Topic: Changing final drive ratio

[1KULSOL]

I have the 3.23 caddy diff as well as a t56 6speed tranny ... DDM installed it with the LS3 ... last road trip I got 26.1 mpg ... cruising at 70mph in 6th gear - rpm's +/- 1700     

[DrJones]

I'm just stealing the ring and pinion out of it since the donor diff is an open diff.

[elff]

Whats the new tire size?

Glad to hear you are keeping the Sol.  It has and will continue to serve you well.

[DrJones]

270/650R18 Avon slicks.

[elff]

Are you finding those work better than the wider hoosiers?
Or is that a new class requirement?

[Arabas]

Very interesting mod! although personally i would prefer a 4 or 4.10 diff for street use.

[DrJones]

Well since these are slicks the sizing description is a bit different.
270 is the tread width in mm (not the section width). This tire's section with is 302 mm
650 is the tire diameter in mm (not the aspect ratio).
and 18 is the same.
using typical DOT tire size designation it would be a 305/30R18 or close.

We are going with a little narrower tire because the new class has a 100 lb weight penalty for rim widths over 10". I'm picking up new inner half section to make my 11" wide CCWs into 10" wide. This will also allow me to get rid of the 1" spacer in the rear and narrow up the rear of the car by 2". This will help greatly with slaloms.
The tire size was chosen based on the 10" rim width.
And a vehicle weight of 2350 lbs will mean I won't need as much tire.

This tire also uses a much much softer compound. They aren't legal for road use (unlike the Hoosier A6's) so they can build them a little more extreme.

a 4.10 gear would make for a lot more shifting than needed. Even on the street that would be a bit much. I think the 3.91 from the base model would be the highest you would want to go.
I'm not sure there's an easy to find 4.10 gear for the Getrag 645 type rear.
Last I checked they had 2.73, 3.23, 3.42, 3.73(GXP), 3.91(Base)

[wspohn]

One issue I didn't notice being dealt with here (unless I missed it) is whether you can just swap the gears rather than the whole diff unit.

Often when going to lower numerical ratios, the diff carrier that the crown bolts to is offset differently as the smaller pinion means that the crown is thicker.  On some cars you need to get the correct differential unit, or stick what you have in a lathe and skim the mating surface with the crown wheel so that everything matches (grinding it off the back of the corn is tedious and too expensive).

Anyone checked the part numbers for the differential cages on these ratios?

[DrJones]

One issue I didn't notice being dealt with here (unless I missed it) is whether you can just swap the gears rather than the whole diff unit.

Often when going to lower numerical ratios, the diff carrier that the crown bolts to is offset differently as the smaller pinion means that the crown is thicker.  On some cars you need to get the correct differential unit, or stick what you have in a lathe and skim the mating surface with the crown wheel so that everything matches (grinding it off the back of the corn is tedious and too expensive).

Anyone checked the part numbers for the differential cages on these ratios?

I'll find out for sure when I do the swap.
But I have direct info saying the ring and pinion will swap in and out with no modification. Might need to shim the pinion to get the proper backlash and mesh. but that's some thing you would have to do even if it wasn't a gear swap.
From what I understand all the 645 type Getrag rear ends are like this.

[Gentleman Jack]

Well since these are slicks the sizing description is a bit different.
270 is the tread width in mm (not the section width). This tire's section with is 302 mm
650 is the tire diameter in mm (not the aspect ratio).
and 18 is the same.
using typical DOT tire size designation it would be a 305/30R18 or close.

We are going with a little narrower tire because the new class has a 100 lb weight penalty for rim widths over 10". I'm picking up new inner half section to make my 11" wide CCWs into 10" wide. This will also allow me to get rid of the 1" spacer in the rear and narrow up the rear of the car by 2". This will help greatly with slaloms.
The tire size was chosen based on the 10" rim width.
And a vehicle weight of 2350 lbs will mean I won't need as much tire.

This tire also uses a much much softer compound. They aren't legal for road use (unlike the Hoosier A6's) so they can build them a little more extreme.

a 4.10 gear would make for a lot more shifting than needed. Even on the street that would be a bit much. I think the 3.91 from the base model would be the highest you would want to go.
I'm not sure there's an easy to find 4.10 gear for the Getrag 645 type rear.
Last I checked they had 2.73, 3.23, 3.42, 3.73(GXP), 3.91(Base)

Ive got to say, i love it when intelligent, well thought out discussion happen on this forum. As opposed to the Crap Some people throw around.

Someday, id love to see Dr. Jones drive in person.

[GXP_Matt]

Great to hear!  Let us know how the swap goes.  I assume you are going to try and get all the weight out you can and ballast it back up to 2350?  Any idea how low you think you can get it?  There is a FP S2000 that runs near me and I think they're below 2k lbs with all the carbon bodywork, but they still run 315 A6s strangely enough.

[elff]

Time to break out the yellow paint.
(Those who have done gears will get that)

Thanks for the info on the tire reasons. I think those will work out well for handling and even increase your traction.

Good luck with everything and kick some AutoX butt!!

[Sol Asylum]

Ive got to say, i love it when intelligent, well thought out discussion happen on this forum. As opposed to the Crap Some people throw around.

Someday, id love to see Dr. Jones drive in person.

I'll second that!

[rlhammon]

Ive got to say, i love it when intelligent, well thought out discussion happen on this forum. As opposed to the Crap Some people throw around.

Someday, id love to see Dr. Jones drive in person.

I'll second that!

Umm... it's not as nice as you might think.    But then again, you'll only be watching... I've had to race against him for the past few years!

Great thing is Alex is a great guy and has helped me make my car and me faster.  I've been trying to close the gap between us, but one thing will be true this year... I'll be faster than him in ASP at least!

[ophidia31]

So the gear in the n/a cars is the shortest you can go? That just burst my bubble.

[DaveOC]

So the gear in the n/a cars is the shortest you can go? That just burst my bubble.

From Getrag's website for the 645 drive axle:
Vehicle ( Platform )     Cadillac CTS, STS, SRX, CTSv, STSv und Pontiac Solstice
Ring Gear Dia.     211 - 215
Hypoid Offset mm     38
Ratio     2,73 / 3,23 / 3,42 / 3,73 / 3,91
Torque Nm     976 - 1.580

[old goat]

Just to throw what very little I know about our rears in the Solstice. Back when folks were still drifting the Solstice they were using the Kaaz diffs that were spec'd for the GTO in the stock GXP housing's, they came in quite a few flavors at the time as GTO's are used in a broader type of racing. I remember that they had I think half a dozen different ratio's. They tried the cadillac diffs and they broke before going to the Kaaz.

[DrJones]

Just to throw what very little I know about our rears in the Solstice. Back when folks were still drifting the Solstice they were using the Kaaz diffs that were spec'd for the GTO in the stock GXP housing's, they came in quite a few flavors at the time as GTO's are used in a broader type of racing. I remember that they had I think half a dozen different ratio's. They tried the cadillac diffs and they broke before going to the Kaaz.

so the diffs for the GTO's would drop in the Solstice housing? hmm... interesting...
I did get the 3.23 caddy gears in last week. Luckily the pinion depth was spot on. but the preload was was to high.
Just for shiggles I bolted up on of the other side covers I had just in case the shims in it were correct for what I needed.
BAM, it was perfect. Lucked out on that one.
And the yellow paint on the gears is a little labor intensive with the way the 645 Getrag works, but it sure helps a lot.

We also braced where the torque tube mounts to the diff and that aluminum bracket as well. I busted the bolt hole in the diff and bracket in 2011... something something sticky tires gobs of torque something something...