[Schurkey]

Originally Posted by PontiacJim1959

Ah, you are an automotive engineer - I say that with no disrespect.

Nope. Former mechanic, long-ago retired.

Originally Posted by PontiacJim1959

So here is the complete info I got this from as the word "catalytic" caught my attention. The topic is headed AFTERBURNERS. My book source is published 1968 and specific to automotive emissions systems of all the US manufacturers and the diagnosing, troubleshooting, calibrating, and repair of the systems.

"The two types of afterburners that have received extensive development work to date are the catalytic and the flame type...
...So, as you can read, it is exhaust heat activating chemically treated pellets - not air injection.

But catalysts were NOT installed on a production vehicle until model year 1975. Everything prior to that was "extensive DEVELOPMENT work". NO cars came with catalytic converters in '68.

As for AIR vs. thermal reactors, your text explains the mixture in the cylinder has to be overly-rich to support the post-cylinder burn in the exhaust system. If the mixture in the cylinder is overly-rich by design, you'd HAVE to add oxygen to continue the burn "downstream". The air added to the exhaust system goes without saying; you can't burn something without enough oxygen.

Originally Posted by PontiacJim1959

Your statement about low power and crappy mileage came about due to the drop in compression that coincided with the open chamber heads is a bit off.

Yes, it's a bit off. Low power, crappy mileage, lower compression and open chambers are all interrelated. But not necessarily in the simplistic way folks expect as presented in this thread.

Originally Posted by PontiacJim1959

I don't think any of the aftermarket aluminum heads are of closed chamber design?

I'm thinking just the opposite. Virtually all aftermarket heads have a quench pad, some have more than one.

Originally Posted by PontiacJim1959

I will agree that the open chamber may contribute to additional heat. Guess what, goes back to emissions again. The open chamber burns better; more complete and efficient burning of the air/fuel lowers emissions out the pipe.

"Additional heat"? Leaving aside the issues of higher-rated thermostats that drive operating temps higher, yes, the open-chamber head will tend to push more heat into the coolant. This is less a matter of "more-complete, more-efficient combustion" and more a matter of SLOW, LAZY burn, so that the heat is absorbed by the combustion chamber and then goes out the exhaust port instead of doing something useful like pushing the piston down. The exhaust port, along with the greater surface area of the combustion chamber increases the BTUs that have to be carried away by the cooling system.

Originally Posted by PontiacJim1959

If you know what a closed chamber head looks like, then you will understand that a good portion of the head has a flat area hanging over the top of the piston which acts as a quench. This was bad news to the emissions police.

Yes, that was the belief at the time. Turns out, it was a mistake in logic. Open-chamber heads were a dead-end technology. Solved immediate problems, was "rushed" into production; then dropped like a broken rubber when the REAL solutions were identified.

Originally Posted by PontiacJim1959

So what did their findings reveal? You guessed it, open chamber designs to reduce the cooling effects of quench. Pontiac was not the only manufacturer to go with open chambers.

Which got many manufacturers past the immediate crisis. Later research brought back the quench pads and higher compression. The real problem was not the quench pads per se, it was the horse-sh!t machining practices where the quench distance was too great to be beneficial, but too small to promote full combustion--combined with poor control over fuel mixtures, poor control over spark timing, poor combustion chamber and port design in general, etc.

Originally Posted by PontiacJim1959

For us engine builders, we actually shoot for a good tight squish on those quench areas (any flat portion of the head above the piston) to take advantage of the cooling properties of the quench area and force the mixture into the chamber in an effort to minimize detonation.

The "cooling properties" are a side effect. The real bonus is the in-cylinder turbulence that proper quench provides.

Originally Posted by PontiacJim1959

I suspect the engineers also learned a lesson in detonation and why compressions were lowered and premium fuel was required so as to have the higher octane to prevent detonation - even with mild cams/engines in the family sedan.

Detonation became a problem because Detroit foolishly gave up the faster combustion of a properly-done quench pad in favor of a slow, lazy burn that left the chamber and piston-top hotter. This did reduce hydrocarbon emissions, but vastly decreased efficiency and therefore power and fuel economy.

Look at the new engine designs: Mopar Hemi: Double quench pads, two spark plugs leading to fast combustion. GM LS: quench pads, high in-cylinder turbulence, fast combustion. GM "performance aftermarket" heads are called the "Fast Burn" heads. GM's TBI Swirl-port heads and the 7.4L "Vortec" heads have ski-jumps cast into the intake ports to promote in-cylinder mixture motion leading to...you guessed it...speedy combustion. Fast, controlled combustion leaves less time for the combustion chamber to be exposed to heat. The combustion starts later, is DONE before the exhaust valve opens, so temperature of the exhaust gas is already dropping by the time the port is exposed to the exhaust gas stream. In short, more power, less wasted heat.

Open-chamber heads tend to be turds. Garlits found out in the '50s that the only way to make a (First-Generation) Hemi provide power for racing was to advance the timing beyond all reason: They burn so slow! They also retain heat in the combustion chamber, so you need high-octane gasoline, or alcohol fuel to avoid detonation.

My point is, it's not "just" the drop in compression that made the mid-'70s engines so pathetic. It's also the WAY they dropped compression, which compounded the power/fuel economy problem The open-chamber head proved to be a short-term fix, and has all but been abandoned since then.