85-91 318 heads, or 318 4bbl heads

ChryslerCruiser

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I have 318 4bbl heads on the car at this moment. It is an 82 Imperial, so those heads came on it from the factory.

I can get some of the higher compression roller cam heads for $50 for the pair.

Is that a good swap to boost compression in a a car that will never see over 48-5000 RPM?
 

Camtron

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Totally! I recently met a guy with a 84 Fifth Ave; thing sounds like a total turd next to my 89, lol.
$50 is a great deal for a set of heads (I’m assuming their good and usable). Would Magnum heads be even better? Yes, but then you gotta change pushrods and shit too and God knows finding a set of Magnum heads without cracks in between the valve seats or around the head bolts and water jackets can be hard enough.
Do it! Do it, Do it! Lol.
 

Mikes5thAve

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Yes that 302 head swap used to be a common thing to do for a bit more power.
They're a bit prone to cracking but I think it was more of a problem with 308 heads.
 

Duke5A

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Are you leaving the factory cam in it? Those closed chamber heads were used with roller cams with different specs. Two things I would look at:
  1. Valve springs might be too much pressure for a factory flat tappet and require changing
  2. Compression boost may result in detonation issues with the factory cam
I don't know for sure if you would run into any these issues or not. It's more of a general principal thing as I don't know the specs of the parts you're looking to put together. Someone else will chime in though who does.
 

AJ/FormS

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Have you calculated the compression difference? I think it comes to about 10%, so from 8/1 to 8.8/1.
With no other changes, I'd have to say; that's a chitload of work, for a modest gain. Glad I'm not contemplating it.
Still the Part Throttle torque will pick up nicely, if you can keep it out of detonation.
You would get far greater results with more rear gear, or a higher stall TC.

If you have 2.20s with the 2.74 low gear, that is a meager 6.03 starter gear. Your 10% more Scr, is not going to hardly be noticed handicapped like that. 2400rpm where the torque begins, comes at about 24mph.
2.45s would punch the starter up to 6.71, still in the doghouse, as the 2400 now comes at ~22mph
Honestly, a much higher stall TC is where I'd spend my money.
Say you had a 2000 in it now, and say your 318 put out 80 hp there, which is 210ftlbs. Into the rear axles ,with the 2.20 gears this would be
210 x2.74 x2.20=1266 ftlbs.
But say you installed a 2800TC and now your engine is spooled up to 300ftlbs. So now, into the rear axles, this would be;
300 x2.74 x2.20=1808 ftlbs, the difference being plus 43%..............
Compare that to the compression change being perhaps 3% or plus 6 ftlbs at 2000rpm; this is a non-contest; 6 versus 90

But say you already have a 2400TC in there, and your torque at 2400 is 225 ftlbs. Then, 300 is only a 33% increase.......... compared to 3% with the compression change..... still no contest.
But, you won't buy a 2800TC for $50,not even a used one,lol.. Not even by the time you include the gaskets and fluids. Maybe you'd break even, if you had to hire a shop to install the heads for you. But for 3% that would be nuts.
The TC should not change your cruise rpm, more than a tiny bit, if at all.
The pressure increase might reduce it a wee bit, but it will also cause your throttle-opening to be a tiny bit less, and so, you might pick up a tiny bit of fuel economy..
 

ChryslerCruiser

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Have you calculated the compression difference? I think it comes to about 10%, so from 8/1 to 8.8/1.
With no other changes, I'd have to say; that's a chitload of work, for a modest gain. Glad I'm not contemplating it.
Still the Part Throttle torque will pick up nicely, if you can keep it out of detonation.
You would get far greater results with more rear gear, or a higher stall TC.

If you have 2.20s with the 2.74 low gear, that is a meager 6.03 starter gear. Your 10% more Scr, is not going to hardly be noticed handicapped like that. 2400rpm where the torque begins, comes at about 24mph.
2.45s would punch the starter up to 6.71, still in the doghouse, as the 2400 now comes at ~22mph
Honestly, a much higher stall TC is where I'd spend my money.
Say you had a 2000 in it now, and say your 318 put out 80 hp there, which is 210ftlbs. Into the rear axles ,with the 2.20 gears this would be
210 x2.74 x2.20=1266 ftlbs.
But say you installed a 2800TC and now your engine is spooled up to 300ftlbs. So now, into the rear axles, this would be;
300 x2.74 x2.20=1808 ftlbs, the difference being plus 43%..............
Compare that to the compression change being perhaps 3% or plus 6 ftlbs at 2000rpm; this is a non-contest; 6 versus 90

But say you already have a 2400TC in there, and your torque at 2400 is 225 ftlbs. Then, 300 is only a 33% increase.......... compared to 3% with the compression change..... still no contest.
But, you won't buy a 2800TC for $50,not even a used one,lol.. Not even by the time you include the gaskets and fluids. Maybe you'd break even, if you had to hire a shop to install the heads for you. But for 3% that would be nuts.
The TC should not change your cruise rpm, more than a tiny bit, if at all.
The pressure increase might reduce it a wee bit, but it will also cause your throttle-opening to be a tiny bit less, and so, you might pick up a tiny bit of fuel economy..

AJ, What is your suggestion for rear gears, that still allow reasonable cruise RPM, and get one off the line a bit better... But I am not interested in buzzing the motor to cruise at 65-70 MPH
 

AJ/FormS

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With 28" tires, and a loc-up TC, for cruising 68mph, the math says;
2.20s=1800
2.45s=2000
2.76s=2250
2.94s=2400
3.23s=2640
3.55s=2900

For every 1% decrease in smaller tires, add 1% rpm. For example 27s are about 4% smaller than 28s, and so, adding 4% to say 2.76s, comes to 2340

The actual formula is ;
(mph x gear x1056)/tire roll-out=rpm
Where tire roll-out is the distance the tire rolls out on exactly one revolution.

As to getting off the line better, this would depend on ;
your current stall-speed
what low-gear ratio your trans has and
how tall your rear tire is.

I suspect you might have an A999Loc-up trans with ratios of 2.74-1.54-1.00, but I'm just guessing. And I suspect you have 28" tires that roll-out to 88 inches. And I suspect your stall is gonna come in at ~2400.

I know that a starter gear of 10/1 is killer nice with a 2400.
I know that 8.5/1 is acceptable with a 2800TC
So working it backwards; 10/1 with a 2.74 would require a 3.65 rear gear, and 8.5/1 would require a 3.10 rear. So, for a 2400TC, your range is 3.10 to 3.65.
Going back to the cruse chart, we see that
3.55s cruise at around 2900, which is way to high,and
3.10s might be 2530, acceptable, for me.

But here's the deal; if you have a loc-up TC, there is no reason not to run a higher than 2400 Stall. A 2800 would allow your 318 to spool up an additional 400 rpm, picking up maybe 20/25 more horsepower. That's probably 20% more power than at 2400. I'm guessing. But if it is , then , that will translate to about 13 ftlbs more and that is Plus7%. So then you can reduce your rear gear by that same 7%; so now we have a range of 3.39 to 2.93.. Going back to the chart, we see 3.23s running 2640
and 2.94s running 2400rpm.
Pick your poison.

Now before you get too excited, remember all the assumptions I made; A999 loc-up,28" tires, etc.
Do your homework, and find out what you actually have.and what your current rpm is, at a true 68 mph.

I can tell you this; if you currently have a 2.20 gears, and a 2400, then a switch to 2.94s and a 2800, and then manually shifting, on your first trip to the top of first, or even second gear,yur gonna have a hard time not to giggle like a schoolgirl. This represents about 42% more take-off torque, and up to 33% more after the stall is used up. Your 318 is gonna feel that much bigger. There is no other bolt-on besides supercharging that can touch this.
I said manually shifting, because your governor is gonna shift way to early with the gear-swap..... but that can be fixed. Actually it has to be fixed, cuz in an Imperial, those early shifts are gonna suck.

If you can only afford one change;
I recommend the TC first, but you really need to make sure you have a loc-up trans first, then add at least 400rpm to the current stall.
If you do not have a loc-up, then consider the 2800 as max.
If you already have a 2600, save your money; 200 rpm is not very good bang for the buck, instead, do the gears first.

I drove 4 years at 56=2600, the max I could stand in my combo. Then I installed an overdrive and now cruise at 65=2240, which is so much nicer. For me, 68=2400 as to NVH, is acceptable. She won't get great fuel-economy there, compared to 68=1800/2000 with the factory tune, but you can work it out.
NVH is Noise, Vibration, and Harshness.
 
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ChryslerCruiser

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Thanks AJ. Good info here to plan from. I think my ideal would be a 3.55 rear end and overdrive.. as I want to keep a comfortable cruise RPM. Have not, until now considered the results a different torque converter would bring..
 

Duke5A

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A/J is right, 3.55 and a 0.69 OD is a sweet spot for 70mph cruising. I miss not having lock up though.
 

AJ/FormS

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Thanks AJ. Good info here to plan from. I think my ideal would be a 3.55 rear end and overdrive.. as I want to keep a comfortable cruise RPM. Have not, until now considered the results a different torque converter would bring..

The TC is an incredible gizmo. It varies the output torqur to input torque depending on how hard the engine is working and the speed difference going in to going out..... and it's all automatic.
It has an internal hydraulic multiplier that can can be as high as 2.0 but more often is in the range of 1.6 to 1.8.
This Multiplier is greatest at zero mph, when the engine spools up to it's stall speed. And it is smallest when just loafing along.
As road speed increases, with a fixed throttle position, the ratio continuously diminishes.
What does this all mean to you?
Well lets take your assumed to be 2400TC.
Letssay your 318LA puts out 100hp at 2400; that would be ~220 ftlbs.
When you mash the gas from a dead-stop, that 220 is gonna go into the TC and the TC is gonna multiply it hydraulically to 220 x 1.6 say to equal 352 ftlbs instantaneously. But this ratio will rapidly diminish as the roadspeed goes up, so that by 5000/47mph (depends on your other gears) the ratio could be as low as 1.1 or perhaps up to 1.2 .. if your engine made 150hp at 5000, that would be ~160ftlbs, so 160 x 1.2say equals 192 out the back. That takes care of First Gear. So remember the numbers; 352 off the line, and 192 at the top of first gear.

Now lets do the same with a 2800.
At 2800 your engine might be up to 124hp or 233ftlbs.
The 2800TC might have a ratio of 1.7 at zero mph, so we get;
233 x 1.7=396ftlbs off the line. Again at 5000, the ratio might be just a tad better so say 1.25, so we get 160 x 1.25=200ftlbs at the top of second. To recap; that is 393 to 200..

Now lets compare them; the 2800 is 11.6% stronger at 0 mph, and 4.17% stronger at 5000. And you never did a thing yet, to the engine!
Notice the above numbers in blue; 100hp versus 124hp that is a 24% boost in hp at zero mph, at 2800rpm. There is no bolt-on you can install, to compete with that, short of forced induction.

Ok now recall that the TC is continuously varying it's output ratio from ~1.8 @2800WOT and zero mph) to perhaps 1.1 at the top of second gear, still at WOT.This translates to every gear having two ratios, the one at low-rpm with lotsa torque input, to the other at max rpm with less torque input. This makes your 3speed, act like a 6 speed.
Lessee how that works out, with an A904, with ratios of 2.45-1.45-1.00.
Off the line your chassis will be seeing.... 1.8 x 2.45@zero mph =4.41
At the top of first the TC may be down to 1.4, so...... 1.4 x 2.45=3.43.
Into second, the TC may fall to 1.3, so.................... 1.3 x 1.45=1.89
By the top of second,the TC might be down to 1.2, so 1.2 x 1.45=1.74
Going into third; the Tc might be down to 1.15, so... 1.15 x 1.00=1.15
At the top of third,the TC will be down to it's minimum so 1.1 x 1=1.10

As you can see;
in third gear the TC has settled down to be nearly the same throughout the run.
In second, the variance is a small but noticeable 8.6%. This is similar to one rear gear change.
But get a load of first! Here we have a difference of 28.6%, which is the equivalent of 3 rear gear sizes!
OK so lets average second and third, then line them up; and add the loc-up, so....
4.41-3.43-1.82-1.125-1.00 Ok count the ratios; I get 5, granted loc-up is only a half gear, and you will never race in it... but then with 2.2/2.45s you never will in "drive" either.
Lets see what your engine sees, which is the splits;
.78-.53-.62-.89
So that's how the TC works.
Now lets add some rear gears; I choose 2.94s to keep your buzzzzz down to 2400. So now we multiply every one of those numbers just above, by the 2.94s and I get
12.97-10.08-5.35-3.31-2.94 That's sweet!Compare to my 8.33/GVod
......10.97-6.82-4.97-3.55-2.77, my ratios.
In your Imperial, you only care about the first three, because top of 5.35 gear with 28" tires is 71 mph@5000
Now, none of these ratios are written in stone, as the TC is continuosly varying it's ratio, depending on how much torque is coming in to the roadspeed and the selected gear. To me this is fascinating stuff.
Lets compare what could be to what is.
Let's say you still have an A904Loc-up, a 2400TC(1.6ratio),2.45 rear gears, and 28" tires. Your gears might look like ;
....... 10.80-8.40-4.62-2.82-2.45 and splits of .78-.55-.61-.87 compare
12.97-10.08-5.35-3.31-2.94 splits of .. .78-.53-.62-.89 2800/2.94s
As you can see, the 2800/2.94s is trading away one gear at the top (2.45) to get one (12.97) at the bottom.
Lets see what that means in terms of torque to the road, first the current, then the 2800/2.94s
Again let's say at 2400, your 318LA is putting out 100hp/220ftlbs, so at zero mph this looks like 220 x 10.8=2376 into the axles.
At 2800, as before,124hp/233ftlbs x 12.97=3022 into the axles.
This is 3022/2376=plus 27%, and you don't even have a 4bbl yet!
Ok lets convert 3022# into the axles to on the road ftlbs and I get 2590ftlbs. This is enough to squeal the tires a fair bit, and when the 4bbl opens up, and if the exhaust gets away, you could have a pseudo-race-car there,lol. At least for a little while.
Ok now the disclaimer;
Notice all the assumptions I made. And how often I use definiteness. This is because I have no way of knowing anything about your combo, cuz it hasn't yet been revealed. For all I know your engine is sick, or you might have an A999, or 2.20 gears,or a 2100 stall; I just don't know.
But the whole point of this exercise is to make you aware of the tremendous potential your car has, to respond to gearing and stall, exactly because it is,now, so far away from performance. You got no other place to go but better.
 
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