Rear end gear?

[keith4909]

Pontiac 474ci
Max rev 5200-5500
Turbo 400
275/35r18 rear tire


Are 3.73's to low for that motor/tranny/tire combo


Found a few 12 bolts for sale it all have that low gearing.
Was playing around with speed calculator and the Rpm will increase from 3000 to 3400 compared to a 3.31 gears. Any thoughts. Thank you

[4mul8ion]

With the 3.73 at 3400 RPM of the engine or a cruising speed of 70 mph you're probably well past peak torque of the motor and about 2/3rds the way to redline. Max speed with your tire size in first gear assuming a redline at 5500 is about 44 mph, 2nd gear is about 73 and max speed is about 109 mph. I don't think the engine, tranny, tire size combo will give you what you're looking for but a lot depends on what you're trying to do with your car.

Where is peak horsepower and torque on your engine? What are you setting your car up to do?

Kevin

[4mul8ion]

Is a transmission swap out of the question? I'm thinking either a 200 4r or 4l80e depending on your power/torque levels to give some more options.

[keith4909]

I was afraid of that gear being to low. Just really a street car. Won't race it but I do want a decent cruise speed. I'm thinking 3.08-3.42 would be better.

[4mul8ion]

Just a few what ifs:
with the 2.48 TH 400 1st gear and a 3.08 - 1st gear max speed about 53mph , 2nd about 89 mph and top speed at 5500 is 132. Cruise is about 2500 RPM at 60 and 2900 at 70
with the 2.48 TH 400 1st gear and a 3.42 1st gear max speed about 48, 2nd about 80 and top speed is 119. Cruise at 60 is 2750 rpm and is about 3200 at 70.

If you change the first gear planetary set in the TH 400 to a 2.75 / 1.57 and go with the 3.08, 1st gear max speed is about 48, 2nd is 84 and top speed is still 132. Cruise stays the same. You'd get a little more leverage against the car's weight in first and second and still have a decent cruise rpm.

[4mul8ion]

I just saw your other posts with your engine specs of about 480 hp 600 tq and a source for a 12 bolt with a factory 3.31. Did you get the factory 3.31? Are you looking to keep the TH 400 and change out the rear gear to drop the cruise rpm? I'd keep the factory 3.31 for how quiet they are if a beefed up 2004R will stand up to your engine since it'll bolt up direct to the Pontiac engine.

With the 3.31 and 2004R: 1st gear max speed at about 45 MPH, 2nd at about 78, 3rd about 123 and top speed of 183. Cruise RPM at 60 is 1800RPM and 2100 RPM at 70.

I thought Quicktime bellhousings made a Pontiac to 4L80 adaptor. Couldn't find the part number.

[keith4909]

A tranny swap isn't totally out of the question. So far the car has been a money pit. You all know that though. Which tranny bolts in with the least mods. What does that typically cost to do? I can do some research on the swaps. The deal fell through with the diff in SoCal.I had a deposit of 500.00 down and was going to pick it up but 5 hours before I was going to leave he cancelled the deal and refunded the deposit. Now it's either scoring a 12 bolt for 2k or moser for 2600 ish. Not sure which direction to go with new or rebuilt.

[4mul8ion]

I think the 200 4R is the more common swap for the GTO as it will bolt directly to the BOP mounting pattern but maybe others have a better suggestion. In the little research I've done, you'll have to really have someone knowledgeable built one up for you to handle the power and torque. Some sources that I saw most commonly mentioned were
http://www.extremeautomatics.com/
http://www.ckperformance.com/List/20...que-Converters
http://www.jakesperformance.com/200-4R.html
http://www.cpttransmission.com/

I would go with a new housing and leave the used housings to the collectors but it's easy for me to spend your money. To answer your original ratio question, given your 5500 rpm redline and 275 35R18 tires, if you go with a 12 bolt, I'd go with a 3.31 gearset if you can find one, 3.08 otherwise. Cruise with the 3.31 and the .67 OD puts you at about 2100 RPM at 70 MPH. The 3.42 is about 2200 and the 3.08 is about 1950. If you go with a Ford 9", I'd go with a 3.25 ratio. Cruise with the 3.25 is about 2050

Just thinking about your engine again, something doesn't seem right. Are you sure about the 5200-5500 Max RPM on your 474 roller motor?

[4mul8ion]

Is your car carbureted or fuel injected? 3.08 cruise speed with the .67 might be too low an rpm for a carb. Q jet might do it.

[keith4909]

From what I understand is the Pontiac big motors do want to rev that high. Peak power will be around 5200 from all the research. I'll look into the 200r4 swaps. Are there any electronics involved?