8.1 Converstion

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Ckeene

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I have a 8.1 2002 6 speed and 05 8.1 Avalanche .....dooooo it!!!!! I personal don't like the truck because it's a 4 door but my oldest boy has already claimed it . I drive it like a vette.
 

DTrain

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Does everything, accessory wise need to be 8.1? Are there parts that are any easier if they are my factory stuff?

Easier put. Do I just need to plan on putting a totally complete 8.1 in, and connect the dots accordingly? Or modify some of my accessories to this motor?

Seems computer and wiring can be done with some head scratching. What about heater core and radiator and stuff like that? I assume the radiator may need to grow?

End of the day, I guess I’m hunting a realistic parts list.
 

someotherguy

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If you could find a 2001-2002 GMT400 3500HD donor truck you'd have a PCM, engine harness, and probably numerous other things you may need for this swap. That's the only factory GMT400 application with an 8.1. Unfortunately the radiator from that truck won't work as it's too tall (different core support) but you could use a radiator from a 2500 or 3500 454 truck.

Richard
 

someotherguy

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Speed parts are not super common, or are very expensive. I've never heard of stock parts being a problem. I would think the sensors are mostly used on other GM engines too.
You are pretty much right, but this is nice to know about: http://www.raylarengineering.com/ - the 8.1 performance guys

They have some swap articles too, but nothing specific to a GMT400.

Richard
 

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Hi I was going to install an 8.1 into my caprice. I had a new crate engine.I sold the engine but still have a computer,
a new painless wire complete swap harness and a set of new conversation motor mount plates.
If interested let me know
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L31MaxExpress

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Does everything, accessory wise need to be 8.1? Are there parts that are any easier if they are my factory stuff?

Easier put. Do I just need to plan on putting a totally complete 8.1 in, and connect the dots accordingly? Or modify some of my accessories to this motor?

Seems computer and wiring can be done with some head scratching. What about heater core and radiator and stuff like that? I assume the radiator may need to grow?

End of the day, I guess I’m hunting a realistic parts list.

The easiest 8.1s to swap come out of a GMT400 or an Express/Savanna van. I used the engine wiring harness out of the van, converted it to stand alone and married it to the L31s harness near the firewall connector. I took both fuse boxes apart and moved the connections from the van fuse block to the Tahoes. Then cut the wires that went to the engine harness of the Tahoe from the firewall connector and soldered the 8.1s matching wires in one at a time. I used a 6.5 diesel pedal and the van TAC module and throttle body. Most of the inputs for the TAC module that come from the body side of the Tahoe already existed in the cruise control wiring. I had to move a few wires around to match the diagrams. The Van TAC module bolted directly in place of the cruise control actuator. My air intake is a late model 6.5 diesel air box with a forward facing air outlet. I used a 90* elbow out of the air box, connected the Tahoe MAF directly to the 90, then connected a straight section of aluminum tubing with a straight coupler to the MAF. At the throttle body end another 90" coupler connected right up. I used a K&N filter for a 6.5 diesel. My exhaust is long tube headers for a 454 TBI, dual 3" piping with thunderbolt 3" high flow cats and a dual 3" in, single 4" out magnaflow with a 4" diesel tail pipe. I have dual batteries and dual AD-244 alternators. I used the stock small block Tahoe radiator because it was massive 34" wide double row anyway and an electric fan setup from a GMT800 Tahoe mounted upside down. I used a 4.3 V6 torque converter in the 4L80E and it stalls about 2,500 rpm behind the 8.1. Works very well with the 420 hp HP2 Marine cam. I can roast the P305/50R20s with light throttle powerbraking despite having 3.42 gears. I need to put together a swap thread and take pictures of it all. That is if I could ever pry my Tahoe out of my brothers hands.

I did a complete front suspension rebuild while I had the truck down. New control arms, 2500 Suburban front sway bar, variable rate HD front springs for listed for a 454 SS truck, inner/outer tie rods and adjusting sleeves, 454 SS steering gear box, new idler and pitman arm, new rag joint, new KYB Monomax shocks all around, HD rear springs and added a rear sway bar setup from a Suburban. I also upgraded the front calipers to SRW 3500 calipers and rebuilt the drum brakes. I also swapped the motor mounts to polyurethane inserts and used a polyurethane trans mount from Energy Suspension. The stock rubber mounts will not last long with a big block sitting on them. I added a massive trans cooler next to the OEM pusher condenser fan and added an OEM power steering cooler from an earlier TBI Z71. When I set the cruise at 85 mph on the toll road it was like driving a new truck down the road. No steering wander, no bouncing, no botteming out, just a nice tight handling truck that rides like an unloaded 3/4 ton. I was at 85 mph for more than an hour and still got better than 15 mpg that trip. I run 1,850 rpm @ 70 mph and 2,300 rpm @ 85 mph with the 4L80E, 3.42s and 32" tall tires. The 8.1 is slightly oil thirsty though. I have found I burn about 1-qt every 1,500 miles which is better than most but I discovered the intake gaskets on both of the 8.1s I have torn apart had come apart the same way the L31 gaskets do. Saving grace on the 8.1 is they do not contain coolant passageways and only caused vacuum leak at the intake runners of the heads. Both engines had multiple cylinders with intake runners coated in oil they were pulling through the gaskets.

Attached is the GM dyno graph for the HP2 8.1 from the now defunct page on GMPowertrains website. 2md chart is the HP1 stock Marine engine which uses the stock truck cam. Since I will never tow more than 7K with the Tahoe, I used the higher HP marine cam. 400 ft/lbs at 1,200 rpm moves the Tahoe effortlessly at lower RPM and when I stab the throttle the converter flash stalls 2,500 rpm which makes the torque difference between the cams at low rpm largely irrelevant. As it is I cannot pin the throttle to the floor from a stop with this setup anyway without ALOT of wheelspin.

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DTrain

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One small piece that may be helpful, and make things easier is my situation, is my truck is as base as it gets. I think the 5 speed, no cruise and some of the other factors in line like that with base will be helpful. I hope anyways.
 

L31MaxExpress

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One small piece that may be helpful, and make things easier is my situation, is my truck is as base as it gets. I think the 5 speed, no cruise and some of the other factors in line like that with base will be helpful. I hope anyways.

Does not really make a difference to be honest. Adding cruise control with the 8.1 would be as easy as adding about 6 wires and a multi function switch for a truck with cruise. 2 of the wires I mentioned above you would need to add to signal the TAC module you are on the brake anyway. The wiring connection to the cruise control on my Tahoe had almost every wire in it that needed to go to the TAC module anyway. I know atleast on the Van wiring harnesses the cruise wiring is already pre-wired just has a block off cover when it was not used. My Tahoe is a LT and the wiring is mostly the same between base and LT.
 

DTrain

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So this swap has been so far outta sight, to the point that I pretty much forgot about the motor and even thought about selling my truck. Back into reality I’ve got the coolest truck I’ve ever had and realize what a tragic regret and remorse I would have the morning after I sold it.

This swap has been on my mind again and with a new year in front of us, I want to get back on track and make things happen.

One thing that has me personally stumped, and something I’d like some advise on is wiring and computer. Originally this 8.1 was suppose to have a full harness with it, that’s not the case anymore. I was confused about wiring with a swap as it was, but with that gone, I feel even more so at a loss. Is there a swap harness or wiring kit to aid in this process? What is best to do and where should I look? Do I need to track an 8.1 harness down and still tie things up with my trucks existing stuff?

Looking for a little insight and some secrets.
 
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