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Old 03-28-2012, 07:17 PM
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Quote:
Originally Posted by Air Randy View Post
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.
This is not theory, it is fact. Cold gas resists atomization and can fall out of suspension easily if the much lighter air makes an abrupt change in direction as it does after it travels through the carburetor and makes the 90 degree turn in the manifold. Compounding it further is the cold intake manifold that then causes it to pool, much like a cold glass of iced tea in the summer. Of course not all of the fuel falls out of suspension or the engine would not run but enough to warrant the use of exhaust as a heater until convection takes over. You may do your own research and you will come to the same conclusion.


Quote:
Originally Posted by Air Randy View Post
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.
Your understanding is on the right track, however incomplete. The cold air is indeed denser and so is the fuel. I have stated the fact that cold fuel resists atomization and does so to a point that it is necessary to add more to keep enough in suspension (because gas must be atomized or a vapor to burn). So you are correct that the mixture must be enriched but it has more to with the gas' resistance to atomization than with the density of the air. The Toyota cold start injectors work for a very brief moment during start up and the injectors working in open loop take over from there. An injector mounted as close to the valve as possible and cold gas being drawn from a carburetor, changing direction several times before arriving at it's destination have very little in common. We can clearly see why fuel injection operates much more efficiently at cold engine temperatures.


Quote:
Originally Posted by Air Randy View Post
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.
To the best of my knowledge all Chevy carbureted V8s had exhaust crossovers in the intake. I know that Ford and Dodge also employed exhaust crossovers on most, if not all, of their carbureted V8s as well. Fuel injection changed all of that, though. Perhaps you are more familiar with the modern stuff.


Quote:
Originally Posted by Air Randy View Post
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.
The amount of oil splash is rather minimal but enough of a concern that drag racers have been using shields under the intake for decades and that spawned the invention of Air-Gap intakes. It has little or no impact on the warming up period but a measurable amount on the density of the intake charge on a warmed up engine. Agreed, a moot point in this discussion.


Quote:
Originally Posted by Air Randy View Post
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.
I agree with everything you said there. But without a doubt, a properly functioning exhaust manifold will heat the intake quicker and thus be more efficient through the warming cycle. It may not be financially viable to most and perhaps not even possible as usable parts dwindle. It is my preference to use a manifold whenever possible.
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