Why Inline 5 Engines Can't Be Carbureted and Why They're So Rare

Here we have a inline five or straight five engine, now gasoline or petrol versions of this engine configuration are very uncommon despite offering some very interesting benefits. But what’s more interesting than their rarity is the fact that they were not made before 1976?

We made gasoline engines in pretty much every configuration imaginable many many years before 1976. Singles, all kinds of twins, Inline three, inline four, inline six, v4, v6, v8, v10, v12, everything was made and mass produced decades before the inline five. We even made oddball stuff like VR engines long before the inline five. In fact the inline five is the youngest gasoline engine configuration, it’s the last to enter mass production.

The question is why? Well the reason is that fuel injection technology didn’t mature before the mid-70s. What does fuel injection have to do with it? Well, the inline five doesn’t really work with carburetors. How come? We carbureted everything else very successfully. From singles to v12s. Is it because it has an odd number of cylinders?

Well we successful carbureted many different inline three engines so why not the inline five.

To understand it we have to visualize it.

So here we have our inline five engine. Let’s start by trying to make it work with a single carburetor. Logic dictates that we should place the carburetor in the middle. Now let’s connect the carburetor to the engine via an intake manifold. Do you see the problem?

It’s the very different length of the intake manifold runners. Because we have both a long engine and an odd number of cylinders we are forced to have a great difference in runner length which can lead to unequal performance of individual cylinders resulting in a rough running engine.

Ok let’s try to fix this by using two carburetors. That’s obviously a waste of time because we end up in a scenario where one carburetor feeds two and the other three cylinders, again we have unequal fueling between cylinders and a rough running engine. We could try distributing the two carbs on a single manifold so that each carb somehow feeds 2.5 cylinders? Well this is no different from a single carburetor setup because we can’t “tell the fuel” where it should go, because it’s all the same fuel in the same intake manifold, in this scenario the middle carburetor gets the most fuel, as both carburetors feed it nearly directly. As long as we have a single intake manifold for all the cylinders the number of carburetors doesn’t matter because we can’t tell the fuel from a particular carburetor to go into a particular cylinder.

How about this. We use one large and one small carb each with its own intake manifold. The larger one feeds three and smaller one two cylinders. If you know a bit about carburetors you probably know that getting this to run right would be extremely difficult and even if you could get it to run smoothly for the particular environment and altitude of the factory, tuning this setup in a different climate or as the engine ages would be nearly impossible because all the adjustments on one carburetor would not be proportional to the adjustments on the other and you would end up with a tuning nightmare and a scenario where two cylinders perform differently from the other three leading to a rough running engine.

So the only way out is to use five individual carburetors. In a way that inline four motorcycle engines use. Yes, this would work and would lead to a smooth running engine but the associated costs and tuning complexity simply make sense in a mass produced passenger vehicle.

Ok, but here’s a counter argument to everything I just said. There have been plenty of single carburetor single intake manifold inline six cylinder engines over the years, and some had very long and successful production runs. And if you look at their intake manifold you can observe a massive difference in length between the individual runners. As you can see the difference in length is pretty much the same in an inline six and an inline five, the only distinction is that in the inline six there’s two cylinders in the middle whereas is the inline five there’s only one.

So why does a single carb single manifold work for the six and not the five? The answer lies in the firing order.

A typical inline six has a firing order of : 1-5-3-6-2-4
Whereas a typical inline five has a firing order of: 1-2-4-5-3

As you can see in the inline six adjacent cylinders never fire one after but in the inline five they do. 1-2 followed by 4-5.

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