The high heat from the exhaust does a couple of things just as you mention, one it helps cold start ups, and two, it keeps the fuel vaporized. I have run engines both ways, with no heat and full heat ( no flapper valve). The full heat was never a problem and always ran better then no heat, even when hot out side. Race cars are a different animal and cannot be included when discussing street driven cars or trucks. I suspect full heat also lets the mixture flow quicker, since hot gases are lighter.
With the small amount of air flow the inline engines make at normal driving speeds, any type of exhaust will do a good job of getting it out of the engine. I built a header type manifold for my 250. Six short pipes into a tapered pipe which ended up being 2 1/2" at the outlet. The front pipe was 1 3/4" which blended to the tapered piece. It was all smooth and nicely done with no sharp corners or jagged welding. It sounded better, seemed to run better, all was good. I had no heat under the carbs ( three of them ). I took it to the local drag strip and made five runs. all with in a short time. All five were very close to each other, 17.98 @ 72.6 mph - 18.0 @ 72.9 mph. It would make a great bracket racer! Next week I went back with a stock 1 barrel and exhaust just as GM delivered it, guess what it ran? 17.97 @ 72 mph, I couldn't believe it. Five more runs all the same.
What I found was the engine had plenty of exhaust pipe and the stock manifold was good enough for how hard I was running the engine. I let it shift at 4500, no need to push it any farther as the carbs were at there limits anyway. From transmission back, I used the same exhaust system, 2" single pipe, through a 2" quite muffler with 2" full tail pipe.
I started thinking about what had happened and went looking for some answers. Using my dad's $400 computer based engine analyzer I started plugging in numbers and data from my truck and recent drag race results. I came up with fairly mild 155 hp flywheel, 125 hp rear wheel engine. This is just what I was guess it had due to mph, and weight calculations I had done. So with this data I started changing the header design. To make the most horse power and torque in the rpm range I had used, the primary header tubes needed to be 1 9/16" diameter, 36" long, going into a 2 1/2" collector about 18" long. The very small and long pipes was key to high torque which is what moves the truck. To get higher hp, I needed shorter larger diameter pipes. The program had cast headers as a choice along with log style manifolds, both of which showed the same out put. On most engines we looked at on the program, cast headers were a option and almost every time, they showed very little gains over stock log type manifolds on V8's. Another factor is the exhaust port, this is what limits flow, not the manifold or pipe. If the exhaust can not get out of the head, pipe size doesn't make any difference. In my case, the head was ported with about a 14% gain in exhaust flow over stock ( I used a bigger exhaust valve).
Joe
Last edited by Joe H; 05/21/2014 3:16 PM.
