Yeah, and if we only had the auto shutoff feature when stopped like my Sierra, think of the gas we could save, hahaha.

 

Exactly my thought re hp power being equal, has to be!. It takes a fixed amount of hp to overcome drag and tire rolling resistance beit 4 or 8 cylinders. That link mentioned less pumping loss with both valves closed. That is no doubt a factor BUT you have to quantify each of the benefits, can't simple say "less pumping loss." How much?

I'll give one I observed an IMO is a factor. Can't quantify for hp but can an issue with cylinder wall heat extraction from combustion!

First an ICE is very inefficient. Much of the energy in gasoline goes into wasted heat! From the pic below, 29% of the energy in gasoline goes into the coolant. That comes from the ~2500 F combustion gases conducting into the cylinder walls and combustion chamber.

Second, I lowered the coolant temp when stopped idling in the ~525 hp 8.2 Liter BB in my '34 Ford with a thicker aluminum rad BUT same height and width as Henry used for his <100 hp 3.6 Liter flathead! Note cooling temps were fine when moving with the aluminum multitude Griffin rad, 6 blade high pitch engine fan and car moving so air was going thru the rad. But even with the two electric fans turned on in front of the AC Condenser helping at very slow speeds or stopped idling coolant was increasing to 220 F on a hot day.

An old Peterson pub re ignition provided the solution! The 502 crate motor used a ported vacuum advance (as most modern cars) versus manifold vacuum. It discussed when that change occurred, it was to reduce emissions. Prior cars used manifold vacuum to activate the distributor advance. Quoting from my website below (that also shows the result.) "Using "Manifold Vacuum" the spark at idle will occur sooner and "less cylinder wall is exposed on the power stroke to the heat (of combustion), thereby lowering coolant temperature." Interpreting their statement, with more advance at idle, the combustion starts on the upper portion of the cylinder on the compression stroke. Flame front travels very slowly at the low cylinder pressure that occur when idling. Therefore the cylinder head and upper cylinder walls will absorb most of the combustion heat and the lower portion of the cylinder on the power stroke will be cooler. This transfers less total heat to the cooling system!

BOTTOM LINE RE AN ENERGY EFFICIENY ADVANTAGE OF V4 0VER V8
So with no combustion in 4 of 8 cylinders there is less energy loss to the coolant as no heat is transferring from those combustion chambers and cylinder walls to the coolant!
How much? Don't know but certainly a factor.

SIDEBAR
These are details from my website:The Chevy Crate Motor instructions say to use "Ported Vacuum" to connect to the distributor vacuum advance diaphragm. This means there will be no vacuum present at idle. This port comes from above the butterfly's in the carb. Therefore until the throttle is open, there is essentially no vacuum in this port to activate the distributor advance mechanism.

However and old Peterson publication entitled "Basic Ignition and Electrical Systems" reviews the history of "Manifold" versus "Ported" vacuum. As they indicate, "Ported Vacuum" came about during the start of pollution controlled vehicles to reduce nitrous oxide emissions with lean air/fuel idle mixtures. They state that by using "Manifold Vacuum" the spark at idle will occur sooner and "less cylinder wall is exposed to the heat (of combustion), thereby lowering coolant temperature." Interpreting their statement, with more advance at idle, the combustion starts on the upper portion of the cylinder on the compression stroke. Flame front travels very slowly at the low cylinder pressure that occur when idling. Therefore the cylinder head and upper cylinder walls will absorb most of the combustion heat and the lower portion of the cylinder on the power stroke will be cooler. This transfers less total heat to the cooling system.

In fact, on this 502/502 motor an additional ~ 10 degrees advance occurs at idle when the "Manifold Vacuum" port is connected to the distributor vacuum advance rather than the "Ported Vacuum" location. This provides about 25 degrees advance in total when idling! The engine idles considerably faster compared to when the "Ported Vacuum" port location is employed. The throttle butterfly's have to be closed about 1/2 turn on the idle adjustment screw to maintain the ~900 RPM idle speed. I consider this decreased throttle opening to be a good indicator of more efficient use of the fuel. All else being equal, less butterfly opening means less fuel consumed which equates to less total heat to transfer to the coolant.

Quoting another source; "Ported vacuum showed up when emissions became a factor - before that time, full manifold vacuum was provided to the distributor. Many emissions engines use a thermal vacuum control switch to deliver ported vacuum to the distributor until the engine reached operating temperature, then manifold vacuum to keep the engine cooler at idle."

Measuring the engine temperature on a very hot day (95 degrees in the shade, with the air-conditioning off and the electric fans on) with the car idling at 1100 RPM in PARK, it slowly rose to 220 deg F when "Ported Vacuum" was used . Under the same conditions using "Manifold Vacuum" for the distributor vacuum advance the temperature maxed out at 210 degrees, a 10 degree lower coolant temperature!

Not on the website but subsequently found some cars when ported vacuum started some cars with AC, when idling had a switch to go from ported to manifold vacuum to lower coolant temp!


A lot of the energy in gasoline is wasted as heat!

 

That's why I put it in manual. Only mode where AFM is deactivated.

 

Heres how AFM works
First, it deactivates your cylinders which saves you $40/year in fuel.
Then, it falls apart inside the engine which costs $1500/year to fix




nailed it

 

Since day 1 on my C7 GS M7 it has never been on Eco which turns on AFM. I only do city and some country roads. I average around 23-25 mpg. Probably well over 30 on the highway. So yes I spend the extra $40 on gas so I do not have to spend $1500 on repairs. Since then I have AFM tuned out in the ECM.

 

Hmm, WRONG! I use Z-Mode Power set to Track (Ride set to Sport.) It shifts up and down at higher rpm, only shifts to 6th after 72 mph and NEVER goes into V4!

Used Manual Mode for a month to get over 62 years of EVERY DD I had being a stands shift! Not good when I leave the stop sign at the end of the street I live on. Have to make a 90 degree turn to merge into often heavy traffic on a 4-lane divided highway. Have to shift out of 1st at ~35 mph or you hit the rev limiter.

With my 5.17:1 dif the 1st gear combined ratio is as 10:1. BUT it gets there in the blink of an eye and the upshift paddle is out of position to pull. No fun having all fuel shut off with cars, RVs, Trucks coming up behind from Myrtle Beach at ~70 mph and crawling up on your ***!

Z-Mode solves that! Shifts at 6500 by itself! For comparison my prior 2017 Grand Sport had only 10:1 and if you had a C7 Z06 that 1st gear dif combo was 9:1! Yep had time to shift! Not the C8 hunting for the upshift paddle!!

 

What you said here is correct, but additionally an engine has its most efficient operating point in RPM, combustion pressure, combustion temperature, coolant flow, etc. This is generally not at very low RPM and very low load, so operating fewer cylinders in a more efficient region of their fuel efficiency curve no doubt adds to fuel economy.

 

Not exactly. Track mode or Z Mode with Track transmission shifting selected will also deactivate AFM.

 

I have never had the gears change when going from V8 to V4 so the RPM's stay the same and the engine is still operating in the same efficiency region.

 

Know few would read my long post #22! But spent over a month researching my street rod cooling issue. The Pederson Publication solution takes several step thinking but as this question has come up before thought I would make a pic! Hope it helps some. Will be useful for similar future question!

 

RPM is only one component. More importantly is load and combustion efficiency. At a given RPM, you have either 4 cylinders or 8 cylinders making the power at a given RPM. Since the RPM does not change and the vehicle speed does not change, the total power output is unchanged. But instead of 8 cylinders making the power, now it is 4 cylinders - you have effectively doubled the power output from the 4 activated cylinders as well as doubled the load. The 4 activated cylinders will be operating in a much better part of their fuel efficiency curve, because you have better cylinder pressures and cylinder temps and more efficient combustion and perhaps even scavenging, this will give more efficient combustion and improved power per unit of fuel.

This is a well known effect in diesel engines, they like to be operated at say 60 or 70 percent of full load for best fuel efficiency.

 

Lots to think about.

Thanks for all the inputs.

 

Yeah, its "wrong", but in context, he was talking about being able to put in 8th gear without activating AFM. I think maybe manual mode is the only way to cruise at reasonable speeds in 8th gear without AFM activating.

 

Nope in Z Mode it's also possible. Depending on your speed, Z Mode may not get past 5th or 6th gear. BUT IF YOU THINK 8th gear, with it's 0,33:1 Overdrive, is needed and really helps mpg (only does if you are also driving as they did in the Mobil Economy Run where in stock class a vacuum gauge was NOT ALLOWED you had to drive as if you had a raw egg between your foot and the throttle) then just pull the upshift paddle and go to 8th gear! Throttle position and changes in throttle position has more to do with mpg than gears!

I'll occasionally do that if in Z Mode where it typically picks 5th at ~60/65 mph on our rural roads if I come up to someone in front going say 50 mph. While I'm having to drive "slow" I use the upshift paddle and go to a higher gear until I'm able to drive at ~60+ mph.

Heck I remember when OD was 0.70:1. With the C8, 4th is 0.88: 1; 5th 0.66:1 OD and 6th 0.51:1 OD! All fine for GM to get max "EPA Test" mpg. But just because it's there does not mean you have to use it! Nor depending on your right foot will it always give you better mpg!

BTW, 1250 rpm in 8th at 60 mph is low IMO NOT reasonable rpm! Member PoorHouseNext posted Speadsheet data of C8 gears, rpm and speed. Don't have it as I put it with my spreadsheet developed for my C6, C7 M7s. It's very close to my calculations.

 

Yeah, I know its possible to get to 8th gear if you are in automatic mode, with engine shift set to track, by pulling the paddle - but technically thats "temporary manual mode". And it will shift back won't it? I think if someone wants to cruise on the highway in 8th gear (as the poster described), at a reasonable speed, indefinitely, without having AFM activate, then the car would need to be in manual mode. And in part I was going by what you said - it will not automatically shift into 6th gear until it gets to 72 mph - if the engine shift is set to track. That corresponds to about 2248 rpm with Z51, and 2219 rpm with non-Z. And is consistent with my experience - with engine/shift set to track it likes to stay up around 2200-2300 or so. In tour mode, it will cruise fine in 8th gear at 72 mph at around 1500 rpm (1521 for Z, 1440 non Z), but AFM will activate. I think to be in 8th gear, at a reasonable speed (72 mph for example), and stay there while cruising steadily, without AFM, one has to use manual mode.

 

Yep in the case I mentioned when the slowpoke in my rural who was going 50 mph turns and I'm say in Z Mode having pulled the upshift paddle to 8th gear, I use the throttle and back to the logical Z Mode! I hear many folks saying the C8 is too civilized! Or recently a ME does not pin you back like a FE car?? It's because of the way they drive NOT the car or it being an ME!

Posted the video below by a Forum member that shows why he ONLY drives in Z Mode. AND he is not driving as aggressively as I do in my rural area. He shows driving on a more populated area.

Now if on a highway (don't drive my C8 on the Interstate- too many issues) I use MY Mode. Have all handling set exactly as Z Mode, i.e. eBoost, NPP set to Track along with Power; Steering and Ride set to Sport. So the car handles about the same BUT have it set to Sport. It uses all 8 gears and V4.

So I don't have to go through Touring I do not get out of Z Mode by pushing the steering wheel button! Without looking, while in Z Mode I put my hand on the Mode **** Wrist Rest and make one hard turn CCW. It's in MY MODE! Want Z Mode? Push the steering wheel button.

This fellow always uses Z Mode and shows why. It's like driving a different car! Not overly civilized! Your right foot still governs!

https://www.youtube.com/watch?v=RI9M...annel=jrock702

 

Very true, but the machismo in us hates to admit that. We are all Duke boys at heart.