Help! Replacing a 3 speed Auto with?????

[manofsteel]

In my '73 vette, I have the turbo hydra matic 3 speed tranny. With a 350 crate engine and getting lousy gas mileage. Have the Eldelbrock 600 and they say I should get between 18-22mpg. But I only get about 10-12mpg.
Questions:
1. All of you with a 350 and auto trans, what kind of gas mileage are you getting?
2. Is there and would it be to my benefit to install a 4 speed with overdrive auto trans to get better mileage and has anyone done this?
3. If so, what did you use and what was the cost and where did you get it?
4. Is there any used trans from a salvage yard that I could use with the same criterea?
Thanks to all who will answer!

 

[Hib Halverson]

Anyone who says you'll get 18-22 mph with that carb on a 73 with a stock auto has been drinking...a lot.

Fuel mileage depends a lot on driving habits, carb tuning, weight and gear ratios.

What's the axle ratio of the car?
What other modifications besides the carb have been done?
What happened to the stock Quadrajet carb?

It might be to your benefit to install a four-speed auto but you'd have to drive the car many tens of thousands of miles to recover the cost of the trans and the installation in better mileage which wouldn't be much. If you are getting 10-12 now, you might see 14-16 with the four speed.

 

[TimAT]

I drove the 77 a touch over 1400 miles (Arizona to Missouri) and averaged 18.5 MPG. That's with 3.55 gears, 26" tires, 350 with an Edelbrock intake and carb, and a 200-4R trans. The engine has a bit of a cam in it, I was told it's more of a torque cam than hi-performance and headers.

The 3.55 rear had some noise on coastdown about 60, so I swapped it for a 3.08 I had. Haven't checked the mileage since the ratio change..

 

[bill81vette]

My '81 with a 3 speed auto,a Quadra-jet gets 17-18 MPG on the high way!!!
and its all original!!!!

 

[damoroso]

My 81 in stock trim with the TH350 got right at 18mpg from Denver to Jacksonville. I know the 81 has a lock up torque converter and that helps economy. Not sure if the 73 had that, I doubt it. The newer 4 speed OD transmissions certainly offer the benefit of an additional gear and overdrive. They will bolt up to your engine, will fit in the car, but you'll likely have to do some modification to your trans cross memeber.

I put a 2004r in my car. I choose the 200 because I like the first to second gear change better than what the 700 offers. I did modify the trans and if it was going behind a stock engine, I wouldn't have done much more than the rebuild with good clutches, and perhaps a mild shift kit.

My cross memeber had to be modified a fair amount (the 200 is seven inches longer than the 350 from the front to the mount), but it wasn't hard, and you can buy a modified cross memeber from Bowtie Transmissions. I wired the lock up torque converter to a rocker switch I mounted in the console, easilly done. With my rear gears, I'm running 75mph at about 1800rpm. I can't tell you the mileage because I had a 2800 stall converter in it (until last night!) that I really didn't like. That completely wrecked the fuel economy, and for what I've built the car for, isn't a good choice (I should have stayed with what I wanted instead of listening to others!!) so I'm swapping it for something more along the lines of stock, say 1900.

 

[manofsteel]

Mileage

My '73 has now a crate 350 gm mexican engine in it with a 3.08 rear and turning the stock tire size of that year. The original q-jet carb was long gone before I bought this car. Had the Elebrock 600 in and I installed a HEI ignition. Runs good and strong but I thought I would have better than 10-12mpgs! Thinking of getting rid of the 3speed auto trans and putting in an auto 4 speed with OD.. Was hoping that from a junk yard, there was a car that had a tranny that would fit in the 73. Any one know? Never heard about the 200-4 tran. Let me know more about it and where and $$$$? I'm quite capable to do the work myself. Thanks guys!

 

[damoroso]

There are two popular 4 speed OD transmission people are using in their C3s. The 2004r and the 7004r. The 200 is a big smaller physically, and seems to fit in the tunnel a bit better on some cars than the 700. The 1st and 2nd gears aren't as far apart on the 200 as they are on the 700 either. The 700 is a bigger transmission and in stock trim typically handles more horsepower than the 200. However either can be built to handle big horsepower numbers. I chose the 200 becuase it fit my car better than the 700, I like the 1st to 2nd gearing better and the OD is .67:1. It just fit my application better.

As for the cost, my core was $40. I'd suspect if you don't have a place that deals in transmission cores, a bone yard would help you out. You can get a good quality overhaul kit for about $200, a shift kit for anywhere from $30-$90. A torque converter is anywhere from $100 and up. Again a couple of notes. First, check your cross memeber, you'll likely have to modify it (or buy a modified one for about $200) to fit either the 700 or 200. If you keep the lock up feature (great on the highway) I'd suggest you wire it to a switch so you can select it in either 3rd or 4th gears. It's easy, if you need help with that, let me know, don't spend the money they get for the switch conversions!!

A ATSG manual is a great help and they're not too bad either. I get parts from Macko distributing on line and have been very happy with their parts, prices and service. https://www.makcotransmissionparts.com/

 

[Hib Halverson]

My '73 has now a crate 350 gm mexican engine in it with a 3.08 rear and turning the stock tire size of that year. The original q-jet carb was long gone before I bought this car. Had the Elebrock 600 in and I installed a HEI ignition. Runs good and strong but I thought I would have better than 10-12mpgs! Thinking of getting rid of the 3speed auto trans and putting in an auto 4 speed with OD.. (snip)

Now that more information has been posted, my opinion is that:

if the car is driven normally
if the engine is in good condition
if the spark curve is correct
if the camshaft is indexed properly
if the Edelbrock 600 is properly tuned

The engine should be getting better mileage than 10-12 mph.

I think you could get 16, maybe 18 but not 22 mpg.

As I said earlier, the cost of installing a four-speed OD trans in your car will probably exceed any savings on gas mileage that you gain from it.

What I'd do is 1) go find a Quadrajet, 2) tune it properly and 3) make sure other tuning aspects are addressed.

 

[c4c5specialist]

HI there,
I can tell you from personal experience.

I did a 350 crate motor, Accel throttle body fuel injection and 700R4 in a 1964, and got 21.

That was 9 years ago.

1981 Corvette, I installed a Ramjet 350, rebuilt the TH350C and am getting 16 mpg highway. With the .70 overdrive, I will probably get 22 mpg if I put a 4 speed in.
The customers next installation will be a 700r4 because all I need to do is to shorted the driveshaft and slightly modify the crossmember.

Allthebest, c4c5

 

[Hib Halverson]

Ok.
I stand corrected. Seems like there's folks out there who get better mileage with 3-speed autos than I think.

Of course, that make the issue at hand even worse.

If you can get highway mileage in the low 20s with a C2/C3 and a 3-spd. auto then there is something seriously wrong with the tuning of "manofsteel's" engine. If that's the case, no transmission swap is going to fix those problems.

My advice is to get back to the basics, check ign, check cam timing, check carb tuning, review driving habits...and so forth.

 

[damoroso]

Agreed, the engine has to be tuned properly. I'd also go to a Q-jet, small primaries and great secondaries. IMHO I much prefer the Q-jet to a holley or Edelbrock, but that's just me. I believe most people don't degree a cam when they install it. That's typically free horsepower left on the bench for sure! The last one I did was actually 4 degrees retarded when I checked it outta the box!!

Still, with highway speeds usually posted at 70-75 and people typically driving +10 or so, another gear really helps.

 

[Koop]

My 69 L48, TH400 with 3.08 did 18+ hwy about 14 city when it was stock. The tech talked me out of the 4spd auto conversion with 3.73's last time I overhauled the drivetrain. I still run the 3.08 and TH400 and the stroker gets HORRIBLE MPG!

 

[spikebot 81]

Well here's my .02 cents and engine combo. Best ever on the highway was 17mpg and I normally get 10-12 in town driving like a 17 yr old would drive a vette. Stock th350c trans 600 edle carb 350ci .040 over, 882 smog heads, no computer, headers, duals. 16 deg btdc timing 34 total at aprox 2900 rpm

 

[manofsteel]

milage concern

With picking of all of your brains for what to do to improve gas mileage has totally confused. Some says that 10-14mpg is all I will get, some says 16+ is what I should get. Some say to go to the 200R tranny and some say for the cost value, going that route with cost and all is not worth it. Go to a Holley carb....go back to a Qdrajet. Geez........ Eldelbrock is what I got, and somehow, with the correct indication, I will improve mpg without spending thousand of $$$$$$. Is there something wrong with te Eldelbrock? I don't know. No one ever answers that question. I've cleaned it out and have it "adjusted to manuf advice". BUT no one can answer why when I turn the right jet in all of the way....the engine does not bog down as when I do the left jet. Do any of you guys have a good answer to that. Do I need to do something else to the carb other than throwing it away and getting something else? It is on an adapter plateon the original intake manifold. can not get eldelbrocck to respond. Thanks

 

[Crimson Thunder]

The reason the Q-jet will get better mileage is because it is a spread bore carb with little primaries and huge secondaries.Your Edelbrock is a square bore carb with the same size primaries and secondaries. If you can turn your mixture screw in all the way without it stalling that could be your problem with your tuning that is causing your poor mileage. Get a vacuum gauge and turn your mixture screws to gain the most vacuum. This should get you in the ballpark.

 

[damoroso]

I agree with sharkcar71. The Edelbrock isn't operating correctly. Also, like he said, it's a square bore carb, you're likely to get better economy (and performance IMHO) with a Q-Jet. If you can find a used one, the kits aren't expensive and their easy to rebuild, if you follow the instructions. Cliff Ruggles book is a great source of information, and you can contact Sensei on this sight, he builds them alot.

The trans fix is really more for getting better mileage at highway speeds. If you plan to spend alot of time at highway speed, I'd consider the trans swap otherwise, there's really no point. After all, if you never use the overdrive, how could it ever help?

 

[THORIN]

I have a 1976, When I bought it and drove it from Oaklahoma to Minnesota, I got 11 MPG. I had a 700 R4 put in. The worst gas milage I get is 18 MPG. I drove it to kentucky and back then Ohio and back. I averaged 22 MPG. One tank, flat land and tail wind, 26MPG. I like the overdrive.

 

[Racer86y]

Tuning for milage

I agree with Damoroso on both counts.I run quadrajets and love them for milage and power.All the rest seam to run rich and shorten life of motor.TMO