Although there are better options…

is there any reason I should absolutely not use ported 243 heads with stock valves on a 388” lsx block if I already have them ?

The reason I ask is simply for budget reasons while wanting a stronger bottom end than my current ls3 with forged rods and pistons.

I plan to go with a forged bottom end lsx 4.125 bore and 3.622 stoke, Johnson lifters, btr 660 springs in the heads, my current ws6store max effort rockers.

The plan would be to put better heads on it later 6 bolts thicker decks and push the power later.

Any reason not to do this ?

 

How much power are you wanting to make with them?

Have they been milled any?

 

They're not going to make as much power, but if you're on a budget, just run them. My only recommendation would be to unshroud the chambers in the heads to closer match the larger bore size.

 

Not milled. Plan is to keep my f1a-94 around 1000 rwhp for a year then go to better heads, f1x, and upgrade fuel system the following year.

Wife approved 12,500 for this year lol. Can’t spend 25g on a motor all in one year. Lsx short block will take up all this years budget. I’m still running my ls3 until I get new short block or it breaks whichever comes first.

 

I make 1000 rwhp with untouched 241’s on an ls3 now so I should be able to make 1000 on a 388” with ported 243’s unless there is something im not considering.

If I had better heads and bigger exhaust im sure I could make 100 more rwhp on the current engine easily but I’m already passed where most say the ls3 block is safe so I don’t want to put anymore money or effort into this motor. I’m just going to run it this summer as I make plans and accumulate parts for the next engine.

 

I would find some used LSA heads and put on a stock LS3 intake manifold. That should probably get you as far as you need... better grade aluminum, rotocast, designed for boost heads and budget friendly. Do some very light cleanup work on them (bowl area and remove swirl vane casting) and they will get the job done. People pay big bucks for those 243's so you wouldn't be out of pocket very much.

 

I want to stick with cathedral ports I already have a cathedral port btr equalizer intake.

 

If the 243s flow more than 241s, a good amount, it's an improvement.

Out of curiousity what 6 bolt cathedral heads are you planning to get?

 

I have not made that decision. I'm looking to build the new motor over next winter. With my previous experience trying to get parts I'm in planning now so I can make sure I have the short block and parts I need to assemble the engine by next winter.

Sometimes things don't go according to plan but the plan is:

Run my ls3 at 1000 rwhp as is this year spring/summer.
Build new motor over the next winter and have it ready to run the spring of 2026 with the current F1a-94
Winter of 2027 switch to bigger blower, install better heads, and upgrade fuel system.

Any need to replace valves with new stainless valves in the 243's and if I did do that sticking with the same valve size could I just lap them in or do I need a valve job?

Looking at 6 bolt heads better than the TSP offerings I know it's going to be rather pricey. Buying a short block of this level along with everything to assemble it plus adding $4000 heads and a $4000 out of pocket plus selling my current head unit to fund a larger head unit is more than I can spend on a hobby in a single year. So basically, it's a 3-year plan considering I'm starting now with no intention to assemble the engine until next winter.

If I don't hurt the ls3 before next winter that is a plus and I will keep it for something else or a back up.

 

Sounds like a good plan to me, is there any reason why you're not choosing the boost friendly 317 heads?

 

The 243 heads will be fine, but the 241's are fine too, since you already have. FYI, I have a new LSX block for sale. I also might have heads for it cheap....DM me.

 

I run E85 and a secondary system of meth on at 4 psi of boost currently so 10.5 to 1 static compression has been no problem. Otherwise, no reason I just don't have 317's. If 317's are what I had I could choose pistons accordingly to get me around 10.5 to 1 with that combination.

I'd really prefer nice 6 bolt heads from the get go, but I just can't spend that much money all at one time.

 

317 heads have lazy combustion chambers. The port cores are otherwise identical. I wouldn't use 317's over 243's in any application. They will make less power even at the same compression ratio. Poor mixture motion. I know that's less of an issue with forced induction, but it still exists.

 

This is a before and after on a sbe ls3 going from 799’s to Brodix br7’s at 15psi. Depending on your camshaft it probably won’t carry rpm the greatest but you can grown into some better heads. I have some tfs245’s coming off a customers engine if that’s something you may be interested in.

 

That is kind of what I would have guessed I'd be giving up going from ported 243's to a $4000+ head. I've always kind of said I'm giving up about 100 rwhp with my stock heads on my current motor with my restrictive exhaust as well. RPM hasn't been a problem at all with the procharger and 232/248 cam, but I shift at 7200 rpm to keep everything safe with ls7 lifters.

One thing to consider if you have a more efficient motor with better heads and keep the psi the same you are actually using a higher volume of air as well. Not a bad thing obviously, but with a procharger given the same pulley combo and shift rpm boost would actually go down with a more efficient combo so if you wanted to keep the boost the same you would have to put a smaller pulley on the head unit to raise boost back up after it dropped due to better flowing heads/exhaust.

I mention that because initially I will be sticking with my current F1a-94 and over spinning it from the get-go so the blower is going to make what the blower is going to make. I don't think I would see as much gain until I go to a bigger blower. If I make 1000 rwhp at 20 psi and change nothing with the blower except better heads I'm likely to make the same 1000 rwhp at say 17 psi. That isn't a bad thing just stating in this case the gains might not be as much as you are seeing until I also go with a bigger blower.

Hard pill to swallow for a guy at my low income to buy a short block and the parts necessary to assemble the long block which would put me at or over $15,000 to also add $4000 heads, $4000 head unit, and another $4000 in fuel system upgrades putting me at nearly $30,000 in one year on a motor. Easier pill to swallow and easier to get the wife on board with $15,000 next winter and another $15,000 the following winter.

You are correct though better heads and everything at once would be much more ideal.

I guess what I'm asking is using ported 243's on a combo like this for a year a huge mistake or going to cause me issues I'm not currently considering?

 

What is the ultimate power goal?

I'd just leave what you have and sell the 243's since everyone wants them. I generally get $400 for a JY set. More for a nice cleaned up set. Then put that towards an aftermarket head with thick deck. I think you are over rating 6 bolt heads. You don't get all that much more sealing power and you don't need to spend 4k on a good thick deck aftermarket head. Guys are going crazy quick on 4 bolt heads with good decks. Let the boost do the work, its a lot easier to run lowish* compression and high boost if on a budget. Heads will seal better as well.

 

I’d like to make over 1200 easily all said and done with the F1x.

 

With a $12.5k budget why not just buy the bigger head unit first and make the desired power with boost instead of the better top end? Could sell the smaller blower to boot. Doing it the other way around is less bang for your buck, no?

 

If they are not in the budget you need to run what you can until maybe next winter. It will run quicker and faster with less boost, less IAT, more timing, rev higher with the better heads as long as the cam, lifters, intake, etc are all up to the task. We did a f1x-94 427 with nice heads and it made 1180whp thru a th400 at 21 psi. Also went 5.48 at 4080 and 5.30 with a 60 shot. Upping the nitrous this year.

 

I don’t think my current ls3 block with forged rods/pistons is up to the 1200 rwhp task. This short block is over 5 years old. Started around 750 rwhp and been bumping it up now around 1000 rwhp. It doesn’t owe me anything at this point but I’d rather stay at this power level until I get a stronger block and internals.