ls/lm head question.

[59weasel]

in the truck is a 2004 LM7 out of a silverado 1500.
i have stock heads but have a chance to swap out 2008 chevy van casting #799 heads which i have read on other forums is the same heads as LS1's except the valves of coarse. but same CC's.

my question is they are both 5.3L engines/heads. so they should be a straight swap correct?

minus new gaskets and head bolts....

any body got some helpful knowledge i can tap into?

[66LSx]

I wouldn't bother with the head swap... not enough of a flow difference.

If your not doing a full cam, pushrods, springs, rockers, intake manifold... etc.... you won't see a difference except in your wallet.

Keep the truck manifold unless your going to an LS6.... don't doubt the truck manifold... makes better low end torque than the ls1 intake manifold


Casting Number 706 and 862
Head: 1999+ 4.8L / 5.3 Liter Truck
Material: Aluminimum
Part Number:
12559862
12561706
Combustion Chamber Volume: 61.15cc
Compression Ratio: 9.5:1
Intake Port Volume: 200cc
Exhaust Port Volume: 70cc
Intake Valve Diameter: 1.89 inches
Exhaust Valve Diameter: 1.55 inches


Casting Number 243, 799
Material: Aluminimum
Part Number:
12564243
Combustion Chamber Volume: 64.45cc
Compression Ratio: 10.5:1
Intake Port Volume: 210cc
Exhaust Port Volume: 75cc
Intake Valve Diameter: 2.00 inches
Exhaust Valve Diameter: 1.55 inches

[NorCalAnthony]

+1 If you aren't going to be swapping in a cam shaft while you're at it then I can't really see it being worth the trouble or money.

The 243/799 heads are the same castings that were originally used on the LS6/LS2’s engines and were later put on the higher output 5.3’s. The higher output 5.3’s also had the flat top pistons though so if you’re planning on swapping them to your LM7 with the dished pistons then you would need to have them milled around 0.030” to get your compression ratio back up near stock. They are a better flowing head than the 706/862 heads and also have a larger intake valve which is all a big part of why the newer engines make 30-50 HP more than the older ones.

FWIW I have a 5.3 LM7 from a 2000 Yukon XL and will be using a set of milled 243 heads to compliment my new cam shaft. Depending on how many miles your engine has on the clock you may want to replace some other wear and tear items while you have everything exposed. My engine has 146xxx miles on it so I’ll also be replacing the lifters, push rods, valve springs, timing chain, and oil pump along with the bolts and gaskets. If I were swapping the engine without making any changes I wouldn’t bother replacing any of that stuff but since I’ll have everything taken apart anyways I think it’d be silly not to.

[59weasel]

ok thanks for the info guys!! pretty much sounds like unless I was doing a full blown rebuild and stepping up on cam selection it really wouldn't be worth my time. oh.. 123K on my LM7. so a little freshening up is just a little ways down the road. i'll probably just have the heads gone through and jump up a little on the cam when it comes time. already have headers and switched to carb'ed intake from Eldelbrock w/ carb.

oh but I do have another question...

which heads are better? rectangle port or cathedral port? I have guys tell me cathedral all the way, but have read they are really the same size just different shapes.....

[NorCalAnthony]

Heck 123K miles is where these LS engines start breaking in so I doubt it’ll need any freshening up any time soon lol. The only reason I’m going through the trouble myself is because I knew I would be kicking myself if I didn’t clean it up and throw some go fast goodies at it before I did the swap. My project K5 runs great as it is so I don’t have a problem taking the extra time to squirrel away parts a little at a time.

When it comes time to do your own cam swap, do yourself a favor and get a custom ground cam. They cost the same as an off-the-shelf cam but will be made for your exact setup with your engine size, heads, intake, trans, stall, preferred RPM range, etc all factored in. I personally went with Martin at Tick Performance for my cam but there are many other people/places to choose from.

The rectangle port heads are what’s being used on the new high performance engines like the LS3 and L92 but require a 4.00” - 4.065” + bore because of the much larger 2.165”/1.59” valves. Besides the larger bore size the other issue you’d run in to is their different intake ports which aren’t compatible with the cathedral style so you’d have to swap over the rest of the top end with them. Also, from what I’ve read at least, they really shine above 4000 RPM but under that they actually lose power vs the cathedral ported heads due to the larger runners. They are badass heads and flow like a mo-fo but wouldn’t work with the smaller 3.78” bore of your 5.3 and wouldn’t really be worth it unless you regularly rev your engine way up there. You can read more about the cathedral vs rectangle head debate here: http://ls1tech.com/forums/generation...ort-heads.html

If you’re going for a budget cruiser then I think your 5.3 will be great just as it is. If you have the money and are looking for a max effort tire shredder then you’d most likely be better off finding an LS2 or LS3 that have much better power potential. It just depends on what your budget is and what you want your truck to be able to do. If you’d like to learn more about the performance side of these engines I’d recommend doing some reading over at LS1Tech, tons of good info on every LS based engine you can think of.

[59weasel]

wow ok then! thanks for the knowledge.

[BR3W CITY]

Its refreshing to see that the concept of "Building as you go" isn't lost on people. As much as I LOVE pre-ordained big budget builds where everything can happen in a vacuum, the "reality" of most projects is far from that.

I will say that if you do decide to do a big-power overhaul, you can ALWAYS build a 6L+ on a stand, and swap it in extremely easily if your already "setup" for LS. Much the way some guys will run a stock 350 till its dead, only to have a built motor waiting in the wings. It doesn't have to be a one-and-done project.