FLATHEADS TO HEMIS

Adam writes...

And Jim, when you're done talking about silly things like museums and crosses and such, tell me something else: It seems like the flat-head engine was used in almost every mass-produced automobile from the 30's to the 60's. And now it seems that most cars have hemispherical or pent-roof heads. So tell me Jim, what in the hilly-dilly heck-hole came before the flat-head?

Well Adam, your question is quite complicated and just a bit ambiguous. You talk about "Flathead engines" and "Hemispherical (Hemi), or pent-roof heads", but these are apple and orange comparisons. I'll tell you what, let me just give a little history about engines and some of their features, and how and why these features were changed. Just bear with me, the less technically minded may get a little lost, the more technically minded may get a little bored.

As you might expect, engines have changed over the years in order to get more power and efficiency. Early gas engines were very simple and not very powerful. The first tact to get more power was to increase the number and size of the cylinders. By the time it was becoming obvious that the combustion engine was going to be the main propulsion system for cars, (as opposed to steam or electric), many cars were sporting four cylinder engines. Chrysler started making 6 cylinder engines by the early 30's and Ford went straight from a four cylinder to an eight cylinder. By this same time, luxury cars such as the Cadillac had come out with engines with up to 16 cylinders. This V16 had a displacement of a whopping 452 cu. In. but with it's 5.7:1 compression ratio, it only managed 165 horse power.

The next tact was to increase the compression ratio. The biggest problem to overcome there was the octane of the gasoline at the time. When a spark lights the gas-air mixture in the cylinder, the explosion travels out pushing down the piston, but also compressing the rest of gas-air mixture. As with any gas, the more you compress it, the hotter it gets. If the gas mixture gets hot enough, it will explode and now you have two explosions in the cylinder going in opposite directions, which is very bad for the engine. The octane rating of gasoline is it's ability to resist that secondary explosion called pre-detonation or "ping".

The third tact for more power was to get the gasses flowing in and out of the engine more easily, thus eliminating resistance to airflow.

So, let's go back to our flathead engine. This was one of the first designs for engines. The main feature of a flathead is that the valves are in the block of the engine. This is a simple way to do it as the valves ride on tappets, which ride directly on the camshaft. The disadvantage is with the valves sitting in the block, next to the cylinder, it is necessary to have a space in the head between the valves and the cylinder for the flow of gasses. This space becomes a limiting factor for compression. The next step was the overhead valve head engine where the valves are in the head directly over the cylinder. Of course, to make this happen you need more parts. You need to introduce pushrods and a rocker arm assembly. When the compression ratio increased to 7:1, the flathead feature was still not an issue, that is why Ford and Chrysler used them well into the 50's. Buick, on the other hand, had an overhead valve engine as early as the 20's, so different manufacturers took different routes to get to the same place.



By the mid 50's octane had increased enough so compression ratios of 8:1 and higher, were not uncommon. This forced the hand of the builders of flathead engines and they died out very quickly.


One trick Chrysler came up with was to put a hemispherical shape on the top of the piston and into the head. This configuration, like octane gave the engine the ability to resist "ping" so they could increase compression even greater. Today's "hemi" is not truly a hemi, it is the pent-roof that you mention earlier, but it is doing the same job with a slightly different shape.


While we are on the subject of octane, the resistance to "ping" is it's advantage and only advantage. It is not a cleaner burning fuel, nor will it give you any extra power, if your car takes a lower octane. Look at you owners manual to see what your octane requirements are. Once you've obtained it, higher octane will do nothing for you.

The way that manufactures opened up airways for more power was to use bigger, wider carburetors. Two barrel and four barrel carburetors were like having two or more carburetors all packaged together in one unit. Of course another way to do this was to actually install two or even three carburetors on the engine, but this creates linkage problems as all the carburetors must be doing the same thing at the same time, but it is probably one of the easiest ways to retrofit an old engine to get more power out of the same unit. Another way to accomplish the same thing is to instal duel exhausts. Many performance cars had and still have dual exhaust for that exact reason.

Recent engines are still trying to work on the airflow problem. That is why they came up with "Dual overhead cam" engines and 24 valve engines. Computer controlled spark systems and dual sparkplug systems are also being used for everyday cars while "turbo chargers" and "blowers" have been used by hot-rodders for years. When you see scoops in the hood of a high performance car, it is there to fit those extra parts (of course, sometimes they just do it for looks).

And then, there's the "V-tech" engine. I have no idea what that is, but I'm sure it is another way to accomplish one or more of those three performance techniques.