Thread: Another 700R4 Question

Tweet

Hello all. I'm new here and I have a question. I have a 30 Model A Sedan with a mild 350 Chevy, TH350, and a Ford rear end with a 3.00 gear ratio. I want to swap my TH350 for a 700R so I can get a little better gas mileage but I'm not sure if my current gearing is suitable for that transmission. Is this too high of a gear to run with the 700R? I would rather not change my gears but I don't want it kicking in and out of overdrive while cruising down the highway at 55 to 65. There is a down shift kit from B&M that keeps it from doing that under 2/3 throttle. Have any of you used it?

My 93 Corvette had 2.89 gears before I changed them out, and it ran fine. 3.00 gears are not a problem at cruising speed.

You can install a simple switch that will lock up the converter in 4th gear, and it won't cycle in and out.

3.00 is fine. A better Street/road gear would be somewhere between 3.4s and 3.7s

Here is a site that you can use to play games with gear ratios and tire sizes. I replaced a TH350 in a Model A with a R700 and changed a 2.73 rear to a 3.55 ratio but I don't have the car on the road yet so I don't really know how it works. Still, I played around with the calculations and tire sizes until I had an overall low gear ratio lower than my personal memory calibration of a flathead Ford with a 3.78 ratio rear and a high gear with a higher gear (lower overall gear ratio) again higher than my memory-calibrated flathead with a 3.78 rear and a Ford three speed. With the low gear of the R700 and the fourth gear OD you can go lower in low and higher in high than my memories of the 59AB-3.78 Ford I remember well. In particular I just loved to hold the car in low gear and wind it up to it's max (just a .030 over rebuilt 59AB-239) and the acceleration was wonderful until you hit a bump in which case it would often remove a tooth somewhere inside the trans and sadly I did this three times and it got to be expensive!

http://home.tampabay.rr.com/1bking/c...calculator.htm

Don Shillady
Retired Scientist/teen rodder

Well I had about decided not to do this swap but now I think I will. There are a few things I'm concerned about though so maybe some of you can help me. The first thing is the width of the transmission pan. Is the 700R pan any wider than the TH350? As I said in my first post my car is a 1930 Model A Sedan with a TH350. My brake master cylinder and booster is mounted on the inside of the frame rail under the floor board and I don't have much clearance between it and my current transmission pan. Tearing that out and re-working the brake stuff is not an option so I need to know how much wider the pan is if any. Another thing I don't know is if the 700 is bigger around in the area between the bell housing and the tail housing. I'd hate to run into clearance problems with the floor board. This is not a car I'm currently working on but one that is completely finished inside and out so I can't change too much. Any info would be appreciated.

a 200r4 will fit better.

some of the C4 vettes had 2.59s in them and the "performance" gear for an automatic vette was 3.07 so I'm sure you're right in the ballpark

Can't help much because I have a Brookville repro frame under a replica '29 body. I can tell you that I started with a TH350 and then decided to use a 700R4 and I had to change the Brookville sling mount under the trans to mount the trans bolts about 3" further back so the bolts are further back. Fortunately, I had not yet set up the drive shaft, so the drive shaft set up for a TH350 may not be the right length for the 700R4, assuming the motor mounts are in the same place. In the Brookville frame the brake pedal and the master cylinder do clear the side of the 700R4 by at least 1" but it is tight on the driver's side. I do have a large vacuum can for power brakes on the master cylinder and that clears OK too. As I recall the stock Model A frame there is a "hat" cross section cross member somewhere in there but the Brookville frame just has a rectangular tubing 3"x1" crossmember at about the same place and the trans sling hangs from that. I am attaching a drawing of the original A frame but the whole problem depends on where your motot mounts are. I had to make the diagram small to get it on here but you can expand it with Paint.

Don Shillady
Retired Scientist/teen rodder

Originally Posted by shine

a 200r4 will fit better.

One thing I forgot to mention is that my TH350 isn't a short shaft, it is the longer shaft so if I went with the 200R4 I would have to modify my drive shaft length. With the 700R4 I don't think I'll have to.

if you have the mid size output shaft the 700 is the same length the mount is 2 inches back from the 350 the 700 is so close to the 350 that a genie or lokar shifter from a 350 can be use with a little bending. If your using a column shifter the manual shaft is in the same spot the cooling lines line up . As far as pan size easyest way to see how it will fit get a 700 pan line it up with the front of the 350 pan or just use apan gasket. I have had customers do this it makes it quite obvious if the will be a problem .700s have a 3.07 first gear 350s have a 2.48 first gear you will get a much better lauch your1st gear ratio at the rear is7.44 it you just add a700 it will be 921 or the equivalat off3.71 I rub a 36 pickup with corvette 307 rear with 700 cruises at 100 mph

My memories are calibrated on a '47 Ford three speed with a 3.78 rear and a 3.114 low gear for a total ratio of 11.77. It was pretty "sudden" in low gear with just the 239 c.i. 59AB flathead but of course the second gear was too high at 1.773 which is why some folks used the Zephyer gears for a better split between first and third. As I calculate it I am using a 3.55 rear with the 700R4 3.07 first gear to give an overall first gear ratio of 10.89, so it should still be pretty sudden in first gear with a SBC 350 but I hope the mpg is reasonable with the OD gear. If/when I get the wiring finished I will find out if it hooks up or just spins the rear wheels in low gear.

Don Shillady
Retired Scientist/teen rodder

Anybody replace the speedometer gear housing on a 700R4 lately? I broke the PLASTIC threaded 7/8" male end while installing a pulse generator. I guess I can chisel it out with a screwdriver around the edge since it is already broken, but there is not enough left of the original coupling snout to get a grip on it. Just chip it out?

Don Shillady
Retired Scientist/teen rodder

To answer my own question and maybe shed some light on one aspect of the 700R4 I include a picture of the problem I have had with the "Speedometer Gear Cover". The problem started when I tried to install the pulse generator for my digital speedometer. Sadly, the width of the transmission mount that came from Brookville for my frame is just not quite wide enough to attach the cable plug on the pulse generator. This was solved by the purchase of a right angle adapter. However the combined weight of the pulse generator and the right angle adaptor caused it to flop backward where it came in contact with the H-pipe of my exhaust system where the wire would eventually short out after the insulation burned off, sooooo I tried to tighten the coupling of the right angle adapter so as to keep the pulse generator away from the H-pipe BUT (!) I used a 12" adjustable wrench and I snapped off the snout of the fitting. After searching the Internet I learned several facts. First there are TWO types of covers. One type is for a larger speedometer gear which is off-center (40-45 teeth) and the other is for smaller gears (34-39 teeth). The smaller gear allows the gear shaft to be centered so when you spin the cover on the gear shaft (in your hand) it does not wobble. However the gear cover for the larger speedo gear has an off-center hole to allow space for a larger gear. Although the gear covers have writing cast into them the easiest way to tell the difference is to spin the cover on the gear shaft when it is held in your hand and the cover will wobble due to the off-set hole. In the picture shown you can see the broken plastic cover with a 44 tooth gear. In the middle you can see the better auminum cover with the cumbersom pulse generator and the adapter with a 42 tooth gear. On the right you can see the cover for the (34-39 tooth gears. Since I do not plan to change the nylon gear on the transmission output shaft I will swap the matching 44 tooth gear from the broken cover and use the middle aluminum cover. I have to chalk up the slight cost of the smaller gear cover ($25) to education. The lesson for others here is twofold. First, avoid the plastic cover and use the aluminum cover whenever possible. Second, if you have to use the plastic cover note that it is fragile and a 12" wrench handle is too much torque for that brittle plastic!

Don Shillady
Retired Scientist/teen rodder

Hey Don,
I had a similar problem with my 34 Ford 4dr. The original builder had 3 senders with the last one attached to a right angle adapter. They were for Computer (LT1), Speedo, and Cruise Control hanging off of my 700R4. Needless to say (but I'll say it anyway... ha ha), is that the speedometer Gear Cover warped with the added weight and leaked. No amount of O ring replacement would resolve the problem. I resolved the problem by replacing the warped gear cover and going to a single speedo sender going to the LT1 computer. The computer has a wire coming out of it for speedo pulse that I tied to the Speedometer and the Cruise Control. It resolved the leaky trans and everything gets the speed pulse it needs to work properly.
Jerome

MRJB1929, Thanks for the warning. I will try to orient the adapter and pulse generator so some of the weight leans against the transmission mount in a forward position away from the H-pipe and maybe put some rubber hose around it to keep it from chattering with metal-to-metal vibration.. With the aluminum cover and some experience obtained the hard way I think I can mount the cover so it is not canted. I noted when I took out the old cover that it is easy to tilt it in the hole so the Y-bracket is all that keeps it straight.

Don Shillady
Retired Scientist/teen rodder