androdz

an upgrade from 3.55 to 3.73 is a waste of money... Not to much of a difference. If you are gonna pay for the install make it worth with 4.10's. A lot of factors make your tires spin. If you still have the stock crapellis you can burn the $$$$ out of them with just 4.10's and bolt ons. On the other hand if you have 270/xx/xx or bigger with 250 or less treadwear trust me they will grip.

Att. Andrew

06hoss

Hey Jay, I don't think Ford Racing sells the 3.90 gear set (that's why you can't find them). If you want to stay with Ford, you'll have to choose either 3.73 or 4.10+. If you really want the 3.90s, you might want to try Motive.

Andrew sorry, but it's a little more complex than just shouting 4.10s at everyone who wants to change their gear ratio. When you change your gear ratio upwards you are leveraging the available torque the motor has, but when you add a SC you are adding a dramatic amount of torque. In my case, the SC I'm looking at will add 90 to 100 ft/lbs of torque with most of it down low. Using 4.10s to leverage that much torque is overkill and will actually make driving the car on the street a nightmare as other SC guys have found out. It's generally accepted knowledge that 3.73s are the max you want to go when you SC these motors expecially if it's a twin screw or a roots blower.

MBDiagMan

I agree with the previous poster. 3.73's are not enough difference to justify the bother or the expense. Don't fear the gear. These engines have a torque peak of 4,500 RPM. They also have a tall tire and a very high top gear. The 4.10's sound like a lot of gear and if you were talking about your '66 Mustang instead of your '06 Mustang you would be correct, but times change.

06hoss

Again, I'm talking about putting a blower on this engine for use in a street car. This blower will make about 360 ft/lbs of torque at 6300 rpms while the stock motor only produces about 220 ft/lbs at 6300. The stock motor does drop off dramatically after 4500, but the motor with a twin screw blower does not. There are plenty of guys out there with twin screws or roots that are running 3.73s since it is the appropriate gear ratio for an engine producing that amount of torque in a street car. If you've ever driven in a car with a blower and too much gear you would know what I mean. I've been there and too much gear absolutely sucks on a street car. Having your tires spin when you don't want them too is a bitch!

AguynamedJay

06 Hoss,

You're correct on the gears I can only find them with Motive. I guess my question is still would the 3.90s be enough of an increase to spend the bucks to do it? I am not doing a blower or anything like that now or in the future and I don't want the issues the 4.10s seem to have.

Maybe I should just forget the gears, pop some headers on and let it be...?

Jay[sm=headbang.gif]

06hoss

If you're going to stay normally aspirated, then I agree with all the 4.10 guys. Go with the 4.10s and go with Ford Racing Performance gears. They only time I disagree with the all 4.10s all the time is when you get into blowers.

mustangman02232

then i might even just leave the 3.55s in but upgrade to a true trac, 31 spline axles, and a girdle to help out, unless you plan on doing lots of suspenition work and some sticky tires, there are a few people wiht 4.10s and KBs around, try talking to them to see how they like em, ive never driven a car with a KB so i wouldnt know

lynwood78

How does this sound like? I'm upgrading my 3:31's to 3:73's Plus I'm looking for a torque converter since my car is auto. Plus I've heard 4:10's are too much for autos. I sometimes take long trips on my stangs take it once in a while to the track. Will the torque converter help out a lot?

jacrockett

I agree with 4.10 for n/a, 3.73 w/ a blower. As it says in my sig, I've got 3.73s, and like them just fine.

When I had a KB blower on my previous 2v motor, I could spin the tires (yes on dry pavement) @ WOT going into 3rd gear with an AOD tranny

I've heard that gear whine can also be caused by not breaking them in properly

06hoss

Here's a response I just got back from KB for the SC I'm looking at...they suggest 3.73 or even the stock 3.55 for the street:
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I have been modifying my 2006 Mustang GT and the final mod is going to be your supercharger for this engine. Before I get that far I am going to change out my rear ring and pinion gears. I am currently running the 3.55 gear, but am going to go to a 3.73 even though a lot of guys are saying go to a 4.10. I have heard that 4.10s are too much for a supercharger such as yours for a street car...too much as in too much torque with the tires spinning when you don't necessarily want them to..I'm not concerned about gas mileage. Can you please let me know what rear end gear ratio you would recommend for your 2.6L blower at 8 psi for a 2006 4.6L running on the street?
Thanks,
>>>
The 3.73 are a good street gear, a sugestion though would be to instal the supercharger first. You may find that the 3.55 are what you want.

Thank you for the interest in Kenne Bell Superchargers!
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I think I'll take their advice and let the gears wait until after the SC.

Mustangman, we've already got the 31 spline axles on the 05+ GTs and I am probably going to stay with the stock 8.5" tread tires, but not sure yet if I'll go to 10"s.

Lynwood78, I've heard the the auto has a steeper first gear than the manual, but It's really up to you on that one. If it was me, I would probably still go 4.10s with an auto if it was normally aspirated. I'm not sure what you mean about the torque converter...are you going with a higher stall speed for the strip? If that's the case, then the trans will just engage at a higher engine rpm, but until it does, it doesn't even see the gear.