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Q&A > General Tech Advice > What should I do with my diff? > Community Forums > Mustang Forum Australia - Mustang Tech

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What should I do with my diff? Reply to topic

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xpconnor
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So I have made the decision to get fat az racing to build the engine for my car just have to make the phone call to lock it all in. With that in mind I am wondering what I should do about rebuilding the diff. Engine will be over 400hp and lots of torque so do I need to go to LSD/true trac or is just a basic reconditioning of the standard diff fine. Will all be hooked to a c4 auto (any advice on what to do to the transmission would be good also.) and car is just for cruising around.

Opinions please.........


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boofhead
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Sorry to ask but why do you need over 400hp to just cruise around?

------------

Any way, what is the specs of your engine?

I will assume 347 230 - 240 @ 50 Cam and largish heads which is what is necessary to get your target...

So firstly you need to match you running gear with the characteristics of your engine. So should be looking at
Need to decent ratio - 3.80 :1 - 4.11: 1 to start. Secondly you will be better off with a locker diff - in most cases a true trac is the best choice for most people though a Detroit locker is stronger.
Next you will likely need a 3500 converter for the C4 and seriously consider a kit for it box to give it extra capacity to hold the 400+ flb of torque you will have. The diff can stay an 8 inch practically if your not going to pound on it while staying auto helps reduce the shocks.

Hope this helps;


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Shaunp
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Boof if the engine is from Woody it will be a dart 363 with around 480 of torque, he tends to make them as street able as he can so a 3.5 maybe ok

XP, ask Woody what stall and diff ratio he recommends for the engine as , they do this stuff every day, As Boof said you want an LSD of some kind, I'd run a truetrac, a Detroit lockers are strong but they can be annoying due to the way they work, kind of, you may love it or you may hate it. They are basically a loaded ratchet to allow the wheels to turn at different speeds, so they are locked until the load on the inside wheel is enough to overcome the ratchet ie tight turns, then the diff makes a clicking/clucking noise as the ratchet jumps.


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boofhead
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Well the bigger the engine the easier it is to get large power figures and retain streetablity. 3.5:1 could work fine as long as the converter is not to tight. Since your going Dart (with its benefits of extra strong block) then you might as well go 9 inch diff [as its the same argument/reason for its use]. Added to the benefits of strength are a far wider range of ratios to select from and the True-trak is a very common center to use.


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Shaunp
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There is a disc to disc Versailles 9" at Gav's he no longer wants he sold the car it was going in. It is an LSD but would need ratio swap.


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xpconnor
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Boof I probably should clarify that by cruising I mean the car won't be going to the track or drag strip but I do want some good hp for when I put the foot down. Also if I go for torque and bottom end power hopefully the engine will be good on the street and in general I won't have to work it hard during normal driving.

In reality with Woody building the engine and from the discussion I had with him it will cost about the same to go with the 363 as it would to go back a bit in size. Cost will only increase if I try to push it out to 500+hp. Also I will admit that like all of us it is hard to restrain yourself when it comes to engines and power but I don't think I have gone too over the top but still will have an engine that will pin the ears back when wanted.

Shaun I had heard the Detroit lockers can be a bit noisy and a pig to drive if you don't like the way they work that is why I mentioned the true trac. I will call Woody to get the engine started soon and ask him about diff and transmission options.

Cheers guys


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xpconnor
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Sorry guys I was posting while your last two posts where going up.

Boof, spot on, big capacity equals easy to use low down torque which is what I want.

If a nine inch diff is necessary then I would consider it but I am a little worried about getting out of my depth when it comes to fitting it all back to the car and the brakes, wheel back space etc. I will speak to Woody and see what he thinks about it all.


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Shaunp
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8" will be ok as long as you don't drive around doing burn outs every where, but the other one is there if you want it, they are touch wider the a 66.


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boofhead
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It is important to know what you want and you do so that's the first step. So you then make the rest work well with and survive those choices.
True a 9 inch is something that can be added later if you choose to go that way. Having said that 480 ftlb of torque is getting to where most would recommend a 9 inch. It is no harder to install a 9 as it is to install the 8 - just a tad heavier. I would get a housing the right width to suite the car then purchase the rest as necessary. If you go with disks on the rear an engineers certificate will be necessary for the brake modifications.


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Shaunp
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Gav just sold the 9".


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cage
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What are the specs on the internals and bolt on's for this engine XP? Please post them when you know if you don't already.


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Shaunp
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Woody's secret formula, it will be a Dart 4-1/8" bore SHP block, scat rotating assembly, not sure what heads he will use, but he is a Brodix agent, in any case they will be 190-220cc inlet port, name brand alloys probably about a 280 deg roller cam, Airgap and 750 carb at a guess. He likes to use all new stuff including the block, less issues long term. A mate of mine has a 383 sbc They did in 28 steel body ford Tudor rod, it's off it's face but drives like a champ, will turn the tyres any time any gear.

He has a pic on his web site of a 363 in a 65, 470 hp/480ftbl torque.


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xpconnor
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Hey Cage

What Shaun said is even more then I know about it at this stage. Still have to call Woody and lock everything in but at this stage I will get it built up to long block stage and then see how the funds are going. If I can afford it I will just let Woody finish it off but if cash gets a bit tight I will get it sent over here and then just get the accessories to bolt on as money permits.


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xpconnor
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I called Woody today and made the first payment on the engine so it is all official now. Told him that I still want a semi sleeper look so it will be a black block with gold rocker covers. The heads and manifold will probably just stay natural finish but nothing overly blingy.

Because I want mainly bottom end power he thought probably RHS 180 alloy heads, hydraulic roller cam, and a 650 dp Holley with air gap manifold. That is with a dart block and 363 stroker kit, probably scat but didn't check. If something changes I'm not too worried as I am confident that Woody will put together the best combination for a really strong street engine.

Going to cost a bit more then I wanted to spend but I am sure the pain of paying for it will be forgotten every time I fire it up and that smile spreads across my face, not to mention every time I give it a boot full.

Anyway getting back to the original thread I have been advised to go with a 3.25 rear end, and a true trac would be nice but to stay away from a Detroit locker. This should cruise along at 100km/h between 1500 and 2000rpm depending on tyre size when hooked up to a c4. Does that sound about right?

Also thinking about trying to save a few dollars I thought I might have a go at rebuilding the diff myself. I have linked the first in a series of 3 videos below and even though this is for a nine inch is this basically the correct process?Looks like I would only need one specialised tool and the rest is fairly straight forward. After the rebuild of the pumpkin then all I would need to have done is new bearings pressed onto the axles, replace the seals and reassemble the whole thing. How much would a full rebuild kit cost on top of the $450 an eaton true trac costs? And I assume I work out how many splines I need once I pull and axle and have a look.

youtu.be/LeWfDTZEW0Y

Lastly I was recommended to probably just stick with the stock c4 get it record and not to go any bigger than a 2000rpm stally. Any thought on this?

Sorry about so many questions in one post and am perfectly happy to go with what I have been told thus far but open to other opinions. Thanks in advance


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Shaunp
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Summit do a full diff rebuild kit for about $90, all bearings crush sleeve etc. Make sure you get the one with large carrier bearings for the True rack. You need a dial gauge to set back lash, a rattle gun to crush the pinion preload sleeve. To measure the preload drag I just use like fishing scale from BCF. The preload is measured in inch pound of torque. Make an 2 inch round pulley/drum thing and attach that to a socket that fits on the pinion nut wrap string around it and pull it with the fish balance scales, this is inch pounds, pounds of force at an inch from the centre, keep tightening the nut till the drag = the correct preload.
True tracks are about $480 I think, gears about $170. You 8" will be 28 spline. Summit have everything you need.

Woody normally likes to build the Cruiser engines to be a stump puller, big cube, streetable cam, and carb. Hence the 3.25 diff an small stall, that's all it will need. He is not keen on vac carbs, they are for girls.


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