Hey guys, I have a 2.73 ratio in my car with an older gmpp 350 HO (300hp) crate engine and 700r4 combo. I'm looking to upgrade to a higher numerical ratio for better performance and driveability. I really don't want to be turning the motor over 5200 RPM because it doesn't make power past 5000 and the max recommended RPM is 5500. My 700r4 is stock besides the vette servo I installed, I havent had it fast enough to see if it shifts into 4th at wide open but I do not want to push it, I'm just assuming that it doesn't. What would be a good ratio for me, I picked up a 3.73 gear set for next to nothing out of an s10 but I'm afraid of putting them in since I feel like I would be turning the motor 5000RPM at wide open in 3rd even though I'm only going like 100mph. I heard of people putting in a 4th upshift valve to combat this but wouldn't that wreck my transmission? I'm also not running lockup, what would be the best ratio for me? I want this car to be able to pull hard on the highway when I punch it going around 60-70.


I know this is a crate engine and all but running these engines at higher RPM is like grinding teeth for me, I just know something bad can and will happen.

 

If your engine makes good mid-range torque, I'd probably go for a 3.42 axle.

I have that ratio in my car with a TH700. At 60-70 mph, if you were go from OD to 3rd, it would hit about ~3500 rpm depending on your tire size. Which is a good RPM to start pulling from...

 

I'd agree, 3.42 is a good match for that engine. 3.23 might be better for ones with lesser-flowing heads; 3.73 or higher for the same motor with some induction system that doesn't choke the motor to death the way TPI does.

TPI is the limit to the whole package. NOTHING you can do will change the torque vs RPM curve of that. Not at all like a carb setup, or lots of other EFI systems that don't rely on the same phenomenon that TPI took advantage of, being all that was available to it back in the day. Best to WORK WITH what TPI gives you, which is, a VERY STRONG torque peak at 3600 RPM or so and RAPIDLY falling-off power above 4800 or thereabouts, rather than HALLUCINATE that you can somehow overcome the laws of physics. As long as those long thin runners are on top of that motor you are BOUND TO the RPM characteristics that they deliver.

Gears are cheeeeeeep anyway, even if you pay full price. You can buy good dependable brands in the low-mid $200 range. By the time you factor in all the costs of the changeout, using used gears IS NOT a winning proposition. Not that they're "bad" necessarily; only, it's almost impossible to know how "good" they really are, and what you save in the cost of the gears, can evaporate REAL QUICK if there's anything even the least bit wrong with them. I wouldn't get too wound up over having a set of used 3.73s on hand at ANY price, even if they were FREE. Get whatever gears you need and don't worry about the sunk cost of the used ones. Save them for another day or sell them to finance THE RIGHT thing whatever that is.

 

Here's something to help with the mathematics.

http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php

 

Budget? 3.23 or 3.42 4th gen rear.

Max with TPI and 26" rear tires? 3.55 aftermarket gears.

Lots of driving on the expressway? 3.23 for mileage.

Keep in mind if you're considering wheel and tire changes in the future, a slightly taller tire may make the little extra gear just right.

3.73 isn't a good match for TPI. Edit: looks like you went carb. Non-emissions, no doubt.

 

You can make a copy of this sheet. Then just change the transmission, gearing, and tire size drop downs to play with different transmission, gearing, and tire size combos.

https://docs.google.com/spreadsheets...yPyuD0huA/edit

 

Question to the contributors. When you're discussing your cruising RPM, what MPH would that be? I can imagine different answers so let's say this is your local interstate and you'll be at the limit and /or your comfort level.
To put it into context, is this part of my country, our "controlled access" highways, aka interstates, have a limit of 110/KPH. (~69 MPH). At the risk of insulting anybody's sensibilities, that's comfortably driven at 75 MPH. Maybe 80. I can spend literally days of driving at those speeds when I'm going cross country.

 

With my gear vendors behind the 700, I'm doing about 2000 rpm at 80 mph. Very comfortable and getting 28-29 mpg to boot.

Even though the speed limit is technically 65, traffic typically moves between 75 and 80.

 

I went from the 2:77 to a 3:23 and it was a different car with that 700r4 tranny. Still good cruising at highway speeds. Is your lockup converter still in the car or replaced with something different? There are aftermarket controllers for the lockup and that does wonders for highway driving.

 

"Cruising speed" around where I live is also usually 75 or so. Until recently I travelled constantly - like, 3 - 4 trips a week - around the entire US and much of eastern Canada, and that seemed to be the norm more or less everywhere.

 

I also agree to not mess with installing used gears. New gears aren't all that expensive and trying to set up used gears to with an old wear pattern usually end up with objectionable gear noise. Just use new gears.

I also agree that 3.42:1 gears are a good match to this setup.

With stock tire size and a 700R4 in 4th gear, a 3.23 rear will be about 2500RPM @ 80mph, 3.42 about 2700RPM and 3.73 3000 RPM.

I did the 3-4 upshift valve years ago on my GTA so I could go WOT in 4th. These days I just don't care to go that fast, so I wouldn't bother.

 

Yup, re. the gears.

Sell those used ones to The Greater Fool. Since I got my latest assignment which is to this miserable planet, one thing I've learned is ALWAYS true here, is that there is ALWAYS A Greater Fool out there somewhere. Just when you think you've seen it all, something/someone even stuuuupider pops up. So, put em on craigslist or something, make a profit, and turn the money into a new set and the 3 series carrier you'll be needing.

 

I lived where 75 was the highway limit between towns.

But now I see a lot of 65mph signs, with a lot of folks ignoring them.

Yes, 75-80 mph when I can control myself is normal.

All my years of having 3.73 gears and autos or manuals from the third gen era are now being updated by driving a 6L80 and 3.08 rear with some non-third gen relevant tires.

 

Thanks for the awesome advice guys, I think a 3.23 gear is more up my alley since this car is my daily and I do drive it between states. I want to do it right the first time, so I'm ordering a full overhaul kit with the 3.23's, yukon posi unit and the master overhaul kit included for $630 off of Quick Performance and I'll be handing the car over to my buddy's shop to get it installed properly. I did go with a carburetor on this particular engine and I do not regret it, I run an AVS2 that's been tuned with an o2 sensor and I'm considering picking up a carb cheater unit from ThunderHead289 just to see what it's about. If I kept the TPI I would've been sacrificing a ton of power potential, plus my intake bolt pattern wouldn't accept it. As easy as fuel injection is, a well tuned carburetor with an electric choke has gotten people where they need to go for ages and the ease of an edelbrock is unmatched. 70-80 miles an hour is not that uncommon where I'm from, people do it all the time on I-495 and It's better to just keep up with the traffic.