schmendog

iTrader: (10)

Would a junk yard 5.3 be the best option for 15 +PSI, or would a newly built 5.3 assembled from GM parts hold up better (see below for internal specifics).

The reason I ask:

Ive been planning to buy a local guys 5.3 long block with 120,000 miles for $200 - $400 (oil pan to intake) and keep the shortblock stock, with the exception of increasing ring gap and adding ARP rod bolts, pushing 15-20 PSI with twins. Today, I found another 5.3 shortblock (only 10,000 miles on the block itself) that has been built with a stock crank, LS6 rods, flat top pistons, new rings, new bearings, floating wrist pins, new lifters etc for a really good deal (about $1000 for the shortblock basically). Rotating assembly has 2000 miles on it.

I want to buy the newly built short block, but am starting to wonder if I should just buy the factory built Junkyard 5.3 instead, because those are proven to take a beating. Which is the better way to go if the $ difference is not a big concern?

schmendog

iTrader: (10)

More details I found out:

The pistons are GM flat top hypereutectic pistons and rings (are these going to be as week as the stock LS1 Hyperuetectic pistons? Are the stock 5.3 any different?)

Since the rods are LS6 rods, are they going to be a weak point also?

Starting to lean more toward the JY block now that Im thinking the internals on this newly built short block may not be as strong as a factory 5.3. Suggestions?

chiaj144

iTrader: (4)

Here's what I would do, did. Get the junkyard one put valve springs, push rods, head studs, gaskets and a cam in it if you want a bit more power. The ring gaps on the higher mileage motor will be fine and the rods will bend before the bolts break.

a4ls2goat

IMO go for the l33 if it is available.
Better rods and heads.

schmendog

iTrader: (10)

^fixed my first post...I think it was misleading you to think that the $200 longblock was an LM7, and the newly built $1000 block was an L33, so I deleted that part. Im not actually sure which variant each one is (LM7, L33, LY5 etc) the $200 longblock with 120,000 miles has flat top pistons, is an iron block, and has 706 heads so I was confused if that was an older LM7 or a 2003+ model with the better rods.

The newer built shortblock ($1000) is built with a fairly new bare block, and is assembled with parts mentioned before.

schmendog

iTrader: (10)

other opinions?

schmendog

iTrader: (10)

Update: going to buy the used longblock for $200.

SSZ

I think you made the right move. I guess in the end it is always a gamble but the guy that assembled the other engine may not know what they are doing. I may be wrong too though. Just do what chiaj said. Pushrods, springs and cam. Maybe head studs and gaskets. Good luck bro.

Ashton233

I just purchased a lm7 with the 706 heads. I am planning on a on3 76mm turbo. So far my parts list contains arp head studs, ls9 head gaskets, ls7 lifters and a ls6 cam and intake along with springs. Am I in the right direction and should I use the 706 heads or find a set of different ones? New to all this and any help would be great. Thanks

schmendog

iTrader: (10)

Same boat here man. This long block has the 706 heads, Ive read in some threads that their were two versions of those heads produced. One of them is cast and has a tendency of cracking under boost, but I don't know how to tell the difference.

copley87blaze

Take off rockers and mount. Look around, their will be what looks like a little battery. If you see that you don't want those heads. I would buy a old jy motor. I have had a few of these now, with great luck. I have pulled apart ones with 300k on them, they look great. what I have found is that ones with around 120k will be around 18 to 20ths ring gap. I open mine up pretty good around 28ths. Keep timing around 15 too 17 deg and around 11.0 afr. It will be fine. With my pt 74 I made 550 and 560ftlbs at the wheels, on 12 psi. It loved the 15 psi. My buddys lq4 with a 88 was nasty but we bent the rods. Good luck.

LS1Adam84

iTrader: (21)

Just FYI if it is an iron block with flat top pistons, you have a 4.8... I has a shorter stroke than the 5.3, I have seena few 4.8's take as much abuse as the 5.3.

The lm7 will come with -7cc pistons.

schmendog

iTrader: (10)

Really? This one is an iron block/ flat top pistons/ 706 heads. Were there any iron 5.3's made with flat top pistons? I hope I don't find out this guy sold me a 4.8 when he claimed it was a 5.3

schmendog

iTrader: (10)

Is the VIN code stamped on the block somewhere?

Sarg

iTrader: (6)

4.8 is not a bad starting point either! Little less displaccement, but it will carry power a little higher with the right cam and turbo. That on3 76 with its smallish turbine will probably be a little better suited to the 4.8 also. So the pistons do not have any dish and are 100% flat?

HRHohio

Like Sarg said, a 4.8 isn't a bad thing. Just need 11psi instead of 10psi to do the same thing. No biggie. Plus shorter stroke means longevity.

denmah

iTrader: (2)

i breezed through this quick but, here is what you have to figure out first, how much power are you looking to make, horsepower is the currency that everything else will be measured with.

HRHohio

Also, another suggestion, take the money you saved on this motor and get the heads done and a custom cam put in it. A head job and custom cam will help you get spooled up faster and will allow you to reach your HP goals with much less psi.

schmendog

iTrader: (10)

That's true, I guess in the world of FI, and for my purposes as well, displacement is king...although it helps. He claims the pistons are flat tops, one of the heads is off so Ill be able to get a look at them when I go to pick it up. Is the cylinder bore the same on the 4.8 and 5.3? If not, I can measure and determine what it is. Ill have to do some digging to find out.



I have a custom cam by Martin in the budget right now, but after seeing Wicked99's results with his twin 0n370's, Im starting to wonder if my old blower cam sitting the current engine would work out well, its almost identical to wicked99's cam. Not sure if I can justify a head job on the stockers...ideally id like to throw my TFS 215's on it, but Im selling those to help fund the new transmission, which sucks cause I really love these things.




Yeah that's the best place to start I guess. I have an idea of what Id like to make, just not sure if its going to happen. Now im starting to wonder if the blower cam I planned to use will work out well now that the engine may be a 4.8.

HRHohio

Cylinder bores are the same between 4.8 and 5.3 (3.78 in Bore). Stroke is different, 4.8L is 9mm shorter stroke.

What are the specs of the blower cam you have?
I'm doing a 72 Camaro w/4.8L and put in a 228/234 .588/.598 114LSA Hopefully will have the car ready to turn the key by July.

I also had a local guy port my 706 heads and put in larger intake valves (2.02), springs, titanium retainers, .010 decked, etc... Didn't break the bank and should flow well. Just saying, it's an option... or throw those TFS 215s on it (didn't realize you had some laying around like that... musty be nice! HAHA) Well, the TFS-215s are about $2500. I have right at $1k into my 706 heads and the guy says they will flow comparable (Maybe 10% less) at my lift.


"I have an idea of what Id like to make, just not sure if its going to happen." ---- Come on now, throw out a number so we can help you reach your goal and give advice.

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