Lizard928

MK,

Unless you use the STII and do it properly by editing the table itself DO NOT do it!!!!

dprantl

+1. It would be absolutely crazy to try and do this without a wbO2, full access to modifying the fuel maps, and a way to monitor knock counts.

Dan
'91 928GT S/C 475hp/460lb.ft

brutus

Perhaps Mark you could just shut off the injection to 4 cylinders with a simple switch. We KNOW the cars will cruise just fine on 4 cylinders just a little hard on automatic driveshafts. Then you have a max 150 hp plus 1/2 the injection events per mile.

Lizard928

Brutus, I expect mark would be needing a drives gift in short order!
This is not a good idea at all, plus you really need a way to hold some valves open so that you are not compressing the air in those 4 cylinders too.

123

It takes "X" amount of horsepower to push a particular car down the road at a constant "Y" mph. It doesn't matter if that "X" amount of horsepower is coming from 8 or 4 cylinders. Think along the lines of each of the 4 cylinders needing to make twice as much power as each of the 8 cylinders of a normally running engine would, so those 4 cylinders would need to use twice as much fuel each. I know that's not exactly the case, but close enough that just running the engine on 4 cylinders instead of 8 wouldn't make that much of a difference in fuel consumption.

Only shutting off some of the fuel injectors will actually increase fuel consumption by quite a bit. Air would still be getting pumped through the cylinders with the disabled fuel injectors. That air would flow through the engine into the exhaust, where the O2 sensor would detect the excess oxygen. The LH would get that excess air reading from the O2 sensor, read it as the mixture being too lean, and increase the pulse width to those injectors that are operating to try to correct that. As long as there's air going into the exhaust like that, the O2 sensor will pick it up, and the LH will keep increasing fuel delivery to the operating injectors until it reaches the limit of it's adjustment range. The result is those operating cylinders getting too much fuel, and mpg plunges.

To really have the best milage increase from shutting down cylinders would require shutting off the injectors, and the valves of those cylinders. That would keep the air from getting pumped into the exhaust, and you'd get some milage gain from the reduction of parasitic loss from not having to operate those valves on the shut down cylinders.

brutus

OK so .."It takes "X" amount of horsepower to push a particular car down the road at a constant "Y" mph. It doesn't matter if that "X" amount of horsepower is coming from 8 or 4 cylinders. Think along the lines of each of the 4 cylinders needing to make twice as much power as each of the 8 cylinders of a normally running engine would, so those 4 cylinders would need to use twice as much fuel each."
Does that mean you get the same MPG running 60 mph in 2nd gear, 3rd, 4th, or 5th ?? Same "X" of horsepower same fuel consumption ? Not hardly. But you are of course correct about pumping losses and the need to shut off the valves like Cadillac, VTEC hondas, or the Hemi all did.

mark kibort

probably a good idea. im really not that interested in wasting a $10,000 motor, on saving 50 bucks in gas a year. BUT, if it could work, and it was easy to get it real lean , i might do it.

what i really want it for is the Chebby Tahoe. that stupid gas guzzler shouldnt get 17 mpg on the highway. it should get 25 !

cant we put a resistor in the O2 sensor circut so it hunts around 16:1 rather than 14:1 ?

123

You don't get the same fuel consumption using the different gears because you're changing the inherent efficiency of the engine when you operate it at the different RPM points to go that same road speed. It requires the same horsepower to push the car down the road at that same speed, but you need to use different amounts of gas to make different amounts of horsepower, to compensate for the different parasitic power losses and inefficiencies that are occurring in those different gears.

Use different gears and the engine RPM will be different at the same road speed, so internal friction in the engine and transmission will be different, the power required to move parts at the different speeds will be different, the efficiency of the cylinder filling due to the cam and intake specifics will be different, air, exhaust, and fluid pumping power requirements will be different, but the horsepower delivered to the rear wheels, which is what pushes the car down the road, will be the same at that road speed, no matter what gear is used.

Just shutting off half the fuel injectors doesn't change the inherent efficiency of the engine. The brake specific fuel consumption is going to be basically the same at the same speed if in the same gear. That means it's going to take basically the same amount of fuel to make the same amount of horsepower, and the same amount of horsepower is going to be delivered to the rear wheels. It doesn't make too much difference if that amount of fuel is coming out of 8 injectors, 4, 1, or 100.

No, but you could do it with a programmed analog output from a wideband O2 sensor controller. You should really be changing the ignition timing then too though, both for engine safety, and to get the maximum mpg benefit out of leaning the mixture.

dfrhodes

okay, wait a minute though - 1 cylinder makes a primitive engine and a V8 is effectively 8 engines ganged together. So yes, there are various factors like fuel, temp, air, altitude, load that impact overall efficiency, but what these variable intake/cylinder guys (i-vtec, AMG) are essentially saying is - there is more efficiency running fewer cylinders per given load – If they are running in their respective power bands - than running more cylinders dynamically rich/lean. And additionally, reference to pumping losses-

http://asia.vtec.net/Engines/RiVTEC/index.html

but anyway, there is no reasonable way to hack the 928 heads to do variable valve tricks so it may be a moot point. But for turning off some injectors (NOT all 4 on one side!!) I’m curious how much a negative factor the dead cylinder compression is to overall power and efficiency. Is that the only reason they redesigned these engines to disable intake valves as well as fuel??

It seems like it would be relatively simple to add O2 sensors for each exhaust port on the manifold if there are worries about excessive O2 from dead cylinders tricking out the LH via the existing central sensor, if that is the primary concern against doing this. I’m sure I’m missing something major here though.

I’m also skeptical that simply remapping the LH to be lean in low-power scenarios is the holy grail. Isn’t this economy already mapped in to IC engines at this point? I suspect it’s not so easy, as the inherent latency imposed by the throttle means - if the brain forces lean running after sensing economy mode conditions, then the throttle suddenly opens…it’s too late to switch to rich and avoid damaging detonation. right?

d

RKD in OKC

Cadillac's 8 6 4 claimed 15% better gas mileage. It used electromechanical actuators to stop the rockers from opening the valves on the cylinders not in use. Failed miserably.

Dodge's MDS Hemi switches from 8 to 4 by removing oil pressure from roller lifters so they don't open the valves. It only activates for 6 minutes at a time to keep heat down and claims are 20% better gas mileage in some conditions. They are getting 15 to 17 mpg on 93 octane, much less with lower octane.

123quattro

Mark, I'm not guessing. I work in powertrain at a large US manufacturer.

If you are just lean, but not misfiring the cat doesn't care. It just doesn't work. Lean misfire is when you lean out the mixture enough that there is combustion instability. Unburned mixture containing both fuel and air get passed into the exhaust stream. The catalyst will still oxidize that mixture. Normally catalysts generate some heat due to normal operation. If you pass an unburned charge into the catalyst it will get a lot hotter. If your engine continues to misfire the catalyst will continue to climb in temperature until it runs away and melts into a big glob of ceramic crap.

When you are talking about coasting down a hill, the engine is in decel fuel cutoff. There is no fuel in the system period. The catalyst just sees air that passes straight through. This cools the catalyst because there is no chemical reaction going on. Decel long enough and the catalyst will get so cold that it won't even function anymore and has to be relit once the engine refuels.

daddyov8

Cadillac's 8 6 4 and Dodge's MDS not the best examples of how this can work. They were very heavy and the power outputs were low not a good combination.

The move forward in technology in particular ignition/fuelling has led to much more refined systems now being available. There are a lot more examples in Europe. Mercedes has done this for years Cl600/500 as examples shutting down from 12 or 8cyl to 6 or 4.
Currently AMG are using it in the new 5.5 v8 the m152.

The technology has been well used in F1 cars for a few years now Mclaren/ and Williams? use it think one of them runs the car constantly changing which cylinders are shut down to help cool the car evenly and produce uniform wear.

Volkswagon are using it in all their new engines from 1.2l upwards the first ones being released in the Polo Bluemotion etc
Bentley are using it on the new Arnage about to be released
Audi have introduced it on the new A1 and plan to on the A3 and 4
Honda have used it for years on various v6 and hybrid models

Our problem is probably more the hardware than the software i.e keeping the valves closed if this was achievable the rest should be possible.

The obvious relationship to this is the stop start technology that almost all new cars have now as std. hardware challenges like the new style starters etc but the remainder would be possible. Following this you could look at the new audi system for only part running the ancillaries and charging them when under freewheel or braking etc which seems to produce quite a large benefit that would work when cruising.

Other route to go is hybrid and get a 918 or add a kers flywheel/electric motor etc

mark kibort

yes, i could see how raw unburned gas, going to the cat on a misfire could cause it some issues. but, i contend that if you did have a misfire, it would be few and far between, and probably wouldnt be a problem. anyway, i dont have a cat, so that wouldnt be an issue for me. also, i think under most operating conditions, any fuel air mixture that didnt combust, would turn to vapor and be ignited in exhaust ports, so it wouldnt be a problem for the cat. ( in most cases)

hacker-pschorr

I just realized this is for your race motor, I'm so glad Todd doesn't pay any attention to threads about better fuel economy....

As most engine builders see their creations as their children, reading this thread would make Todd feel like a father watching his son score a goal on the wrong end of the field.

mark kibort

nah, wouldnt do something like this without his blessing.