New Engine for a GMC Motorhome
#1
New Engine for a GMC Motorhome
Hello,
First off I'm not an engine guy, but I have a 78 GMC with a 403 and bits got in the engine from a botched distributor swap. I want to go multi-port fuel injection so I was thinking GMC 8.1 from a 2003 Suburban but its hard to shoehorn those into the TH425 based GMC MH so....I saw on Mondellos site I can get a multi-port 455 engine. I know I want multi-port fuel injection (no more carbs for me) and single port FI is just an electric carb to me. What is the biggest baddest engine I can put in the GMC that is at least as good or better than an 8.1? This is our cross country family cruiser so I want it to run smooth but have plenty of power. I always believed beefing up an 8.1 from an 03 suburban was best. Any thoughts? I'd really love it if I could keep it Oldsmobile like it is now.
Thanks,
Steven
First off I'm not an engine guy, but I have a 78 GMC with a 403 and bits got in the engine from a botched distributor swap. I want to go multi-port fuel injection so I was thinking GMC 8.1 from a 2003 Suburban but its hard to shoehorn those into the TH425 based GMC MH so....I saw on Mondellos site I can get a multi-port 455 engine. I know I want multi-port fuel injection (no more carbs for me) and single port FI is just an electric carb to me. What is the biggest baddest engine I can put in the GMC that is at least as good or better than an 8.1? This is our cross country family cruiser so I want it to run smooth but have plenty of power. I always believed beefing up an 8.1 from an 03 suburban was best. Any thoughts? I'd really love it if I could keep it Oldsmobile like it is now.
Thanks,
Steven
#2
The main problem you are going to have is bolting something up to the TH425 trans ? Unless I am wrong it uses the BOP bolt pattern so a Chevy engine like the 8.1 wont work without some type of adapter.
I don't know how much room there is in the engine compartment but I would think a well built "torque monster" 455 Olds would be the easiest swap. With the Olds engine most of your accessories, brackets etc might be easier to fit. I also don't know how much torque the transmission can handle so you would have to be careful there too.
As far as Fuel Injection goes I think the Holley systems seem to be pretty decent and have a good support network.
I don't know how much room there is in the engine compartment but I would think a well built "torque monster" 455 Olds would be the easiest swap. With the Olds engine most of your accessories, brackets etc might be easier to fit. I also don't know how much torque the transmission can handle so you would have to be careful there too.
As far as Fuel Injection goes I think the Holley systems seem to be pretty decent and have a good support network.
#3
Stroker small block Olds, I have the Stroker kit and everything will bolt right up.
As far as EFI goes, the Holley stuff is good but their tech line is typically backed up. I prefer the AEM stuff. Lots of tunability, and a wicked fast processor.
As far as EFI goes, the Holley stuff is good but their tech line is typically backed up. I prefer the AEM stuff. Lots of tunability, and a wicked fast processor.
#4
The main problem you are going to have is bolting something up to the TH425 trans ? Unless I am wrong it uses the BOP bolt pattern so a Chevy engine like the 8.1 wont work without some type of adapter.
I don't know how much room there is in the engine compartment but I would think a well built "torque monster" 455 Olds would be the easiest swap. With the Olds engine most of your accessories, brackets etc might be easier to fit. I also don't know how much torque the transmission can handle so you would have to be careful there too.
As far as Fuel Injection goes I think the Holley systems seem to be pretty decent and have a good support network.
I don't know how much room there is in the engine compartment but I would think a well built "torque monster" 455 Olds would be the easiest swap. With the Olds engine most of your accessories, brackets etc might be easier to fit. I also don't know how much torque the transmission can handle so you would have to be careful there too.
As far as Fuel Injection goes I think the Holley systems seem to be pretty decent and have a good support network.
The easy solution is another Olds motor. The 455 is a bolt-in. And frankly, you'll never recoup the cost and pain of the multi-point injection conversion. If you've talked yourself into EFI being a must-have, then consider a throttle body unit. Frankly, a properly adjusted Qjet will be fine in that motorhome.
#5
![Smile](https://classicoldsmobile.com/forums/images/smilies/smile.gif)
#6
Word of note. If you go with multiport EFI on these it allows you to use a std style throttle body, which is about 1.25” shorter than a carb or TBI unit. That’ll help in overall clearance under the cowl etc.
#7
You guys all missed one very important comment in the original post. Mondello. If it is the one in Paso Robles with Lynn Whateverhisnameis, he needs to be warned to run in the opposite direction.
#8
I was going to post the same warning.
Search this site and the internet to be fully informed prior to conducting business with them.
Search this site and the internet to be fully informed prior to conducting business with them.
Last edited by Fun71; September 17th, 2021 at 06:28 PM.
#11
#12
Plus you can throw a halo on the 403 and it should be just fine in this application. As mentioned, rpm will be at a minimum so that will help too.
#13
The 403 might be lighter, but on a 5 ton motor home the 50 pound weight saving of a 403 is nil. Manifold choices gain most in the upper RPM ranges and not so much in the lower torque range.
Seems like a lot of extra work to get a 403 to a power level on a par with a 455. Oh, and you could stroke a 455 to gain more torque yet. Torque is what gets a motor home moving and keeps it moving. Torque also makes a difference in 2nd gear. high gear and uphill grades.
.....Just my two cents worth.
.
#16
I bet that is so much fun. I am curious, how is the fuel mileage on a high output motor like that with EFI? The 403 can be built more with improved torque, mileage and drivability over big horsepower. 600 ft/lbs should be no issue with 470 ish cubic inches.
#17
At what RPM? Look, that's a great motor, kudos on the build. It's not what you want for a 7,000 lb motorhome that rarely sees over 3,000 RPM - I bet your dyno pull didn't even start until that point. The MH application wants to maximize torque between 1500 and 3500.
#18
At what RPM? Look, that's a great motor, kudos on the build. It's not what you want for a 7,000 lb motorhome that rarely sees over 3,000 RPM - I bet your dyno pull didn't even start until that point. The MH application wants to maximize torque between 1500 and 3500.
![](https://cimg2.ibsrv.net/gimg/classicoldsmobile.com-vbulletin/1629x983/dyno_3_0c7d4bc5be2bc8a6c330e7c3ed01e9b0a3cc357a.jpg)
Last edited by cjsdad; September 18th, 2021 at 01:05 PM.
#19
I have MPFI 455 in my motorhome using the stock factory iron toro intake and a dummy Q jet as the throttle body.
crank trigger with a coil pack. Tri-y headers 3:08 gears and a switch pitch trans
It goes pretty good 😎
crank trigger with a coil pack. Tri-y headers 3:08 gears and a switch pitch trans
It goes pretty good 😎
#22
My suggestion for the engine would be to go 455 minimum cubes. If you want badder then the 496 eagle stroker
use the aluminum version of the TORONADO intake made for the motorhome and convert it to port injection welding in bungs for the injectors.
some aftermarket controller with throttle body on the stock intake. There are a few to choose from for multi port
tri-y headers and a good flowing exhaust system
edelbrock heads would be fine as is. A mild hydraulic roller.
this conversion to big block allows you to use most of your existing parts.. and accessories
There are transmission options too and gears
use the aluminum version of the TORONADO intake made for the motorhome and convert it to port injection welding in bungs for the injectors.
some aftermarket controller with throttle body on the stock intake. There are a few to choose from for multi port
tri-y headers and a good flowing exhaust system
edelbrock heads would be fine as is. A mild hydraulic roller.
this conversion to big block allows you to use most of your existing parts.. and accessories
There are transmission options too and gears
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