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Old 01-31-2014, 03:14 PM   #29
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1984 31' Airstream310
Honokaa , Hawaii
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There are at least a couple of threads here about swaps like this, search for threads by choctawmel and bkahler. After reading them, I decided to live with my stock tranny and just drive slow. Replacing the carb with a tbi setup sounds like as good a step toward better mileage, and might actually be within my skill set.
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Old 02-01-2014, 07:12 PM   #30
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1986 34.5' Airstream 345
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Thinking about doing a trans change out to one with and overdrive . My mechanic was saying an early 90's trans has a manual overdrive and would a simple change . But the cost and time . . . what I am going to get one of those light up signs that says " bite me " and put in back window for those getting all clogged by yours truly
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Old 02-01-2014, 09:55 PM   #31
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You must have just missed the 4L80E. My 92 300 came with one.
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Old 02-09-2014, 07:18 PM   #32
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1989 34.5' Airstream 345
Saint Petersburg , Florida
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Raveson did the TBI swap last summer, just before he took off for a 4 month cross-country trip. He spoke to me about what an amazing difference it was having the TBI on the 454. Of course, he towed a Jeep Wrangler the entire trip, mpg surely increased, but can't remember by how much. I've priced the TBI, and thinking that at some point before I retire, it will be a definite upgrade.
Good Luck, Derek
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Old 02-10-2014, 10:48 AM   #33
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1989 34.5' Airstream 345
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Hi Derek and others here. I took over from Raveson as caretaker of his 345 about three months and three thousand miles ago. While I don't have experience with these motorhomes with the stock engine set up, I can say that the engine with the TBI is easy starting and smooth running with it. Also Richard said the power doesn't fall off as the altitude climbs. It does struggle a little in climbing but is dragging a lot of weight up them mountains. As for MPG, I've been getting around 7.5. Up a bit from the 6.5 that Richard experienced in his trip towing the Jeep. I think the real advantage is the clean running , ability to self adjust to altitude, and engine longevity.

Back to the OP's question about the overdrive. The real noticeable advantage for me is the more relaxed feel of the coach at speed. The overdrive seems to add about 10 MPH in top gear at a given RPM. While this may not make a big difference on lesser roads where the average speeds are lower. While driving on interstates with average traffic speeds of 70/75mph, being able to cruise at 65/68 mph. Vs 55/58 mph makes better miles per day and a much more comfortable driving experience in relation to the other traffic.
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Old 02-15-2014, 05:37 PM   #34
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1991 30' Airstream 30
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I have read through everything I can find on Gear Vendors and trans swaps, and my conclusion is that the GV is the best solution. With my 91 there is a way to swap the ECU and wiring harness to use a 4L80E, but that sounds like a pain, and my trans was rebuilt 4k miles ago. I can install the GV myself and I get overdrive on the highway and second with OD for those long hills that kill me at the moment. The TH475 that our RVs came with is one of the most durable transmissions ever made. I have been looking on eBay and CL for months and have not found one for sale, but I'm really just waiting till I have cash for a new one and find a good price.
I was down in San Diego the other day, and dropped by the GV headquarters...nice folks, gave me a tour, and explained some of the issues they had in the past, and how they are all fixed for years now. I'm sold.
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Old 04-08-2014, 11:31 AM   #35
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1984 34.5' Airstream 345
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Smartstream - I have an Isuzu Powered 345. I struggle with the same RPM issue at highway speeds. Do you know the part number and gear ratios of the Gear Vendor unit you purchased? I would like to look at one of those units for my rig. Thanks, Tim
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Old 04-09-2014, 12:01 AM   #36
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Smartstream - I have an Isuzu Powered 345. I struggle with the same RPM issue at highway speeds. Do you know the part number and gear ratios of the Gear Vendor unit you purchased? I would like to look at one of those units for my rig. Thanks, Tim

Hi Tim, all Gearvendors are .78 to 1 ratio. They only sell one unit. The difference in application is the front housing. On the TH400 you remove the tail shaft housing, bolt on the Gearvendors adapter and bolt on the Gearvendor OD unit. Normally you have the driveshaft shortened so everything fits. On the TH400 in the Airstream the rear housing of the Gearvendor also is changed to accept the driveshaft mounted parking brake. If you acquire a used Gearvendor unit they will sell you the adapter to mount to whatever trans you have. For the rear housing for the parking brake they want you to send your unit to them because you have to dismantle the unit to change the rear housing. If you have one of their units and want to change from one car or truck to another they are very helpful. Last I heard I don't believe they have an adapter for an Allison trans if that is what you have.


I hope that answers your question. If not let me know and I'll do my best to help.
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Old 01-20-2015, 10:12 PM   #37
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very interesting stuff as I've just driven my newly acquired 1991 350LE from Seattle to Austin, Texas (see link 350LE Seattle to Austin in the on the road forum). I too am concerned when running at between 2700 and 3000 rpm in top gear, my over the ground speed on straight roads, 45mph up the hills, was around 60 mph which was and is way to slow for modern highway cruising. So, do I shell out $3300 plus labor for a Gear Vendors overdrive unit or put this money towards an engine overhaul (out to 496 cid) which would include a new head, roller cams, hydraulic lifters, fuel injection, multi-spark ignition, high volume coolant and oil pumps, maybe Banks etc. etc.
This would produce a high performance engine, smoother revving to sustain 4500 rpm highway cruising in top gear with the existing 4.56 rear end gearing ???
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Old 01-20-2015, 11:26 PM   #38
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"More POWER!"... well, I would consider the O.D.... revving even a well built motor, under load, will have it's downside. Having the OD means you CAN shift into it if NEEDED... so, you will have 'granny gear' when slow work needed.... and can drop the R's when underway..
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Old 01-21-2015, 12:20 AM   #39
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Back in the day I had a Corvette with a big block that would rev well over six or seven thousand rpm, and it was basically the same motor that we have in these coaches. The biggest difference is that it was pushing 10-12-14 thousand fewer pounds, using a radiator of about the same size to cool it, and dealing with a dramatically smaller frontal area that had to be pushed through the wind. At highway speed it ran at 170-180 degrees doing 2700-3300 rpm.

The Cummins in my 325 with Gear Vendors runs at 190 degrees at 60 mph, but quickly heads up over 200 degrees when pushed harder.

The temperature of the 460 Ford motor in my recently acquired 370 also rises similarly when pushed above 65 mph, and I have never driven it in weather above 80 degrees. I'd love to put an overdrive into it as well, but I really wonder how much difference it would make with coolant temperature. Theoretically, while an overdrive does lower your rpm's at a given speed, it is also working your engine against a less advantageous final drive ratio.

Just as a point of interest, I think I once read that 90% of the energy used to push a bicycle above say 30 or 40 mph, is required to overcome wind resistance. So given the frontal area of our motorhomes; at highway speed on flat ground, that motor thinks it's pushing uphill.

So since we can't make these things any smaller, lighter, or more aerodynamic, I guess my thinking is that engine temperature might best be managed through enhanced cooling (and I doubt that turning the AC off, and the heater on, will be of much help).

Has anyone experimented with any supplemental cooling? Perhaps in place of the front air dam, whether it be water or oil? Just curious.
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Old 01-21-2015, 12:30 AM   #40
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1982 28' Airstream 280
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very interesting stuff as I've just driven my newly acquired 1991 350LE from Seattle to Austin, Texas (see link 350LE Seattle to Austin in the on the road forum). I too am concerned when running at between 2700 and 3000 rpm in top gear, my over the ground speed on straight roads, 45mph up the hills, was around 60 mph which was and is way to slow for modern highway cruising. So, do I shell out $3300 plus labor for a Gear Vendors overdrive unit or put this money towards an engine overhaul (out to 496 cid) which would include a new head, roller cams, hydraulic lifters, fuel injection, multi-spark ignition, high volume coolant and oil pumps, maybe Banks etc. etc.
This would produce a high performance engine, smoother revving to sustain 4500 rpm highway cruising in top gear with the existing 4.56 rear end gearing ???
Remember what you are driving. Somebody put an aluminum can on a truck chassis designed back in the 70's. The brakes and suspension aren't like your modern car or pick up. Also you are driving a "recreational" vehicle. Slow down and enjoy the ride. Let the guys in a hurry go around. As said above the big power modified engines don't do well for the long haul.
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Old 01-21-2015, 01:36 AM   #41
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1991 30' Airstream 30
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I have driven a 345LE with a Gear Vendors, and I can tell you that it's really nice for highway cruising. At 65 you kick it in and drop down around 500 rpm, quiet and smooth. Cruising at 70 is no problem at all. Nice to have area gears for hill climbing as well. I have on in my garage slated for install on my 300LE next month...can't wait!


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Old 01-21-2015, 05:00 AM   #42
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Ive got the Isuzu diesel with the 4sp Allison tranny. I run at 2600rpm traveling 62mph. I was concerned about the rpm when I first got the coach. But Someone pointed out on this forum that these rigs were designed when the speed limit was 55mph. Im with Dan, go slow and try to stay out of the way. Even at 62, with these disc brakes, I need to anticipate far in advance to feel safe on the highway.
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