8 3/4 Sure Grip ratio change

[sireland67]

I can tell you from experience.
I had a Challenger in my late teens, I would change the 3.23 out on friday after work, stick the 3.91 in for the weekend and sunday evening stick the 3.23 back in.
3.91 suck on the highway engine is just screaming, but are way fun around town.
If you make the metal brake lines on the rear end long enough, you can slide the axles out enough to change the center sections without bleeding the brakes.

 

[AJ/FormS]

4.86s are the right ratio for a streeter not like yours, assuming a cam of 230* or less. The engine will power peak at about 5200/5400 maybe a bit more. The shift rpm will thus be 5800 or a bit more. Assuming 5% slip in the TC, 4.86s will get you about 63 mph topped out, for a 2-gear run for fun.The starter gear will be 4.86x2.45=11.91, which will be all but useless. So.....For a 318 or a 273' sure, 4.86s are the ones.
But for you;
For a one gear run;
to 51mph,3.55s are the ones.
To 56mph,3.23s are the ones,
to 62, 2.94s are the ones.
If you have to shift, The Rs will drop to 1.45/2.45 =59%, or to 3400. Does your engine have power there? Probably, but not nearly as much as at peak.
If your 417 cannot smoke 3.23s to 56mph, on the street with street tires, then there is a problem.Find it and fix it. Also with a 417, and good heads, you can stretch them out a bit; There is no reason you can't hit 60 at 6200 with those 3.23s.Or 65 at 6500.All with the right gear.
Before swapping gears, I would spend a few hours tuning what you got. IMO a 417 with 3.23s could be an awesome street combo. The engine has enough torque to annihilate any street tires,with a reasonably low-stall, yet cruise at about 65=2600. Puuuuuurfect.

 

[brodphish]

OK so it is time to refine my question. I am taking the car to the speed shop in a week or two. Gonna get a variable flow PCV installed to take care of the small rear main seal leak while the engine is running. I'd like to have the shift points sorted at that time as well.

As I mentioned earlier the 727 shifts around 3400 rpm under light to mid throttle. Under heavy throttle the shift points are higher but the tires are spinning. Lots of fun but not ideal for driving. I have the dyno report max torque 533 @ 4000 max hp 471 @ 5200. What shift point should I be targeting?

Sorry but this issue is well beyond my working knowledge. We've had a ton of fun with the car just the way it is. However if it would rev higher at partial throttle before shifting it would be even more fun.

Thanks for the help!!!

PS thanks to the help on this thread I now know the issue is not the rear end gearing but sorting out the shift points.

 

[AJ/FormS]

You will be able to determine the exact shift points from the power curve.
The 727 ratios are 2.45-1.45-1.00 and the rpm drops are to 59.2%, and 69%. This means that whatever shiftpoint you use,the rpm will drop by those percents.The two different drops means you will have two different shift points.
As to the 1-2
If your power declines very slowly, after peak, then selecting a 6500 shift for first will drop the Rs into second, to 6500x.592=3848, and the powerband requirement is 6500 less 3848 =2652. The centerpoint of that powerband would be 2652/2 +3848= 5174;pretty close to 5200. Now if the outgoing power at 6500 is very different than the incoming power at 3848, then you adjust the shiftpoint up or down until they match. Generally on factory small blocks, the power falls off a little more rapidly at the top the it builds at the bottom. I imagine 6500 will be a little high. Check your data.
I can guesstimate,perhaps 6300 for the 1-2 shift.
As to the 2-3
Here the rpm drop is to 69% so the shift will occur much sooner.Since I can't see your graph I arbitrarily select a number, and do the math. I choose 6100.
6100x .69=4200, for a powerband of 6100 less 4200=1900. And the centerpoint is 1900/2 +4200=5150; gee that worked out pretty close. So again, refer to your dyno chart. If the outgoing power at 6100 matches the incoming power at 4200 then 6100 it should be. If not, adjust up or down until the powers match. Every time you change it, you have to recalculate.
So, the preliminary points are 6500 and 6100, which you will have to adjust to match your curve. Then some shift loops at the track will dial it in.
Now if your TC is stalling over 3850, this will throw a monkey wrench into the calcs

Now having said that, those points are to generate the maximum average horsepower during the run and will generally get you the lowest ET.Those therefore are track shiftpoints. On the street with 3.23s, 6500 will be about 63mph. So basically you just shift it whenever,lol, cuz to run it up any more mphs is speeding. With 533 ftlbs@4000, you're probably gonna want to shortshift anyway, cuz it's like you said, the tires are spinning. You won't get to 60 any faster spinning the tires at 6500 than spinning them at 4000. So save your engine.
With a 417, traction is sure to be an issue.You probably want to address that as soon as possible.
enjoy

 

[AJ/FormS]

The part throttle upshift is controlled primarily by the governor pressure, fighting the line pressure and delayed by the KD adjuster.
Reset your KD to the max. If the upshift is still too early, you will have to get inside the tranny.
If it busts the tires loose on the shift, delay your secondary opening, or do something about your rear suspension.533ftlbs @4000 is a heck of a problem for street tires,lol

Now, IMO, trying to solve an oil leak at the rear mainseal, with crankcase evacuation is not a very good idea. Your system will be pulling in unfiltered air and circulating it all through the engine. And the air is coming from just about the dirtiest place imaginable, right next to the pavement.