Custom 4 Speed OD trans build

[jeff york]

Tonight I spoke with a custom build shop in Georgia about building a one off 4 speed OD transmission for my 77" Airstream Argosy 28 motor coach. It will be based on either a 700R4 or a 4L80E. I am torn between having a shop build it or building it myself. My target is a getting my 3200 RPM cruise down to 2200 RPM. And being back up and running by spring. Probably taking my last camping trip of the year this weekend before I take the AS down for the trans build and EFI/TBI conversion

[jeff york]

I want to be sure those in this blog understand. My rig is an Airstream motorhome. Not a airstream trailer being pulled. So my set up is a 454 thats rebuilt, stoked and bored with custom grind cam to achieve target powerband at a lower RPM then a common 454. This is because of the low end rpm load and cruise of a typically heavier MH vs a truck. My current trans is a rebuilt TH400. As one person pointed out the comment of a TH400 with OD..... thats great and I would do it but thats cost prohibitive at more than 3 grand for a Gear Vendors. OD unit. As far as pointing out the load weight to a trans shop to build it strong enough.... I will be building the trans myself and yes, it with be built with clutches and bands strong enough and it will be built with additional cooling. It will have a torque lock up. Im an engineer, not mechanical, aviation and electrical. So engineering this will be within my ability and building it will as well. As for the TBI/ EFI. Thats mostly for dependable starts but mostly for precise AF ratios and the benefits of which are numerous. Engine life mostly.

[richard nance]

another advantage of the 700r4 is the low first gear.you will be able to pull away from stop lights faster.be sure the trans shop knows how much your rig weighs so he can modify the trans so it will hold up in overdrive.of course the 4l80e is a better transmission but much more expensive and needs a aftermarket contoller to operate it.if you go with a 700r4 get a 1987 or later which are improved over the earlier ones.

[david craft]

Does the 4 speed have a converter that locks up?  I was wondering how that tranny would compare with a th400 with an overdrive giving you 6 speeds.  If you were not towing, a 350 would probably do the job, given the vehicle and building for a specific powerband.  I had a customer with a small class A, and an overdrive, and he claimed 15mpg with michelins.  Later, he switched to another brand and claimed he lost 3 mpg.  I like the idea of the EFI to get that fuel atomized at low rpms...Sounds like a great build...

[jeff york]

Rich, I have done a lot of research on EFI/TBI and 700R4 trans and overdrive. In my case I am not looking primarily for a dollar value on the return on investment. I am looking for a indirect value on ROI. Let me explain. My Airstream is a 1977. Its beautiful and nicely restored. The motor has been modified with the intention of going to an OD trans. In other words, its modified to get in a power band below where the current TH400 3 speed trans runs. 

So, whats my point and whats the indirect ROI ? First the EFI/TBI is for reliable starts. Quadrajet carbs can be notorious for difficult starts when they sit and you can also flood them. I am not here to debate that subject. But, EFI / TBI is a way to finely tune the air fuel ratio to get that peak A/F at any condition, temperature load and altitude. What that means is your motor doesn't end up getting its cylinder walls washed with to much fuel which will contaminate your crank case oil leading to oil break down. My Mercedes is rated for 12,000 miles between oil changes instead of the typical 3,000. Part of this is due to the precise metering of the fuel. What does this mean ? Much longer engine life because there is much less wear due to carbon and oil contaminated oil. 

Part 2: The 4 speed over drive trans. Again, modern cars take advantage of OD transmissions. This means a car that used to run at 3500 to 4000 rpm at 75 mph can today run at 2200 rpm at 70 mph. This give you two over the top benefits. It reduces your fuel consumption by as much as 40%. To use with the weight of a camper it can still mean a reduction in fuel consumption of 20% or maybe a little more. The second benefit is this. Less wear on the motor because its turning 1/3 less revolutions to go the same distance. We typically think of motor wear by the number of miles on it. This is really an indirect association to the number of times a motor turns in revolutions. You add EFI/TBI and an OD transmission and the result is a cooler running motor, that uses less fuel and goes a lot farther between major overhauls. It starts more reliably even if its sits for long periods of time in storage and you know it will start when your deep in the mountains and a long ways from home. It is a sort of indirect ROI. I love my nearly 40 year old Airstream motor coach. I want it to retain its vintage look but, I want to make it as reliable as possible, especially since I my children travel with me.

So, if a person wants to look at just fuel econ as a ROI, then what I am doing may not be for them. But, by the way, my engine mods and such are already netting my 12 mpg in a MH that typically gets about 6 mpg. There are nay Sayers that may doubt my fuel econ and all I can say is, I'm an engineer, I have applied my aircraft race engine building experience to this and if thats not enough, I welcome anyone that wants to take a trip with me and see it for themselves. Note: I tried this on my Fleetwood MH and didn't get the same results. In fact, I am not sure I netted any improvements at all on the Fleetwood.  But, I learned, I am using a platform in an Airstream that is pretty ideal weight wise and even drag wise and I pinned in most of my undercarriage on the AS to reduce parasitic drag. ( thats from air racing and I also applied it to our sports racing car too. )   

[Rich Thomas]

Are you looking at a particular set up on your EFI/TBI. I've been tossing that around with my sons.They like the idea but, I'm on the rail. I must admit every time I read of some one doing it, it puts me one step closer. My biggest deterrent is the return on my investment vs the age on my camper. will it pay off over time? The over drive is an interesting project in its own right. good luck on both.

[James Ferrando]

That is a great idea.  I'll be looking for that.