Rudi wrote:

Alessandro wrote:

Thanks guys you are great ! So many info for me !
Free beer in my garage !

 Castello Di Udine?

 
I live in THE motorvalley .  Ferrari ,lamborghini,Maserati ,pagani,ducati  all settle here around my town  .Modena !

Alessandro wrote:

Rudi wrote:

Alessandro wrote:

Thanks guys you are great ! So many info for me !
Free beer in my garage !

 Castello Di Udine?

 
I live in THE motorvalley .  Ferrari ,lamborghini,Maserati ,pagani,ducati  all settle here around my town  .Modena !

 And you drive a classic Ford Mustang! I love it!

TKOPerformance wrote:

Mountain carving IS a drag race from corner to corner.  An old sprint car builder told me that.

You need to look at the RPM potential of your engine too.  A 331 making decent power should probably have a redline of 6,500RPM, which is a good 1,000RPM higher than a stock 302/289 that the early Mustangs had (excluding Hi-Po, Boss, etc.).  The Z-spec trans and a 3.73 gear would be perfect for your setup.

 
Well it depends . ...  As icon  American drag race are straight to the point in shortest time .
Mountain carving or GTO driving are little different from 1. 2. 3  finish  quarter mile.
I dont Need to beat the time from corner to corner up on a mountain .
But For Sure i will use 2 3 and 4 gear for more time engaged in mid rev .
For example a drag race car will never suffer the engine brake of a downhill,
All the post are very good to clear my mind for my necessity .
During this month i realized that what i wanna do ivolves not only a part ad the car ,but like cascade one after another they are connected .
Speed and brake , suspension and upgrade, trans and rearend .
The unrealistic first budget was pass on in a blik.
As i have downsized my hp first madness , i Belive and agree with you that t5 z spec will match my request .
What you think about a 3,40 ?  Too long  ?
Thanks !

TKOPerformance wrote:

With the Z-spec I would really shoot for that 3.73.  I think a 3.40 is a little too long for a trans with a 2.95 1st gear.  With less horsepower on tap you are also going to have less torque to bridge a less than ideal gear ratio.  I think a 3.73 with the 0.68 5th gear gives you reasonable highway RPM, even at 80mph, and your 1-2-3-4 shifting up and down will be very responsive and keep the engine in the sweet spot.  BUT, the key is rear tire size too, because a short tire will make it act like it has more gear, so with a short tire you could probably get by with a 3.40.  The problem is most performance tire sizes are going to be tall when you start talking about like 17" wheels.

 
Ok lesson learned about rear gear !  About tire size  i cant do nothing except use the OEM size  205 x70x15  . here is not legal change tire size .

TKO  you have a PM

TKOPerformance wrote:

IMO that trade off is not worth it for the reduction in acceleration in 1-4.

 
Please tell me in a different way ... Whats IMO ?

At 2300 rev in 5 th on highway is a good range or should better lower it ?

What I'm saying is that a 240RPM reduction in highway cruising speed at 80MPH is not worth slower acceleration off the line and when running up and down through gears 1-4.  I would still run the 3.73 gears given the choice.

The 8" rear has to go too.  Its not up to the task either.  A 9" would probably be simplest, but an 8.8 is also a good option. 

If you decide to run the 8" anyway, don't say I didn't warn you when it breaks, but the practical difference between a 3.73 and a 3.80 is pretty much meaningless, so use the 3.80 gearset.  Run a solid pinion bearing spacer and studs for the caps.  It will probably break the main case at the pinion pilot bearing bore anyway, but that will give it a chance.  There's a reason Ford put the 9" under every Mustang they built that had any kind of engine in it.  The 8" was fine for mild 289s and 302s, but much beyond that they just didn't last. 

I respect TKOPerformance's opinion and he generally gives good advice, but you can buy 4-pinion 8 inch center sections that are rated up to 550 or 600 horsepower.  If you are not planning on more horsepower than that I wouldn't worry about a 9 inch... unless you are planning on racing it and really abusing it.

Last edited by Michael H. (9/16/2016 2:16 PM)

The differential may be rated to 550-600HP, but that's not what's going to break in an 8".  I've rebuilt a lot of rears over the years of all makes and models, and I've got a very good handle on what the strengths and weaknesses of each design are.  The Ford 8", and 9" too, have the pinion bearings too close together.  To "solve" this problem Ford added a pocket bearing and a nub on the end of the pinion to increase stability.  The problem is that the area in the case that supports that pocket bearing is thin and weak.  What will happen with a stock crush sleeve is that it will oscillate at speed.  Eventually the pinion will move in an elliptical orbit and put too much stress on that pocket bearing and bang, the case will fail right where the pocket bearing is.  You can improve the situation by using a solid spacer instead of a crush sleeve.  I never use crush sleeves.  But the weakness is there, coupled with the fact that 8" cases are all gray iron; it just won't last.

When I build a car I build it to take whatever abuse the engine can throw at it.  I usually build the drivetrain first now, because I've had too many vehicles where in my younger years I went into the engine first and ended up with a busted trans or rear and no cash to fix it.  Drivetrain strength isn't sexy, but the car that's on jack stands with a blown rear or busted trans isn't very sexy either. 

Sometimes there's not much you can do other than a custom rear, or trick custom parts, and I've built rears like that out of necessity.  But early Mustangs have a lot of bolt in and near bolt in options.  I'd bet you can find something in a junk yard over in Italy that has a Ford 8.8" or 9" rear in it and a shop that can modify it to fit your car.  IMO that's a better route than trying to make something live that won't, or worse, having to be easy on the car because you're afraid to break it.  I get the distinct impression you're building this car to run it hard.  There's nothing wrong with that, and its a lot more honest than a lot of people are when they build a car.  But being honest about it means addressing these things now, rather than the dishonest guy who addresses them later after something breaks.  Of course you'll be laughing at that guy when you drive past him...

Michael H. wrote:

I respect TKOPerformance's opinion and he generally gives good advice, but you can buy 4-pinion 8 inch center sections that are rated up to 550 or 600 horsepower.  If you are not planning on more horsepower than that I wouldn't worry about a 9 inch... unless you are planning on racing it and really abusing it.

 
Thanks for advice !  This relief my wallet from estinguish ....
American and european peaople are different when talking about hp
Everyone has his point of view and goals .
Mostly i want to build a mustang that run fast , not a monster .
I have driven only a tired 66 fastback that can give , IMO  at least 160 hp .
I dont really know what will be have a 350 hp at the wheel .
The 4 pinion center section is the inside part of differential ?
Maybe is the same thing i wanna change from the beginning of this post .
I should go with powertrax grip pro that is a limited slip diff .
This if i have well inteded 
Thanks

No parts interchange between 8" and 9" rears.  Is not like you can just pop a 9" center section into an 8" housing and have a 9" rear.  You have to swap the whole thing from brake drum to brake drum.  Swapping a bolt in 9" made for your Mustang is not hard at all, just unbolt the 8" and bolt in the 9".  The issue with that approach is cost.  A bolt in 9" is going to run you around $2,500-$3,000 US, not including shipping and taxes.  That's why I'm suggesting looking for something in your area that can be modified to fit.  Its more work, but it saves considerable cost.

Don't look for high performance cars.  I'm sure those are rare.  Look for trucks and big sedans.  Spend some time on google and learn what an 8.8" and 9" rear look like.  Ford built tens of millions of each, some of them wound up in Italy.  You'll have to get a competent shop that can modify the housing to work in your Mustang, but its not that hard.  Plenty of amatures have successfully narrowed rears, and cut off the old brackets and welded leaf spring perches onto them for use in a different car.  Any competent welding and fab shop should be able to do it.  One that specializes in automotive fabrication would be preferred, but with proper guidance all you really need is someone who can measure and weld. 

Alessandro wrote:

I should explain me better my mistake !
I have found one center section 9 inch 28 spine and one axle case 9 inch .resonable price .
Just wanna know if my 28 spline axes could be re used but i think had the same answer . no mix
Ok i will search something , no problem for welding here !
Thanks

Not so.  It gets a little complex, but... 
If your 8" axles have the same spline count as the 9" center section, then they will work together just fine. In your case both have 28 splines, so no problem.  And, if you don't bang shift the trans, the 28 spline axles are strong enough for your application.

The 9" housing must have the same width as your current 8" housing.  It can be shortened by a competent shop.  Do not try to do it yourself unless you have excellent fabrication skills. The spring pads must be repositioned to that of the 8" housing. (new ones can be purchased from most any Mustang house.)  The cutting and welding will throw the housing out of alignment, so you must re align the housing after each weld cycle.  The alignment process requires some special tools to make sure everything is correct.  Don't try it yourself.

And, finally, The axle bearing size must match the 9" housing bearing size.  9" axles came with two different bearing sizes.  They are very different, so you will know when you match them up.

To summarize.  The good news is that the 9" housing can be shortened to the size of the 8".  And if the bearings are different sizes, the end of the 8" housing where the bearing races are can be grafted on to the 9".

Alessandro,  it must be tough having this conversation with numerous Americans in a different time zone and using a language that is not your first language.  You've been doing a great job though.