Years-ago, when my Z28 had the original 3.42 gears with a decent 350" SBC, it ran no-better than 14-Forties @ 97+ MPH with open-pipes @ OEM tires (225/70R-15 UNIROYALS):
a switch to 4.11s, with nothing-else done, resulted in 14-Teens at the same MPH.

The closer to the optimal 1/4-gearing you begin with, the less drastic performance gains you'll see with a gear-swap
(i.e. - a switch from 3.73 to 4.56 may provide little, or NO, reduction in E.T., depending upon the combination)
I knew a young fellow who had dropped a stout Olds 455 into an '83 Cutlass Brougham, originally equipped with a diesel and geared with 2.41 or 2.56 ring-pinion, running low 15s, and reported that a swap to 3.73s put him into the high-13s, over a full-second of E.T. reduction.

I believe that BKbroiler gained so-much simply because his 3.08s were so-far from optimal (similar to the Cutlass mentioned above), and he and I have discussed that 4.11s might be too-much gear for his current application, as he is presently trapping @ his maximum RPM with the .73s.

 

Thanks to all that responded.....
Still not absolutely sure which way to go.... but leaning towards sticking with what I've got.

I gather a 3.7 diff would liven the performance as long as I can maintain traction..... I guess I could always learn to use throttle control Or get bigger boots for the rear.

By the way, 1st gear is 2.87 ratio, and with 3.36 gets me out to 45~50mph.
I figure the 3.7 would still get me to about 40 mph, which isn't too short.

I have a slight annoying "driveability" issue at present, which I am assuming is due to the cam specs... 242*/240* @ 0.050 on a LSA of 112* (won't buy into argument about LSA's at present )

Currently with the 3.36, I need to drive around in 3rd gear around town (speeds 30~35mph) as motor doesn't like maintaining constant speed below 1500 rpm. It's OK if under load, and will pull without complaint in 3rd gear from 900 rpm..... it just doesn't like constant rpm below 1500 though. It jerks and shudders and just doesn't like it.
It's especially bad prior to motor reaching operating temp..
It's very good above 1500 rpm though. No issues at all.

I figure a change to 3.7s would bump up rpm and allow motor to run better around town.... should be OK in 4th at 30~35mph.

63Mako asked about timing....
currently it has 12* initial, 34* mechanical advance all in by 2500 rpm
I think vac adv adds about 10* whilst cruising.
Could my timing curve be causing my shuddering below 1500rpm at no/little load? There's about 9~10" vacuum at idle. Carb is a Holley 4150 HP Street.... and dizzy is HEI style with inbuilt coil and mechanical tach drive.

If I can eliminate the low speed shuddering through tuning, I'll more than likely keep the 3.36, as it does suit my driving style.

What ARE the symptoms of too little or too much ignition advance at lower rpm?

 

I would look more to a vacuum leak causing your poor low-RPM performance. Since you already have a vacuum gauge, disconnect ALL vacuum lines and hook-up only the gauge. Then start the [warm] engine and see what vacuum level is. Add in the dist. vacuum advance system and check it again. If it didn't change, the vacuum advance is not leaking. Now, adjust the distributor back a forth just a bit to see where you get max. vacuum on your system. That will be the best you can do with your engine...and that timing point will be the best idle setting for it, as well. If you get much more than 9-10" vacuum, you have a vacuum leak somewhere in the system. You can add-in the vacuum systems...one-at-a-time...to find which circuit is the problem. Vacuum level would be the easy one to fix.

 

Too little, low rpm power issues, too much, detonation. I would bump initial up 2 degrees and make sure you have no detonation issues. If you want to really get into it bump the initial to 16 degrees and limit your mechanical to get both all in no more than 36 degrees @ 2500. It will help your low RPM driveability issues.. Or go to a 3.73.

Edit: vacuum leaks are real common on our cars. I agree with 7T1 vette. Rule that out first.

 

thanks for the prompt replies guys...
There's no leak at manifold. 9~10" is all the vacuum there is at idle.
Cam has around 80* overlap.

I will try advancing timing at idle.... If it picks up some torque down the bottom, it may feel better. In gear from rolling start, there's no real "kick" in the back till about 2000..... It absolutely "zings" when it hits 3000 and holds strong to 6000.

Here's an example .... half way through this clip I do a stationary takeoff from the lights.....



3.7's or 3.36's

There are times I think maybe I should have opted for the slightly lower 460hp option. It had a 230/230 @ 0.050 cam spec, with the Etec-170 heads Hard to make educated decisions when dyno reports don't show details below 3000 rpm.

 

Didn't know you had 80* overlap. Your car needs gears. Timing will help a little but with that much overlap gears are your friend. You will be closer to your sweet spot at cruise too. JMHO. BTW, Your car sounds Bad a$$. 3.70 gears will make you glad you went with the motor you chose. Matches your build better.