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Old 12-13-2014, 11:59 PM   #29
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They ride like a Cadillac, and 'float' down the highway. There is no real need for an entry step, although mine had a Ragusa folding step which was very nice.
The interiors are def space challenged. They were not advertised or intended for full-time, or even extended stay camping use. Back in the '70s, road trip vacations were far more prevalent than today. I remember cruising across the country every summer with my parents in our '72 Olds 98 2-door HT; had the 455 and netted 11-12mpg hwy. GM promoted the the motorhomes to replace large cruiser cars for short term vacation use. Makes great sense for the time. Get very close to the same economy (I netted 10-10.5mpg consistently with mine) and have home-like amenities. And no more listening to "I have to go to the bathroom" every 5min from the kids.
I wonder if anyone has swapped a LS 5.3L or a 6.6L duramax diesel engine into one? it might get close to 15-16 MPG with the 5.3L

We have a Small school bus on a 4500 Van chassis in the fleet that i drive.
It Weighs about 12-13K lbs.
It was averaging 8.5MPG and the dash showed a average speed of 16 MPH
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Old 12-14-2014, 06:29 PM   #30
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I wonder if anyone has swapped a LS 5.3L or a 6.6L duramax diesel engine into one? it might get close to 15-16 MPG with the 5.3L

We have a Small school bus on a 4500 Van chassis in the fleet that i drive.
It Weighs about 12-13K lbs.
It was averaging 8.5MPG and the dash showed a average speed of 16 MPH
Doubtful you'd get anywhere close to 15mpg with a 5.3L. What economy do the 5.3 Tahoes net? Maybe 16hwy, and the GMC is 3x the weight. Plus, the 5.3's only made 315-335lbs/ft torque; the 403ci in my '78 made 320, so the 5.3 wouldn't be an upgrade (the '73-early '77 455s were 10-15lbs/ft more). The 5.3's do have a higher HP rating, but much like boats, motorhomes rely on torque, horsepower really means very little.

There are quite a few that have been converted to a diesel powerplant. I'm not sure which diesel platform is generally used for the conversion. I don't know that a Duramax would bolt up to a TH425, or clear the side/under trans configuration.
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Old 12-14-2014, 07:53 PM   #31
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I wonder if anyone has swapped a LS 5.3L or a 6.6L duramax diesel engine into one? it might get close to 15-16 MPG with the 5.3L

We have a Small school bus on a 4500 Van chassis in the fleet that i drive.
It Weighs about 12-13K lbs.
It was averaging 8.5MPG and the dash showed a average speed of 16 MPH
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Doubtful you'd get anywhere close to 15mpg with a 5.3L. What economy do the 5.3 Tahoes net? Maybe 16hwy, and the GMC is 3x the weight. Plus, the 5.3's only made 315-335lbs/ft torque; the 403ci in my '78 made 320, so the 5.3 wouldn't be an upgrade (the '73-early '77 455s were 10-15lbs/ft more). The 5.3's do have a higher HP rating, but much like boats, motorhomes rely on torque, horsepower really means very little.

There are quite a few that have been converted to a diesel powerplant. I'm not sure which diesel platform is generally used for the conversion. I don't know that a Duramax would bolt up to a TH425, or clear the side/under trans configuration.
The Bus I just mentioned has a 6.0L LS engine and it does NOT have the cylinder deactivation on it. For the speed it driving and with all the stops it does its getting great MPG.
The B-O-P to Chev trans adapters are every where its just a matter of time before you see one swapped.

I wonder if GM built some test units years ago with the V8-6-4 Cadillac engines. it would have had to make enough power on 4 cylinders on flat ground for it to help.
Not sure if it would have.
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Old 12-14-2014, 07:59 PM   #32
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I had a '98 Aurora with the 4.0L baby Northstar for several years. One of my favorite cars, but with 196k, it was only a matter of time before the head gaskets went.
A V8 dropping 2cyl while steady cruising would be a good option.
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Old 12-15-2014, 02:51 PM   #33
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That's why the 8-6-4 didn't last, I Believe.
I've got the 4.6 Northstar in the concours and she's going on 196,000 Still will blow off a few.
Tim
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Old 12-15-2014, 05:25 PM   #34
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There is a GMC motorhome restoration shop in Florida that has put Duramax diesels in two GMC's, but had to build new frames to accommodate the front wheel drive Allison transmission set up. Several owners have swapped the larger Cadillac 500ci engines along 454 and 8.1L GM motors. I doubt there is any gas mileage improvements with those swaps. There is an interest in putting rebuilt 6.5L GM diesels from 1990s GM pickups in the GMC's also. These could provide some fuel mileage improvement, time will tell if this becomes a popular swap for the GMC owners.


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Old 01-06-2015, 09:05 AM   #35
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That's why the 8-6-4 didn't last, I Believe.
I've got the 4.6 Northstar in the concours and she's going on 196,000 Still will blow off a few.
Tim
196k miles? Just about time for that North* to shotgun a head gasket (unless you've been judicious about changing the ^$#% Dexcool regularly). It's too bad, the N* is such a nice powerplant, but virtually unrepairable once a head gasket goes. I really love the way they engineered the water pump & drive, easiest FWD WP replacement on the planet.
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Old 01-06-2015, 09:07 AM   #36
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I just ran across this; makes me really miss mine, especially seeing the bump/RR tie section comparison. I don't think I'd want to try some of those tight turns with the Sun Stream.
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Old 01-06-2015, 02:16 PM   #37
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There is a GMC motorhome restoration shop in Florida that has put Duramax diesels in two GMC's, but had to build new frames to accommodate the front wheel drive Allison transmission set up. Several owners have swapped the larger Cadillac 500ci engines along 454 and 8.1L GM motors. I doubt there is any gas mileage improvements with those swaps. There is an interest in putting rebuilt 6.5L GM diesels from 1990s GM pickups in the GMC's also. These could provide some fuel mileage improvement, time will tell if this becomes a popular swap for the GMC owners.


Mike H
You will get some mileage improvement. I went from just over 8 to just over 9 swapping to the 502 with FI. Realistically, I would probably get more, but I've bumped my typical cruising speed up since the install. For gas, I would expect the most improvement with the 8.1, as it has fast burn heads. Revcon had a diesel option. It easily got around 14 mpg. Its was a dog, so not a popular option.

Realistically, to realize much improvement, you need to change gear ratios. The design goal is use a more powerful engine, so it is still tolerable to drive with taller gearing. The biggest losses are heat and pumping losses. As long as you are using the same size engine, you are having no impact on your biggest loss. The only way to reduce pumping losses is to reduce the RPM. There is only a small amount of loss in burn efficiency, so you can only gain so much with modern FI and better head design. Most of that has already been achieved. If one were starting from the ground up, you use a tiny little engine, so there is no pumping loss, and then turbo charge it, so you can get enough HP out of that tiny little motor.
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Old 01-06-2015, 02:36 PM   #38
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If one were starting from the ground up, you use a tiny little engine, so there is no pumping loss, and then turbo charge it, so you can get enough HP out of that tiny little motor.
The only problem with this for MH use is the complete lack of torque. My Saab 93 Aero has a 2.0L, turbocharged 4-banger. With the stock tune, she yields 210hp/220tq; the TQ doesn't really kick in until ~3k rpm. With the Stage 2 tune, she puts out 260hp/285tq, dropping the torque range to ~2800rpm. The output is impressive for an otherwise bone stock 2.0L, and the TQ number looks good on paper, but reality is, the rpm is just too high for useable torque.
By comparison, my BMW 850Ci had the 5.4L V-12; TQ was rated at 361, all-in by ~2200rpm, and fairly flat almost to the redline.
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Old 01-06-2015, 08:04 PM   #39
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196k miles? Just about time for that North* to shotgun a head gasket (unless you've been judicious about changing the ^$#% Dexcool regularly). It's too bad, the N* is such a nice powerplant, but virtually unrepairable once a head gasket goes. I really love the way they engineered the water pump & drive, easiest FWD WP replacement on the planet.
Boy I hope your wrong.
We have never even flushed the radiator
Have always changed the oil @ every 3,000 miles and rotated the tires every other GOF.
Most of the miles are HWY. She has started to use a quart every 1500 miles. Thanks for the video I really liked how they covered a MH to keep the identity a secrete.
There are a couple of those GMC sitting in a old used car lot I looked at one of them and had to look in the windows but I might go out and look again. See if anyone is around. I remember the rear end was pretty close to the ground.
I really like the style of them, plus a pretty good load carrier.
Thanks,
Tim
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Old 01-06-2015, 08:55 PM   #40
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Had one of the GMC's next to me for the last week. Talked to the owner and he mentioned how the interior was redone and such. Not great mileage so I don't think any motor swap was done on it or anything. Really clean looking for being as old as it is.
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Old 01-06-2015, 11:29 PM   #41
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The only problem with this for MH use is the complete lack of torque. My Saab 93 Aero has a 2.0L, turbocharged 4-banger. With the stock tune, she yields 210hp/220tq; the TQ doesn't really kick in until ~3k rpm...
That is a result of the design choice. I was judging by my Cooper which is ruler flat from 1500 RPM all the way up. Yes, there is turbo lag in the low end, but once it spools up, its fine. If I were driving an RV with the same characteristics, I would be very satisfied with were the power was.
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Old 01-07-2015, 06:28 PM   #42
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Had one of the GMC's next to me for the last week. Talked to the owner and he mentioned how the interior was redone and such. Not great mileage so I don't think any motor swap was done on it or anything. Really clean looking for being as old as it is.

Nice, I haven't ever see one in green.
Thanks,
tim
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