Compatible blocks

[Mike534x]

While my Trailblazer is in the body shop, my mechanic friend gave me a ring yesterday with an opportunity I'm finding very hard to pass up. He's selling his TB SS, only thing wrong with it is that it spun a bearing. I know he doesn't abuse it, since its mostly a hauler for his boat. I was searching up prices for replacement blocks, and the LS2 is still a bit on the pricey side. I did some digging, and I'm trying to figure out what the best option would be for a replacement engine. So far, the LY6 seems like the perfect candidate. I'm looking to stick with something that would be a direct drop in, and hopefully just plug n' play. The only thing about the LY6 is that it has VVT, so I'm not sure whether the trucks PCM would support it, or what would need to be changed to make it work.

I'll need to check back with him, but I believe its an 07 or 08. While I would like to stick with a 6.0, I'm not entirely against finding a compatible 5.3 donor that I could just do a cam/lifter swap on, and swap over the cylinder heads and intake. I plan on doing the work myself, and thankfully, I have a co-worker whose had some experience doing engine swaps, so he's onboard with joining me on this adventure if I pull the trigger.

Any advice, or suggestions on engine choices? Thanks!

 

[Mooseman]

Although I have my ideas on what to do, I posed your question to ChatGPT and it pretty much matches what I'm thinking:

That’s a very interesting opportunity — a TB SS is a rare and desirable truck, and picking one up with a spun bearing is often the cheapest way to get into one, provided you’re ready to deal with the engine swap/rebuild. Let’s go through your questions step by step:

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LS2 vs LY6 — Big Picture​

• LS2 (Trailblazer SS version)
  
  • 6.0L aluminum block (lighter than iron)
  • 10.9:1 compression
  • Rectangle port heads (243/799 cathedral-port style heads on the TBSS LS2 are actually a little different than Corvette/GTO LS2s which had 243’s)
  • No VVT
  • Drive-by-wire, truck intake
• LY6
  
  • 6.0L iron block (heavier, but very strong — often used for boosted builds)
  • 9.6:1 compression
  • Rectangle-port L92-style heads (better flowing but require matching intake)
  • Has VVT (can be deleted)
  • Typically truck intake manifold, truck accessory spacing

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Heads Compatibility​

• LS2 heads vs LY6 heads
  
  • LY6 heads are rectangle port (like L92/LS3).
  • LS2 TBSS heads are cathedral port (243 style).
  • Your TBSS intake is cathedral port, so if you want to keep it, you’ll want cathedral-port heads.

Yes, you can bolt your TBSS’s LS2 (cathedral port) heads onto the LY6 short block. Compression ratio will change slightly depending on gasket/piston dish, but it’s a common and workable combo.

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VVT Issue​

• The LY6 block has VVT, but if you drop it in as-is, your TBSS PCM won’t know what to do with it (no native VVT control).
• Solution: Delete the VVT and install a non-VVT camshaft (plenty of aftermarket “VVT delete kits” exist — new timing chain, sprocket, front cover, etc.).
• Once VVT is deleted, the PCM can be tuned just like a standard LS2 with a mild cam upgrade.

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Practical Drop-In Factors​

• Mounts/accessory drive: Same as LS truck family — will fit fine.
• Electronics: Since you’ll delete VVT, the PCM won’t need modification outside of tuning for displacement/cam.
• Weight: LY6 iron block is ~80-100 lbs heavier than the aluminum LS2. Not a dealbreaker, but worth noting.

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Pros/Cons of LY6 Swap​

Pros

• Dirt cheap compared to LS2 long blocks
• Strong iron bottom end (loves boost if you ever go that route)
• Easy to delete VVT and swap cam
• Can reuse your TBSS heads and intake to keep everything plug-and-play

Cons

• Heavier than the stock LS2
• Slight compression drop with LY6 pistons (but cam swap can help wake it back up)
• Extra work if you keep rectangle-port heads (would require LS3/L92 intake, bigger injectors, retuning, etc.)

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Recommendation​

If your priority is direct drop-in with minimal wiring/PCM hassle, the cleanest route is:

1. Use the LY6 short block.
2. Transfer over your TBSS LS2 heads, intake, and accessories.
3. Delete VVT and install a performance cam while you’re in there.
4. Tune PCM for cam and displacement as needed.

That way, everything bolts up, your intake and exhaust match, and the truck stays “plug and play.” Later, if you ever want to build it nastier, the LY6 bottom end can take a lot more abuse than the stock aluminum LS2.

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I didn't ask it about the 5.3 simply because that would be a downgrade and you could be then accused of having a clone. It would suck to just be left in the dust by lesser vehicles unless you un-SS it. I'd go with the LY6.

I also thought about the LQ4, basically the same but some differences:

Yes — an LQ4 block will also work, but there are some differences you’ll want to keep in mind compared to the LY6. Let’s run through it:

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LQ4 Basics​

• 6.0L iron block (like the LY6 — very strong, but heavy)
• Found in ¾- and 1-ton trucks, vans, etc.
• No VVT (older Gen III design, 1999–2007 classic)
• Usually came with 317 cathedral-port heads (big chambers = lower compression)
• Stock compression ~9.4:1 with 317 heads

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LQ4 vs LY6 for Your TBSS​

1. Electronics Compatibility

• LQ4: Gen III, so the block itself has no problem being used, but you’ll want to swap over your TBSS front cover, sensors, reluctor wheel, etc. (early LQ4s had 24x reluctor, later LY6s have 58x).
• Your TBSS is 58x crank reluctor / Gen IV PCM, so you’ll need a late-model LQ4 block (2005–2007) or you’ll have to swap reluctors and cam sensors.

2. Heads & Intake

• Since the LQ4 already came with cathedral-port heads, your TBSS intake bolts right on.
• However, the 317 heads that normally come with it have big 71cc chambers → lower compression than LS2/TBSS heads.
• Best bet: use your TBSS LS2 heads on the LQ4 block to keep compression closer to stock and make more power.

3. Strength

• LQ4 bottom end is stout (same iron block family as LY6).
• Not as strong as an LY6 with its beefier rods/pistons, but still capable of plenty of power for a street build.

4. Cost/Availability

• LQ4s are usually cheaper than LY6s since they’re older and more common in junkyards.

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Pros/Cons of LQ4 Swap​

Pros

• Cheap and easy to find
• No VVT to delete (saves you some work vs. LY6)
• Cathedral-port by default, so your TBSS intake bolts right up
• Strong block for boost or heavy builds

Cons

• Heavier than aluminum LS2 (like LY6)
• Compression lower than LS2 (unless you use LS2/TBSS heads)
• Earlier models are 24x reluctor → more work to make compatible with your TBSS PCM

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Bottom Line​

• If you can find a late-model LQ4 (2005–2007, 58x crank), it’s basically as easy as an LY6 swap — maybe easier, since you don’t have to mess with VVT.
• If you find an early 24x LQ4, you’ll need to either swap reluctor wheels and front cover, or run an Lingenfelter converter box to make it work with your 58x PCM.

Best approach: LQ4 short block + your TBSS LS2 heads, intake, and accessories. That keeps it “plug-and-play” like with the LY6, just with slightly less compression unless you mill the heads or go aftermarket pistons.

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[Mike534x]

Thank you @Mooseman ! For once, ChatGPT provided some useful info!


I checked eBay, and they are grossly overpriced on what they want for both blocks. Going off of car-part, the LQ4 and LY6 are almost similarly priced. Finding a x58 LQ4 is not as plentiful it looks like, so that might be crossed off. The LY6 seems to be the best match, its pretty close to the HP and Torque of the LS2. For about $140 shipped, I can get a delete kit (cover, sprocket, and cam sensor) from Texas Speed, and if the cam is still good from the LS2 I can just swap that over to the donor block then.


You're right, I don't want to be that guy that has a poser SS. Lol. I don't know if I've seen it mentioned here, but has anyone managed to swap the block out, while just unbolting the oil pan and leaving it in the truck to avoid yanking out the diff/axles?

 

[Mooseman]

It's all in how you ask it the questions

I've never seen it done here but I can't see it being done easily. You have to remember that the oil pan is sealed with RTV so it could get messed up during the reinstall. If going this far, might as well just yank the axles. The diff can come out with the engine on the pan. That's how I did it on my EXT with the 4.2. Shouldn't be any different on a V8. Remove the intake manifold before pulling to give extra room.

Be prepared to deal with broken exhaust manifold bolts in both engine's heads. Check the manifolds for cracks. I would replace the rear main seal at the same time. You should replace all the seals on the diff and refresh the grease in the "disconnect". Another reason to pull the axles. Not sure if everything will be plug and play, minus the VVT. There may be different connectors, injectors and sensors between the two.

 

[Mike534x]

You've got me there! I haven't had much use with it, so I'm happy with the results its provided. Helps save a tad bit of time at least!


That's a solid point, and one I'm definitely taking into consideration. Oh! that's good to know at least, so its really just the axles. Hoping they'll come free without too much trouble at the very least. You read my mind! I was planning on doing the rear main seal with it out. It looks like the fuel injectors are plug and play, this chart shows it uses the same style connector. I may just leave the heads and intake as is, the throttle body is a few mm smaller, but the LS2 is a direct fitment on the LY6 intake so that might be all I would need to swap over. Internally it looks like I just need a non-VVT cam, sprocket, and the LS3 timing cover kit.

 

[Mike534x]

Alright, so I spoke with my buddy today. I'll know for sure this week or next regarding getting it. Sounds like he's going to see what the Stealership is offering on trade, otherwise I got first dibs.

So onto the new info. The SS is actually a 2006. Which means this repair will be much cheaper/easier if I can get it. To those looking to snagging one, and do the engine work, everything above in this thread is applicable to the 2007+ model year.

After some more digging, the LQ9 would be a viable candidate for the SS. Good ole Iron Block, and 24x reluctor setup. Seems it's the "Vorex Max" model and the HP/TQ are pretty dang close to the factory LS2. I think with a mild cam swap, it'll go from the rated 345 HP back to the 395.

 

[Mike534x]

Did not purchase it sadly. He got a steal on the trade in value, that dwarfed what I offered significantly.

Well, that idea is shelved. I guess I can fall back on just freshening up the Envoys engine, and just doing an upper half rebuild with new cam, lifters, and valve springs. Though the info I got about the LQ9 does make it pretty tempting to snag one on the cheap, and swapping it in.

 

[Mooseman]

Another engine that could have been a candidate is the L77 in Avalanches and Caprices (added for future reference).

If you were to swap to a 6.0L in your Envoy, the tranny and rear diff would need to be beefed up to SS specs to survive. It would be tempting

 

[Mike534x]

I did forget about L77 as well, that's a good suggestion!

Shoot. I forgot about the rear end, I thought they were the same for the 5.3 and 6.0? Decisions decisions.