3.73 rear end

[try2pas]

I have an '80 corvette with a 3.07 rear end. I am just putting in a 700
R4 trany which has a 20% or so lower first gear than the TH350 trany that was in the car. Now someone says to really make it "go" to put in a 3.73 rear end. Will it kill my gas mileage? Will I really jump off the line? Will it be way more reponsive in 2and gear? What are the pluses and minuses?

 

[Hib Halverson]

If you are going to both change to the 700R4 and go to a 3.73 axle, your mileage will probably be the same or worse than it is now and you'd better put some big, sticky tires on the back because with a ZZ4's torque, you'll be spinning the rear tires a great deal of the time.

I'd put the trans in first, then drive the car a while to evaluate it's performance and fuel mileage.

Then make your decision about axle ratio.

 

[Rodney Gold]

At the moment your effective 4th gr ratio will be a 2.14 and if you running 235/50/15 tyres , you are geared at 57 km/1000 rpm which is most likely way too high to be able to achieve top speed or accelerate nicely in 4th. At 60 mph you will be less than 1700 rpm.

At a 3.73 diff ratio , your effective top gr ratio is 2.65:1 which is still lower than a 3.07 so you will still have an overdrive in top , but will be able to accelerate decently or actually USE 4th gear , yes , you will have a short first and will have traction problems if you hoof it from standstill....but that depends on your right foot

So the issue is whether you want to effectively use 4th gear ...cos if you dont change the ratio , it's relatively useless.

So to answer your questions - it depends on you as to whether your gas mileage will be worse - but if you use 4th a lot , it wont be.
Yes , your car will launch better (if you get traction) and will be a lot more responsive in 2nd and 3rd.
If you do the change , you would most likely get to 5500 in top and that will give you a top end of around 150 mph and you will be doing 27 mph/1000 so at 60 mph you will be at 2300 rpm and at 75 , 2800.
Why did you do the tranny swap anyway? For economy?

I swapped my rear gears in my 80 for 3.55 and put in a 2400 stall , but im using a TH350 box and have a hot 350 ...I did it mainly for more power and for short acceleration bursts and limited drag use,

 

[Peer81]

The 700R4 has a 3.06 first gear ratio. With your current 3.07 diff your total first gear will be 9.4:1. When you change to a 3.73 your total first gear will be 11.4:1.
The optimum for street performance is about 10:1 total first gear ratio. So I would say keep the 3.07 gears so you can still use the first gear, changing into the 3.73 will be nice if you like smoking the tires but I think your first gear will be difficult to control if not totally useless.

Also keep in mind that the C4's also had a 700R4 with a 3.0. diff ratio.
Good luck,

Groeten Peter

 

[Rodney Gold]

In terms of a 1st gear being useless if a higher diff ratio is selected is not quite the full story. Yes , you might get more spin , but the spin is easier to control and to stop , as the tire and wheel is actually going "slower" than spinning in a lower diff ratio. The spin is easier to stop as there is less inertia and a lower circumferal speed. I spin more uncontrollably with a 3.0 than a 3.75 in my El camino. You can of course moderate the spin very easily with the loud pedal and your right foot...

 

[Evolution1980]

Try2pas...you're gonna end up with the same setup as me: ZZ4, 700R4, stock rear gearing (3.08). Don't change it. You'll still light up the tires in first gear if you mash it down and you'll get a solid chirp into second. Once or twice I even chirped into 3rd, but that's when the engine was fresh. You'll especially want to keep the stock 3.08's if you do any highway driving.

Lighting up the tires is fun, but it really offer no benefit whatsoever. You lose traction and time if you can spin the tires. If you really want to be able to light up the tires from a stop at a moments notice, installing a line-lock kit is a pretty inexpensive and easy way to accommodate that. (Which I also did.) :cool

 

[Evolution1980]

If you are going to both change to the 700R4 and go to a 3.73 axle...and you'd better put some big, sticky tires on the back.

Adding a bit to this idea, if you have the stock 15" rims, you ain't getting big AND sticky tires on the rear. They just don't make 'em in that size. That's partially what lead me down the path to getting 17" rims.

 

[Rebelrob]

Heres a handy link , just plug in your wheel/tire size and this will layout exactly what your rpm speed will be at X MPH.It has a list of several trans, including the 700r4 with the .70 4th gear OD. Remember to put 3.73 and not 3:73 when you enter your gear ratio. Bowtie Overdrives

Evolution. Please provide some info on the front end line lock you mentioned. I had a friend with a late model mustang that had that years ago. It was pretty fun.

 

[Evolution1980]

Evolution. Please provide some info on the front end line lock you mentioned.

Line Lock Kit
SS Lines to custom bend

I got kinda sneaky and hooked up my line lock switch to my cruise control button on the stalk. I removed my CC looong ago but left all the wiring and everything in place. It works out rather nicely from a 'sneak' standpoint because I have no obvious switch for it. And it works from a safety standpoint because I never have to take my hands off the steering wheel to engage/release 'em. :cool
In the first link, if you don't need the switches, fuse, and all that and want to wire it up yourself, you can save a few bucks and just get the solenoid itself. The link lists both with and without the extras.

If you want to know more, we can create a new thread and I can post some pics in it.

 

[Rebelrob]

Yes, I am interested in this. I like the cruise control idea.
Please start a thread and info. That would be great.

 

[Art Jett]

I started out with a 700R-4 and 3.08 rear. Made for good mileage but a real dog on the highway. When cruising 60 mph @ 1400 rpm if you need to accelerate, but not enough to kick it into passing gear it had no balls whatever. And I'm not running a stock L-48 either. I have an injected 383 with a fairly radical cam. I switched to 3.70 gears and love it. My cruising rpm at 60 now is 1800. I still get over 20 mpg average and it has plenty of scat. I would never go back!

 

[Rodney Gold]

I implemented a line lock that works differently albeit not on my vette. I just looped the brake line to the back brakes into the cab and used a plain old hydraulic lever type stopcock
All i did was reach under the drivers seat , swing the lever closed and tromp on the brakes. This left my back brakes free so I could do a decent static burnout at the start line.
I do prefer the solenoid thing tho , cos with my manual system, if you dont remember to turn the lever back you have no back brakes --- this caught me out a few times on the dragstrip where we have limited stopping distance.

 

[Evolution1980]

I started out with a 700R-4 and 3.08 rear. Made for good mileage but a real dog on the highway. When cruising 60 mph @ 1400 rpm if you need to accelerate, but not enough to kick it into passing gear it had no balls whatever. And I'm not running a stock L-48 either. I have an injected 383 with a fairly radical cam. I switched to 3.70 gears and love it. My cruising rpm at 60 now is 1800. I still get over 20 mpg average and it has plenty of scat. I would never go back!

Huh??? I find it strange that our speed:RPM values are so different. I have pictures of the my car at various RPM on the highway. (They might be in my Flickr account, but I can't get to it here at work to confirm.) Around 70mph with the 700R4 and a 3.08 rear, I'm just about 2000 RPM. I don't know about 60mph because on the highway I don't drive that slow. (I woulda kept the old L48 if that was my driving style )

As far as downshifting, even at 70mph, if I mash the GO pedal, it will usually downshift. Once in a while it doesn't. But who cares as I can just manually drop the shifter down into -3- and there ya go! I guarantee at 60mph if I mash the pedal, it's gonna downshift. I wonder if your TV cable where it connects to the carb wasn't properly adjusted.

 

[Art Jett]

As far as downshifting, even at 70mph, if I mash the GO pedal, it will usually downshift. Once in a while it doesn't. But who cares as I can just manually drop the shifter down into -3- and there ya go! I guarantee at 60mph if I mash the pedal, it's gonna downshift. I wonder if your TV cable where it connects to the carb wasn't properly adjusted.

If you would have read my reply, you would have seen that I said if I had to accelerate, but not enough to kick it into passing gear. Of course if I wanted, it would kick in and accelerate with no problem. The TV cable is adjusted properly. There are times however, that you just need to ease around someone--not blast past them. It's much easier to build speed in the gear you are in if the engine is in the power band, even if it is the lower end. My cam builds power from 1800-6000 rpm. If I'm below that, it lugs pretty bad. The speed limits here are generally 60 mph and I prefer not to give all my money to the police departments. Not to mention the raised insurance rates from multiple tickets. If I feel the need for speed, it's there. The car shifts into 4th at around 115 mph if I'm into it.

 

[Evolution1980]

If you would have read my reply, you would have seen that I said if I had to accelerate, but not enough to kick it into passing gear. Of course if I wanted, it would kick in and accelerate with no problem.

Ma bad :eyerole...I did read it a couple of times, but for whatever reason, I was comprehending it differently. Regardless, I still wonder why our RPM:speed ratios are so different. Oh well, whatever. You know what your car was reading and I know what mine still reads.

PS - The speed limit isn't 60mph everywhere I drive.

 

[Art Jett]

Don't know on the difference. I've checked mine with the digital readouts the cops put on the side of the road that read your speed and then say ---Slow Down---. I've used mile markers and also my wife's Infiniti and they all say my speedo is accurate. I wish the limits here were higher. Hell, my car idles at 850. 1400 rpm isn't enough engine speed to do squat if your in 4th gear. Art

 

[Peer81]

A very simple solution to this problem. First change the transmission and drive it for 6 months. If you don't like it, change the rear gearing. Of course there are some in between gearings between 3.07 and 3.73.

Groeten Peter

 

[Evolution1980]

A very simple solution to this problem.

...is sometimes the easiest!

*DING DING DING*

 

[Rodney Gold]

At the end of it all , one has to know why the original poster did the 4 speed swap. I don't quite understand the reason cos if its for economy , why put the zz4 engine in and why go to all the trouble and expense of a 4 speed -- the time spent in overdrive in normal traffic is probably minimal and with a 3.07 diff ratio the car isnt exactly screaming along at highway speed with a th350 or 1:1 final gear.

If it was for more utility and performance a higher diff ratio is essential to utilise 4th gear.