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

 

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.

 

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.

 

^^^THIS. The TH425 has a BOP-only bolt pattern. People have adapted the BBC to that trans, but it is a LOT of work. The bellhousing adapter is the easiest part of the swap. You will need to fabricate mounts for the engine in that chassis. You will need to fabricate brackets to hand the differential and axles off the block (the Olds brackets won't bolt to the Chevy motor). You will need to fabricate custom headers that clear the transaxle. If you are paying someone to do this work, expect to pay well into five figures.

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.

 

There you go Will the stroker kit work in the 403 block ? That would be perfect.

 

Yep, I know someone who did one for a ‘76. Came out great.
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.

 

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.

 

I was going to post the same warning.

Search this site and the internet to be fully informed prior to conducting business with them.

 

A 403 stroker would make equal torque to 455. If you can swing the cost of EFI, do it. The shorter height vs a carb will definitely make using the RPM intake easier.

 

A well built 455 would fit right in, and they used them in motor homes for years. The 403 has that weak windowed block to contend with.

 

It does but in a motor home, it will be lower rpm for the most part.

 

True, but the small block will be lighter and shorter, allowing for better manifold choices.
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.

 

Joe P in post #4 pretty well covered it. Then we had a suggestion to put a $2000 stroker kit in the 403. Now, we have a $500 "halo" (and studs) to bandaid the weak windowed 403 block to make it live. People have said the "halo's" are minimal to useless strengtheners in the past. If the 403 block wasn't so weak, people would be building them for their Cutlass instead of the 350's.
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.

.

 

Stroker small-block Olds with multi-port EFI, AEM engine management. Over 500 HP, over 550 torque, but beware of sticker shock! This thing runs like stink!

 

Cjsdad. Looks great. Can’t wait for ours

 

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.

 

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.

 

You're right Joe, the dyno readout starts at 3500 rpm where it produces over 550 lbs. ft. of torque. I think that is where most dyno's start data-logging anyway so that comment is redundant. The torque is already on the way down at that point so there is plenty under the start point for the needs of an RV. It also stops at 5500. But the OP asked specifically about multi-port EFI on an Olds motor. There aren't many outside the ones used in the mid 80s Cadillacs and I doubt those would pull a motorhome up a driveway, let alone Donner Pass. This is just a modern example of what can be done in the pursuit of that goal, and a very expensive alternative to your 455 example. But it is an alternative that answers his questions! I also mentioned sticker shock on something like that! Would I recommend it for an RV? No! It is extreme overkill for that. A well built 455 is reasonably affordable and produces the gobs-o'-grunt that will make driving a motorhome almost pleasant. The throttle-body FI on the factory replacement 454 in my motorhome ran reasonably well when I drove it from Albuquerque to Virginia Beach two months ago but it saw well into the 4000 rpm range quite often. I wanted more power many, many times. You're not gonna get 20 mpg no matter which option you choose so go with the most economical option that will fulfill your needs! Or wants!!!

 

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 😎

 

Nice!. What fuel mileage increase did you get when switching to MPFI?

 

never had a carb on it so I don’t know.

It’s also the rare short 23 footer which is much lighter

 

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