Cadillac News: 2020 Cadillac CT4-V and CT5-V Aren't As Powerful As Their Predecessors

[Drew Dowdell]

@dwightlooi - I had an off-topic question for you.  

Why couldn't a pushrod engine or even a SOHC engine still have 4 valves per cylinder?

[dwightlooi]

1 hour ago, Drew Dowdell said:

@dwightlooi - I had an off-topic question for you.  

Why couldn't a pushrod engine or even a SOHC engine still have 4 valves per cylinder?

Simple Answer: Yes it CAN. The Duramax 6.6 Turbodiesel in the current generation and previous generations are a Pushrod 32-valve V8.

Complex Answer: The push-rods go from a  cam in the valley of the vee to the inner side of the cylinder heads. These obstruct where the intake passages in an OHC engine would normally be. The intake passages hence goes around the pushrods offset to the right or left of the cylinders in the space not obstructed by the rods (and their channels). There is barely enough space to serve ONE intake valve per cylinder. If you put four pushrods in an engine you'll obstruct all the approaches to the cylinder from the intake side! Doesn't do you any good to have increased valve areas if you squeeze out the intake ports and runners does it?

So how does the diesel 6.6 do it? Well, it still has two rods per cylinder. Each operate two tandem valves using a bridge between the valves. The inner and out valves on the left are both intake and the inner and outer valves on the right are both exhaust. This is possible because the roof of the head is flat and all the valve stems are parallel, allow one rocker to open both valves via the bridge. It is meaningful because it is a DIESEL engine and it doesn't rev past the mid-3000 rpms anyway so one single intake and one sn=ingle exhaust passage is enough to not be the airflow bottleneck. Had it been a pentroof combustion chamber like that found on gasoline engines, the bridge design would be impossible and having a single passage serve two valves would have defeated the intent of using more valves to increase the airflow potential at high rpms.

[Drew Dowdell]

1 hour ago, dwightlooi said:

Simple Answer: Yes it CAN. The Duramax 6.6 Turbodiesel in the current generation and previous generations are a Pushrod 32-valve V8.

Complex Answer: The push-rods go from a  cam in the valley of the vee to the inner side of the cylinder heads. These obstruct where the intake passages in an OHC engine would normally be. The intake passages hence goes around the pushrods offset to the right or left of the cylinders in the space not obstructed by the rods (and their channels). There is barely enough space to serve ONE intake valve per cylinder. If you put four pushrods in an engine you'll obstruct all the approaches to the cylinder from the intake side! Doesn't do you any good to have increased valve areas if you squeeze out the intake ports and runners does it?

So how does the diesel 6.6 do it? Well, it still has two rods per cylinder. Each operate two tandem valves using a bridge between the valves. The inner and out valves on the left are both intake and the inner and outer valves on the right are both exhaust. This is possible because the roof of the head is flat and all the valve stems are parallel, allow one rocker to open both valves via the bridge. It is meaningful because it is a DIESEL engine and it doesn't rev past the mid-3000 rpms anyway so one single intake and one sn=ingle exhaust passage is enough to not be the airflow bottleneck. Had it been a pentroof combustion chamber like that found on gasoline engines, the bridge design would be impossible and having a single passage serve two valves would have defeated the intent of using more valves to increase the airflow potential at high rpms.

Thanks for the explanation.  In my head I was thinking about the design you described for the diesel, but I forgot that valves are at an angle in gasoline engines. 

[balthazar]

Another way around the pushrod/runner territorial fight is canted rockers. Old Chrysler hemis have valves perpendicular to the crank rather than parallel- they used dual rocker shafts & canted rockers. Still only 2-valve of course.

[dwightlooi]

20 minutes ago, balthazar said:

Another way around the pushrod/runner territorial fight is canted rockers. Old Chrysler hemis have valves perpendicular to the crank rather than parallel- they used dual rocker shafts & canted rockers. Still only 2-valve of course.

Actually, no. Not really. You cannot have four push rods per cylinder and still have room for the intake runner to reach both intake valves. The Chrysler Hemi design simply allow for the intake and exhaust valves to be opposed instead of side-by-side. The purpose of that being the theory that two opposed valves and a hemispherical combustion chamber is more knock resistant and hence capable of higher compression and output than the heart shaped combustion chamber with side-by-side valves. However, with the current generation of Hemi vs non-Hemi V8 engines, this theory does not seem to yield real world performance superiority.

[Drew Dowdell]

9 minutes ago, dwightlooi said:

Actually, no. Not really. You cannot have four push rods per cylinder and still have room for the intake runner to reach both intake valves. The Chrysler Hemi design simply allow for the intake and exhaust valves to be opposed instead of side-by-side. The purpose of that being the theory that two opposed valves and a hemispherical combustion chamber is more knock resistant and hence capable of higher compression and output than the heart shaped combustion chamber with side-by-side valves. However, with the current generation of Hemi vs non-Hemi V8 engines, this theory does not seem to yield real world performance superiority.

I thought the current generation of Hemis were Hemi-in-name-only and didn't actually have a hemispherical combustion chamber.

They also have 2 spark plugs per cylinder, so putting more valves in there would be challenging. 

[balthazar]

Valve placement in a true hemi head (parallel or perpendicular) may not be that crucial for airflow/combustion, but the perpendicular placement was to accommodate the splayed valve stems, making the old hemi heads incredibly wide. They would not have fit set parallel to the crank. 

A similar bridge arrangement could also work with 2 pushrods and 4 valves, tho the result may become physically cumbersome. It would require something not already engineered (to my knowledge).

[dwightlooi]

4 hours ago, balthazar said:

Valve placement in a true hemi head (parallel or perpendicular) may not be that crucial for airflow/combustion, but the perpendicular placement was to accommodate the splayed valve stems, making the old hemi heads incredibly wide. They would not have fit set parallel to the crank. 

A similar bridge arrangement could also work with 2 pushrods and 4 valves, tho the result may become physically cumbersome. It would require something not already engineered (to my knowledge).

Splayed or not, one rocket actuating one valve is no problem regardless of how the valves are oriented. But if you use a bridge to open both valves, they have to be parallel and the rod/rocket has to be perpendicular to the bridge. Otherwise as the rocker depresses it'll skew to one side of the bridge and open one valve more than the other -- like stepping on one side of a see-saw.

The Hemi head is generally taken to mean ANY two valve head design where the intake valve is on one side and the exhaust valve is on the other side. This is irrespective of whether the roof is actually a dome or more of an oval. All the modern ones are oval allowing the flat decks on the sides to form a squish deck with the top of the piston to allow higher compression ratios.

Twin sparks are very common with two valve and three valve designs -- OHC or Pushrod. If you want a REALLY HISTORIC example, go Google Junkers-Jumo 213A. That is a Supercharged 35 liter SOHC 36v inverted V12 cylinder engine used in the late WWII Focke-Wulf 190-D9 fighter -- it features three valves per cylinder, twin sparks and Bosch Direct Gasoline injection to generate 1,750hp @ 3,250 rpm (2,100 hp with 50% Methanol-Water injection in emergencies).

Edited September 10, 2019 by dwightlooi

[dwightlooi]

(1) Valve and spark arrangement in modern Hemi pushrod cylinder head.

(2) Flat Deck 6.6 Duramax turbo-diesel pushrod cylinder head.

(3) Rocker and bridge arrangement on 6.6 Duramax turbo-diesel engine

(4) Junkers-Jumo 213A engine showing off -- Direct Injection, Twin Spark, 2-intake / 1-exhaust valves per cylinder and a variable drive ratio supercharger (hydraulically coupled)

[balthazar]

De-couple :
https://newatlas.com/camcon-digital-iva-valve-system/55827/

[riviera74]

12 hours ago, balthazar said:

De-couple :
https://newatlas.com/camcon-digital-iva-valve-system/55827/

A large necessary step towards a cleaner future with ICE.  It will take a few years to make this viable from a mass production standpoint.

[G. David Felt]

16 hours ago, balthazar said:

De-couple :
https://newatlas.com/camcon-digital-iva-valve-system/55827/

Very cool tech, be interested to see which engines they incorporate this into.

[G. David Felt]

I know people have not been impressed with the New V series so far, but over at the Cadillac Society, someone took the latest CT5-V and made a station wagon and I have to say they have some photoshop skills. I like what i see.

http://cadillacsociety.com/2019/09/13/cadillac-ct5-v-wagon-rendering-has-us-drooling/

[ccap41]

It's hilarious that their renderings only emphasized the blank window piece to look even worse as a one square foot blank now. 

[Drew Dowdell]

[ccap41]

There was ONE thing people REALLY harped on and they only made it worse... 

[G. David Felt]

5 hours ago, ccap41 said:

There was ONE thing people REALLY harped on and they only made it worse... 

True and your right it is worse now, but ignoring that ugly mark, I do still like the over all station wagon vibe.

[Potluck]

That wagon is sharp with the escalade tail lights, but we all know that Cadillac styling has moved on from the purely vertical blades these days. 

[dwightlooi]

5 hours ago, Potluck said:

That wagon is sharp with the escalade tail lights, but we all know that Cadillac styling has moved on from the purely vertical blades these days. 

Cadillac is lost on styling. Their new stuff looks "Mazda Inspired". But, they have also shown a lot of inconsistencies and indecision, hence who knows if the next vehicle will go back to the previous gen cues, stay the course or go in a totally new direction.

[Drew Dowdell]

51 minutes ago, dwightlooi said:

Cadillac is lost on styling. Their new stuff looks "Mazda Inspired". But, they have also shown a lot of inconsistencies and indecision, hence who knows if the next vehicle will go back to the previous gen cues, stay the course or go in a totally new direction.

They haven't kept a President very long lately either.