Need advice on rear end gear...is it worth $700 to go from a 3:23 to a 3:91?

[AKcharger]

Guys need some advice. I was planning on having my 3:91SG chunk rebuilt this spring to replace my 3:23 SG that's working fine, Problem is the estimate for a rebuild is around $700...I was thinking around $300-$350  . So 3 questions
1) Is the diffrence in that extra gearing worth it?
2) I can afford 2 of my 3 planned upgrades this year 1) Carb (800 to replace a 600) 2) MSD 3) Chunk rebuild...which should get cut?
3) Last, is $700 right for a chunk rebuild? seems kind of high to me, I was thinking 3-4 hrs @ $75 per hr + $150 for parts


Thanks

Oh, for a '72 Charger 470 stroker

[71383bee]

Guys need some advice. I was planning on having my 3:91SG chunk rebuilt this spring to replace my 3:23 SG that's working fine, Problem is the estimate for a rebuild is around $700...I was thinking around $300-$350  . So 3 questions
1) Is the diffrence in that extra gearing worth it?
2) I can afford 2 of my 3 planned upgrades this year 1) Carb (800 to replace a 600) 2) MSD 3) Chunk rebuild...which should get cut?
3) Last, is $700 right for a chunk rebuild? seems kind of high to me, I was thinking 3-4 hrs @ $75 per hr + $150 for parts


Thanks

Oh, for a '72 Charger 470 stroker

1) VERY noticeable difference in 3.91 vs 3.23.  the car will FEEL faster and it probably will be down low at the start.  3.91's are on the border as far as highway though...depends on what your doing with the car though. 

2) If it were me I would cut the MSD.  I have had good luck with the standard LX-101 boxes from bumper to bumper.  Good plugs/wires and be sure your timing is on and your good for a stockish build.  The carb is WAY too small for a 470.  Not so sure on gear.  It depends on your combo really...auto/stick, cam, intake, etc.

3) $700 is actually not a bad price.  The $300 days are long gone.  I had a 3.73 et built last year and it was around there.  Not unusual. 

[cpthowdy1369]

700 isn't that bad.  my stepfather was quoted a little over 1000 (so i did it for him for 500)

[68X426]

I vote 3.91 first (you'll love the difference), then the carb as a close second. MSD will find it's way under the hood eventually.

[firefighter3931]

I would do the carb first....that 470 is just starving to death with a 600cfm carb ! Next would be focusing on the distributor and making sure the advance curve is dialed in for best performance. Those two changes will make a lot of difference in how the car performs. 

With a stroker, you have LOTS of torque so gearing isn't as critical. Optimize the engine combination first then decide if you want deeper gears. 

How tall are the back tires Bill ?



Ron

[AKcharger]

Thanks for the input guys

...Ron, tires are "massive" 14 inch 70's. I forget the metric number but their tall.

Copy on carb, some say (and they're probably right) the 800 is still too small, but I'm familiar with the Edelbrocks and since it's street 95% of the time, dependability is more important than a few extra horses...plus I think the 800 will be fine.

Now about the 'advance curve' tell me more. I have a stock MP electric distributor with the vacuum advance disconnected. I set timing with a gun, but ended up going "by ear" since it seemed to run better. is there anything else I should be playing with on the distributor?

Really, I want the MSD for the rev limiter, for some reason I'm paranoid about blowing the motor since it's the original block/trans

[71383bee]

I would do the carb first....that 470 is just starving to death with a 600cfm carb ! Next would be focusing on the distributor and making sure the advance curve is dialed in for best performance. Those two changes will make a lot of difference in how the car performs. 

With a stroker, you have LOTS of torque so gearing isn't as critical. Optimize the engine combination first then decide if you want deeper gears. 

How tall are the back tires Bill ?



Ron

That is a good point.  Carb is a must...WAY too small fora  470. 

[0X01B8]

before you spend any $ on gears use this -->

http://vexer.com/68rt/speed.html

[AKcharger]

Hey...now that is cool!

[firefighter3931]

Now about the 'advance curve' tell me more. I have a stock MP electric distributor with the vacuum advance disconnected. I set timing with a gun, but ended up going "by ear" since it seemed to run better. is there anything else I should be playing with on the distributor?

Bill, most engine's want lots of base timing and will respond to that change, favorably. Try bumping the distributor up to 16* BTDC at idle and check the total advance. Ideally you want to see no more than 36-38* all in by 2800-3000 rpm.

First thing you need to do is replace that carb.....that will make a significant difference. 



Ron

[Sixtyeight]

I'm thinking about the same change, but I thought 3.55 was a better option, don't you agree? Compromise between wheel spin, 1/4 mile time and highway rpm

Jeroen.

[firefighter3931]

I'm thinking about the same change, but I thought 3.55 was a better option, don't you agree? Compromise between wheel spin, 1/4 mile time and highway rpm

Jeroen.

If you're asking me Jeroen ; the 3.55 is a descent all around gear but it also depends on the tire diameter....which can significantly influence the final drive ratio. Also relevant is the engine combination & camshaft. You want to keep rpms in the powerband so the motor isn't lugging at freeway speeds.

I run 4.10's in the Charger but i also use a 29.5in tire so it's like running 3.55's with a stock height rubber. The old 446 seemed very happy cruising along a 3000 as long as i wanted it to. 



Ron

[R2]

Really, I want the MSD for the rev limiter, for some reason I'm paranoid about blowing the motor since it's the original block/trans

You can "wire" in a MSD "soft touch rev limiter",,,,
I had the same concerns,,,,i run a stock style distributor,,,( and had a chrome box,,orange box,,etc ),,,, so i put a MSD rev limiter in with the stock distributor and orange box ECU,,,just for some insurance....

Eventually changed everything over to a MSD6AL,,,,b/c i kept having troubles with the MP ECU's.

I agree with Ron,,,curving that distributor will be a big help,,, 

DOUG.

[GreenMachine]

Guys need some advice. I was planning on having my 3:91SG chunk rebuilt this spring to replace my 3:23 SG that's working fine, Problem is the estimate for a rebuild is around $700...I was thinking around $300-$350  . So 3 questions
1) Is the diffrence in that extra gearing worth it?
2) I can afford 2 of my 3 planned upgrades this year 1) Carb (800 to replace a 600) 2) MSD 3) Chunk rebuild...which should get cut?
3) Last, is $700 right for a chunk rebuild? seems kind of high to me, I was thinking 3-4 hrs @ $75 per hr + $150 for parts


Thanks

Oh, for a '72 Charger 470 stroker

Was the $700 just to replace the clutches in the Suregrip? I just bought a used 3.91 SG 741 case this morning to that has never been apart to replace my 3.23 Locker 489 case, so I'm in the same boat as you only I have lots of bodywork to do before I can get to the rearend.