Concerning 1960s Suspension and Carburetor Tuning:
So it was brought to my attention that I am fallible person and said something erroneous on these two fronts which I was aware from the start of this was probably going to happen. Thus, I had do some investigation.
And that investigation led me down a bit of a rabbit hole that needs more than just a few quick sentences to do justice.
The big scruple here is progressive versus standard rate springs and its not exactly a simple question that has straightforward answers, if for no other reason than most discussion around this subject has to do with modern hot rodding and off-roading. But these things themselves are a discussion worth having and shed light on the true history of these design choices.
First off. What are progressive and standard rate springs?
Standard rate springs can be most succinctly described as springs which are described perfectly by Hooke’s Law - F = kx. The non-technical version of that is that the spring rate is fixed and the displacement of the spring is directly proportional to force applied. If you apply say 1 N of force to a standard rate spring and it compresses 1 cm, then if you apply 1 more N of force it will compress 1 more centimeter. So with this same spring if you applied 50 N of force, it would compress 50 cm. Simple.
Progressive rate springs are springs which are NOT described perfectly by Hooke’s Law. Their spring rate varies throughout their travel, getting stiffer as they are compressed more. This means that if you apply 1 N of force and the spring compresses 1 cm, it will NOT compress exactly 1 cm more if you apply 1 more N of force; it will compress somewhat less.
- Most springs used in automotive are not truly progressive, meaning their spring rate does not vary continuously. Reason being, true progressive rate springs are hard to manufacture. Instead, most “progressive” springs have two or more set spring rates built into one spring and these engage at set points in the travel.
But anyways. Why would you use one versus the other?
Progressive springs are very suitable to off-roading, commercial trucks, and heavily comfort-oriented vehicles. The reason is that spring rate changes with the mass of the vehicle. With standard springs, the locked spring rate must be made hard to cope with heavy loads or soft to give comfort but these two are mutually exclusive. Comfortable suspension can’t take loads and heavy duty suspension can’t be comfortable. But with more than one spring rate, this allows for an unloaded vehicle to maintain similar ride quality to a fully loaded one. The variable rate also allows a vehicle to take small bumps with and irregularities on the driving surface with a nice soft spring rate and switch over to harder rate when pushed for handling or into more adverse conditions.
Standard springs are cheap and easy to manufacture. And they also tend to give more consistent handling since the multiple rates of progressive springs makes them hard to correctly dampen. Also, with a switch between softer and harder rate, progressive springs often upset weight transfer and make for a more vague road feel.
And finally the question to be answered. Which to use?
In general: standard rate springs
Progressive springs would be seen as an option on trucks and heavily offroad-oriented vehicles. Sometimes also on luxury cars. But standard rate springs were – from what I can tell – by far the most common spring type in suspension design until the 1980s, when they started to become standard equipment on premium and luxury cars. In fact even on cars today, standard rate springs are still very common.
Its also worth noting here that leaf springs lend themselves to easy manufacture of a progressive rate, which is I suspect is part of why Chrysler held onto their leaf spring designs into the 1970s.
Just a few other fun tidbits here:
Part of the actual theory of suspension design in the early to mid 20th century was the theory of so-called “natural frequencies”. It was assumed that by approximating the oscillation frequency created by walking, people would feel very secure. Most humans walk at between 70 and 90 paces per minute creating a side-to-side oscillation frequency of about 1.1 to 1.5 hz, hence the very soft tuning of many old suspension designs.
It turns out that humans are actually quite a bit more tolerant and can find up to and including 2.0 hz full body oscillations perfectly comfortable which is part of why newer designs don’t go to the same extremes of softness.
Motion sickness on the other hand is induced by oscillations between about 30 and 50 cycles per minute or about 0.5 - 0.9 hz. So make sure not to go below 1.0 Hz on suspension tuning or the ride will be somewhat… vomit inducing.
Like springs, this also gets into the whole theory of operation on this system and there are never straightforward answers. There is one thing that I want you to remember though before we even get started:
Carburetors virtually always run rich.
How rich? The simple answer is VERY! The long answer is well… this is where the theory comes in to play.
So for performance tune 4 barrel carburetors, its not actually out of the question for AFR to reach as high as 12.5:1 at wide open throttle. Dead serious. Some tunes will go all the way to 12 and above, which depending on your engine and application could be perfectly acceptable.
The bold bits about four barrel carbs and wide open throttle are especially important here because the actual truth about carburetors is that they are NOT fixed AFR machines in multi-barrel configurations. See, the reason you add barrels onto the carb is to make them work better at a range of throttle positions and engine speeds. To understand why, lets talk about single barrel carbs for a second.
Single barrel carburetors are as basic as carbs get. They are a single venturi with a single jet system and because of this they pretty much do have one AFR. This is because the size of fuel jet in the venturi of a carburetor is what determines the AFR, so if you only have one jet, you have one AFR. The problem with this is that during hard acceleration, you want a rich fuel mixture for more power, and for cruising you want a lean fuel mixture for economy. Well, there is only one set of jets so you just sorta have to compromise with a single barrel carb and go for the middle ground between stoichiometric optimal of 14.7:1 and acceleration optimal of around 12.5:1, probably somewhere around 13.4:1.
Now, how do you overcome this limitation? What if we had a separate set of jets for acceleration and a seperate set of jets for cruising? This is the wisdom of the 2 and 4 barrel carburetor. At idle and at cruise, half of the barrels of the carburetor are not open. The primary openings have a set of jets optimized for crusing, producing an AFR usually somewhere between 14.0:1 and 13.0:1 – and yes its still a little rich; I will get to that in a second. Meanwhile the secondary openings will open when the engine is pulling a lot of air into it like when you smash the accelerator and they have a different set of jets which will push the AFR up the desired 12.5:1 range for acceleration. So you get the best of both worlds: economy and power.
But carburetors have one important thing they still need to overcome - intake manifolds. See, fuel delivery is to cylinders as a hose is to a bucket. If you want to fill the bucket accurately and precisely, bring your hose close to it. Carburetors are not close to the cylinders though; its like trying to fill a bucket with a hose from 5 meters away. A lot of fuel just condesates on the manifold. Furthermore, some cylinders are further from the carburetor than others meaning they receive a leaner mixture. So in order to keep every cylinder happy about its AFR and keep it from detonating, you have to run your mixture rich. This right here is why carburetors almost always run rich.
Stoichimetric carburetor tuning is strictly speaking possible but its impractical especially in road car applications where the a perfect AFR would lead to bad acceleration and stalling due to one other critical hurtle for carburetors. When the throttle blades first open, fuel doesn’t just instantly move. Suction from the air has to overcome the inertia of the fuel which leads to an instantaneous lean-out of the mixture. This lean-out condition will cause a drop in power or even an engine stall. Accelerator pump circuits, which add fuel in a quick burst during initial acceleration, do alleviate this problem but not entirely and the ultimate solution is just to keep running a little rich.
So given all of these things, what does that mean for realistic carburetor tuning? You should generally be sitting close to the 13.0:1. Guidelines:
- 4 barrel performance tuned carbs will be pushing DEEP into 12:1 AFR territory. 12.5:1 is a sorta hot rodder optimal.
- 2 barrel and 4-barrel economy carburetors will probably see AFRs somewhere between about 13.5 and 12.5 to one. Generally a 2 barrel should use a less aggressive AFR than a 4-barrel since it sacrifices more by having a rich fuel mixture than a 4 barrel does
- Single barrel carburetors will somewhere around 13.5:1 but probably never go into the 14s.