Hi everyone!

I'm searching for the right GMC Motorhome to restore and use as my sweet mobile mancave. Many of them came with, and still have, the Olds 455. I'm guessing whatever one I find will need engine work, and that's fine. But my questions for you guys who know these engines well:

Should I replace it with a different engine entirely? I've read that the Caddy 501 is a popular replacement choice. I don't know the 455, or the different iterations of the 455, well enough to be confident that I should keep it. I have no opinion on it at all, no brand loyalty. All I want is an engine powerful enough to accelerate up hills and reliable enough to drive around without worrying about being stranded.

My other question is, should I use an EFI conversion kit on it? I don't want to mess around with adjusting a carburetor. For that reason, I would like to bolt on the EFI kit. BUT, there are possibly reasons unknown to me that this would not be a good idea. What are the reasons to install EFI, and to not install EFI on the 455, or 501, or whatever engine you recommend.

 

The Olds 455 is an excellent engine. I would not change it for something else that may be marginally better or marginally worse. It may be easier to find parts for the Olds. I'd assess the condition of the engine you get thoroughly via a cylinder leakdown test, oil pressure test and confirm that it does not overheat, especially under load with the A/C working.

I'd also consider replacing the timing chain due to age and/or mileage.

About EFI, I'll leave that to someone else that has made the conversion. I just don't know about the conversion..which are best...etc.

Good luck!!!

 

You can do a stroker crank in the 455 if you want more torque. I believe there is an expensive aftermarket aluminum intake with bungs for mpfi for the 455. http://appliedgmc.com/prod.itml/icOid/568

 

Well this must becoming a trend!
I’m doing an EFI’d Stroker small block Olds for a guy in CA with a ‘76 and exact same application.
We’re doing EFI and a Stroker small block because it’ll save weight over the big block, we can use a better intake because of the added height, it’ll still make really good tq(the first one of these I did made 508lbft at 3200rpm), and he can drive if from sea level to 7000’ and not have any running issues. Not cheap though, but should prove to be the ultimate in combinations.

 

Thanks guys!

Cutlassefi, that sounds great. Is this something you do as a business? I'd like to have that done to mine when I get it.

 

Yes I do. But a build like this isn’t for the faint of heart if you want to do it right.

 

Don't consider stroking the motor. A 455 is plenty of motor for one of those GMC RVs. An engine refresh is all you would need. The 455 is preferable to the Cadillac 502 because you would need an adapter plate to mate the 502 to the BOP transmission. Plus, as others said there's more aftermarket support for the 455.

Self-learning, throttle body EFI would work finefor the RV you buy. Holley Sniper and FiTech seem to be the best rated systems available now.

Not to kill a sale for cutlassefi, but there's no need to stroke a motor or consider multi-point efi unless you're racing or wanting to build serious HP.

 

I couldn’t disagree more, those statements are 100% your opinion with no facts to support them.
Any idea how the “self learning” systems compensate for altitude? Tell us that and I’ll tell you why it’s not the best arrangement by any stretch.
And I guess all the “stock” engines produced today don't need EFI either, not even from a performance or longevity issue?
I get that he could do something ordinary. But when modern tech allows you to make something noticeably better, why not at least consider it.

 

actually the 500 caddy will bolt up to the BOP transmission, its the oil pans and motor mounts that are different, and EFI doesn't really increase the horsepower much if at all, its more about drive ability and tune ability.

 

Mark, will a RPM intake fit with the SB in a motorhome? The EFI stroker small block sounds awesome.

 

A 455 with a TBI on a dual plane manifold (probably already in the MH) would be terrefic. The TBI compensates for altitude with the O2 sensor(s). The sensor reads the mixture, and adjusts it automatically. Carbs must be rejetted to change altitude to any degree, because they lean out as the altitude increases. MPFI is a little better, because it feeds each cylinder as it needs it, where the TBI feeds more like a carb with auto altitude jet change. The TBI is a wet intake manifold system, and the MPFI is a dry intake manifold system. They make the TBIs for 2 and 4 bbl intakes (spread bore is not supported).

Any intake manifold can be made into a MPFI manifold by welding on bungs for the injectors, and drilling them out to the proper size. You will need fuel rails (fuel rails can be used for bung spacing) for the injectors.

Either system self learns with a basic set of parameters that are inserted into the computer before startup. It is amazingly easy to install the EFI units, especially if you are upgrading from a carb. Some just bolt on where the carb is, and you run a couple of wires.

 

True but only in steady state mode.
Let me tell you the real problem with some of these "self learning" systems on an application like this. Most do not have any way to discern the difference between full throttle at sea level or full throttle at 5000'.
Let's say the MAP and Baro both read 103ish KPA at sea level key on engine off (which they should). That equates to a ratio of 1.0 between the MAP vs Baro. Now lets say you're at 5000'. Your MAP and Baro would most likely read about 85 KPA key on engine off but still have a 1.0 ratio. When you're setting your Air/fuel/Lambda and timing tables you need to have a common sense number for load, whether it be at sea level or 5000'. The Holley HP, Terminator and Dominator for instance have an internal Baro and the option to build a MAP vs Baro table but you have to do it yourself from scratch. The AEM does it for you already. If you tune in MAP vs Baro mode a 1.0 ratio is full throttle at any elevation. Otherwise the Holley and others will continue to have to learn. The downside to that is that takes time. Look in the Holley manual and it will tell you "it may take driving the vehicle for a couple of hundred miles before it has totally learned the fuel map". Do you want it to have to do that every time you change elevations significantly? Plus are you going to have the same timing and commanded full throttle air/fuel for all elevations? Hmmmmm, don't know about that. Overall the Holley is a decent system, just not sure it's the right one for this application. Plus I can monitor AND influence both sides independently with the AEM using 2 O2's. Can't do that with any of the mainstream self learning systems.
And yes EFI on it's own seldom results in a gain in hp, but it goes a long way in increasing overall efficiency and engine life. Ever wonder why today's oil change intervals are longer than before? Some of it is the pieces being used internally, some of it is the quality of the oils etc. But a good part of it is modern EFI. Engines run way more efficiently and just as importantly are only in cold start (yesteryear's choke mode) for a very short period of time, some for only 30 seconds or so. That greatly reduces oil contamination.

Lastly, I really wish those who really can't explain their own posts just wouldn't post. I don't post in the transmission section because I'm not nearly as well versed in that area as I am the engine. Not sure why guys do that.
Nonetheless I hope this helps.

 

The system will make adjustments to the fuel and air mixture because of the wide band O2 sensor. If O2 sensor senses a lean mixture, it will correct the mixture almost immediately. It stores the variation in a second table, and combines it with the base table until it is manually updated permanently. My Holley system is set for continuous learning, and will have changes depending on the weather on a given day. The system has a mixture table as well, and will try to optimize the mixture to that table at all times.

 

Correct, but that’s not the whole picture, that’s what I’m trying to tell you.
It takes time for it to learn and modify the table correct? And actually you should’ve narrowed your learn and correction percentages by now. They tell you to do that after a while. Do you know why? Plus in order to make the changes permanent you need to do that manually. Are you gonna whip out your laptop or even stop to do it on the handheld every time you drive up and down the hill?
Yes it’ll correct, but as you know once it makes those changes permanent and limits correction, it runs better doesn’t it? See my point?
And all that still doesn’t solve the issue of potentially needing different air/fuels and/or timing at different elevations and loads without constructing another table. Again you only have part of the equation, sorry.

 

As altitude changes and the mixture needs to be adjusted, the O2 sensor will adjust it on the fly. Timing will not affect power a lot at different altitudes, and you will get substantially more power than the carb in the same situation. It may not be perfect but the result is a much more drivable car as altitude increases.

 

I get all that, again you’re missing the point. Without the ability to have predetermined maps for all conditions it’ll be correcting continuously. That’s NOT the best scenario, nor is assuming that the application wouldn’t benefit from different timing and/or air/fuels at different altitudes. Apparently this is past your comfort zone regarding EFI and its inner workings.
Forget it.

 

Right you are. I didn't know Cadillac went to the BOP bolt pattern in later years. Check it out!

https://www.jalopyjournal.com/forum/...nsaxle.641980/

As far as EFI is concerned, one of the "off-the-shelf" systems would give good drive ability. I've heard that a properly tuned carburetor can make better HP though. I'm sure cutlassefi can build a great engine. I guess the OP's wallet is what determines the build. MAW stroke a SBO, use head studs and a girdle and bump up the compression too.

 

You guys are awesome! I appreciate all the input. In searching for the right Motorhome to buy and restore, I found one that seems to have a ton of very fine mechanical work done to it already. Please advise if this description sounds like nonsense, or what additional upgrades you would recommend.

Cutlassefi, my heart is not faint. I understand these upgrades can cost serious $$ to do correctly.

Here is the description from the post, what questions would you recommend I ask about these mods?

"3500 miles on custom build dyno’d Olds 455, 10,000 miles on Manny Trovao TH425, Thorley ceramic coated headers, cold air intake, custom single muffler 3” exhaust, 3.70 limited slip diff, aluminum intake manifold, aluminum radiator, throttle body fuel injection, in tank fuel pumps, quadrabag air ride rear suspension, new front torsion bars, KYB shocks, rebuilt front end, steering box/cv joint, bearings and CV drive shafts,"

 

Sounds pretty good but get a dyno sheet and exactly what’s in the engine.

 

The gmc rv motor is susceptible to vapor lock. Recirculating FI is not a bad idea. 455 is probably easier since there are more in existence. Stroke, sure why not? There was a gentleman here in Tulsa selling a nice GMC with FI and all the work done for $20k a couple months ago. http://www.gmcrvforsale.com

Did you see the guy on YouTube with a turbo 5.3 LS style motor. In a Winny or something like that. He's got some torque with a 4l80e. But of course, it is rwd.

 

Thanks for the heads up!

 

It sold right away! Bummer.

The gentleman told me, when I asked him if there was anything he would do differently, that he should have swapped in the Caddy 501 (502?) instead of rebuilding the 455. He hears from other GMC RV owners that uprade improves power and they love it. Just his opinion of course. And that the quadrabag airbag upgrade was his favorite upgrade because the RV no longer felt like a "marshmallow" on the road, and he can change a rear tire without a jack.

 

Caddy 500. Not 501, not 502.

Added displacement should be good for ~60 ft/lbs of torque, and somewhat improved horsepower. Downside: Goodby, RPM. Anything over 4500 RPM on stock 500s is not recommended. (I'm not saying the 455 is a rev-happy monster, just that it's better than the Caddy.)





"I" would be looking into a Duramax conversion. The only downside would be that you'd probably have to limit the torque output to keep the transaxle alive. So--probably--you wouldn't be able to use full-throttle. Even so, it'd kill the Olds and Caddy engines for usable power.

 

Yeah, a duramax would completely ruin the classic gmc. That swap is best suited to any old RWD RV and might actually increase the value of the rig. Kinda like the turbo 5.3. I think it might have been in sn early 80’s dodge. Pretty funny to see the look on people’s faces. On YouTube somewhere.

 

The cad 500 opens up a lot of cans of worms if you not familiar with them, a good 455 olds built for torque would be your best bet