OP, on an honest dyno NA, with a TH400 & 9 inch, I think the car will make 365 whp or less with stock 853's heads & LS6 intake. Not being able to lock the TH400 will probably cost 25 hp or more depending on the converter. The TH400 isn't a high dyno number friendly transmission. TH400 do rock at the track and run well. It's possible if you have a tight converter and a,TH400 with light weight internals etc that I'm wrong and the combo will hit ~400 whp or so.

SRT8,
99-00 LS1 853 heads typically flow ~235 cfm on the flow benches I've seen. ​​​​​265 seems very high for a stock 853 head. Maybe maybe it had a valve job or some basic porting, or someone forgot to put a spark plug in the cylinder being flowed. When a plastic intake is added the 853 will usually lose 25-50 cfm of air flow. It's very possible for the flow to be ~200-210 cfm with the typical intake in place.

Link to AI CNC details for 853/241 heads & flow data



Agreed 100%, my 416 is very responsive and rev happy.

What part of ls3 heads will have shrouded valves with a 4.030 do people not understand? The only ls3 head that would work is small bore ls3 heads and those are $$$. A 225cc ported ls6 or trick head is the way to go. Especially with a fast and hyd roller cam when peak rpm will be 6400. Just because it "dynos" higher doesn't mean it will run better or faster.

Actually cylinder filling/emptying is probably at the top, A small high velocity port will make less power if it's not filling the cylinder completely, This is why the right combination of velocity and total flow is desired and that involves everything from beginning to end. Throttle body/intake/heads/piston design/camshaft/bore/stroke/exhaust all effect cylinder filling/emptying. As the saying goes bigger isn't always better but the opposite is also true. You can't have a restriction no matter how high the velocity is because it will cost you power. The LS3 head is a cheap alternative to high dollar aftermarket heads, They flow well in the stock configuration and are a awesome head with a little port work.

What part of best bang for you buck don't you understand? Do you really think that he choose the 853 heads over ported ls6 heads or trick heads because he thought the 853's were a better head? My guess he used them because that's what he had to use at that time. The shrouded valve issue is really not an issue in a case where you have shitty heads restricting power versus cheap LS3 heads shrouded valves and all!

I would think shrouded valves in a 4.030 bore is a very minor tradeoff.

It is not the best bang for buck. 600 for heads, 200 for rockers, 800 for intake, rails and injectors. Or you could get 243s for 400 and use stock rockers. Ls7 lifters for 100 and have tsp port them for 750. It would run a whole lot better everywhere. It may not Dyno as high, but if we are going to argue that Dyno numbers means a faster car then we are too far gone to bother continuing.

Not taking sides because i like both cabothrals and rectangles. Each has they place in my eyes. Lets talk pricing.... im now running a LS3 headed LS2. My heads costed me 600 bucks *821s with rockers and i added Tooley valvesprings then i did a basic trunion kit upgrade for 150 bucks. Found a L76 intake on here for 200 dollars then i found some LS3 injectors for a 150 bucks. Lifters i went with SLR lifters which was 200 bucks. The LS3 head is good budget but it takes the RIGHT CAM to make them work great. The LS3 head is the most misunderstood cylinder head it is when it come to specs.

Well aright then, I have already talked to numerous head pros about this because I wanted to run ls3 heads and they educated me about why I shouldn't. (402 ci) I then did hours of research to try and disprove them but couldn't. You want the smallest port with the highest port velocity needed for your given engine size and the correct port design for your desired rpm range. A small bore cathedral will give us this in this situation. Ported ls6 being great and aftermarket 225 being better. If he had an ls3 than my opinion would change. And yes a .03" difference of bore size is a big difference in the perspective of a cylinder and a valve that opens a hundred times a second. Shrouding is also a big deal.

I gave $350 for low mileage ls3 heads with upgraded springs with rockers and see them stock at that price regularly. I gave $400 shipped for a new takeoff intake with injectors and throttle body included on e-bay. So $700 total. I have no idea where you're getting your information but considering those heads come on 6.0 trucks and they tow better than previous 6.0 models they certainly don't lack low end torque and if you've driven a Camaro or Vette you'd also know they don't lack top end.

Aren't the Gen IV L76 & L98 engines 4.0 bores with rectangular heads direct from GM that were installed in cars and trucks?

I think the 4 inch bore with rectangular heads might be a little tricky to cam and tighter on piston to valve clearance with big cams. Even with some shroud they will be better than the sucktacular stock 853's.

You realize you're talking a small amount when looking at the overall picture, shrouding at that level is only costing you a few ponies down low and more than making up for it on the top end where it counts. When I jump on the throttle the RPM's are never in the 1500-2500 range.....it's 3,500 to 7,000 and that's where budget LS3 heads shine. I don't street race but if I did I could care less that I was giving up a little under the curve as long as I keep making steam up to redline when I need to.
Just because someone talked you out of LS3 Heads doesn't mean diddly. I have fully ported AFR cathedral heads and they flow great but at $2300 for the heads and $1300 for the port work and shipping back and forth they are hardly budget friendly. Yes they will make a lot more power than stock LS3 heads across the board but at that price they better. On the other hand I have a two other engines with stock LS3 heads that cost less than those AFR heads alone and at cost per HP the LS3 heads win hands down. If you have the budget there are better heads...that's the reason you got the answer you did...but that does not mean that it's money well spent.

Exactly, You can take a stock bottom end throw on some LS3 heads/decent cam and 3500-4000 stall and headers and make it scream on the street or the track at a fraction of the cost of a full on build. I won't say what my 408 cost to build... I honestly don't want to know lol - Iron blockPinned billet mains/ Callies crank and rods/ Ported AFR heads/ Morel link bar lifters/ Ed Curtis cam/ARP everything lol. Honestly it's enough to take the fun out of it knowing that if goes boom you just took a match to thousands of dollars lol. I know guys that run sprint cars with engines starting at $35,000 to over $60,000 and they have two or more of them. They get a few nights out of them and have to have them refreshed, I could only dream of having that kind of a budget!

Someone needs to call GM and tell them what idiots they are, they're putting rectangle heads on 4" bore motors in the 14+ 2500 and 3500 trucks. Man if someone would just tell them the valves are shrouded maybe they could get some low end out of those heavy duty trucks that are built for towing and moving around literal tons of weight.

^^^^^^^^^^^ My thoughts exactly!

:thinker: I agree GM has it going on...but these engines also have VVT do they not?

Would 1 7/8 headers be enough for a 350 hit or should go to 2 inch