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

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.

Subscribed for edification.

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.

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.

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.

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

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.

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

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

XFi cam is advanced vs non-XFi
 

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

Two weeks after I had the timing belt replaced on Bacon it began driving very roughly. The shop that did the work said that the timing belt was off by one tooth and it was "firing on two cylinders." I do not want any part of that ever again.

degree per tooth
 

You have to double that because of the 2 to 1 ratio cam to crankshaft, makes 1 tooth effectively 18 degrees.

The Metro guys actually do have an adjustable cam pulley. I think you can get it in variants of +6, +8 and even +10 which seems to be quite popular as 2000mc posted. I don't know what I'd go with.

It also doesn't look like it would be hard to modify the stock cam sprocket.

Got it - thanks. I'll edit my post.

Yeah, thats kind of what I had in mind. At least give it a try.

Spitballin late at night, I thought it was weird people would pay for a 10deg mod if they could get 9deg for free... Obviously not firing on all cyls over here

I deleted my earlier post because you are asking about cam timing and I wrote about ignition timing. However, if you adust the cam timing and your distributor is connected to the cam, I suppose you'll want to adjust ignition timing too, unless the cumulative effect works for your plan...

Yep, after cam timing has been adjusted, ignition timing should be updated to optimize the change.

As it is, I'm running stock ignition timing and that could definitely be changed for a bump in efficiency.

Sounds cool. If I did things like this to my car, and it sounds interesting, I would likely fail California emissions testing. I am not complaining about emissions, just observing the limits for anyone in a US state that has adopted Cali standards (about 30 states, I am told...).

FYI, I changed the Firefly's cam to the XFi version (that trim was never available in Canada, so there's no emissions profile for it here) and advanced the ignition timing quite a lot and the car always passed emissions (sniffer/tailpipe gas analysis type) with flying colours.

They check ignition timing here in Cali. So I would assume most if not all states that adopted the Cali law also check. I don't know about WI, but Canada is among the states exempt. ;)

Wisconsin hooks your car up to an OBDII scanner and pretty much as long as there is no check engine light you're fine. Anything pre-OBDII isn't even checked.

Hey, realize this is an old thread and probably not relevant to the originator any longer so I guess this is just a little FYI. I had an early Geo metro I wanna say it was a 1990 or 1991 and the cool thing about these older models was that Suzuki had an adjustable cam sprocket on that G10, if you pull the cam gear off and inspect it, you will find it has an extra notch that is if I remember correctly 3° offset. I don’t know if they still do but Suzuki used to race these three cylinders and the other notch is the setting that used for racing ;) . As far as mpg I never clocked it, so I don’t know if it improved or gotten worse. Either way I was still getting killer gas mileage, and with the offset cam timing once that thing hit 90 did not want to slow down. I had a lot of fun with that car.

Now part two is that I arrived here in the first place, because I am trying to figure out the answer to that same question that started this thread. If I’m trying to increase my mpg, should I retired or advance from what I remember on the metro I believe I need to go 3°, but I can’t remember which direction or if it would improve gas mileage at all, but my assumption is that the more efficient burn That this adjustment I believe creates would increase gas mileage as well. So if anybody has the answer, let me know please. My current project is not a metro by the way, but a single overhead cam YJ so I’m assuming the results would be ballpark similar.

When I rebuilt the 1972 VW Super Beetle engine I had the cam ground for a close to stock cam timing and lift that was also compatible with a 10:1 compression ratio running regular gas. I'm not sure how much power it made, but that Super Bug felt like it was quite a bit faster than the 1985 VW Golf I had that was suposedly more powerful even though everything else on the Bug was stock (carb, air filter, exhaust, pistons, heads except machining for higher CR) except the custom ignition timing. I could climb any pass at the speed limit in the Bug whereas the Golf wouldn't. I also got a rock solid 30mpg average in the Bug.

Kinda like a manually-activated VVT? Interesting, even though it's often pointed out that DOHC valvetrains are better suited to feature VVT.

I have no idea, but thanks for reviving the thread.

I know all about VW flat fours, but the Metro is allien technology [to me].

https://ecomodder.com/forum/member-f...-metro-xfi.jpg

Wouldn't the composite headlamps be supposed to be better suited to the XFi, as they're more aerodynamic than the recessed sealed beams? Well, at least until the FMVSS 108 is not repealed...

It's a thousand-dollar car. [shrug]

I've got some good used side marker lights, have been thinking about stainless steel mesh covers.

a cheap upgrade on the old metro would be a plexiglass cover over the headlight openings.

Another tip for metro enthusiasts is to check out Suzuki Racing Development.
suzukird dot com
Now these parts are high $$$ but if that doesn't bother you and you want some awesome mod parts or just want to drool a little LOL, they sell aluminum pulleys, flywheels, high compression piston set, and lots of other cool stuff that will make your metro better.

Back to that discussion on cam timing if you are not lucky enough to have the early cam sprocket or do but want more adjustability SuzukiRD also sells an adjustable cam sproket for the G10.

I was going to give you the link but apparently you have to have 15 posts before that is aloud :( there site is easy enough to search though.

https://suzukird.com/

These guys?

yep that's the one.:)

Now that would be quite cool to make a sleeper...

That was what I was planning with mine, but never did finish it. I also had made an adaptor to put 3 Mikuni carbs on it.

Motorcycle carburettors? I have already seen some makeshifts resorting to a single motorcycle carburettor on compact cars, but not as a performance trick. When it comes to the Metro/Swift, I don't remember any version from the '90s factory-fitted with a carburettor, at least none of the ones imported to my country.

Back when i was doing that, a 6pack of strombergs were the setup to beat for inline 6s. Mikuni were for rich extremely people. So why not?

They have also been used on "cheap" motorcycles :confused:

Yup had 4 on a honda 750. The variable venturi ones were bullet proof and very efficient for the times. Making the custom manifold was usually the expense

yes, I had 3 mikuni carburetors off some 350cc?? maybe single cylinder dirt bikes or quads. cant remember for sure its been to long. they seemed like a good option because they are super easy to re-jet and tune.