on my coupe with a 98 cobra engine and trans...it seems to move pretty good and i do about 70 mph at about 2400 rpms...the irs i have has 4.10
 

Speedtospare

Well-Known Member
Going to have to disagree with your post. The above information is good for drag cars. Road cars or street only cars are different. Too much gear on a road course plays hell on your exit speeds. Roots/Screw superchager cars need less gear because of massive bottom en torque. Centri blower 4V cars reall need help on the bottom end so 4.10 is minimum.

Running a Mach 1 motor, Vortech V7, and T-56 in my fox. 4.10 is my starting gear and I will see what it traps vs RPM in the 1/4 and report back in a month or so.
 

87_notchfox

justr another notch among the rest...
Donator
i will like to know the results of that...i just put a 01 IRS with 4.10 gears havent taken it out but not sure if i want gears that tall for a regular street car
 

Speedtospare

Well-Known Member
If your car is NA and t45, then I would personally go 4.10-4.30. I tend to lean towards the drag side of things.
 

87_notchfox

justr another notch among the rest...
Donator
yeah its NA DOHC and t-45...but not sure how well this trans will do with these gears because the SRA i had on it before had 3.73 and 1st gear sucked lol
 

Bill

Well-Known Member
Staff member
Donator
i will like to know the results of that...i just put a 01 IRS with 4.10 gears havent taken it out but not sure if i want gears that tall for a regular street car
I meant to look at your build thread before I posted, but didn't. Which IRS did you use, the 99-01 or the 03-04? I have the early set-up for my current build. If you want the real deal from the guys who are winning at NASA, check this bunch out. http://www.fulltiltboogieracing.com/index.htm They have some of the best how-to stuff on the web for the IRS.
 

87_notchfox

justr another notch among the rest...
Donator
I meant to look at your build thread before I posted, but didn't. Which IRS did you use, the 99-01 or the 03-04? I have the early set-up for my current build. If you want the real deal from the guys who are winning at NASA, check this bunch out. http://www.fulltiltboogieracing.com/index.htm They have some of the best how-to stuff on the web for the IRS.
I know it's old...but I used a 01 irs for my old coupe. Never got around to upgrading it. Was more expensive then a SRA. But I miss that car...[emoji30]
 

whiteponygt

Well-Known Member
Everyone keeps saying4:30s that just sounds really low I had a 2valve with4:10 and on the interstate with o/r x pipe and flowmaster tacking 2700 we couldn't barely hold a conversation,is there a big noticeable between the 10s & the 30s
 

Speedtospare

Well-Known Member
After driving the mine on the street all summer 4.10 is great of your setting up for drag. I cross the line at 6300-6500 in 4th. The problem is that car is getting faster and faster. Soon i will be going through the traps at 7000. Gears will need to be swapped out again for a 3.73.-3.90. If your staying NA definitely stick with the 4.10-4.30's. 4.6 4V bottom end torque just isn't there. I use to bog my car when it was NA
 

Bill

Well-Known Member
Staff member
Donator
I agree. 4.10 to 4.30 is really good for NA. I'm not a drag racer, but that is perfect for autocross. Most courses are too short to get past 3rd year. Open track gives you plenty of power for most courses. Watkins Glen back stretch will let you hit 5th if you carry the speed out of the curve. Hallett is a bit different due to the vertical climbs and drops. You 'll have plenty of gear there, but brakes might be an issue for most street cars with modfox setup.
 

rcustomz

Active Member
i'm running 4.10's in my 8.8, figure that with the 5 speed would let me have fun at the track but still could drive it on the highway
 

bc474

Member
Donator
What gear would you suggest for a stock Mark 8 4.6 with the auto tranny. My plans for this car is to be my DD. I might take it to the track once or twice just to see the times but that is not my priority at all (I have a different car for that). The 8.8 I swapped in had 2.73 so I switched in some 3.73s I had sitting around. Will this be a good choice?
 
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