Diff gears swap.

[boxhead]

Hey there, just doing research at the moment.
Looking to change out the 3.45:1 diff gears I currently have in my car (Classic Revival with AU IRS), I am reading alot of other forums saying to use the skyline 3.9:1 or 4.11:1 (I think I will go 3.9:1).
Does anyone know for sure if the crownwheel and pinion are a straight swap (I realise I will need to reset backlash and so forth) but I am wanting to be sure that the crown wheel bolts straight onto carrier and that pinion gear is correct length, splines on end are correct for Ford uni joint mount, shaft diameter is correct for Ford bearings etc.
A mate out here who rebuilds all the drag car diffs has offered to lean over my shoulder while I do mine and assist with tools and shims etc.

So any info is appreciated.
It will save me joining a few forums and asking questions. I really dont like to join a forum and first post be asking how I do something.

[Gav]

You a free loader Dave?
Never!

[boxhead]

Not sure if that is sarcasm or if I should be offended?

[Jaydee]

That's a matter of a pinion [and crown wheel] sorry bad pun.
The ea diff being a salisbury diff it will need a diff spreader. And an expert to set up. Proper gear mesh, bearing preload, backlash. I would take it to a diff specialist.
Why do you want to go that tall a ratio?
JD

[Modena]

4.11's will mean tyre smoke galore

[boxhead]

Jaydee, with the 3.45:1 AU LSD centre, and the 6 speed gearbox behind the LS1, I am pulling 130kph at 1300rpm, great for fuel economy (and I should know having driven mine from Alice Springs to Cairns and back), but really not a great setup.
The 3.9:1 will give me a better ratio for drag racing, but still give me a decent ratio for when in 6th gear driving long distances.
You will discover this when yours is mobile.

[boxhead]

Ben, that is what puts me off the 4.1:1 idea, just too low I reckon.

[Gav]

Originally Posted by boxhead

Not sure if that is sarcasm or if I should be offended?

Just sarcasm Dave

[Jaydee]

Do you have 2 od gears. What's your top speed in the outback?
I'm looking at a tremec tr 3650 5 speed. It says it's for a BA 5.4l. Are they a different bellhousing to a 5.0l windsor? It's hard trying to locate a complete setup.
JD

[boxhead]

Yep Jaydee, I run the T56 6 speed manual.
5th and 6th are both overdrive.
Top speed I have reached going home from work is 240kph.
Jaydee (I have a short memory) is your car a square tube or round tube?

[Jaydee]

I'ts 2003 square tube. Very solid. 140% beyond specs.
If you have 2 od's I agree with a taller diff. But you'll only be in first for a second. Man, 240, 144 mph. How long to get to that speed?
I've had a Ferrari up to 220, and that seemed scary, especially when it went really light on a bump, and the air got under it.
JD.

[Modena]

Dave look at what Commodores are running with your engine/trans combo, almost no-one goes 3.9's, most go for 3.73, and that is with 600kg more.....

4.11 would be silly.

[boxhead]

Jaydee, I know all about the CR specs (I have one too) mine is a 2004 car, what number chassis is yours?
Do you have the in depth build manual?
I asked what chassis you have because there are many tips to make it better.
Ie lower the steering rack, talk to Aussie Mike about a rear bump steer kit (single most biggest improvement).

Ben, I had not thought to check on the Holden forums, will do that.

[boxhead]

Jaydee, the pic in my signature line is one I took last year driving home from work.

[DaveW]

Hi Dave,
I' just whipped up a little excel calc if you want it.
Put in your tyre size, gear ratios, diff ratios, and a speed and it will tell you what revs.
Probably others on the net.

For me, 3.7 Diff, TR6060 box with 0.84 5th and 0.57 6th with 295/35R17's will give me 2582rpm in 5th at 100kph and 1752rpm in 6th.....give or take.

Let me know if you want to play with some figures,

Dave

[boxhead]

Cheers Dave.
I will need to do some measuring at home.
But ideally I wiould like to be crossing the finish line at about 5500rpm in 4th gear.
The tyres I am looking at are Hoosier*D.O.T. Drag Radial White Letters&partnum=76R5DDR&tab=Sizes for the rear end.
The 275/60 R15 is a 27.99 rolling diameter.
Or they also do a 275/50 R15 with rolling diameter of 25.83

Not sure if you can work out an "ideal" ratio with those numbers?

[spookypt]

Boxy, I understand this is for the d/strip? I'm not saying you would or are planning to but I know these tyres (drag radials) have caused some pontification amongst some with their use not being allowed for circuit / sprint events....

If you wanted to do both there are a fair few guys running Hoosy A6's which are technically a gymkana style tyre that gets super sticky really quick. Perhaps not quick enough for drags though.

Spookypt

P.S. I finally found those genuine chrysler mud flaps at home on the weekend. Will bring them down the Nats for you.

[Aussie Mike]

First gear in the T56 is very tall @ 2.66:1 so going to 2.73: or 3.91:1 final drive really wakes them up.

The HSV Clubsport came out with 2.73:1 gears as an upgrade over the stock 3.46:1

The HSV GTS got 3.91:1 gears to wake it up even more.

I've got a Hydratrak diff center from a GTS in mine and the few times I've driven it on the street and the track it was great. I was very happy with the ratio for the power and weight I have. Cruising is still about 1800RPM at 110.

Cheers

[stiffy]

Aussie,

I've got a Hydratrak diff center from a GTS in mine and the few times I've driven it on the street and the track it was great. I was very happy with the ratio for the power and weight I have. Cruising is still about 1800RPM at 110.

That would have been when petrol was 95 cents a litre

STIFFY

[boxhead]

Cheers Spook, the drag Hoosiers are only for the strip.
I hope to run Billboards for street use and circuit racing (although I also have the 17's)

The local tyre shop has offered to swap the Hoosiers on whenever there is a drag meeting that I am running at.

I most likely wont make Nats this year (unless I fly down just for teh weekend) as we will have only just gotten home from UK a week or so before.