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Re: new engines for the GMCs [message #316561 is a reply to message #316482] Mon, 24 April 2017 17:40 Go to previous messageGo to previous message
TR 1 is currently offline  TR 1   United States
Messages: 348
Registered: August 2015
Location: DFW
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Mike,

I'm going to preface my reply with this statement: I too love LS motors. I have an 89 Mazda RX7 that I swapped in an LS6 a few years back. I also daily drive an 05 GTO, (LS2) and my wife daily drives a G8 (L76) that has 200K miles on it and counting. They're great motors. That being said, I don't see any real advantage at this time in swapping one into my GMC. You can rebuild a 455 with all new parts except for the block and crank. Both of those items are able to be inspected and deemed "Good" or "Not Good". There is no advantage to a brand new block over a good 40 year old block. When my 455 needs a rebuild, I will rebuild the 455, and anticipate having no issue doing so for some years to come. (FWIW, I'm 41, and plan to have our GMC for at least the next 25 years...) If 455s/455 parts become scarce, I would probably then switch to a Caddy engine. It is a true bolt in, and you should be able to reliably get parts for them for a very long time, as Cadillacs have been, and most likely will continue to be collectible.

If I had the $ now, or if gas prices go through the roof, I would opt for a diesel. The 6.5 swap Manny and other such as Hal have worked out, looks to be a great option. If the 6.5 is not refined enough for you, Cummins is planning to release a line of crate engines. The 5 liter Cummins V8 looks like an awesome fit for the GMC, but I'm sure it will cost boatloads for a Cummins crate.

Also, if you go this route as opposed to the 6.5, you would be pioneering literally everything. Having done a few engine swaps in addition to the LS motor in my RX7 over the years, I can tell you there is a lot to it. Lots of detail work in addition to the big things like getting it to fit. And at least for me, to go through all that, there have to be some significant gains... And I don't see that with an LS over the 455.

For example, LS motors, if taken care of are easy 200K motors... -IF- they are pulling around a truck that weighs 5K, and sometimes tows a trailer. GMC is 10-12K pounds, and has a pretty large frontal area to boot. I would guess they are more like 100-150K in motorhome service. 455 is similar... Maybe a little less, but not enough to warrant the effort of putting an LS in my GMC. And a good LS rebuild is not much less than a good 455 rebuild. Say 5K for a decent LS rebuild. Yes you can do it cheaper. I can spend less rebuilding a 455 as well. But you'll get what you pay for either way.

Now, if you absolutely have to have an LS in your motorhome, you want to start with a truck motor. The 2 crate engines you mention above are not really for HD truck use... They're more light duty truck or muscle car. If I were to LS my GMC, I would at least go with a Gen IV, iron block LS motor. The rectangle port heads (often called L92 or LS3 heads) are damn near magic when it comes to flow, torque production, etc. I would probably opt for the 6.2 so look for an L92 or L99 engine. Even better would be the Gen V L86. These add direct injection, variable speed oil pump, etc.... Lots of little tweaks to increase efficiency. This is arguably (arguable with Ford and Mopar guys Wink the most efficient gasoline engine available for motorhome use. And even with all that, it's only marginally more efficient than a 455 when pulling 12K pounds.

Now, for your swap, you mention physically bolting on the trans, and modifying the oil pan and engine mounts, modifying the interior step and maybe raising the hatch height. When forcing an engine into a vehicle it was not designed for, that's only the beginning:

Finding Radiator/heater hoses that work with this swap
Adapting the gauges, buying new gauges to work with the LS
Adapting the LS wiring harness to "interface" the LS brain with the GMC. ignition switch, etc.
Buy new ECU ($$$$), or modify truck ECU. Disable anti theft key, (VATS) other ECU changes to allow the engine to run without the rest of the truck it came in
Are you going to run a catalytic converter? If yes, modify GMC to run cat. If not, reflash ECU to remove cat emissions system from diagnostics.
You going to run stock manifolds or go with headers? Either way, you need to modify the exhaust on GMC
No evaporative emissions system on GMC. Modify GMC to add it or reflash ecu to remove it from diagnostics
Modify GMC fuel system. LS motors are high pressure, returnless fuel systems stock.
What are you going to do with engine oil cooling?
Gen IV and up LS motors use an electronic throttle, you'll need to figure that out, though there are options (Electronic gas pedal or switch to older ECU and reluctor wheel with mechanical tbody)
How do you run the mechanical TH425 on an engine designed for a computer controlled transmission? Again, there are options, but it needs to be considered
Modern truck is going to generally have a higher amp alternator (150 amp or so?). You'll need to verify your GMC can handle that current and modify accordingly
AC system will need custom hoses made. LS AC runs through ecu, so you'll need to modify GMC AC system with necessary sensors.
I love my cruise control on long trips. I consider it a must. Need to figure that out.
Will need some way to interface with the ECU... OBDII connector?
Hal mentioned cooling fan, shroud modification, etc above.

So after all that, you end up with a GMC with an LS motor. You get 8-10 MPG, with an engine that lasts ~100K miles, has around 400hp and tq, and costs $5K to rebuild.








Mark S. '73 Painted Desert, Manny 1 Ton Front End, Howell Injection, Leigh Harrison 4bag and Rear Brakes, Fort Worth, TX

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