GM High Value LZ9 Swap? (Page 11/11)
Will JUN 15, 01:38 PM
The LZ4 & LZ9 use 2.250" rod journals. That's the same size as 4.3litre V6 journals. 4.3 V6 rods can be narrowed at the big end for use in LZ4/LZ9 engines, just like small journal Chevy V8 rods could be narrowed for use in the older V6/60 engines.
zkhennings JUN 15, 01:50 PM

quote
Originally posted by Will:

The LZ4 & LZ9 use 2.250" rod journals. That's the same size as 4.3litre V6 journals. 4.3 V6 rods can be narrowed at the big end for use in LZ4/LZ9 engines, just like small journal Chevy V8 rods could be narrowed for use in the older V6/60 engines.



Thanks for the info, I went digging for aftermarket rods with the correct journal size and did not find any, are forged aftermarket versions available?

To add to that the LS1 journals are 2.1" and the SBC journals are 2.0"
Will JUN 15, 03:14 PM

quote
Originally posted by zkhennings:


Thanks for the info, I went digging for aftermarket rods with the correct journal size and did not find any, are forged aftermarket versions available?

To add to that the LS1 journals are 2.1" and the SBC journals are 2.0"



This list actually shows "big end bore diameters" of 2.125, 2.250 & 2.375... I'm not sure if they actually mean bore diameter or if they mean design journal size. Here's the list:

https://www.summitracing.co...6?N=engine-type%3Av6
Will JUN 19, 09:44 PM

quote
Originally posted by ChuckR:

Here is a question going through my head, has anyone looked at works best on this engine to increase compression ratio? By modern standards 9.8:1 that is stock on this engine is pretty low.

Can you deck the heads? if so how much before you need new pistons with reliefs?

Is there a domed piston that would work on this engine?

are there thinner gaskets?

This engine is designed for 87 octane, but running premium 93 pump gas in my area, 91 for some or E85 gives a lot of room for a high CR and avoiding det.



It has the same bore as an LS1 and the 0.943 pin size, right?
ChuckR JUN 20, 07:17 AM
I have read that the bore is the same. I have to take your word on the pin size. You guys have shared some good info on the pistons. This is great to know, but what about the other side of the question? Can the heads be decked and by how much before there is a clearance issue with the valves? This seems like the most effective way to gain a few points of compression in the least invasive manner. That is assuming there is room to do so without having valve clearance issues.

Do you have a good reference on the 3.9 that lists the details of the engine like pin size? Also do you not also have to worry about how tall a piston is and its location of the pin? Not just diameter and pin size?

Maybe I need to read some engine building books to better understand the question I am asking.
zkhennings JUN 21, 03:50 PM

quote
Originally posted by Will:


It has the same bore as an LS1 and the 0.943 pin size, right?



Yes that is correct.

Aftermarket LS1 pistons come in varying compression heights, which is essentially the distance from piston pin axis to the top of the piston. You could also get 4.3 V6 rods if they sell a slightly longer rod, that also has the correct 0.943 floating wrist pin and get the rods narrowed. However the quick search I did on Summit showed only 5.7" long rods (vs stock is 5.9" according to manual) and they all had the 0.927" floating wrist pin small end. But more research could be done.

I do not know how far you can deck the heads until you have valve clearance issues to the pistons, you may run into clearance issues with the valve seat first. It is pretty simple to cut deeper valve reliefs in the pistons though, you can make a cutter from an old valve. Check the youtubes for tutorials.

You may want to deck the block instead of the heads if you do go down that route. Heads warp a lot more than blocks do, I wouldn't want to remove too much material from the aluminum heads.

I bought a factory service manual pdf for Impala/Monte Carlo off Ebay and it has a lot of detailed information on the motor specs, it was like $9.

[This message has been edited by zkhennings (edited 06-21-2021).]

Will JUN 22, 08:46 AM

quote
Originally posted by ChuckR:

I have read that the bore is the same. I have to take your word on the pin size. You guys have shared some good info on the pistons. This is great to know, but what about the other side of the question? Can the heads be decked and by how much before there is a clearance issue with the valves? This seems like the most effective way to gain a few points of compression in the least invasive manner. That is assuming there is room to do so without having valve clearance issues.

Do you have a good reference on the 3.9 that lists the details of the engine like pin size? Also do you not also have to worry about how tall a piston is and its location of the pin? Not just diameter and pin size?

Maybe I need to read some engine building books to better understand the question I am asking.



I think google will have all the numbers you need, one way or another.

Decking the head makes the intake manifold not fit right, which requires cutting the intake at weird angles on all three mating surfaces... You're probably better off with drop-in pistons. Replace the rod bolts if they're torque-to-yield, though.

EDIT: as noted above, get a service manual covering this engine if you're going to start tearing into it that far.

[This message has been edited by Will (edited 06-22-2021).]

ChuckR JUN 22, 08:58 AM
I plan to get the service manual once I purchase the car, but until I know the donor I don't want to get just any manual for a car with this motor. I may end up getting the flex fuel version etc. Thanks for the info.
ChuckR JUL 01, 01:39 PM
https://wot-tech.com/forged-3900-lz9-pistons.html

If you dont mind forking out the money for forged pistons you can order a set from WOT and choose from 9 10 or 11 CR. I hadnt checked their page in a while and I dont think this was there the last time I looked.
Joseph Upson JUL 01, 03:45 PM

quote
Originally posted by ChuckR:

I have read that the bore is the same. I have to take your word on the pin size. You guys have shared some good info on the pistons. This is great to know, but what about the other side of the question? Can the heads be decked and by how much before there is a clearance issue with the valves? This seems like the most effective way to gain a few points of compression in the least invasive manner. That is assuming there is room to do so without having valve clearance issues.

Do you have a good reference on the 3.9 that lists the details of the engine like pin size? Also do you not also have to worry about how tall a piston is and its location of the pin? Not just diameter and pin size?

Maybe I need to read some engine building books to better understand the question I am asking.



Valve clearance shouldn't be a problem, it's a VVT motor so it has extra in addition to about .1xx piston to head clearance. As mentioned, it is more practical to purchase pistons in the compression range you are aiming for. When I built my motor, there was a lot I did not take into consideration on the metrics side, that I would now. Ebay is probably the best place to search for LS1 pistons and is where I found my production Mahle forged options remarkably cheap. I recall many options to choose from with the range enhanced by small over bores. The listings were very informative by posting all of the important measurements and all options available in the listing.

Be careful with the 4.3L rods, I recall mention of some being offset in a discussion on this topic some years back. If you're interested in stroking the motor, you may want to check with phoenixbeginning on the forum. He has my first offset ground forged crank which was machined to the wrong stroke of about 3.41". The crank was radiused and was a nice piece without a purpose at the time.

[This message has been edited by Joseph Upson (edited 07-01-2021).]