What means
"not going
"ham" on camshaft selection"?
This does not seem to fit together well with
"max effort
daily driver 318"
The 75 Dart is a fairly lightweight car, and for a DD, all you need is a hi-cylinder pressure 318. This will provide all the performance you will ever need in a 3.21 geared automatic daily-driver car.
But I sense from the way the thread went, that you are not fully committed to Daily-Driver status, but are leaning more towards building a modest performance car with daily-driver as an option. Am I right? Cuz why else would you even consider a fat-bore short-stroke 400 cuber?
If I'm right, here are My thoughts;
In this thinking, the overall combo is the big deal; and you will want to always keep fuel-economy in all your thoughts. So then;
The very first thing that you will want to do is select a stall-speed, and the Second thing is the camshaft. After that you would select the grade of fuel that you are willing to pay for, and finally back-calculate the Compression ratio, to achieve the pressure that you can safely run without detonation on that grade of gas.
That' all there is to it.
What I mean is this;
if you intend to run 3.21s and a stock-stall automatic without a Lock-up, this will require a different engine than if running 3.91s and a 3500 stall.
By choosing the 3.21s, you have already limited the rest of the combo. Why? Because the engine is gonna be handicapped by lack of torque-multiplication , and the Mopar"s rather large 1-2 shift split, of 59%.
Lets say that you are gonna run 27" tires.
With 3.21s, and in first gear, your speed is gonna run about 9.2mph per 1000 rpm, so 4000 would be 37mph, 5000 would be 46mph, and 6000 is 55mph. The point is this; that is a lotta lotta mph to be stuck in one gear. So then, the engine build will need to reflect that, as this is gonna be very nearly the only gear of choice at WOT. At the shift into Second Gear, the Rs will drop to 59%, which from 5000 is to 2950, so again, the engine build will have to reflect that. That is a Powerband requirement of 2150 rpm, well beyond the usual 1500 rpm band of optimum average power.
To get the transmission required powerband, to better fit the engine, would require a hi-stall convertor. How high? Well that would depend on the rpm of peak power, which depends on the cam....... but it quickly gets North of 3000, which is usually not the best for fuel-economy.
Furthermore, your take-off performance depends entirely on how much torque gets to the tires at zero mph. This depends a lot on the various gear ratios and the basic engine torque.
Your ratios are already selected as 2.45 x 3.21=7.86.. And at zero mph the Torque convertor will multiply that by about 1.8.... but this number quickly diminishes towards something like 1.1 as the car starts to move.
So whatever torque is coming out of the crank, gets multiplied by these numbers.
Say you have a stock stall of 2000ish rpm, and say your engine makes 200 ftlbs at 2000. Therefore 200 x 7.86x 1.8=2830ftlbs briefly, diminishing to 200 x 7.86 x 1.1=1730 ftlbs as the rpm starts to build. These are lackluster numbers. And you are not a happy guy. So you swap out that convertor for a 3000, and at this rpm say your engine is now making 327 ftlbs. Your new numbers are 4626 diminishing to 2827ftlbs which is a chitload better than 2830/1730. In fact, as you can see, the max at zero mph and 2000 of 2830 ftlbs, is now about equal to the minimum achieved by the 3000 stall TC@2827.
2827 is not that great, but not so bad either, and for a DD I would gladly be happy with that. Now, I'm not saying that you need a 3000 stall Convertor. I'm just presenting a thought-process.
Lets move on; With 27" tires and 3.21s, 65=2600 at Zero slip, could be 100/150 either way on the tach. So for optimum fuel usage, the thought is to have a stall no higher than this. But if your leaning more towards performance, I can tell you that I really really liked my 2800 Turbo-Action, which I purchased back in the late 70s and at that time, it had the name of Dirt-Jerker. Great TC. I still have that TC today, still installed, having been thru many combos.
If yur still reading;
Now we have the basic combo worked out which is ;
318/904/3.21s/2800 stall.
Next up is the gas, what octane are you willing to pay for. This will determine NOT the compression ratio to run, but the maximum cylinder pressure.
Here are the typical pressures to octane numbers, for an iron open-chamber :
87 at 150psi
89 at 155psi
91 at 160psi.
A Quench-design will usually tolerate about 5 more psi.
For max-torque, you would build to the highest pressure
For max-Power at high rpm, whatever octane is sufficient to suppress detonation.
For max fuel economy, you would build to the cheapest gas
Lets say you choose 89, and 155psi, as a happy medium.
Now we're getting into the nitty gritty.
Now is the time to pick the cam.
Getting back to the 3.21s being a handicap, and the 2800 stall, we are looking at a power peak of ;
2800 plus about 1500=4300 This is just about exactly equal to the stock 318 cam.
But this is a DD, so it doesn't have to be a super gas-sipper.
Lets stretch the power band out to fill up all of First gear and if it falls on it's face on the 1-2 shift, we won't lose any sleep over it....... cuz it's a DD; ok?
So then you gotta chose an rpm limit, at which the engine will survive say 100,000 miles, cuz longivity also plays into fuel-economy. Ok then, we know that Mopar small-blocks tend to throw rod bearings with stock oiling systems, after 6000, so lets sneak in at a 5500 rpm shift.
Allowing just 300 rpm past peak for the factory heads, this gives us a power peak of around 5200. Oh well that's just swell, this allows a cam of up to
[email protected] which for a 318 is really big and NOT suitable for getting fuel mileage.
Ima gonna arbitrarily cut that back to a
[email protected] cuz I know I can push that to close to mid 20 mpgs, with iron heads, from point-to-point.
Ok , see how this is working out?
Ok now, using David Vizards "128rule", for performance, this cam should be on a 108LSA. But we're NOT targeting performance exactly, so a 110 will be fine. And we don't want to get into a messy intake exhaust system with a lot of overlap, so at
[email protected] and a 110, we should be good at 65=2650
So this dials our cam in at 218/110 so far.
From experience, I know building pressure with a 318 is a pile of work, so I'm gonna want to run this cam advanced And I know the factory heads need all the help they can get, so I'm gonna recommend a split pattern with about 8 extra degrees on the exhaust, giving us 218/226/110, Awright now we're getting somewhere.
Now, you can get this cam with acceleration ramps from say 58 degrees down to maybe 44 degrees lets go with a typical 48* and that, makes advertised numbers of; 262/270/110
Next lets build the cylinder pressure up to a minimum of 155psi with that cam . In at 109, the Ica is 60* and at 800ft elevation, the Wallace kicks out
[email protected] Scr; BadBoom!
Here are all the events;
262/120/114/270/46 in the order of
intake/compression/ extraction/exhaust/overlap
what do they mean?
Well you can't mess with the 120 compression, cuz that is making your 155psi. Tthe 114* extraction is a generous amount with which to make fuel economy. The 46* is a modest amount of overlap, which won't muddy up the exhaust, yet will put a modest yank on the plenum.
Finally, this is an install at 109, just 1* advanced. To fine tune it in your combo, you can advance it 4* to increase the pressure, if you change your mind and are willing to run 91 gas at WOT, should you need it. Or you can retard it say 4* which will decrease the pressure and move the power up a few rpm. Point being, this size of cam is tuneable to your combo. When it gains pressure, the engine will gain a lil take-off performance. When it loses pressure, it will trade low-rpm torque for a lil high-rpm power.
Now
Before somebody says that AJ said that you gotta run a 228/110 cam, read the post again. I arbitrarily reduced the permissable 230* cam, to a 218* cam. And altho I recommend this size of cam, it only applies to all the assumptions I made in setting up the combo.
If you swap out the 3.21s for 3.91s moving your combo OUT of the realm of DD, then the 218/110 cam is wrong!
If you stick with the factory 2000 stall, I think this cam will still be right, but if you get into lo-rpm detonation, yur gonna have to deal with that.
If you want more performance, this engine build is wrong. Well the whole combo is,lol.
If you get a trans with a Lock-up convertor, then you don't have to be limited by a 2800 stall.
If you are not limited to the 2800 stall, then the 3.21s can move down a size, without compromising the as-built combo.
And finally;
I don't see much value, in terms of fuel economy, in running a smaller cam. The 262 is a nice all-round performer.
The next bigger cam of same FTH design, will definitely suffer a decrease in fuel economy.
If you are willing to upgrade to a Solid lifter cam, there are gains to be had either way.
If you are willing to install alloy heads, and a Quench chamber, you can safely run the pressure up to 200psi on best gas, decreasing about 5psi per grade. And you can upcam one size without sacrificing fuel economy, so long as you maintain the pressure. In fact, if you do both alloy heads and a Solid cam, you can upcam two sizes, and move the LCA back to 108 where it should be. I only chose the 110, to not have the overlap that a 108 would bring, a concession to fuel-economy.
Ok, those were my thoughts.
)
