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Old 12-22-2005, 05:53 AM   #1
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My old El Dorado was built in the last year of production for the original El Dorado Motors Corp. They went under in 90 and were taken over by Honorbuilt. Most likely as a cost cutting idea the little class C has a turbo 350 transmission. With this tranny and fairly low geared differential, at highway speed (60 mph) it is turning a lot of RPMs.

Question is would a turbo 400 transmission make any real difference and is it a bolt up swap or would I have to change bell housing ? drive shaft? ???

Appreciate any input.

Terry
90 Eldorado G-20-I (20.5)(with 20,348 miles on it)
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Old 12-22-2005, 05:53 AM   #2
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My old El Dorado was built in the last year of production for the original El Dorado Motors Corp. They went under in 90 and were taken over by Honorbuilt. Most likely as a cost cutting idea the little class C has a turbo 350 transmission. With this tranny and fairly low geared differential, at highway speed (60 mph) it is turning a lot of RPMs.

Question is would a turbo 400 transmission make any real difference and is it a bolt up swap or would I have to change bell housing ? drive shaft? ???

Appreciate any input.

Terry
90 Eldorado G-20-I (20.5)(with 20,348 miles on it)
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Old 12-22-2005, 06:52 AM   #3
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The tranny won't make any difference unless it has different gear ratios (and that will depend on the configuration of that particular transmission). It seems to me that a rear axle change would have the same effect and be easier/cheaper. A numerically lower rear axle ratio will reduce engine RPMs.

However, your engine has to have the guts to turn the wheels at the new ratio, whether you achieve by changing rearaxle or tranny. At the very least, acceleration will suffer.

A beter solution (though pricey) would be to add a Gear Vendors Over/Underdrive (auxiliary transmission). That way you keep all your existing gear ratios but can shift to an Overdrive when at cruising speeds. You also get additional gears in between the existing one, which lets you better match engine RPMs to the load. Takea look a the Gear Vendors web site for more info.
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Old 12-22-2005, 03:44 PM   #4
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Gary
Thanks for the info. The main reason I was considering a transmission change is that the turbo 350 GM is only a 3 speed automatic versus the turbo 400 is a 4 speed with torque converter lock which serves as a virtual overdrive. I think the final drive on the 400 is higher than the 3 speed 350. It is also a heavier duty transmission.

The coach is not real heavy weighing 7800 lbs (dry weight) but the 5.7 V-8 is not real big either for an RV of any size othe than the micro minis..

The main area I am looking to reduce RPMs is on basically level highway travel as it needs the lower gear ratio especially on long hills. I don't tow with it.

I figure if I can get the Rs down the mileage will go up. The differential may be the best way and least expensive way to go if I can find something in the mid to low 3 range as it has to be in the 4 range now, maybe 401 or real high 3s.

Thanks
Terry
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Old 12-23-2005, 09:59 AM   #5
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The torque converter lock-up on the 400 should help mileage and will probably reduce RPMs a bit as well.

I think the GM trannys are all bolt-pattern compatible. Obviously GM wants to be able to use their trannys interchangeably at the factory, so it makes sense for them to make them fit the same pattern. However, I don't know this for a fact - it's just what I have heard elsewhere.
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Old 12-23-2005, 10:30 AM   #6
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Thanks Gary, I have a friend in California who is an old Chevy guy who will know for sure. He has a 64 Vette he has had since 66 and won't own anything but a Chevy (Except for his Harleys and his DP) even had a small block in his 23 T.

Terry
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Old 12-23-2005, 03:40 PM   #7
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OK, my Chevy Hot Shot says to go with a Gear Venders over/under drive unit, says I'll drop about 800 rpms in top gear.

Terry
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Old 12-26-2005, 03:51 PM   #8
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I KNOW the Gear Vendors will do as he says - it is designed for that and they are supurb units. It might be a bit more expensive than swapping in a used turbo 400, though.
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