Help me understand cam timing, advancing or retarding a SOHC for mpg

[Daox]

I've been reading up a little on cam timing. I know that advancing the cam timing pushes the powerband to a lower rpm, and retarding it pushes it to a higher rpm. However, I'm not really interested in my powerband as much. I'm interested in getting good fuel economy. Most of the info out there is mostly on how to make more power... bleh.

The wikipedia on variable cam timing says that advancing AND retarding intake cam timing can be good for fuel efficiency. Thats not super helpful. It seems to indicate that advancing the exhaust cam is good for fuel economy, and retarding it is good for power.

I'm also mainly thinking about the Metro which is a SOHC. I don't have the ability to advance just the intake cam because there is only one cam.

So, I'm thinking I want to advance the cam timing, but not 100% sure why, and I'd really love to fully understand it. I know the atkinson cycle engines retard it so it pushes some of the intake charge out the valves, but thats retarding the cam, not advancing.

Does anyone have some good resources, or a great explanation as to why advancing or retarding is good for fuel efficiency.

[MetroMPG]

Subscribed for edification.

[2009Toyotoad]

Too much advance creates a detonation issue, which on most modern powertrains means the ECU/ECM will automatically force timing retard to protect the engine.

The others options to play with are throttle control and cam lift/ profile. Cam Lift which modifies both the size of the valve openings and the duration. Note this part becomes tricky and varies from one engine design to another. Basically, some engines by their design won't support cam modification and will perform poorly in both power and fuel economy on any profile other than OEM Spec. The ford cologne 2.9 V-6 is a well known example of this.

Throttle control come in two flavors mechanical and electronic. Electronic works with drive by wire and requires the purchase of throttle controller unit and a bypass or boomslang wiring harness to intercept signal from sensors and recalibrate responses to those signal as outputs the the throttle control itself and the relative throttle range. Basically, if someone mashes the gas pedal they will only get a percentage of full throttle and rather than instantaneous response to the sharp stab at the pedal, the throttle only opens at the speed the throttle controller is programed to allow.
Mechanical throttle controllers are for cars without drive by wire technology and requires the use of governors, stop steps screws or bolts, and heavier throttle springs. The most common usage of this type of control was in commercial trucks and 4X4 vehicle where excessive throttle input... "stabing the pedal"... created safety or economy concerns.

VVT or variable valve timing takes advantage of valve open/close duration timing. The valve does not open any farther but it stays open or closed longer in the piston cycle.

These engines compensate for possible timing retard by the ECU/ECM by allowing the position of the VVT sensor to control timing advance or retard.

"To a certain degree" add on electronic throttle controller devices can be used to "tweak" the timing advance to optimal fuel economy over / power and performance. However, it is a rather narrow range of adjustment in which to play. Too much advance and the Knock sensor signal to the ECU to retard timing regardless of the input from the VVT sensor.

The electronic add on throttle controller can't force the ECU to ignore the sensor signal all together. If it could the ECU would start issuing DTC codes and light up the MIL on the instrument panel. i.e. "Not Good"

Trying to achieve the same kinds of control via mechanical throttle controller is a whole book or series of articles and debates within itself. Let's just say there are a myriad of ways to achieve that type of control and each has its benefits and pitfalls. Nobody ever gets it perfect and it always needs tinkering and modifications. As always successful mechanical throttle controlling varies by weather, driver, driving conditions and the vehicle itself.

Atkinson versus Otto cycle engines: the key here is where in the piston position cycle is the fuel/air charge ignited by spark. There are actually very few true Atkinson Cycle engines in cars. hybrids like the prius are actually "Atkinson like" cycle engines rather than true Atkinson engines. Otto engines ignite the fuel charge just before top dead center on the piston stroke, Atkinson at top dead center or slightly after TDC. For more of modern transportation the Otto theory was preferred other the Atkinson. Ignition at TDC added a stress load to the crank shaft and wrist pins, rings, piston, and bearings. Modern engines using the "Atkinson like" offset the position of the crankshaft from the piston to lessen these stresses and increase engine durability and lifespan. There is/was a larger risk or concern of pre-inginition or "dieseling" in an Atkinson cycle but this is still hotly debated. As the control over a pre-ignition condition have advanced so much since the original Atkinson designs.

I hope that a little more clarity to your question about using timing advance. If you really want to get even more into understanding of how timing advance works look into the Wankel engine design. Once you get a sense of how timing works on that rotary design it makes for a easier to understand piston engines designs.

[oil pan 4]

There was some one on here years ago that had a DOHC engine and tried retarding the exhaust cam all by its self in one test. If I remember correctly it made almost no measureable difference.

If you have a single over head cam you are more closely related to an old school in block cam push rod engine then you think.
Since the beginning of gasoline engines people have advanced the cam timing to increase fuel economy and low end response and torque of a daily driver car. But advancing the cam makes it harder for the engine to breath on the big end.
Retarding the cam is good for high speed, high horsepower and at the expense of low end performance. Retarding the cam may provide a fuel economy improvement only on the high RPM end of things as far as I can see.
As far as I know the only people I know of who run retarded cam timing don't care about fuel economy (drag race engines and circle track racers) or the people who don't understand engines read something to the effect of "retarding the cam timing will produce more horsepower" and put any and all considerations aside and go for it.

I advanced the cam timing one tooth on my Toyota 1.8L DOHC that I had when I lived in japan when I was replacing the timing belt. I wanted more low end response since japan was all city driving and the highest speed limit in the country is about 50mph.
It worked for that reason, but I was not measuring fuel economy at the time. So I don't know what effect it had there.

[cRiPpLe_rOoStEr]

Retarding the intake timing in those so-called "Atkinson" engines is often pointed to increase their efficiency due to a reduction of the pumping losses, but the low-end response gets awful. The static compression ratio is also usually raised in those engines, but the dynamic compression is decreased due to the decompression resulting from the retarded intake timing, so in theory they don't become more prone to detonation issues. Meanwhile, advancing the intake timing seems to make sense due to the improved low-end response, thus benefiting the use of higher gearing at lower RPMs while cruising.

[2000mc]

3 Tech Performance
Guy that sells or used to sell modified cam sprockets. reviews / discussion seem very positive about its affect on power. not a lot of talk on mileage ,but from what I saw they thought it was the same or better

people seem to like the +10deg.... But I think that would be roughly 1 tooth

[oldtamiyaphile]

A mechanic once fitted a timing belt to a Mazda SOHC I was driving, apparently he put on on 'out by one tooth'. I don't know if it was advanced or retarded, but it drove like a dog and had massive backfires. I suspect you might need a finer range of adjustment than just +/- a tooth. Adjustable cam gears are unlikely to be available for this application off the shelf.

[user removed]

Lets say the cam sprocket has 24 teeth. Every tooth would be 15 degrees, but then you must double that amount since the cam is turning at half the RPM of the crank (30 degrees effective).

To make one tooth 10 degrees, the sprocket would need at least 72 teeth or some multiple of that.

Making blank statements about the effect of cam timing, must include the duration of the cam timing before assumptions are made about the effect of the opening and closing point of the valve.

If the valve opens after TDC then compression is effectively reduced, if you are talking about valve opening at precisely TDC which is seldom the case. Inertial forces of the incoming air charge, the inertial forces of subsequent charges and the harmonization of those charges all effect the total volume of that charge. Other factors like bore to stroke ratio, and exhaust tuning also affect the efficiency of the volume of the intake air charge.

EGR percentage, as well as residual exhaust gases remaining (effectively uncooled egr) also are changed when you change dimensions outside of the opening and closing of the intake valve.

Overlap of the intake and exhaust valve closing (intake) and opening (exhaust) will change significantly when intake valve timing is changed by whatever method.

regards
mech

[user removed]

MIVEC | Technology Library | Automobile Technology | Mitsubishi Motors Automobile Manufacturing | MITSUBISHI MOTORS

Not only intake cam timing but other potential, but nice graphs of cam timing, intake and exhaust overlap and their effect on efficiency and combustion stability.

regards
mech

[MetroMPG]

For consideration: 40 teeth on a stock 1998 Metro/Suzukiclone G10 1.0L SOCH cam sprocket = 9 degrees per tooth. Correction: effectively becomes 18 degrees per tooth due to the 2:1 ratio of cam:crank sprockets.

Also: the more efficient XFi cam is reportedly "6 degrees advanced from the stock cam, based on measurements of intake centerline," according to the resident cam guru (superf1y) at teamswift.net.

But the XFi cam lobes also have lower lift & duration.

International heart transplant: the Blackfly gets an XFi cam - MetroMPG.com