Originally Posted by Tommy the Cat
I mean no disrespect, however some of the stuff reported here has some flaws.
A V8 engine, with a few exceptions (Ford FE comes to mind) do not have a sharp 90 degree bend before the valve such as the 2F. And you forget the exhaust crossover on most factory intake manifolds whose job it was to heat the intake. Not to mention the hot oil that splashes the underside of a lot of V configuration engines. I never stated the fluid heat riser didn't work, I stated it is less efficient (slower) than the factory exhaust manifold. It is a band-aid. I have used them and yes they are better than nothing but they are less than ideal. However the 2F was not designed to be pinnacle of automotive engineering and comes with MANY inherent flaws, the intake arrangement being just one of them.
Also, liquid gas isn't ignitable by spark. It must be converted to vapor or atomized. That is the function of the carburetor. If you doubt that then refrigerate some gas in a cup so that it is not giving off vapor and throw a match at it. Yes you can put a match out in liquid gasoline. It must be properly atomized to burn correctly. If liquid fuel makes it to the combustion chamber it will not be burn correctly and has the potential to wash the oil off the cylinder walls and contaminate the oil in the crankcase. I also stated that the amount may be academic and it depends on the condition of the rings. When enough liquid gas makes it to the cylinder to wash oil off the walls it will increase the wear. Again, this may be academic.
I did misread what MDH33 stated about vapor locking while cold. I read it too fast. My apologies.
No one takes any of this personal, it's just an interesting discussion.
I have to take exception to some of your theories. First, if the incoming fuel/air charge was hot and the intake manifold was cold I might accept the condensation theory. Problem is, when the engine is cold everything from the carb to the engine block is the same temp, so not much chance of condensation. As the engine warms up the internal parts will be warmer than the incoming fuel/air charge so again no chance for condensation.
My understanding of why engines start harder and don't run as well when cold is because the cold air is denser and thus has a higher oxygen content. That makes the fuel/air charge too lean to run properly. Thats why on carbed engines the choke closes and richens the mixture to compensate for the dense air. Once the engine warms up (and in the process warms up the incoming fuel/air charge) the choke opens. Even on MPI injected engines the ECM adds more fuel when cold starting. Some of the Toyota's even have dedicated special injectors that operate only on cold starts.
If your condensation to liquid theory was correct, richening the mixture via choke or injector would make the problem worse and potentially flood the cylinders. Then the engine wouldn't start at all.
Contrary to your claim there are few engines I'm familiar with that have exhaust gas routed through the intake manifold. However virtually every engine other than some of the old straight sixes have coolant passages through the intake manifolds, so the OEM designers were concerned with warming the instakes up but also maintaining controlled temps.
You may get some oil splash on the manifold base on V8's but it's minimal, the older engines have valley plates to minimize that splash and most of the new engines don't even have open lifter gallies. besides the oil only heats up a little faster than the coolant does. The heat in the combustion chambers will have much more impact on engine warming than lifter galley splash.
I would also add that most of these 35-40 year old exhaust manifolds are so rusted the heat risers stopped working long ago. They're either stuck partially closed and robbing power or stuck wide open and making no contribution to warm up. Not to mention that the cast iron manifolds are prone to cracking and they contribute to vapor lock in the summer because they don't dissipate heat as quickly as the headers do. Oh, and it's a common problem on F/2F engines to develop a vacuum leak via cracks in the bottom of the intake manifold that are caused by the severe heat cycling that occurs over time with the exhaust manifold bolted to the bottom of the intake.
Don't get me wrong, I am not saying a header is the panacea answer to everything. Headers can crack too and you have to be precise with how you install them to make sure you don't get exhaust leaks.
But I do believe if you install a header correctly, then properly re-jet your carb to compensate for it, you will see a small increase in performance, throttle response and fuel efficiency. In some engines headers can add 10%-15% improvement in those areas. In the F/2F it is a much smaller gain, maybe 2%-3% at best. If you add a liquid heat riser with a properly functioning choke you won't have any issues in cold weather and I believe your chances of suffering vapor lock in the summer is lessened.
My summary would be, if your existing exhaust manifold is not cracked and you're happy with the way your engine runs, don't touch it. I certainly wouldn't spend the money to add a header just for the performance gains if everything else is ok. But, if you have to replace your exhaust manifold anyways because it is broken, a header is a viable option. Cast iron manifolds in good shape are getting harder to come by too. There are trade-offs for any changes you make.