I have a 12 bolt posi with a 273 gear. ... . I want to know if I should put in a 350 turbo or a 700r . I have asked the few people hear that I trust and have gotten both ways. That I don't need a overdrive with that rearend, and that the 700r has a lower 1st gear, and I can hold the tranny in 3rd until I am on the interstate and then I can Idle at 80 mph. I just don't know which tranny to put in. I have a 400 sb with aluminum heads and a single carb. Yet I am not building a race car, just something that sounds good, has good pickup, but will run on the highway all day and not kill me at the pump. A traveling car basicly, that can cruise in town and the interstate.
In running a 2.79 rear, I found that an overdrive trans was simply too tall. If you have a wide ratio type of trans gearing that would be a good way to go.
A 2.73 posi was standard for a 700r4 equipped Corvette in the mid-80's. Top speed of 150, mid-twenties mpg on the highway, and quicker than most factory cars of the time. I think you need to consider how much your car weighs and how aerodynamic it is. Along with that, what is the induction system? How cleanly will the car run at 1,000 RPM or so? One of the advantages of the OD is being able to run the motor real slow, but that's only an advantage if your induction/metering is good enough to allow the motor to perform well running slow. If your project is HAMB traditional you probably want the TH350. If it's a mid-80s EFI Corvette the 700r4 may be perfect.
No need for an overdrive with that tall of a gear. If it was a 3.73 your into OD territory. A lot of GM cars used a 3.42 rear gear with a 700r4, that tells you something. You have to consider that they build the entire drivetrain to be compatible so a stock combination is targeted for around 2,000 rpm at 70 mph in overdrive. Frank
My thought also, my 06 GMC loves putting @ 1800. But my 56 forget it. You never mentioned tire dia. and motor set up.
GM also put 700R4's behind 4.3's in S-10 pickups with 2.56 gears. I had one with the optional 3.08 gear, it would blow the doors off one with 2.56's but got the same mileage. If it's a fairly light car with a mild cam I'd say run the 700R4 with the 2.73's. Put it in 3rd on the two lanes and pop it into overdrive for interstate cruising. It will launch better than the TH350 and mileage at 55 won't be any worse, interstate mileage should be better. Plus, if you decide to swap to lower gears later you already have OD.
TH350 and 700R4 gearing: http://www.smokemup.com/tech/700r4.php A 700R4's low 3.06 first gear will make up for that 2.73 out back. I think a 2.73 + 2.48 gear and 28's could be a real pooch from a dig. I ran 4.11's, 4L60E, 26's and it only did about 2500 rpms at 75mph... And cut 1.4's at the track.
BRANDON...a turbo 400 would be my suggestion, man have you priced a 700. and by the way, don't expect any combination to idle at 80 m.p.h. what dimention are you from? hey! it's a fair question...POP.
A 700 will be better off the line than a 350 due to the low first gear but the high gears almost make the OD useless. I have a 700 R4 in my 38 Chevy coupe and was running 2.80 gears. While I lived in the flat lands of West Texas it was great. After moving to the hills of north Texas I had to run it in 3rd most of the time. If it put it in OD the the tranny was always downshifting on the least hill. I went to a 3.50 gear and it love it. Great off the line and gets just as good gas mileage as the 2.80's. Since I'm using a vacuum switch to lock up the convertor it will unlock on the hills but the tranny stays in OD.
A factor NOT YET mentioned is the cost. A 700R4 costs a good bit more than a Turbo 350. Plus they are more difficult to adjust ( the cable ). Length can be also a issue. Turbo 350 ( 3 different ones ) 27 and 11/16 30 and 11/16 33 and 27/32 700R4 ( 2 different ones ) 30 and 3/4 29 and 7/8 ( Corvette ) The Turbo 350 is easier to work on and has a mechanical drive speedometer. The 700R4 comes either way ( mechanical or electric ) but most I have seen are electric. They can be converted to the mechanical ( more $$$ ) The Turbo 350 will fit in tighter places ... when compared to the 700R4. The Turbo 350 is less senitive to dirt, trash and such. I have owned Hot Rods with both. When I built the 32 3W, I used a 9 inch Ford and a 3.50 rear end ratio. So I got a 700R4. BUT if I had a 2.73 I would use a Turbo 350. BOTH of the above 32's have the same engine combination ( 350 cubic inch, aluminum heads and intake, hydraulic roller camshafts and make 430 HP ). The coupe has a 700R4/3.50 rear end and the roadster a Turbo 400/3.00 rear end A 400 Turbo has the same high gear ratio as a Turbo 350 ( 1 to 1 NO OVERDRIVE ) Even with the coupe being a few hundred pounds heavier ... the roadster and the coupe get the same mileage ( within 1 to 1and 1/2 MPG ) depending on HOW aggressive you are with the gas pedal. The roadster is actually as quick off the line ... as the coupe because it does not want to spin the tires as much. VERDICT ... I use the Turbo 350
A 2.73:1 rear gear, through the 0.70 4th of the 700R4 is like a 1.91:1 through the TH350 in 3rd. 700R4? If you are headed to Bonneville with a lot of horsepower, maybe. For the street, no.
Okay, bout what I am getting here. six one half dozen the other. engine 400 small block, edelbrock Alum. heads, Dual plane high rise intake, 750 is cfm. RV cam. I have a 700r4 rebuilt and waiting in the wings. I will have to change the drive shaft and cross member to use it. I have a friend with a hotrod shop who has them rebuilt for $400 + $100 or so in parts. I have a 350 turbo that was in it on its way to the tranny shop since it will get used soon with one of my 4 car projects. Rebuild price of about $200 + parts. I live in South East Tx with few hills, but do travel. Heavy car of around 3500lb. P 235/15 or so. 273 rear gear.
I'd run the 350 for now. If you don't like the lazy acceleration, then swap out the rear gears to something in the mid 3s and put in the 700
Assuming a 235/75-15. At 65 with the 700R4 in 4th you would be turning 1450 RPM. I set up most of my SBC's to cruise 65 at around 2300 RPM. At 2300 with that setup, in 4th, you would be humming along at 103, assuming enough power v.s weight to do so. If you could red line it in 4th, at 5500 you'd be at (theoretical) 247 MPH. You would be in 3rd almost all of the time on the open road. With the TH350 and the 2.73's, in 3rd, you'd be at 2300 RPM at 72 MPH. 2075 RPM would be 65, 1450 would be 46, 5500 would be a theoretical 172. The TH350 would be more practical. Yes, the 1st gear is not as low, but don't forget, a torque converter is a torque multiplier.
I have had both in full sized Chevies that both had a 2.73 rear ('83 Caprice, 350/700R4 and a '73 Bel Air, 350/TH350). Very comparable cars. There's no way I'd ever do that with a 700R4 again. It was useless. Sure, you're only spinning 1500 or something at 80, but how often do you really drive that fast or faster? The other problem, and I'm not an expert but I did use a vacuum gauge for a while, is that at 1500 RPMS at 80 your engine vacuum is much lower because of lower RPMs, so any input at all from the throttle brings the vacuum right to zero, which I think just dumps gas into the carbs, killing your gas mileage anyway, plus with the high gears (2.73s) unless it downshifts to third it didn't really move that well at highway speeds. I got around 15 mpg in that '83 caprice which was probably around 3500 lbs. It ran a 16.2 at Firebird in Phoenix. (Sadly, yes, that IS a quarter mile time). The '73 was trouble free, worry free 200K miles with that TH350. With the same 2.73s it was spinning 2400 ish around 75 or so, and with the same carb got 17 mpg highway on it's last trip from Georgia to CT on one leg, and that car is listed as being 4100 lbs. (Ran 17.5s but is heavier and this 350 had much less power). Not to mention, it's supposed to be very easy to rebuild a TH350, and much cheaper than a 700R4. Not to mention that 700R4 is a stressful horror show, half the people you talk to tell you you're going to burn it out if you look at it funny because of the TV cable and that it won't take any power, the other half say it's bulletproof...either way it's stressful to even own the damn thing, constant worrying. I'm keeping my 700R4 because I now have low gears and to be able to use it on the highway, too. Ditch the idea of the 700R4, or get a lower gear to take advantage of it. I've done both with the same gears you are talking, and I would not do the 700R4 again.
A agree with your overall advice, although this doesn't sound quite right: Running at lower vacuum and lower rpm because of gear ratio will give you better mileage (assuming the engine is set up to run well at low rpm). I got much better highway mileage with a crown vic with 3.08 gears and AOD than I do with the camaro with 2.73 gears and powerglide. both have about the same size engine.
Just my opinion. The 700r4 is a good trans but I have found it will not hold up or handle much abuse or HP if you want it to last.
Maybe mine isn't set up for lower RPM but the way I picture it, when you are completely off the throttle in third, the vacuum goes way up, to say, (don't recall the real number but for example) 20 inches. When you ease onto the throttle it starts to go down, closer and closer to zero, using more fuel in the process. When you are in OD and take your foot out of the throttle, it's only around 10 inches, so when you crack the throttle the needle immediately goes to 0, and in that case the secondaries would be open whenever the carb's linkage allowed it even though you aren't moving much faster. I always thought that the more vacuum the engine was pulling, the less fuel it's using, since it's at zero when you hit WOT, too...obviously the amount that it's open makes a huge difference, too. Like I said, I'm not an expert, but I don't think my mileage was as good as it would have been with lower gears.
Sir this is what I am talking about. Put some practical Numbers to it and go. Okay, So no 700r4 with the 2.73:1. Sounds like it will not do me much good. Now lets throw this out there. Is there a difference in the 1st gear in a 350 turbo and the 400 turbo. I may have one of those laying around, also. I do know that they are able to handle a little more HP. But, also take more HP to work. What is the idea on this.
Yup. First in a 350 is 0.04:1 lower than a 400, same with second. It ain't much, but it is different. I think you will be fine with the 350. Yes the 400 is stronger, but, as you have noted, they take more power to run. Just make sure you have a good cooler on it for those Tejas summers.
Vacuum is affected more by throttle position than RPM. As the throttle blades open vacuum will drop, when you intially mash the thottle vacuum will drop to Zero and then climb, when you let completely off vacuum should shoot up to 25 InHg regardless of the engine RPM and then start coming back down until it levels out or you open the throttle again. I have a 318 in Dodge PU with 2.94 gears with a vacuum gauge mounted right next to the tach. It idles at 19 InHg at 600 RPM and if you bring the idle up to 900 RPM it pulls 23. At 55 MPH it's turning 2100 RPM and vacuum is about 17 InHg on level ground with no wind. The slightest grade or head wind drops that to 13-15, a slight downgrade or tailwind can bring that up as high as 20-22. I have tried driving at 55 in 2nd gear to see what effect the RPM increase had on vacuum, I got 17 InHg. At 45 MPH I can cruise at 21 InHg, I think it's more because it's not pushing as much air than a reduction in RPM so it doesn't require as much throttle opening. A vacuum gauge can be a great tuning and driving aid, it can also drive you up a friggin' wall.
I guess I hadn't thought about that whole part of the equation, that when you tune the carb initially you're supposed to hunt for the highest vacuum and that if it isn't depended on RPMs at that stage in the game, why would it be when actually moving? That reminds me, I wonder where I put that vacuum gauge... I'll need it when this truck is running and I'll (at least at first) want something else to drive me nuts!
As mentioned above,the vacuum thing. On engines with carbs ,all of them pretty much have a hi speed jet or power valve that responds to vacuum.At lower vacuum,5-10 inches depending on design,the jet opens to let in more fuel for what the engine see as a heavy load.With brutally high gears in the 2.60 or lower numerically range,the engine rpm is low,load is high,the throttle will be opened quite a bit and the hi speed jet is open ,you get shitty gas mileage.Maybe worse than turning a higher rpm with more "normal" rear gears. Modern engines can get better mileage at 1400 rpm at 70 MPH cause the electronic engine management is way more adjustable than a simple carburetor . But.....generally speaking,lower rpm's means better fuel mileage.
It's actually an enrichment circuit, and can be tuned. Holleys use a power valve and are available in different values, they are marked with a number that is somehow related to the vacuum at which they open. Edelbrocks have a rod that is controlled by a spring that can be changed, a tuning kit contains several.
I would like to add that a turbo 350 is an excellent tranny. The only weak point is the second gear sprag which will split the ring under full throttle/ high torque situations, and a hardened ring is commonly available. The last one I bought was about $40. These trans respond very well to a shift kit. A turbo 400 is an awesome tranny. Period. A 700r4 and the later designation 4L60 and 4L60E are more complicated to build mostly because of running design changes, but someone that knows the ins and outs of them can build one to handle a stout big block. My buddy has one behind an L88, but then he also has a 3.73:1 rear gear AND has a partial ownership of a trans shop.
The Jalopy Journal
