the quench pads have nothing to do with gasket sealing. it is something they use to cause a good mixing of the fuel mixture. right as the piston comes to the top it forces some of the air/fuel mix forcefully into the center of the chamber causing it to mix up better. you end up with a better burn since things are mixed more evenly.

that is prety simple and how it was explained to me.

 

I would leave the large heads on. The gains you can make with uping the compression, can be made with upping the boost. If you swing the compression too hi, you are going to run into detenation problems and driveability issues. The lower compression stuff can drive on any fuel, and when it comes time to play you just need a little better gasoline, or meth injection and your covered.
another thing
Large volume in the cylinder as a whole when filled has a higher volume of explosion or bigger stick of dynomite. The small tighter camber will squeeze it harder and have a faster flame travel, which can get into problems really quickly.
You know how your truck runs already so save the money, and just up the boost a smidge.

 

Jim is on the right track. It has nothing to do with sealing the gasket. Quench is the tight space between the piston and the flat part of the chamber when its at TDC. The 5.3 and 6L heads have one quench area while the LS6 heads have two. They effectively squish the air fuel mix and flame into the other parts of the chamber to ensure a good burn.

 

From what I understand, the small chambers (quench, squish area) also slow the flame speed and keep it from smacking the cylinder walls (detonation) before the piston has a chance to gain speed away from TDC.

 

Back to the original question. As I understand it FI doesn't need the intake ported so much because it is overcome by the additional pressure increase anyway, but the exhaust could benefit from pocket porting and basic clean-up. If you have a friend or know someone that can do this and flow them for a reasonable fee, then it should be cost effective. I'm definitely not an expert on this subject, just had a buddy with an 8-71 blown big block altered car and I bracket raced another altered for a few years. Hope this helps.

 

With your cubes i'd keep the compression low, allthough 9.4 isn't exactly low for a 408. The 5.3's don't flow quite what the ls6 heads do either. I had 8.9:1 with your turbo set up on 370 ci's and I had really no low rpm tq issues. You have another 1000lbs+ weight but that turbo ought to be spooling really quick on those cubes.

Grow some ***** and boost that baby. I wasn't happy with mine till I was at least 10psi or more.

 

I have two problems with the heads that are on there right now:

1. They burn oil. That is why I want to send them back to TSP.
2. I get audible detonation at 8psi (no KR on the scanner) at WOT. I should not be getting any detonation with 9.4:1 SCR and 6.9:1 DCR on only 8psi. I'm wondering if the heads weren't cleaned up very well after the CNC program was run, so I have some metal flashing left over that is causing hot spots. It goes away when I spray meth.

Both of these problems could be remedied by a different set of heads. The DCR problem could be remedied by a cam swap, so maybe that's what I should focus on to bring that up.

I don't want to pull the truck down for 2-3 weeks again until the heads come back from TSP. I think what I may do is buy a set of stock LS6 heads to swap in place of the ported ones, send the ported ones back to have checked, then sell them when I get them back. So....my setup, stock LS6 heads vs ported LS6 heads is now the question. Sounds like I won't lose much by going back to stock heads.

 

Ok, here is my opinion on the subject. I've ridin and driven Chris's truck and honestly, for a turbo 408, its a slouch. Especially in lower rpm boosted or not. I think the problem is the big heads and the cam. I think the cam is letting to much boost blow right thought he motor in the lower rpm and there isn't enough comp to make decent power. Plus I think that the blow though is probably affecting the WB and not getting a good reading

Here's my theory. You need big heads and cam on a NA motor because you have a constant your dealing with. NA=14.7 psi of natural air pressure, and you can't change that, so you need head and cam to make the motor operate as efficiently as you can with with that constant. Meaning you have to make the motor work more efficiently in a higher RPM range to get the power out of it.

So, you make it so you can change the constant, with a turbo, you now have a very good way to control the atmosphere in the combustion chamber. You want more bang, you turn up the boost. You build a motor with decent comp and a medium sized cam (like a Z06 cam) so the turbo doesn't have to work so hard and keep it in it efficiency range and then you control combustion pressure with the turbo so you don't have to sacrifice low end performance, for high end. So you still make good power down low and have a MUCH broader torque curve and a lot more usable power, not just high power in the high RPMs. High HP power makes light cars go fast. These heavy trucks need high average torque number to move them.

Here is my suggestion. Some 5.3 heads with your pistons would help get you some more comp (your pistons are already HUGE and will give you plenty of room for the combution gases) and a Z06 cam will allow enough flow for the turbo to push good air without sacrificing much on the bottom and with the wide lob separation, wont let it blow the boost though at low end either.

What do I have to back up my theory? Just a time slip of my truck and Chris's truck running our best times where my setup (stock 4.8 motor with basically a Z06 type cam) ran a 12.79@105 with 11.5 lbs of boost and Chris ran 12.72@106 with 10lbs of boost. Ya, he has a bit more drive train loss but the weight it pretty much the same, 60' where really close, but he also has 115 MORE cubic inches. Also, Dusty has ran mid 11's in the same type truck with a STOCK 6L on 15lbs, short shifting at 54-5500rpm.

Stock 5.3 heads, and Z06 cam on your setup and I'd bet you'll run 11's on 12-14lbs pretty easy. Its all about getting the broadest power curve and maximizing it.

 

i agree with the cam but not the compresion. you might give up a little down low with the lower compresion but will more than make it up with the extra boost you can run.

i have a 234/232 114+1 cam and i think it is too big. probably a good cam for dyno numbers for kind of a slouch below 4000rpm

 

To answer the original question - let me ask a few questions.... and give my answer.

When you are flow benching a set of heads, aren't you seeing how much air they can pass through at a given pressure? If you increase that pressure, wouldn't the flow results be different? What is boost?

I'm going to simply define 'boost' as a measurement of pressure in the intake, which is created by forcing the air into a restrictive engine. Compressing that air generates heat and makes the air less dense. OK -> For the same exact amount of air flow, how can you alter the boost pressure? Either make the complete system more efficient (less restrictive) so that it can process the same amount of air without building as much pressure OR make it more restrictive so that it builds more pressure. The same amount of air is getting forced in either way.

Lets apply that to cylinder heads, which are a restriction to the motor. A ported set flows more air under the "pressure" of a flow bench, why wouldn't they flow more under the pressure of a blower? By running a ported head, you open up both your intake and exhaust flow allowing air to enter and exit more easily. Easier to get air in = less restrictive = drop in boost pressure, which means you take in the same amount of air with less heat generated and have a more dense air charge. That means more power. If you then decide to turn the boost back up to where you were before the heads, you'll be pushing more air into the intake, and more air is more power. If it were me, I'd want the ported heads.

Obviously, this all depends on the whole setup. If its a mild configuration where you can easily just turn up boost to reach your power goal, then thats probably your best bet. If you are running into limiting factors and restrictions, then ported heads will help. The Radix is a good example since its easy to push it to its limit with a small pulley on a 6L motor. Air temps can get really high and power falls off in the upper rpms. Cutting out restrictions (ported heads, longtubes, catback, etc...) will allow the same amount of air at a lower boost pressure (less heat) and net more power.