Milling the heads on 4.8/5.3 for more compression

[Matte454]

I have been doing some research on the 4.8/5.3 trying to find out what heads we have. Finding out that they are identical to ls6 corvette heads (same castings) was pretty interesting. I came across a lot about running thinner gaskets or milling heads to raise compression on corvettes for more performance.

 

Thinking with the 4.8 and 5.3 the benafits would be bigger since they are lower compression to start with. Ls6 is 10-1, 5.3 is 9.6-1, 4.8 is 8.8-1. Have any of you guys done or thought about this? Especially on the 4.8l With very low compression, you could bring it up to match the 5.3 and still be on regular gas. Also appealing as kind of a stealth mod, probably near impossible to tell if it was done to void your warranty.

[TooTall1]

Raising the compression and running regular gas could cause excessive knocking under load. You'd probably need higher octane gas.

[Badass69]

GMT900 5.3's are 9.9:1 compression with the 243/799 castings. The older 706/862 equipped Gen III 5.3's were around 9.5:1.

 

The current 4.8 is a turd with only 8.8:1. The stroke of the crank with the same head as the 5.3 is the problem there. 5.3 is the same stroke as any 6 liter or 6.2 at 3.622" where as the 4.8 is only 3.27" stroke.

 

The heads are supposed to be 64 cc but the two sets of 243's I have both cc'd around 66. It's around 0.006" per cc that you need to shave. So if you go 0.030" you are down in the 60-61 cc range. This would jump a 5.3 substantially... basically a 60 cc head on a 5.3 is around 10.5:1 and 4.8 would be in the 9.5:1 range.

 

GM changed pistons between the older Gen III 4.8 and 5.3 to keep the compression ratio about the same using the same 61 cc heads. 4.8's always had a flat top and the 5.3 was dished. Now on the Gen IV's the 5.3 and the 4.8 both use the same flat top piston and the same heads........ thus the 4.8 gets to be a low compression slug and the 5.3 got a nice bump.

 

The L33 5.3 in the Gen III's was the first one to get flat tops and 243 heads.

 

243's and 799's are the LS6/LS2 casting btw........ which now adorn every 4.8 and 5.3 built.

[Chris]

But the walvetrain is different for the LS2 as it is different for the LS6

 

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[Badass69]

But the walvetrain is different for the LS2 as it is different for the LS6 Sent from my Samsung GS4 using Xparent Red Tapatalk 2

 

Well if we talk your exact terminology here being the "valvetrain" you are correct. The only difference between a 243 on an LS6/LS2 or a 5.3/4.8 version is the sodium filled valves and higher rate springs that the 5.3 and 4.8 lack. Head castings are identical regardless across the board which honestly is all that anyone should really care.

 

Some say the 243 is actually a nicer casting than the 799.... like it's more cleaned up inside but I really don't know that for sure. There was a noticable cleaner finish to my 243's versus the 317 heads I have on my 6 liters which are the same head again but with a larger combustion chamber. The 317 was very rough cast in the chamber and even the ports whereas both sets of 243's I have ( 1 unknown source and the other off a 2007 Gen IV 4.8) have a nice smooth finish in the chamber and smoother ports..... as cast, stock, untouched.

[Chris]

LS2 didnt have sodium filled or hollow stem valves

 

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[Badass69]

My bad for not clarifying that but who cares honestly lol. Its the same head casting and the valve material means nothing. Gm did away with the ls6s sodium filled valves but yet offered the ls2 with a little more power and a similar cam to the ls6 with no ill effect. The 4.8 and 5.3 use the same steel intake valve and stainless steel exhaust valve just with weaker factory springs than the LS2. None of this discussion effects compression ratio or the fact its still the same head casting.

 

Don't pay more for LS6 heads...... because they likely will be the 5.3/4.8 version anyways and you still have inferior valves to anything aftermarket if they are the sodium filled ones........$100 a head used..... max.

[Chris]

I was going to buy new from GMPD

 

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[Matte454]

Wow notamember, thanks for the info I was looking for. You really know your stuff on heads. Thinking on my gen Iv 4.8 I could easily go .030 or .040 and get it in the 9.5 to 10 range. Priced out milling, head gaskets, head studs, beer for mechanic friend and seems like a really cheap mod. 300 bucks or so. I'm wondering how much my power and efficiency would go up if it would be worthwhile. Also this would be pretty stealth, hard to immagines it being caught with my factory warranty.

[Badass69]

If you aren't camming it or doing anything else to it I don't know...... especially with the stock tune. It would improve throttle response and probably bump the mileage a bit but whether or not you will actually notice it I can't tell you. I look at it this way.... stock for stock with a good tune in either I can't tell really any difference in power between an LQ4 and an LQ9...... the only difference between the two engnines is just over half a point of compression (0.6:1). Heads, cam, intake and injectors are the same. Once you start really modding the LQ9 does better because it is 10:1 compression over the LQ4's 9.4:1 but with all the other stock stuff installed

 

At least though the LQ4 is 9.4:1...... unlike the 4.8 at 8.8:1. Give it a try and see what happens.