The big takeaway here is this- Can some ported cathedrals beat the LS3? YUP! But here is the real deal; LS3 top ends are available CHEAP and are VERY commonly available, making them the best deal, bang for the buck wise, if you are looking for good flow (and HP) from an economical source.

 

Funny how I can call multiple cylinder head pros and they all say run the ported 243s on LS2 for a street/strip car, but when you come on here you all think your smarter then someone that has been doing this for over 30 years day after day... Yes, they will say LS3s can possibly make a extra 10hp but the extra expense and loss of low end under 4k is not worth it.. And yes I have called them all and was never told to run LS3 by any of them, they say run a good ported 243 or just go aftermarket.. so take it for what its worth. Oh, and when you all say its cheaper to run LS3, yeah if you want to buy used crap instead of new, I never run someone elses junk in a new engine, but that's me.

 

All good, but how do you explain the LS3 in late Vettes and Camaros running as strong as they do? They are NOT very weak on the low end.
It is true, bolting an LS3 top end on a 6.0 that formerly had cathedrals WILL lose some low end. But very much? And for the price LS3 heads can be had for, is that such a bad tradeoff?

 

To start the test off, we configured the 408 stroker with the 317 heads. After a break-in period, we let the hammer fly and dialed in the air/fuel and timing curves using the FAST XFI management system. After dialing in 30 degrees of timing and getting the air/fuel ratio spot on at 13.0:1, we were rewarded with peak numbers of 543 hp at 6,300 rpm and 526 lb-ft of torque at 4,800 rpm. Torque production with the 317s exceeded 500 lb-ft from 4,300 rpm to 5,400 rpm.

Off came the 317s to make way for the LS3 heads. Would the 408 respond to the increased head flow of the rec-port heads?

After installation of the heads, offset rockers, and FAST LSXR intake, the LS3-headed stroker pumped out 581 hp and 543 lb-ft of torque. Note the LS3 heads increased both peak power and torque, though the rec-port heads did lose out to the 317s below 4,000 rpm. Despite the slight loss down low, torque production with the LS3 heads bettered 500 lb-ft from 4,100 rpm to 6,100, both lower and higher than with the 317s. This test clearly demonstrated that if you are looking to make your 6.0L (or stroker) factory fast on a budget, nothing beats a set of LS3/L92 heads.

So stock 243s were down 40 HP, So a Ported stage 2 243 with 320 cfm would make that up and maybe a little more. So if we are going to talk CFM numbers there isnt much difference from running either one. So this all comes down to price not power, in my situation a max ported 243 is cheaper then LS3 conversion (i dont run used parts) and will run just as hard.....

 

First and foremost you need to read the quote below...That is the topic at hand.... You really think with that setup he give a flying **** what it does below 4,000rpm lol
Second, You can buy a set of used LS3 heads and have them cleaned and checked plus get a good three angle valve job cheaper than you can buy new 243 heads (remember you don't want to run anyones used junk lol) then you have them ported to be better than the stock LS3's. Forget about good aftermarket heads...price wise they aren't reasonable for the added power on budget build.
Remember nobody is saying unported stock LS3 Heads are better than a good set of 243/799 ported heads, We are saying for a budget build they are a much better option and certainly will run circles around the stock 853 the OP has now.

 

Yes I saw the quote which reinforces my desicion to recommend a 243 head. I don't know how "budgets" came into play here and budgets are purely subjective. Look at what he has already, he has a th400, Ford 9 and may be running nitrous. This is not a car we skimp on especially with heads. He wants to move, I just want to guide him in the right direction. I have already called a few experts in regards to this combo and asked what I should run given a few different points of use. I was told "243s are the way to go if he wants to drive it on the streets." Why? "Because torque." Don't forget a world exists beyond that of wot tuning. If it's a race car, this changes of course and he would be more suited for ls3 style head, high rpm oriented intake and a solid cam. The last bit is my opinion.
We could use all the torque we can get (in this scenario) because 1st on a th400 is horrid anyways.
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And I'm not pretending to know more than gm engineers. They have a lot more to work with than we do here. These trucks have tiny cams that make tons of low rpm power as well as VVT. They have 6+ speeds and can be geared short and still be driveable and do well on emissions. Not to mention they wouldn't want to retool for heads if they didn't have too. They were already making them so why not make them work and save on costs.

 

Jesus..... You just want to keep beating a dead horse lol.... The car is running headers/dumped exhaust/turbo 400 and a 9 inch with nitrous....yep that sounds like a typical street car to me lol I'll bet you a dollar he's not running a stock stall or gears ratio either so between that and the N2O he's not lacking any low end torque!

PS running stock "853 heads' Nobody with a decent budget would choose them unless they didn't have the money for better heads....so yeah that would be where the budget talk came from.

 

.025 milled L92's on my SBE 6.0 (runs high 9's)
Building a 398" and its getting FED LS3's (will be in the 8's)
So yeah, Rec ports for me!

 

For the OP needs, I would go LS3 heads, cam it accordingly, and spin the thing 7200 rpm minimum. If it was a street car primarily, then I could see doing some ported cathedrals...but since this engine is going to live above 4k rpm, it's hard to beat the LS3 stuff for the cost.

 

SLP L92 Heads vs. AFR/SLP 225cc Heads on 402 stroker, 236 single pattern cam

Good article worth reading.

We all know the LS3 style head likes +6 to +14 degree more exhaust duration. In this article the L92/LS3 head is stuck with a 236/236 .590 lift single pattern cam on 4 inch bore. The AFR's used a Fast 78 intake & the L92 Heads used the L76 intake.

Flow test data & chassis dyno

Test is on the same 402 engine same cam both sets of heads. I think both do pretty well.

Guess which heads made the most power...hint the same heads that made the most at 3,000 rpm...

 

What's the argument about here? Gm laid it out for you guys simply i thought. 6.0 and below cathedral heads.....6.0 and above ls3 heads....7.0 and above ls7 heads.

Of course some of this depends on what you're building for. But if you're racing your engine at 3k rpms you're doin it wrong.

 

I'd rather see a test with stock unported 243 and LS3 heads on a 400"+ engine with a cam with specs that compliment the heads then the same heads CNC ported. I think stock the LS3 heads will take it but ported it might be closer.

 

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LS3 heads love big split cams and nitrous

 

 

LS Cathedral vs Rectangular ='s

Big Block Chevy oval port vs rectangular
NFL Redskins vs Cowboys
1863-1891 Hatfield's vs McCoy's

I'm a big fan of the 243's and prefer them for daily driver type cars for all under 3,000 rpm reasons. The correctly ported 243's are amazing.

If there engine is kept in the ~6,500 range or less and ~400 cubes or so. Use whatever one has and optimize. Either can run well.

In the SLP test, link above, the stock LS3 won by ~10 hp on the top end with the deck stacked against them with a single pattern cam vs an AFR head on a 4 inch bore at ~6,000 rpm.

Raw hp, spinning the high rpm and the LS3's will rule the day vs 243's.​​​​​​ Have a master head porters like Darin Morgan or Larry Meaux ported both sets and the LS3's will win again.

When flow testing various ported LS heads 706's, 243's & LS3's, one cylinder head guru said, "Not saying you will make this but that Reher-Morrison LS3 is quite capable of making 750+ hp as is with stock valves on the right engine."

For cost of a set of take off LS3 heads and good porting from various sponsors and others for ~$1,600 to ~$1,800 one can have a set of heads with the capacity to make around ~750+ flywheel hp. That's very cost effective.

For the same cost $400 for cores and $1200 to $1400 of porting and hand finished etc the typical very good 243 could maybe support ~700 flywheel hp on the ideal engine set up. Some of TEA/BTR's or similar experts best tricked out 243's might do better ~725 flywheel hp.

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Ultimately, it comes down to airflow.

​​​​​​The valve size will determine max airflow. The LS3 has an edge in valve size. The stock valve seat limits the 243 heads to a smaller valve size than LS3 head. The stock 243 valve seat limits valve size to about ~2.04 max. Why drop $500 or so replace stock seats in a 243 to run a 2.08 valve? The LS3 stock head has a 2.165 valve.

If all else was equal, the 243 is down $500 for new seats plus valve job ~$400 plus the cost of new valves.


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I don't think ANYONE who knows this stuff would argue that point. Besides the port shape difference, is the port size difference.
I would never put 243's on a 6.2 and expect it to make decent power on top. NOR, would I put LS3 heads on anything smaller than 6.0 (that's IF it were even physically possible, but it's not) as it would have a weak low end.
All the above has been beaten to death here and elsewhere. Let common sense prevail.....