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Swap LLT for LFX

20K views 32 replies 15 participants last post by  Ryan W  
#1 · (Edited by Moderator)
Hello newish CTS owner and I have very recently been having very harsh misfires on multiple cylinders. I have been toying with the idea of getting a newer LFX for the added power gains and hopefully a better timing chain and I was wondering if there is anything, besides a tune, that would be required to swap these engines.
 
#3 ·
Unless your engine is beyond repair, and you're getting some sweetheart deal on an LFX, IMO it's not worth it to attempt this swap.

For the power gains, we're talking only 2 lb-ft peak torque difference, you'd never feel it. The increase in horsepower only comes near redline. 318 @ 6800 instead of 304 @ 6400. How much time do you spend above 6000rpm?

As for the better timing chain, issues can still occur with LFX engines or even LLT engines with the newer parts. If oil level and life are not maintained, they wear out prematurely.

The LFX does away with exhaust manifolds and the collector is cast into the head, so some exhaust work would likely be in order. Other electrical/sensor differences would need to be researched.

I believe at least one person here has put a modified LFX intake on their LLT, you could look into that if you're interested in more power.
 
#4 ·
I imagine if you search online you'll find that the ECU is different, engine wiring harness would be different, interface with other harnesses and control modules (ABS, TC, TCU, BCM, etc) may also be different. You'd have a lot of homework to do to figure out all of the implications.

I'd say a much better idea would be to follow Tbbt's advice - find the cause of your misfires and correct that.
 
#17 ·
Can this be done without a tune? I have 2012 llt with a trashed crank/rod bearing. I have a good 2013 lfx shortblock. I was hoping to swap everything over from the llt. Would it need a tune, was hoping for a plug and play. I was a gm tech for many years, quite comfortable doing this work on my own car. Just wondering if anyone has swapped a llt for an lfx shortblock only without having to tune, thanks
 
#14 ·
Yes you will need a tune to get it to run after. I’m in the process of doing this now. You will use your hpfp, injectors and rails, throttle body and map sensors. After that it’s pretty straight forward. Get with Will at Overkill, if he’s going to tune for the swap you might as well have him do a perf tune while you’re at it.


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#15 ·
Yes you will need a tune to get it to run after. I’m in the process of doing this now. You will use your hpfp, injectors and rails, throttle body and map sensors. After that it’s pretty straight forward. Get with Will at Overkill, if he’s going to tune for the swap you might as well have him do a perf tune while you’re at it.


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Thank you I will get with him this week on it. looking forward to it.
 
#20 ·
The pistons are different. I see pix on RockAuto of the "Enginetech" branded ones for both the LLT and the LFX. The LFX looks like the crown may be taller, and the valve clearance recesses should be a little bigger on the LFX pistons because the LFX has 1.510" intake valves and the LLT has 1.456" intake valves.
 
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#24 ·
More good information has been posted.
Just one other thing that's already been stated in the earlier post from @Joseph Upson , but to draw attention to it again in case you missed it, you use the fuel system from the LLT with the LLT PCM. The LFX has a different high pressure pump and different injectors. Using those would require some changes to the PCM calibration maps (a "tune" for the LFX stuff).
 
#23 ·
I had to make a correction toward the end of post #12 where I mistakenly stated the LLT piston was heavier than the LY7 and GM shortened and tapered the pin in the LLT piston to compensate. The LY7 pin is the heaviest of all of the 3.6L builds. The LFX has a slightly lighter rotating assembly as a result of the single groove harmonic balancer, but that's negligible as far as performance is concerned, particularly when you bring inertia into the calculation. The connecting rods between the LLT and LFX are the same. The pistons are only slightly different in that the LFX pistons have no exhaust valve relief, which is probably what accounts for the slight bump in compression.

The LGX has the heaviest piston at 400 g, the LLT and LY7 are 390 g and although I don't have an LFX piston on hand, I'd expect its wt to be 390 g also.
The LGX piston pin weighs 116 g, the LLT and confidently the LFX weigh 120 g with the LY7 pin coming in on the fat side at 142 g.
The connecting rods for the LY7, LLT and LFX weigh 639 g, although the LY7 rod is shorter and the small end is not tapered.
 
#26 ·
All the more reason you should swap the components matched to the crank as well. GM may have changed, or improved the balance on the rotating assembly (trimmed more from the bob wts), requiring a different part number. If the ring packs are lighter that may factor in as well given the LFX has a higher rpm limit than the LLT at about 7200 rpm.
 
#27 ·
Based on everything I've seen, including some good information on this thread, I'd swap the short block.

If you're really concerned with the slight compression bump, you could keep the LLT block and pistons and rings (that have already been "mated" and run together), and change out the crankshaft with all the rods from the LFX. But swapping the short block is so much easier without disassembling all the bottom end stuff. Fewer opportunities for mistakes that seriously shorten the remaining service life of the parts that way.
 
#28 · (Edited)
We have completed 2 short block swaps so far, one was an LLT, the other an LY7. Joesph has more data on the actual components than I do but the LFX is superior to the LLT (even if only slightly on a technical level). The biggest upgrade reason from a LLT to an LFX short block is the fact that you're putting in a much newer, lower mileage engine. Because you're only using the short block itself, you can harvest the engine from a transversely mounted car which make the cost of the engine drastically less than one that came from a longitudinally mounted vehicle (CTS, ATS, Camaro, etc.).

The wrecking yards price the Camaro / ATS / CTS engines at a much higher price point because they're going to assume you are swapping the engine directly in. Those are much more valuable cars so the engines are priced accordingly. GM was still making the LFX engine all the way up until last year (maybe they still are). So you'll want to find the newest, lowest mileage engine from something front wheel drive. Impala, Terrain, Equinox, LaCrosse, etc.

You're going to swap in your front timing cover, oil pan and heads. You'll need a top end kit as well (head gaskets, head bolts, etc.), I would consider getting your LLT heads looked at as well to check over the valves and guides and any other wear you might have.
 
#33 ·
So, I swapped the LLT top end for the LFX short block about 10,000 miles ago. Recently sounded like I head detonation and started using the highest octane gas I could find. It decreased what sounded like detonation/knock, had no way to test it. The octane was 91 to 93 as that is the highest I can find. Last week the car going up a hill lost power and the check engine light started flashing. Needless to say my mechanic said #4 cylinder had very little compression. Any ideas what I should do, this is the second Māoris and I am tired of the car. Just want to get some money out of it and be done and not sell a piece of junk to someone else. I ah e thought about even just making a race car out of it and going that way as well.