340X heads vs late model LA heads vs Magnum.

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340dartley

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I am thinking of changing the heads on my bracket racer 1969 Dodge Dart. It has a set of ported X heads. The car runs mid to low 12 seconds at 108. I just purchased the car but I have been familiar with it for more than 20 years.

I ran a best time in the car of 12.00 back in 1994 or 95. I dont remember my mph.

What I know about the motor is it is a 340. Cast iron stock intake. Holly 750 dp. Ported x heads. Locked weights in the distributor. Electronic ignition. Msd 7AL. Unknown cam. Unknown compression ratio. Headers but not sure of diameter.

With my mile per hour at 108 I am making around 300 horse power.

Was thinking of going through the engine just to update it. Maybe some late model heads and either a Victor 340 or performer Rpm air gap manifold. I think mid 11s are possible.

How much is gained from switching from ported x heads to 1991 318 heads or 1992 and newer Magnum heads. I am basically looking at gaining some quench with a closed chamber head. I am on a budget so that eliminates aftermarket heads. I own a 1991 318 so I have those heads. I can pick up Magnum heads easily as I work at a Chrysler dealer. I am aware of the changes I will need to make to run the Magnum heads on the LA block.
 
340dartley said:
Unknown cam. Unknown compression ratio. Headers but not sure of diameter.

Was thinking of going through the engine just to update it.
Click to expand...

It seems to me, answering the unknowns would be helpful for starters. Running a bore scope in the cylinders may be worthwhile (looking at accumulated carbon & cylinder walls). Before you laugh to quickly at expense, you can get one that works with a smart phone for under $100 at Home Depot (Ryobi Phone Works Inspection Scope).

Not sure what you mean by the last statement quoted. Think there's more power to be had by increasing compression. By all means, update the manifold to an Air Gap.

IMO, there's no air flow to be gained by going to a late model or magnum head. Suggest a look at KB243 pistons for improved quench.
 
I have wanted to stay away from kb Pistons at least there hypernuetechnic or how ever it is spelled. I want to run a forged piston in case I ever get the urge to run some nitrous. I have a bore scope. I will bring it home from work.
 
I would just stay with the x heads. You might gain a bit more adding 2.05 valves with a little more bowl blending.
 
the 318 heads have small ports and will choke the 340 ...

magnum heads do offer better flow put the close chamber heads could be a problem with a 340 if the 340 has positive deck height pistons...you will need thicker head gaskets which will negate the quench...

probably need to tear the end down and see what you got before making any big decisions...especially if you are on a budget...

if the 340x heads ported have they been on a flow bench?....new 11/32 valves...2.055 and 1.625 valves with the bowl work done might be your best bet....

eddy air gap is a good choice...
 
I don't know for a fact but I believe the current Pistons are forged. is there any performance benefits to running 273 adjustable rocker arms. I happen to have them but not sure if the extra weight would be detrimental.
 
I am not sure but I thought late model 318 heads used the same port size as 360/340. I know the valve sizes are smaller but that could be changed.
 
340dartley said:
I am thinking of changing the heads on my bracket racer 1969 Dodge Dart. It has a set of ported X heads. The car runs mid to low 12 seconds at 108. I just purchased the car but I have been familiar with it for more than 20 years.

I ran a best time in the car of 12.00 back in 1994 or 95. I dont remember my mph.

What I know about the motor is it is a 340. Cast iron stock intake. Holly 750 dp. Ported x heads. Locked weights in the distributor. Electronic ignition. Msd 7AL. Unknown cam. Unknown compression ratio. Headers but not sure of diameter.

With my mile per hour at 108 I am making around 300 horse power.

Was thinking of going through the engine just to update it. Maybe some late model heads and either a Victor 340 or performer Rpm air gap manifold. I think mid 11s are possible.

How much is gained from switching from ported x heads to 1991 318 heads or 1992 and newer Magnum heads. I am basically looking at gaining some quench with a closed chamber head. I am on a budget so that eliminates aftermarket heads. I own a 1991 318 so I have those heads. I can pick up Magnum heads easily as I work at a Chrysler dealer. I am aware of the changes I will need to make to run the Magnum heads on the LA block.
Click to expand...

Hey man what does your dart weigh? With out driver
 
340dartley said:
I don't know for a fact but I believe the current Pistons are forged. is there any performance benefits to running 273 adjustable rocker arms. I happen to have them but not sure if the extra weight would be detrimental.
Click to expand...

if you are running non adjustable rockers..the 273 adjustable will allow you to set the correct pre load on the hydraulic lifters ....plus they are probably closer to the 1.5 ratio then the non adjustable...
 
I have not weighed it yet but I believe it is around 3000 lbs. I will find out for sure when I get a chance.
 
I am estimating weight and horsepower. The Et is verified. The car is tubbed with 31x13r15 slicks. It has a 4.30 to 1 rear axle ratio. It has a high stall converter. I believe it is a 4000 but not sure.
 
That's awesome sweet combo the performer rpm is what I use and 2 tenths is what I gained I'f you want you can go to the racing form 340 4 speed dart I thinks that's what it is and post your combo I got mine up too thanks
 
70aarcuda said:
the 318 heads have small ports and will choke the 340 ...

magnum heads do offer better flow put the close chamber heads could be a problem with a 340 if the 340 has positive deck height pistons...you will need thicker head gaskets which will negate the quench...

probably need to tear the end down and see what you got before making any big decisions...especially if you are on a budget...

if the 340x heads ported have they been on a flow bench?....new 11/32 valves...2.055 and 1.625 valves with the bowl work done might be your best bet....

eddy air gap is a good choice...
Click to expand...

340dartley just some food for thought. IF you need new guides,Valves,springs?,harden seats?.....Porting????? it may be cost effective to just buy some RHS Heads........

If you port them more, the head could crack in side the intake port(all your $$$$ wasted.

The RHS will probably flow more air, but you won't know that until you get yours flow test done. aka more money with "0" gain.

I'm not saying that 70aarcuda is wrong, in fact i thing it's a good idea! Just get a good Quote before you jump in with both feet.
 
70aarcuda said:
magnum heads do offer better flow put the close chamber heads could be a problem with a 340 if the 340 has positive deck height pistons...you will need thicker head gaskets which will negate the quench...
Click to expand...
Does not compute... The quench gap will be there whether the piston tops are at zero deck or above deck. Just set it with the head gasket thickness. We have piston .005" out of the hole and a .045" thick gasket for a quench gap of .040".
 
The motor runs well now. No complaints. Just was thinking of updating some of the technology with modern components. I do plan to take the heads off and either check and or replace the valve Springs just because of age. They have been on there for more than 20 years.
 
IMO any gains you "might" see will not be worth all the money and labor.
 
I will have more exact information after I lay down some base line runs. I seem to remember shifting at 6000. The previous owner said he shifted at 5500. Much higher than that and he said it will float a valve.
 
nm9stheham said:
Does not compute... The quench gap will be there whether the piston tops are at zero deck or above deck. Just set it with the head gasket thickness. We have piston .005" out of the hole and a .045" thick gasket for a quench gap of .040".
Click to expand...

dont see anything where he says the pistons are .005 out the hole...what if they are .020 out of the hole...so until he pops the heads off and measures the deck height...any guess is good as mine...

4.30 gears with 31 inch tall tire.....i would add more gear with those tall tires...
 
RustyRatRod said:
IMO any gains you "might" see will not be worth all the money and labor.
Click to expand...

Labor is free as I will be doing almost all the work myself. If I use the heads I have either the 318 swirl port heads or my 340x heads the I am out the cost of valve springs and gaskets. More on the 318 heads as I will need larger valves and seats installed.

I plan to update the intake either way.

Does anyone have a way to check valve spring tension with the heads still on the car?
 
340dartley said:
Labor is free as I will be doing almost all the work myself. If I use the heads I have either the 318 swirl port heads or my 340x heads the I am out the cost of valve springs and gaskets. More on the 318 heads as I will need larger valves and seats installed.

I plan to update the intake either way.

Does anyone have a way to check valve spring tension with the heads still on the car?
Click to expand...

I wasn't talking about the cost in money. It's not free, even if you do it. You spend time and energy doing it. You will have that in it. Knock it out. Let us know what happens.
 
340dartley said:
Labor is free as I will be doing almost all the work myself. If I use the heads I have either the 318 swirl port heads or my 340x heads the I am out the cost of valve springs and gaskets. More on the 318 heads as I will need larger valves and seats installed.

I plan to update the intake either way.

Does anyone have a way to check valve spring tension with the heads still on the car?
Click to expand...

Check out Moroso part numbers 62390 & 62391....
 
As was mentioned, the 318 heads have smaller ports than any 340/360 heads. Even with bigger valves in them, you'll be going backwards.

Stay away from the Victor intake. Way to much intake for what you have or plan on. RPM Air Gap is very good.

Instead of going even bigger with the valves on the X-heads, you can revive the valve job & improve port flow with new seats. 2.05" valves are pretty big for your combo. Bigger is NOT better. Just make sure the machine shop knows what they are doing.

Magnum heads are decent, especially with a little work & good valve job. But don't go over 2.02. May not be better or cheaper than going through your X heads. Check the piston deck height first for clearance. While the heads are off, check ring/bore wear also.

The engine is pretty old and you don't know anything about the cam & springs, which may also be old tech & well used. It would be better to just get a modern grind (properly degreed) with springs recommended for it.

Lots of performance can be found in carb tuning and don't go bigger than 1 3/4" headers.

"High stall" converter basically boils down to another mostly unknown component where more performance might be found.
 
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