Intake hot spot

[jaekl]

I never got around to 'tuning' and have notice in various posts the mention of the hot spot in the intake manifold. I take this as an area of the intake manifold is exposed to excess heat from the exhaust manifold resulting in lower intake charge density. Now go back to Series III cars that have the two piece intake/exhaust manifold that are bolted together. In the area of the four bolts there is a chamber cast in attempting to surround the intake manifold with exhaust temperature. What was the thinking at BMC at the time? Why was it done?

[minor_hickup]

To speed up the engine warming up time and reduce the need to use choke. Performance was considered ample at the time so they kept it.

[bmcecosse]

It probably helps fuel economy slightly, by ensuring any droplets of fuel are vapourised before getting into the combustion chamber, and therefore allowing a slightly weaker mixture to be ignited - but this is at the expense of performance. The 'extra' power on the twin carb 948 engines in Spridget was almost ALL down to losing that hot spot, and very little to do with them having 'twin' carbs - because the standard twin manifold is a disaster from air flow point of view!

[jaekl]

Thanks for the explanation. Fuel economy is a good thing then and now.

[Stig]

Then you want a water-heater manifold. Decent intake charge density and good fuel economy too.

[bmcecosse]

If you want max power - you should NOT connect the water heating! However - the heat it adds is nothing like as great as the heat from the 'hot spot'.