Powertrain swap with something later model
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This question comes up about once a week. The best way to get an answer is to search this forum or the old forum for past discussion threads. Short answer: It's hard to beat the stock 403/455, and it'll cost a lot to try.
There at least six diesel GMCs out there and the unreported cost is really high. Hell Man, I know about what Dave charges to put in an 8.1 and it borders on crazy.
What you are saying actually makes more sense than trying to use a SB Chevy for the job.
The FWD Dodge vans could make a nice donor, too. I would love to see any of this done, but it be done by me. I may have the knowledge to pull this off (based on past crazy things completed), but I have nothing like the resources and energy for such.
Matt
What you are saying actually makes more sense than trying to use a SB Chevy for the job.
The FWD Dodge vans could make a nice donor, too. I would love to see any of this done, but it be done by me. I may have the knowledge to pull this off (based on past crazy things completed), but I have nothing like the resources and energy for such.
Matt
Today's automobile engines spec at higher HP, but they do that with higher RPM. They are not built to deliver a sustained high HP output for long periods of time. Same goes for the tranny. If you put the money into having a new drivetrain installed, I don't imagine it would have a long life. I would also expect it would rarely get into high gear.
My latest GMC pickup has direct injection, 8/4 cylinder, 6 or 8 speed transmission and all those goodies and yes it will get 25mpg hyway. But I also have a utility trailer with ramps (for some wind resistance). Even when the trailer is empty, the mileage drops to 15. Put a load in it (under 3,000 gross) and it will drop to about 12mpg.
The Olds engine was made when they built engines like a battleship. Nowadays they are engineered for the least cost vs warranty claims cost. Least cost to get the job done vs warranty claims for failures.
My latest GMC pickup has direct injection, 8/4 cylinder, 6 or 8 speed transmission and all those goodies and yes it will get 25mpg hyway. But I also have a utility trailer with ramps (for some wind resistance). Even when the trailer is empty, the mileage drops to 15. Put a load in it (under 3,000 gross) and it will drop to about 12mpg.
The Olds engine was made when they built engines like a battleship. Nowadays they are engineered for the least cost vs warranty claims cost. Least cost to get the job done vs warranty claims for failures.
The Hydramatic 4T65E-HD used in the LS4 equipped Impala SS would only last a few thousand miles in the GMC, it is a good tranny but just not engineered for the extreme weight of the GMC. I have seen someone in Europe swapping the whole front sub frame/drive train from a Fiat van into a GMC, but can't find the link right now.
The TH425 really is the best tranny for the GMC, and the only engines that easily mate to it are the Olds and Cadillac engines. If you want more power the easiest route is a pre-1972 version of the Olds 455.
The TH425 really is the best tranny for the GMC, and the only engines that easily mate to it are the Olds and Cadillac engines. If you want more power the easiest route is a pre-1972 version of the Olds 455.
With that in mind, have people gone to better fuel management systems like Holley EFI or something similar?
I understand that for the time, they were very well designed. I have also noticed people have changed final drive ratios either for a better highway gear or better pulling gear.
I grew up driving many cars from the 70's and more than familiar with Q Jet carbs. That's why I would look for something different.
I understand that for the time, they were very well designed. I have also noticed people have changed final drive ratios either for a better highway gear or better pulling gear.
I grew up driving many cars from the 70's and more than familiar with Q Jet carbs. That's why I would look for something different.
Steve,
If you have been a member of the GMC community for long, you would have heard the very standard line - 8 ~ 10MPG - depending on towing and driving style. This comes from a sample of several hundred that are running EFI closed loop and with knock control spark. They start better than the old coaches and don't take such beating over 8KMSL.
That goes for everybody except the diesels and there are not enough of those to make a consensus. The roughly dozen 8.1 coaches are there too (port fuel, drive by wire and all) and they don't any better, but then again, the owners will admit that they drive faster than they used to before the new engine.
If MPG is your goal, then motorhomes are not your target.
Matt
If you have been a member of the GMC community for long, you would have heard the very standard line - 8 ~ 10MPG - depending on towing and driving style. This comes from a sample of several hundred that are running EFI closed loop and with knock control spark. They start better than the old coaches and don't take such beating over 8KMSL.
That goes for everybody except the diesels and there are not enough of those to make a consensus. The roughly dozen 8.1 coaches are there too (port fuel, drive by wire and all) and they don't any better, but then again, the owners will admit that they drive faster than they used to before the new engine.
If MPG is your goal, then motorhomes are not your target.
Matt
There are many engine options to mate to the TH425 that have been successfully used, or certainly could be used with mixed success. SBC, BBC, 6.5L Diesel, Cadillac engines, Other Buick, Pontiac, Olds engines. The engine isn't the problem. The tranny is the problem. They aren't made anymore. Luckily they are tough as anything and quite a few went into automobiles. But when they are gone......
Find a Front Wheel Drive transmission rated for around 15k lb, that would be the ticket.
The Sprinter / Mercedes / Ram Promaster FWD vans are using a beefed up Chrysler Minivan transmission iirc. The weight rating is there, almost, but the history on that tranny isn't great. The upside is you could get it fixed, there are parts available. The engines mated to that however, might put you in the Toyota motorhome realm as far as acceleration and hill climbing. They work, sort of, as long as you're not in a hurry. And forget the Toad.
Find a Front Wheel Drive transmission rated for around 15k lb, that would be the ticket.
The Sprinter / Mercedes / Ram Promaster FWD vans are using a beefed up Chrysler Minivan transmission iirc. The weight rating is there, almost, but the history on that tranny isn't great. The upside is you could get it fixed, there are parts available. The engines mated to that however, might put you in the Toyota motorhome realm as far as acceleration and hill climbing. They work, sort of, as long as you're not in a hurry. And forget the Toad.
Matt, Better MPG is not the goal with this topic. My Winne gets a whopping 6.25MPG no matter what I do so I know all about sucking some gas. I brought his up for more modern technology and reliability.
Todd, thank you for that information. That is what I was researching. What are other options people have tried on this platform. I love the motorhome and would really like to own one someday.
Thank you all for your input on this topic!
Todd, thank you for that information. That is what I was researching. What are other options people have tried on this platform. I love the motorhome and would really like to own one someday.
Thank you all for your input on this topic!
As far as just sticking with the original engine, which is at least a fine option (and many people will argue it is the best option period, and they may be right), it's a great motorhome.
I came from a 31' 1990 Holiday Rambler AlumaLite XL, Chevy BB 454, TH400 tranny. We loved it very much but I like to keep moving and after even a 6 hour day of driving, I could feel it. It was tough on the driver. The HR was probably better for setting up and camping in one place for several days, bigger everything, better bathroom arguably
We haven't done any long trips with the GMC yet, b/c of covid, longest is maybe 4 hours but I know for a fact that it goes down the road a lot faster for the same amount of driving effort. It's easier to go 70mph in the GMC than it was to do 60mph in the HR.
70mph in the GMC I'm getting 8.5mpg, 60mph in the HR I got 7.5mpg. Both vehicle fuel use is here https://www.fuelly.com/driver/tmsnyder
If the motor ever did poop out, and I wanted to have it back on the road as quickly as possible then the easiest fix would be to rebuild the Olds 455 or find another similar block and put it back in. Everything will bolt right back up obviously, since that what came out. That's the 'easiest' route, and you end up with a really fine option (arguably the best option)
Other options would be basically any engine you can get to work with the TH425 transmission. LS engines don't work without pretty drastic modifications.
OMG I did not realize you could share your Fuelly profile! Here is mine https://www.fuelly.com/driver/76spirit
I have averaged 8.1 MPG in my GMC over the past 4700 miles, which I am fine with since I have a lead foot and only use regular E10.
Yeah I'm a geek, I like data.
I like fuelly, still just using the text messaging to log fuel use. It's handy b/c people are always claiming fuel mileage, and I've seen it happen, they get incredible fuel mileage once because of any number of reasons (wind blowing them down the road, they kept the speed under 60 all day, they underfilled the tanks accidentally , ..._ ) and then they claim they get that mileage forever no matter what speed. Yeah right, show me every drop of fuel you ever put in it and then I'll believe your mileage claims. Here's mine - fuelly link
Oh wow, your mpgs are very consistent. Must be filling very consistently.
I installed a Howell TBI injection system back in 2009. The carb had been changed to a Holley carb and it had the usual carb issues... choke won't come ON right, or go OFF... stuck on high idle... long time cranking before it would fire up etc.
EFI makes it start and run like a new engine. I have added EBL system to it so now I have spark control, lean cruise, deceleration fuel cut-off which Howell did not have, so I get a bit better mileage, But I also likely drive it a bit harder.
The Howell system is basically a GM TBI adapted to the 455 so parts are available anywhere. The newer FiTech, Holley etc system are proprietary so if something goes wrong you may have to remove and send the whole thing back to the manufacturer for repair.
EFI makes it start and run like a new engine. I have added EBL system to it so now I have spark control, lean cruise, deceleration fuel cut-off which Howell did not have, so I get a bit better mileage, But I also likely drive it a bit harder.
The Howell system is basically a GM TBI adapted to the 455 so parts are available anywhere. The newer FiTech, Holley etc system are proprietary so if something goes wrong you may have to remove and send the whole thing back to the manufacturer for repair.
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Bruce, Good points. I have the GM TBI on my 91 Winne and it works great. that I'm sure would be a nice simple update instead of the carb. Its pretty much a stand alone system and can be adapted anywhere.
Those FWD dodge vans are using a beefed up Dodge/Chrysler minivan transaxle, and are rated up to ~9k lbs? Same 62TE that's in the Chrysler Sebring Pacifica and 200.
Then what engine? The Pentastar 3.6L V6? 260 ft-lb of tq, 280 hp. I'd like to see it done by someone, I think it could technically 'work' in the same way a Toyota motorhome does. It'll get you down the road and camping, just take it easy on the hills.
Has anyone ever actually put a pre-2000 SBC in a GMC motorhome? I have been unable to find anyone that has. I know the Coop is working on a Olds 350 but that's a different animal. A 1999 5.7L C2500 is rated at 8000 lbs for towing. It weighs about 5000 lb. That's 13,000 lb and I don't think anyone would say that a 3/4 ton Chevy truck isn't capable of adequately towing a 5th wheel trailer down the road. It's capable of pushing that amount of frontal area through the air. I'd like to hear real world stories of someone that put a 5.7L in their GMC. I think they are out there. Maybe even some 383s.....
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Todd,
You need to talk to Dolph his company has had good use of some of the heavier Dodge Ram Vans. one of them was up in our weight rage and with diesel. That is just about all I know and I do know that Dolph doesn't read this board.
Matt
You need to talk to Dolph his company has had good use of some of the heavier Dodge Ram Vans. one of them was up in our weight rage and with diesel. That is just about all I know and I do know that Dolph doesn't read this board.
Matt
Ram dropped the diesel, now only available with the V6 gas engine. Wouldn't surprise me if the tranny was the weak link, probably couldn't handle the tq.
The heaviest duty one they make is the 3500. It weighs about 5000lb and is rated to tow up to 6800lb. That's 13,800 lb, so it's right there at GMC levels.
I don't doubt it could work, if the transverse engine could be made to fit where a longitudinal engine used to go. That would be a significant amount of work and at the end someone would end up with a GMC that would have trouble merging onto a highway or climbing a hill. Toads would be right out.
The heaviest duty one they make is the 3500. It weighs about 5000lb and is rated to tow up to 6800lb. That's 13,800 lb, so it's right there at GMC levels.
I don't doubt it could work, if the transverse engine could be made to fit where a longitudinal engine used to go. That would be a significant amount of work and at the end someone would end up with a GMC that would have trouble merging onto a highway or climbing a hill. Toads would be right out.
Tom,
Your friend is not mistaken, and that used to be more true than it is now, but the painful part here is that diesels just don't like torque converters very much. One would that think the converter would smooth that out, but it does not do much that way.
Matt
Your friend is not mistaken, and that used to be more true than it is now, but the painful part here is that diesels just don't like torque converters very much. One would that think the converter would smooth that out, but it does not do much that way.
Matt
Back in about 1990 we put a 5.9 Cummins with a 4L80e trans with a Gear Venders overdrive in a 1984 GMC 3500 dually pickup. The 4L80e was a new trans at the time and using the GV let us split gears and provide 8 forward gears. It was a real road runner empty but towing with a gross of 20,000 proved troublesome. After destroying two tranny's Chevy said enough and the third trans would be their last freebie. We finally figured out that the constant hammering of the diesel at the lower engine speeds under load was killing the trans. Learning to NOT use overdrive in high gear solved the problem and all lived for another 300,000 miles.
As for engine swaps, my 6.5 TD Inter cooled 4L80e driven 1977 Royale has worked very well for us, although not a project for the faint of heart.
Hal
As for engine swaps, my 6.5 TD Inter cooled 4L80e driven 1977 Royale has worked very well for us, although not a project for the faint of heart.
Hal
