C-6 vs. AOD

[mcne]

My prerunner is a street truck. I would like to have overdrive. What are the pros and cons of swapping out my C-6 for an AOD? How does an AOD hold up in offroad driving compared to a C-6?

[ntsqd]

The AOD can be a stout piece. It has a few weakness' that need addressing though. I replaced a C-4 with one. That OD ratio is REALLY tall. Baumann Engineering used to be on the cutting edge of AOD development and may still be, search them out.
I know some would consider it sacriledge, but I would probably go with a 700R4 instead. You can buy one tricked out & ready to install for enough less than what a built AOD will cost you, and with that savings you can easily pay for the adapter plus have some lunch money. The 700 has a lower first gear and a not quite so tall OD. Have a look at 700R4.com

[mcne]

Thanks for the info. What is wrong with a really tall overdrive? That means I can run lower gears and still have decent rpm's on the highway?

[murphco]

whats firt gear on a 700r4? first gear on my aod is 2.46.its actually a 4r70w but they are basiclly the same thing.anybody set one of the 4r70w's for paddle shift,i'd like to do that.

[jamesjones]

3.06

[ntsqd]

[ QUOTE ]
Thanks for the info. What is wrong with a really tall overdrive? That means I can run lower gears and still have decent rpm's on the highway?

[/ QUOTE ]

It's a .675:1 OD ratio. At 3200 RPM in direct shifting to OD put me at about 1900 RPM. If your engine has the grunt to pull that it could work. The 700's OD ratio is more like .7:1-.75:1 which means you won't be as likely to fall off the bottom end of your power band.

[IanDixon]

I had the AOD transmission behind a 5.0L engine in a Ranger with 33” tires and 4.56 gears. I didn’t like the overdrive at all. You couldn’t stay in OD even on the freeway.

After reading the baumannengineering.com website about the AOD, it looks like you should think about a 4R70W transmission for overdrive.

[steveG]

Lentech builds super-strong AOD trannsmissions. I'd give them a call before making a decision.

[Ando]

OD on the 700R4 is .70 to 1, so not too much different from the AOD. Big question is, are you going to run a lock up converter or not? Both the AOD and the 700 use lock up converters and both will have to be modified pretty extensively if you are not going to use the lock up, alternatively the transmissions will both have to be wired for lock up control.
700r4.com has really good prices on quality transmissions.

[ntsqd]

Au contrair, the diff btwn .7 and .675 is huge in practice, if not in numbers. The difference to look at is how much RPM drop there is btwn direct and OD and where your engine's power band is. To not lug the engine I had to wring that 302 out a little b4 letting it shift into OD.

The AOD (not the 4R70W) uses a hyd lock-up. No re-wiring possible. Maybe with some non-stock parts?
If you're not using the lock-up in OD why even bother? I know of some people who use the locked/unlocked t/c in the 700 as a fifth 'gear', but only when getting rolling - not unlocked in OD in steady state cruise.

[Ando]

The locked/unlocked converter is important because the 700r4 in overdrive under acceleration with an unlocked higher stall converter generates a large amount of heat. Typically more then it can cool even with the very large aftermarket coolers. 700s built to run non locking converters (I think) are setup to flow much more fluid in OD then stock transmissions to stay cool.
If you do manually control the locking converter, as I do in my truck, I would highly recommend a transmission temperature gauge to avoid overheating.

A truck running at 6000 rpm in third gear shifting into fourth will have the rpm drop to 4050 with an AOD and 4200 with a 700R4, assuming similiar torque converter slipage.
If this truck has 4.56s gears and 33" tall tires then this shift would occur at 130 mph assuming no torque converter slippage.
The hot setup is the 4L80E with its .75 to 1 overdrive.

[mcne]

I understand that the overdrive might cause the engine's rpm's to be too low. Why can't I just change my rearend gearing to achieve a nice overdrive rpm? I have always been told that 2,000 rpm is a nice number for the engine to be cruising at. Not to mention good gas mileage. Should I aim for something a little higher?

[TumbleBug]

2,000 RPM's will lug your engine, and you will actually get a little better mileage if you aimed a little higher, say at least 2,500. That is what I'm shooting for with my overdrive.

[ntsqd]

To get it right you need to learn at what RPM your engine's Torque peak occurs at. Then gear it to cruise at or slightly below that RPM at what ever speed you normally drive.
One way to find the torque peak is to try different RPMs when on long drives ans see what RPM gives the best mileage. The best BSFC (Brake Specific Fuel Consumption) happens at peak torque. BSFC defined is the fuel required to produce the most torque at that RPM. If you want an equation: (HP@ X RPM)/(MPG@ X RPM) = BSFC

Here's where a really tall OD ratio hurts you. Let's say you've geared the vehicle to go 65 MPH at 2200 RPM and 65 in direct is 3500. Now some moron is dilly-dallying along in the fast lane and you drop out of OD to pass them. If the OD ratio is really tall dropping out puts you pretty high up in the power band (typically from peak torque to peak HP). That doesn't leave you much RPM left to accelerate with. A numerically larger OD ratio gives you more of the engine's power band to pass with.

[mcne]

Your example of passing on the freeway is exactly what I have now, only with my three speed! I have not regeared my rearend since I added the 35's. My truck drives fine when driving on city streets. When I am on the freeway I can cruise at 65-70 if it is flat. Whenever I need to pass or I have to go up a hill my truck downshifts into second and the engine is redlining. I haven't regeared it yet because of the transmission swap idea. Now that I realize that the overdrive can be a pain in the [I have been warned] I might keep the C-6. Does anyone have any ideas on gearing for a F-150 with 35's and a C-6? I would like to be able to cruise at 70 mph on the freeway and not rev the engine too high.

[Sites]

My C6 + 4.86 gears + 37's = 80MPH at about 4200RPM. Not exactly a "cruiser", but it gets up and goes nicely. If you are looking for a 3 speed transmission that cruises on the freeway at a nice RPM, your gonna have to either give up your bottom end performance (unless you have power) or just deal with the high RPMs.

[EQuin]

Here's a link to a tranny shop that builds high performance AOD's and sells lots of performance goodies for it:

http://www.transmissioncenter.net/AOD.htm

[EQuin]

Here's a link to the Ford Tranny Forum. Lots of good info on Ford trannies:

http://www.network54.com/Forum/260730?it=0

Hope this helps.

[Ando]

It sounds like you may need to just play with a calculator a little bit.
Speed = (RPM x Tire Diameter) / (Overall Gear Ratio x 336)

For Example
Speed = (2000 RPM x 35" tires) / (4.56 rear gear x .675 OD x336)

Speed = 70000 / 1034.208
Speed = 67 mph

You have to decide first off, what is a good cruising RPM, what is your truck's usable power band. Then just make some calculations.

Transmission Gear Ratios
C6 AOD
1st 2.46 2.46
2nd 1.46 1.46
3rd 1.0 1.0
4th - .67

[mcne]

Wow. I did not know that the AOD had the same gear ratio's as the C-6. Now that I learn more about each tranny I think I prefer the Gear Vendors setup. Six gears vs. four gears. I can run a lower rear end gear and still have a decent highway rpm. The cost is more, but I also get more. And I think an extra thousand is worth two extra gears! Thanks everyone for all the information you have provided.

[ntsqd]

The old Doug Nash unit could be ordered as an OD or as an underdrive. Wonder if the GV unit can be as well? That would allow you to run a taller rear end gear, which is stronger, and not turn a set of OD gears, which is less parasitic loss.