What if GM goes back to 2-valve per cylinder?

[dwightlooi]

What if GM goes back to 2-valve per cylinder -- either with a Pushrod design in a V6 or SOHC design in I4s and/or I3s? With today's technological content, the engines will perform more or less like this:-

• 1.5L SOHC 6-valve Inline-3 w/ VVT & Direct Injection -- 111 bhp @ 6000 rpm / 113 lb-ft @ 4600 rpm

• 2.0L SOHC 8-valve Inline-4 w/ VVT, & Direct Injection -- 149 bhp @ 6000 rpm / 151 lb-ft @ 4600 rpm

• 1.5L SOHC 6-valve Inline-3 w/ VVT DI and turbocharging -- 170 bhp @ 5200 rpm / 180 lb-ft @ 2600~5200 rpm

• 2.5L SOHC 8-Valve Inline-4 w/ VVT & Direct Injection -- 183 bhp @ 6000 rpm / 185 lb-ft @ 4600 rpm

• 2.0L SOHC 8-valve Inline-4 w/ VVT, DI and turbocharging -- 237 bhp @ 5200 rpm / 240 lb-ft @ 2200~5200 rpm

• 3.6L Pushrod OHV 12-valve V6 w/ VVT & Direct Injection -- 266 bhp @ 6000 rpm / 269 lb-ft @ 4600 rpm

This is based on the same power density as the LT1 V8 (74.65 hp/L) and a fuel expectation that is 91 Octane Recommended / Not Required. We are keeping displacement as is compared to the DOHC mills. On turbocharged engines it assumes the same boost level and turbine/compressor efficiencies as on the Malibu's 2.0T engine (for the 1.5T I drop it a little to account for the fact that small turbos tend to be slightly less efficient. We are also assuming that we are not using a cam-in-cam dual VVT setup which is technically a feasible option.

It is actually... not that bad even compared to today's typical DOHC engines. Definitely on the lower end of the spectrum, but not embarrassingly bad. Fuel Economy numbers though can be expected to be superior to GM's or the competition's existing DOHC engines.

This doesn't surprise me, and it shouldn't surprise anyone. Think about it... most passenger car DOHC fours and sixes reach their peak torque in the mid 4000s and their peak power rating between 5800 and 6400 rpm. That tells us two refutable facts -- they reach their peak volumetric efficiency around mid-4000 rpm range, and they reach a point where volumetric efficiency falls off faster than RPM is rising at around 6000 rpm.

Whatever theoretical advantages of a DOHC 4-valve design might bestow, most modern passenger car engines are certainly not capitalizing on it since a 2-valve setup can already achieve the same approximate performance. The reasons may vary from not wanting to compromise the idle or low rpm torque, wanting to keep intake velocity high (hence narrow intake passages) for emissions reasons and/or simply not wanting to warranty a 8000 rpm engine or transmission. This then begs the question... why put up with the increased complexity, additional friction and added cost of a DOHC design when you are not going to build a Honda F20C (S2000; 2.0L 240 bhp Inline-4) or BMW S65 (E90 M3; 4.0L 414bhp V8)?

[G. David Felt]

Interesting idea Dwight, something to think on.

[riviera74]

Perception seems to be the real issue here. Why go DOHC at all if the benefits do not consistently exceed the cost? Some people still believe that pushrods are obsolete and that DOHC is inherently better. 1970 called and they want their misconceptions back. I have wondered why GM (and Ford too) have essentially ditched pushrods for all non-BOF vehicles (Corvette, Camaro, Mustang aside). I am not sure if there is an inherent advantage to DOHC anymore. I do believe that Honda had SOHC engines as recently as the 90s, but I am not sure if they still use an SOHC rather than DOHC design.

[Robert Hall]

Seems like it would be a retrograde move...can't imagine any legitimate business reason to do so.

[Drew Dowdell]

to what advantage?

[dwightlooi]

Perception seems to be the real issue here. Why go DOHC at all if the benefits do not consistently exceed the cost? Some people still believe that pushrods are obsolete and that DOHC is inherently better. 1970 called and they want their misconceptions back. I have wondered why GM (and Ford too) have essentially ditched pushrods for all non-BOF vehicles (Corvette, Camaro, Mustang aside). I am not sure if there is an inherent advantage to DOHC anymore. I do believe that Honda had SOHC engines as recently as the 90s, but I am not sure if they still use an SOHC rather than DOHC design.

Honda uses SOHC designs on the Civic 1.8 and the TL / Accord 3.5 V6es. They use a DOHC design in the 2.0L and 2.4L Inline-4s you'll find in the Civic SI, CRV, ILX and 4-cylinder Accords. They also use a SOHC 2-valve per cylinder / twin spark 1.3L engine in the 2nd Generation (2009~present) Insight Hybrid and the Civic Hybrid (Designation: LDA). The selection of a SOHC 2-valve design in the Insight gives it slightly better fuel economy than a DOHC engine would, hence the design choice.

to what advantage?

Somewhat lower cost, slightly better fuel economy and -- only in the case of pushrod Vee engines -- notably more compact packaging and lower weight. But, all of these come at the price of slightly lower performance. Let's put it this way, a Pushrod V6 of 3.6 liters (266 hp) will essentially match the Toyota Camry's 3.5L V6 (268hp), but it'll be cheaper to build and give slightly better fuel economy. The use of a DOHC design in such powerplants are technically difficult to justify. In GM's case for instance, they took the middle road. GM's LFX V6 revs to 7000 rpm and peaks at 6800 rpm with ~90hp/L. Not quite full DOHC potential, but somewhat better than a SOHC or Pushrod 3.6 could achieve. Their 2.5L I4 peaks at 6300 rpm with 82 hp/L and is only slightly above the potential of a SOHC 2.5 design.

The moral of the story really is this... If you use a DOHC design better wind it to 7500 or 8000 rpm and get about 100+ hp/L out of it like the various engines of this genre (Eg. Honda F20C, B16A, B18C, H22A, Toyota 2ZZ-GE, BMW S65, etc.). It will do that for you. But, if you are going to temper that screamer with narrower intake passages, lower valve lift and minimal overlap -- like most "mainstream" DOHC engines of today does in the interest of low end torque, idle civility and emissions (because fat intakes, valve overlaps and low intake velocity is really bad for hydrocarbon emissions) -- you are better off using fewer cams and lesser valves. Doing so will lower cost and reduce parasitic friction, giving you better fuel economy and still meet the airflow requirements of an engine with a 6000 rpm power peak as well as a DOHC layout will.

Edited August 17, 2013 by dwightlooi

[regfootball]

the reason DOHC motors became popular (or at least one of them) was that they didn't display the trait of running out of breath.

so if you can limit the valve area and reduce the rpms but still get it to feel like it has endless revvability, more power to you.

[Drew Dowdell]

A fairly false impression Reg. Much of that was caused by the transmission they were mated to which, while smooth, didn't match the performance characteristics of the higher power pushrod V6es.

The first gen Equinox has plenty wrong with it, but the pushrod V6 plus 5peed auto was plenty class competative.

The reason the 3800 would feel breathless is because it was a low end torque engine and the transmission didn't have enough gears to keep the rpms low.

[dwightlooi]

the reason DOHC motors became popular (or at least one of them) was that they didn't display the trait of running out of breath.

so if you can limit the valve area and reduce the rpms but still get it to feel like it has endless revvability, more power to you.

When you trim valve lift, passage sizes and overlap periods back so the DOHC motor reaches it's power peak at around 6000 rpm, it is no longer able to rev without running out of breath any better than a SOHC or Pushrod motor with a similar power peak. Volumetric efficiency peaks at the torque peak on all engines and volumetric efficiency falls off faster than rpm is rising at around the power peak. This is true of every internal combustion engine regardless of the number of valves or cams. It is even true for a wankel. Hence, 24-valve V6es like the Altima's 3.5 (270 bhp @ / 251 lb-ft @ 4400 rpm), the Toyota Camry's 3.5 (268 bhp @ 6200 rpm / 248 lb-ft @ 4700 rpm) or the Honda Accord's 3.5 (268 hp @ 6200 rpm / 248 lb-ft @ 5000 rpm), no longer have any statistically breathing or volumetric efficiency advantage over a Pushrod or SOHC 2-valve design with a 266bhp 3.6L V6.

Now, a little history how this came about... back in 80s when DOHC 4-vavle designs first entered the mainstream, the engines typically revved close to 7000 rpms and made about 80 hp/L. An example of this is the Toyota 4A-GE used in the high performance versions of the Corolla and Levin, as well as the MR2. If this doesn't sound all that impressive, remember that this is an age when many cars are still carburetted, fuel injection is primitive, compression was low, lead just got removed from gasoline and finite element analysis of combustion is something done on supercomputers for the Steath Fighter. In anycase, even at 80hp/L the engines won't idle right, exhaust would have been sooty and the engines would stumble under 1500 rpm if you floor the gas pedal if not for devices that blocked off half the intake to increase swirl (see Toyota T-VIS). Even so low end torque and civility is is marginal. Some time in the very early 90s, Honda popularized cam switching in form of VTEC. Basically, this allows the engine to run two sets of cam lobes, one for high rpm power and one for proper low rpm running. Specific output went to 100 bhp /L and low rpm character was no worse than the 80s DOHC engines -- which is to say passable but not all that great.

The late 90s saw an interesting trend... Everybody went DOHC because the past decade and an half of marketing had backed the manufacturers into a corner they created. Marketing and alphabet soup now drove specifications as much as engineering realities -- perhaps more so. Yet, specific output fell. A 1998 Maxima had a 190 hp DOHC 24-valve 3.0 V6. The Accord had a 200 hp 3.0 V6. That's about 67bhp/L or less. Why? Because the automakers realized that the typical family car driver has no interest in winding the engine to 7500 rpm, while they appreciate an engine that pulls from a stoplight with smoothness and authority. By narrowing the intake tracts, reducing valve lift and getting rid of much of the valve overlap, they could defang the DOHC mills such that they performed just as well as the SOHC or Pushrod 2-valve designs in the regimes which customers and emission regulators cared about the most. In short, the priorities of the engine's performance characteristics no longer call for the airflow potential only a DOHC 4-valve design could have provided. But, because they and their customers are so used to DOHC setups by then -- and they didn't want to appear retrograde -- they stuck with it and made 4-valve engines that performance like 2-valve engines instead. Yes, it cost more, it took longer to build and its more of a hassle to service. No, there were no technical benefits, and in fact fuel economy is a little worse. But it kept the marketing department happy and the engineers were too lazy to argue or were overruled by the MBAs who ran the company that didn't know any more than what the brochures said. In the meanwhile, the convictions of the DOHC proponents were reinforced by lackluster market performance of those US brands which stuck to pushrods and SOHC modular engines. That this has more to do with complacency, bean counting and a lack of technological content in their 2-valve engines rather than the 2-valve layout itself is lost to the decision makers and the consumers alike.

At some point in the 2000s all the US makers relented to the tide. And this brought us to today. All the mainstream consumer engines are essentially all DOHC 4-valves -- not because they have to be, but because things when done long enough take on a life of their own. There are some exceptions of course... Honda in particular fancies SOHC 4-valves as a compromise and dared to use a 2-valve SOHC design when it comes to delivering the best fuel economy they can in their Insight and Civic Hybrids. GM soldiers on with the Pushrod LT1 V8, but only in the high performance Corvette where the buyers either know better or subscribe to a different tradition, and the engineers know that it is the smallest, lightest and most fuel efficient path to 460 hp.

Edited August 18, 2013 by dwightlooi

[regfootball]

i tend to think more that if all the manufacturers thought it was a dandy idea to go back to low revving 2 valve motors they would.

[balthazar]

Reg, did you read the post above yours?

[loki]

i'd rock that 3.6L as a daily driver if offered.

be a great smaller truck engine(if the 4.3L wasn't used)

remember this?

2009–2012 3.7 L (223 cu in) LLR I5 242 hp (180 kW) @ 5600 RPM

242 lb·ft (328 N·m) @ 4600 RPM^[

slap a 6-8 speed behind it.

Dwight, because of weight/size i'm guessing a possible extra mpg above the LFX could happen in something like the impala?

do you think your 2.0L would replace the 1.8L in the cruze and such?

[dwightlooi]

i'd rock that 3.6L as a daily driver if offered.

be a great smaller truck engine(if the 4.3L wasn't used)

remember this?

2009–2012

3.7 L (223 cu in) LLR I5

242 hp (180 kW) @ 5600 RPM

242 lb·ft (328 N·m) @ 4600 RPM^[

slap a 6-8 speed behind it.

Dwight, because of weight/size i'm guessing a possible extra mpg above the LFX could happen in something like the impala?

do you think your 2.0L would replace the 1.8L in the cruze and such?

The LFX is a 84hp/L engine. It peaks at 6500 rpm. You cannot quite match that with a 3.6L pushrod or SOHC 2-valve engine. To get about 300hp you'll need about 4.3 liters from a pushrod 2-valve engine optimized for 87 octane. The new Ecotec3 4.3L Pushrod engine will more or less match a 3.6 DOHC if it was tuned for a car. This engine gets 18 (City) /24 (Hwy) mpg pulling the 5000 lbs brick that is the full size Sierra 1500 pickup. I will think that it'll at least get better than the 18 mpg (city) rating the Impala gets with the 3.6L LFX. But, if you simply drop a modern 3.6 Pushrod in the Impala, it'll be down about 34 hp although fuel economy will go up. If you are wondering why the GM 3.6 is slightly worse than even the competition's 3.5 and 3.6L V6es in fuel economy... well there is usually an inverse relationship between specific output and fuel economy even amongst DOHC 4-valve engines. The fat intakes, high lift and generous overlap periods required to get you high specific output through good high rpm breathing also equals inferior charge mixing and homogeneity at low rpms. It means that more fuel needs to be burned to make the same power at cruise and consequently inferior fuel economy. GM is actually near the top of the classin the specific output of their DOHC V6es, they are also near the bottom of the class in EPA fuel economy with these engines (Eg. LF1, LFX).

What I was getting at is that engines like what you find in today's Accord and Camry really don't need to be DOHC 4-valve. They would have delivered the same performance with better economy and lower cost if they used a 2-valve setup. Conversely of course, you can use a 4-valve setup to build a 360hp 3.6L V6, but fuel economy will be inferior. It is also debatable whether a larger displacement 2-valve engine would have served up 360 hp with better economy and lower costs.

Edited August 19, 2013 by dwightlooi

[dwightlooi]

But... if I am to write the specifications for the most economical (gasoline) engine for the Chevy Cruze, it’ll probably look something like this…

Configuration: 70% Pseudo Atkinson Cycle Inline-4

Valvetrain: SOHC 8-valve w/ cam-in-cam independent intake/exhaust VVT

Bore x Stroke: 94 mm x 97 mm (Same bore and piston dimensions as 3.6 LFX V6)

Displacement: 2.7L (2693 cc)

Fuel Injection: Direct Gasoline Injection

Ignition: Coil-on-spark direct ignition with dual iridium plugs per cylinder

Power: 140 bhp @ 6000 rpm

Torque: 142 lb-ft @ 4600 rpm

Redline: 6200 rpm

This is the absolutely lowest specific fuel consumption 140hp 4-cylinder engine you can build (in theory at least) that doesn’t involve a hybrid drivetrain or compression ignition. For marketing reasons, you may want to pull a “Mazda” and call it a 1.9L engine by calculating displacement based on the fraction of the actual piston stroke AFTER the intake valves have actually closed*

*Mazda pulled that fast one on the 1998~2001 Millenia / Euros. The so called 2.3L Miller Cycle V6 is actually a 3.0L V6 but because the intake valves remained open for the first ~20% of the compression stroke, Mazda states the displacement as 2.3L. With a Lysholm screw supercharger recovering some of the lost power it made 210hp. This is exactly the same as the 3.0L Mazda V6 of the period, but it was about 13% more fuel efficient and torque was slightly higher. Apparently US, Japanese and Europeran regulators either didn't know any better or didn't care.

Edited August 19, 2013 by dwightlooi

[G. David Felt]

Interesting Dwight on the Mazda motor.

[loki]

actually... perhaps 'derate' the 3.6 torque peak to 4K and the more normal 5.2K and.. would you lose more than 5% ft/lbs or hp? if this is easy to calculate, to more directly compare to the 3.1 3.4 and 3.8 generation

[dwightlooi]

actually... perhaps 'derate' the 3.6 torque peak to 4K and the more normal 5.2K and.. would you lose more than 5% ft/lbs or hp? if this is easy to calculate, to more directly compare to the 3.1 3.4 and 3.8 generation

Actually, if you pull back the torque peak from 5200 rpm to 4000 rpm while keeping static compression constant you'll probably GAIN a few ft-lbs of torque. Let me explain...

What are normally done to help an engine breath better at high rpms include increasing the valve lift, increasing the valve size, increasing the intake passage sizes and, perhaps most prominently, increasing the duration the intake and exhaust valves stay open. Unless you are making a tractor motor the duration of the exhaust valve opening will be longer than the exhaust stroke itself and that of the intake opening longer than the intake stroke. There will also be some period during which both are open towards the end of the exhaust stroke and the beginning of the intake stroke. For an engine to breath well at high rpms, it is beneficial that the exhaust valve open before the piston reaches the bottom of the travel -- you sacrifice some energy recovery for a little extra time for the exhaust to leave the cylinders. You also open the intake valves early before the exhaust stroke is complete and do not close the exhaust valve for some time while the piston is going back up during the compression stroke. You do this because at high rpms air is being sucked in at a high rate and they "stack" momentarily behind the intake valves when they are closed. When they open the air is actually coming in at a slight positive pressure. You will have the ability to expel exhaust gases before they fall to their lowest pressure at the top of the piston stroke and you will be able to continue to aspirate air into the cylinders for a short time after the piston has already started to come up. In short, long duration and overlap is key to proper engine breathing at high rpms. These tuning techniques is how naturally aspirated engines can actually achieve OVER 100% volumetric efficiency.

Now... what will those same cam lobes that open the valves in the aforementioned manner do at low rpms? Well, first it'll waste some propulsive force tom the pressurized combustion products by wasting the last 5~15% of the power stroke opening the exhaust early. The overlap is horrendous for emissions at low rpms and reduces engine output. This is so because at low rpm, air is not flowing all that fast and when the intake valves open early the lack of stacking pressure behind the valves means that exhaust simply regurgitates into the intake -- making them dirty and make the engine suck back some of its own exhaust rather than fresh air at the beginning of the intake stroke. Also keeping the intake valve open past bottom dead center of the intake stroke turn the engine into a pseudo Atkinson cycle unit -- basically you push some of the intake charge back out the intake causing you to lose "effective" compression ratio and "effectve" displacement -- except that unlike an engine designed to function in this cycle you do not have a very high static compression ratio to compensate for the compression loss. All of these means that you LOSE torque at low rpm and hence lose horsepower at low rpms. In fact sometimes it's so bad the engine stumbles because there is so much EGR that the mixture fails to ignite properly. Emissions also goes to sh!t because now you have less complete combustion before you pop the valve and sending the gases into the exhaust.

Variable Valve Timing (VVT), especially independent exhaust and intake timing control, allow you to dial out much of the overlap. But when both cams are of a longer duration that their respective piston strokes whatever dialing you do is a compromise. Open the exhaust earlier and you increase hydrocarbon emissions. Close the intake late and you lose compression and reduce effective displacement. You usually end up doing both but more of retarding the intake than advancing the exhaust -- because the latter is means you'll fail SMOG. You are not so much restoring power lost -- you really can't with those cam durations -- but rather adjusting the cams such that the engine doesn't stumble and doesn't fail emissions. If you ever wonder why high revving engines (which doesn't switch cam profiles) like the M3's S65 are somewhat soft under 4000 rpms, it's because they have long duration cams and the VVT system is retarding that intake cam way way back to reduce overlap, causing the engine to lose compression and displacement until intake velocity builds enough with rpm for it to start dialing the intake cam forward! Another example is the Nissan VQ35 in the 350Z. The engine debuts at 287 hp @ 6,200 rpm / 274 lb-ft @ 4,800 rpm. It was later updated to improve specific output to 308 hp @ 6800 rpm, but the extended cam durations caused torque to fall to 268 lb-ft @ 4,800 rpm.

So much for all that... basically, the LFX is a relatively high revving tune with 321 hp @ 6800 rpm and 275 lb-ft @ 4800 rpm in the ATS. The transverse engines are slightly worse off mainly due to the asymmetrical exhaust contortions. If you dial it back to have a torque peak at 4000 rpm by using shorter duration cams, you will gain a few lb-ft of torque. If you do it by doing that and making physical changes to the head and intake to narrow the ports and runners you'll gain even more. How much? At least 4~5 with just the former, perhaps as much as 10 lb-ft by doing both. So a properly "defanged" 3.6 will probably be about 280 lb-ft @ 6000 rpm 280 lb-ft @ 4000 rpm. Which is about the same result as you'll get with the Camry or Accord engines if you simply add Direct Injection and crank up the compression ratio by a point or so.

Edited August 20, 2013 by dwightlooi

[loki]

that's awesome Dwight. guessing "about 280 lb-ft @ 6000 rpm 280 lb-ft @ 4000 rpm", 280@6K is HP though.

may not be high reving horsepower, but if it's more civil idling and low rpm torque (like the 3.8 is famous/infamous for)...using good transmissions would pay dividends for this engine, more so than the LFX.

seems like why some others on here loved the high-value engines, just hated they didn't get life with a 6speed auto behind it to show they could.... live with the competition just fine power and MPG wise...and while being smaller/lighter than the competition too.

i've not driven the 3.5L or 3.9L more than once, and that's been awhile.

your technical insight is always an interest to me.

[Drew Dowdell]

actually... perhaps 'derate' the 3.6 torque peak to 4K and the more normal 5.2K and.. would you lose more than 5% ft/lbs or hp? if this is easy to calculate, to more directly compare to the 3.1 3.4 and 3.8 generation

So a properly "defanged" 3.6 will probably be about 280 lb-ft @ 6000 rpm 280 lb-ft @ 4000 rpm. Which is about the same result as you'll get with the Camry or Accord engines if you simply add Direct Injection and crank up the compression ratio by a point or so.

That's actually already pretty close to the 3.6 in the Traverse. 281 horsepower @ 6000 RPM / 266 lb-ft @ 3400 rpm

[dwightlooi]

Well, back to the point of this thread...

The point isn't 2-valve or 4-valve being "better". They have their own characteristics and better is subjective. The point is just that... with a 260~270hp 3.6L engine -- which seems perfectly acceptable to Toyota and Honda -- there is no advantage to using a DOHC 4-valve setup. You'll just end up with inferior fuel economy and with not much else to show for it. Whatever the theoretical potential of a DOHC 4-valve design, such engines are not taking advantage of them. In fact, they are deliberately NOT taking advantage of them.

Edited August 21, 2013 by dwightlooi