[steve25]

Low power and crappy mileage came about due to the drop in compression that just so happened to come along with the open chamber, but it does not have to be that way!

And you have something way the heck wrong, as closed chamber heads require more timing then open chamber heads to make max power, its just that the open chamber heads can be more prone to ping and knock if compression ratios your looking at are are kept equal.

Anyway have we gone off the rails in regards to the OPs original question here, sorry about that!

[mgarblik]

I wide ranging thread with all kinds of emission and good info about the Pontiac 428, 455, and even the Oldsmobile V-8's! It's an interesting look back. To the OP's original question, "why was the 428 discontinued?" I think the advertising and marketing group had more to do with it than engineering. Pontiac simply wasn't going to be left in the "displacement dust", by the 4 other GM divisions. Cadillac was moving toward 500 CU IN. Olds, Buick and Chevy were 454,455. Pontiac stuck at 428 would have been a sales disaster in that time period, no matter the HP. Pontiac had to be equal or larger. The move from 428 to 455 was an easy deal. No clearance issues in the bottom plenty of cylinder wall, same engine package, easy change. BTW, I have owned numerous 421's and ran them as 428's, as well as a few stock 428's. I can't fully explain why, but that was a real sweet spot in cylinder head flow, rod ratio, bore and stroke. They just ran beautifully and had superior power to a 455 from 4000-5500 RPM stock. A very nice engine package. 455 was essentially a smog package big engine with the exception of the high compression 70 455. The 71-72 HO had the better top end to help utilize the displacement and of course we know how that was improved more in the SD-455, allowing higher RPM's with a better bottom end and even more improvements up top. Other than that, a stock 455 is just a big smog engine. At my school, years ago a student brought a really nice Grand Ville in for a run on the chassis dyno. Stock engine, about 80K miles, 455. Ran it several times, sounded pretty good too, Q-jet sucking all the air available. When the dust settled, it had made peak power at the wheels of just under 140 HP!!. The owner was ready to cry. This was the reality of cars from that era.

[77 TRASHCAN]

Originally Posted by mgarblik

I wide ranging thread with all kinds of emission and good info about the Pontiac 428, 455, and even the Oldsmobile V-8's! It's an interesting look back. To the OP's original question, "why was the 428 discontinued?" I think the advertising and marketing group had more to do with it than engineering. Pontiac simply wasn't going to be left in the "displacement dust", by the 4 other GM divisions. Cadillac was moving toward 500 CU IN. Olds, Buick and Chevy were 454,455. Pontiac stuck at 428 would have been a sales disaster in that time period, no matter the HP. Pontiac had to be equal or larger. The move from 428 to 455 was an easy deal. No clearance issues in the bottom plenty of cylinder wall, same engine package, easy change. BTW, I have owned numerous 421's and ran them as 428's, as well as a few stock 428's. I can't fully explain why, but that was a real sweet spot in cylinder head flow, rod ratio, bore and stroke. They just ran beautifully and had superior power to a 455 from 4000-5500 RPM stock. A very nice engine package. 455 was essentially a smog package big engine with the exception of the high compression 70 455. The 71-72 HO had the better top end to help utilize the displacement and of course we know how that was improved more in the SD-455, allowing higher RPM's with a better bottom end and even more improvements up top. Other than that, a stock 455 is just a big smog engine. At my school, years ago a student brought a really nice Grand Ville in for a run on the chassis dyno. Stock engine, about 80K miles, 455. Ran it several times, sounded pretty good too, Q-jet sucking all the air available. When the dust settled, it had made peak power at the wheels of just under 140 HP!!. The owner was ready to cry. This was the reality of cars from that era.

The SS GT racers have latched on to the 428, with the legal Edelbrock heads that can be used, for what is essentially some of the reasoning stated above.
Robert Cruzen's Cavalier is super competitive. He never did say who he had build his engine, but is ws a shop that primarily built Chevrolet stuff, even they were impressed with his Pontiac mill...

[PontiacJim1959]

Originally Posted by steve25

Low power and crappy mileage came about due to the drop in compression that just so happened to come along with the open chamber, but it does not have to be that way!

And you have something way the heck wrong, as closed chamber heads require more timing then open chamber heads to make max power, its just that the open chamber heads can be more prone to ping and knock if compression ratios your looking at are are kept equal.

Anyway have we gone off the rails in regards to the OPs original question here, sorry about that!

We haven't really gone off topic as in a sense emissions requirements did shape engine sizes with the lowering of compressions.

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.

Open chambers were introduced to Pontiac in 1968. Are you saying that the 360HP 400CI or 370HP RAIV engine with open chamber heads had low power?

Crappy mileage? What do you think these cars got for mileage back in the day? The GTO's, depending on engine option, transmission, and gear ratio got crappy mileage from the start. 10-13 MPG's for many if you ordered one that has some snap to it. Now if you ordered one with an automatic and 3.08 gear, you got a little more. Keep in mind these were 55MPH speed limits back then. Many of these cars would have suffered going 70 MPH.

Open chambers can make the same compression as closed chamber heads. It is all about the chamber cc's, and Pontiac did use different thickness head gaskets as well. So you could get an 8.0 compression or 10.75.

The closed chambers used less initial timing, total timing was in much earlier, and seemed to handle more total timing.The open chambers, as compression dropped through the years, used more initial timing, and total timing comes in much later up the RPM scale with factory set-up cars. The Royal Bobcat treatment on open chamber heads dialed in the factory timing much tighter, more closer to a closed chamber head but the best Sunoco 260 had to be used to prevent detonation. Too low of an octane for any head/high compression combination, closed or open chamber, will get it to ping/knock. Keep in mind things like cam choice (ie cylinder pressure), engine heat, grade of gas, distributor advance curve, type of trans, gear ratio, weight of the car, etc. also has bearing on timing. So singling out the open chamber as using more/less timing as compared to a closed chamber head has to also takes in other factors than just the chamber type. I don't think any of the aftermarket aluminum heads are of closed chamber design?

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. Most here probably understand the "squish/quench" area found between the top of the piston and bottom of the head. Having a tight squish/quench is what many try to shoot for, right? However, those good old test engineers trying to reduce hydrocarbons studied the quench areas of a cylinder. It was found that when the air/fuel mixture was ignited by the spark plug, it traveled out towards the cylinder walls, and the cooler cylinder wall (ie cooler than the ignited gas mixture) acted as a quench area and snuffed out the flame and left a minute quantity of the unburned mixture to be sent out into the atmosphere through the exhaust.

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.

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.

Head gaskets were also designed to follow the contours of the cylinder. Pontiac used scallops in the top of the block for valve unshrouding. It would have been easy to just cut a round diameter gasket the size of the bore as measured by the outside edge of the scallop and be done with it. But the round gasket would have been seen as oversized and left a small gap between the mating surfaces of the head and cylinder bore along the top of the cylinder bore. This small area not covered by the head gasket was considered a quench area. So head gaskets were designed to follow the general size of the cylinder's bore, and then incorporate the needed shape to follow the contour of the valve scallop - and why head gaskets can be specific to its engine.

So open chambers do promote more heat due to design of the open chamber and minimize the cooling effect of a good quench area. 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.

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.

[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.

[242177P]

Originally Posted by PontiacJim1959

Open chambers were introduced to Pontiac in 1968.

Actually, the 061 head in 1967 has that distinction. But yeah, in 1968 the open chambers were used across the board.