#2 Eco-tune carburated V8's - fast and accurate

[DavidVizard-GFN]

#2 Carb & Ignition Eco-tuning Made Easy.

By

David Vizard

The problem with carb and ignition eco-tuning is that it can take a lot of hours tinkering and a lot of miles verifying results – and that burns a lot of gas in the process.

But-
here is a GFN shortcut to optimal results about first time around.

Here’s the problems:-

• You make and adjustment – and it could be a week or more before you have any idea if the mileage is better or worse.
• You make another adjustment and rack up more miles to see the results – and still things could be better - or worse.
• So you adjust again ---- and so it goes on – and on.

Here’s the questions:-

• Would you like to get it right about first time around?
• Would you like to know it’s right before you even think of testing it?
• Would you like to be sure that the adjustments are about as accurate as can be expected?
• If so read on for the answers!


I finished PEG #1 with the following -
Let’s start by looking at ways you and I can save fuel as well as enjoy a measure of fun from increased performance. That, armed with the worlds cheapest octane booster/fuel supplement, is what I intend to wade into real soon - or should I tackle the subject of the great 4 valve per cylinder rip-off that has been perpetrated on 95% of highway only drivers the world over???? – but I’m not going to stick with that. Why? Because a package from the good guys at Auto Meter turned up for our Camaro drag racer and as I unpacked what they had sent I realized that I could start saving you carbureted V8 guys some gas cash right away – so being impatient and compulsive (and why wouldn’t I be – I’m a racer!) I thought it best, because the other subject will hold a week or so longer, to get this story out right away. So here goes.

WOT it is Not!

With a V8 powered vehicle most street driving is done at small throttle openings that involve the carbs idle and transition circuits. Any adjustments involved will only be done on the idle fuel jet, the air correction jets, and the idle mixture adjustment screws. Along with this there may be some main jet changes for Holley style carbs to cater for the higher highway speeds Along with this there is ignition timing to consider. The problem here is that most hot rodders are fixated on total timing and what it must be for maximum output. The reality is that this has near nothing to do with the timing required for best Brake Specific Fuel Consumption (BSFC) at part throttle and low rpm conditions.

Mixture Requirements.

For maximum power an engine needs to have a an air/fuel ratio of around 12.8/1 to 13/1 but for best fuel consumption at cruise speeds this needs to be much leaner. Just how much leaner depends on the engines ‘lean burn’ capability. Most domestic V8’s will run as lean as 16 to maybe 17/1 without ignition/combustion related problems. That does not mean you will be able to run the motor this lean in practice if some other issue takes charge of events. Sometimes we can run at a steady throttle opening flawlessly with a leaner than expected mixture but run into a transitional problem when the throttle is opened further. Here’s an example of what I mean. Many (many) years ago when testing an engine/ fuel injection system for lean burn capability we found that the combination would run better than 20/1 mixtures and cruise at a constant throttle opening without misfire indefinitely. But open the throttle just a fraction and the engine would die. It seems that we could not get that accelerator pump action to work even half way fast enough to cover the hole. The only way we could make this work was to have the pump jet action come in before the throttle blades actually opened any further. This is doable with a fly-by-wire throttle system but not with the mechanical injection system we were experimenting with. (That tells you how long ago it was right there!) So what we need to find here is the leanest mixture possible consistent with the retention of acceptable drivability during transitional throttle opening situations. The bottom line is then that the quest for mileage, at least from the fuel mixture side of things, will be based on leaning out the mixture until it is just short of lean misfire or drivability problems.

Left is a Holley Spread Bore. This was designed as a replacement for the Q-Jet carb (type shown center). I had good results both power wise and for fuel economy with these Spread Bore carbs but they are not so easy to get these days. The center carb is one of a number built for me by The Carb Shop in Ontario California. This particular one was for a racer but The Carb Shop does a nice total rebuild on these carbs with performance and eco upgrades that makes them a very worthwhile choice for hot rodders looking for economy and performance. On the right is a Barry Grant carb equipped with a vacuum secondary. For an economy carb that is still required to make good power small primaries and a vacuum secondary is the way to go.

The Edelbrock carb is in essence an update of the carter AFB carbs of yesteryear. The carb on the left is Edelbrock’s small block friendly 500 cfm eco unit. The carb on the right is an 800 cfm unit with the 'Endure-shine' finish (looks really sharp in the flesh). You might want to read Tony Brown’s economy test on this at http://www.gofastnews.com/board/tech...r-economy.html

For a Holley or Barry Grant carb that means working the idle adjustment screw, idle jets and air correctors and finally the main jet which in actuality is not the main jet but a higher speed cruise jet. Just for reference here the real main jet is the Power Valve Restrictor Channel (PVRC). That’s the hole under the power valve and that is what you should be calibrating for optimizing WOT output. As for Quadra-Jet and Carter AFB (that’s now Edelbrock) it’s still a question of the idle circuits for the initial stuff but for highway cruise it’s a case of getting the right sizing on the needles. Here it’s worthwhile pointing out that the Carb Shop does a very nice Q-Jet rebuild (you can buy outright from them) that restores a Q-jet to better than as new. They can upgrade the airflow about 100 cfm and install a part throttle mixture adjustment screw which makes calibrating highway cruise mixture an absolute breeze.

Mixture – I Know What I want but What Have I Got?

The biggest problem adjusting the mixture to the needs of the engine is knowing what it actually is in practice. Forget exhaust temperatures – this is the great miss-leader of intake experiments. The best way to know what is going on is a wide band O2 sensor in every cylinder. This is a little expensive but it is worth every penny of any expenditure involved for the pro engine builder.

For the typical performance enthusiast a single wide band O2 sensor system in one of the collectors is the way to go. Just to make sure you understand what I am saying here do not even start to go down the plug reading road. Reading plugs is at best an art and at worst as misleading as exhaust temperature sensing. Sure the likes of Smokey Yunick could do it with a high degree of skill but it took even Smokey a lot of years to acquire that skill level. We are talking about achieving results right now - not twenty years down the road.
So here is where we are at right now. Your first move toward a top notch eco-tune is to get a good wide band O2 sensor mixture analyzer. Shown above and below is a unit that is currently in or going into one of our GFN team race cars.

Ignition Timing.

If you ever need to fixate on one simple factor toward getting MPG here is one simple rule for you to do just that. The less dense the charge is under throttled conditions and the more residual exhaust that charge contains and the leaner the mixture the slower the charge burns. What this means is that all of these factors weight in to make the optimum timing for the situation involved more advanced. Without that extra advance the amount of fuel needed to get the job done goes up dramatically. Let me put you in the picture here as to what a typical sporty cammed small block needs for advance while shuffling down the freeway at say at 2500 rpm and 70 mph. Normally the mechanical (centrifugal) advance would be putting in about 20 degrees (crank) advance plus whatever the initial was. 10 degrees would be a good figure for initial so total advance would be about 30 degrees. But the engine would be showing a considerable vacuum here so the cylinders are not any where near filled. Plus the percentage of residual exhaust will be higher than if the engine was at WOT. All this means that the optimal timing would need to be somewhere around 50 degrees total. Without that extra timing the throttle has to be opened wider for the cruise speed being used and fuel consumption goes up. Where is all that extra timing going to come from? None other than a vacuum can that’s where. What this should be telling you is that no street driven machine should be without a vacuum advance system. Not using such will cost at the very least a mile per gallon and more likely 2 miles per gallon! You have been warned!

I don’t plan on going into detail on vacuum canisters here for various types of distributors. Suffice to say that Steve Davis of Performance distributors and I are working on a big rundown on what’s available and what is needed for an HEI equipped domestic V8 and that includes Fords as well as GM motors.

Suffice to say at this point that vacuum is a prime requirement. The graph below shows how the steady state freeway cruise mileage changed with the part throttle total advance (centrifugal plus vacuum) on my 350 small block Chevy truck. These tests were done mid 1980’s and the results hold just as good now.

I did these figures by running one quart of fuel through the engine as I cruised down a given piece of flat freeway just outside of Riverside CA. I had a measure of control of the timing by means of an MSD electronic Advance/Retard unit that could override what was in the distributor. The points to note here is that a) going from 30 degrees to 55 was worth 2.4 mpg and that the mileage gains, for this engine (which was a typical mildly modified truck motor of a little over 360 hp and 440 lbs-ft on my dyno) leveled out about 47 degrees and held nearly level for about 10 degrees on from that. This means (assuming no detonation) that a little too much advance is better than too little.

As you can see from the graph the significance of adding vacuum advance was an improvement of steady speed highway mileage of some 23%!

At this point we have seen the value of leaning out the mixture and also giving the engine the optimum ignition advance for the circumstances under which it is running. But achieving that is normally a long winded process of adjusting - testing - adjusting - re-testing and so on. I promised an easy way to do this way faster so here it is.

Gauges.

At this point I should have convinced you of the value of an O2 mixture analyzer. The next step is that you not only need to get a mixture analyzer but also a vacuum gauge. With these two instruments you can tune for a smother running, more responsive engine that is also cleaner and much more fuel efficient.

Here’s what you get with the Auto Meter vacuum gauge. If you do not have a suitable location for the gauge check out the mounting panels and the windshield pillar mounts that are shown in Auto Meter’s on-line catalog.

Now we know what’s needed let’s look at the surprisingly simple way in which we use these instruments to get mileage. It is not my intent here to explain the in’s and out’s of your particular carb. You will need to know your way around the idle circuits as well as what ever circuits control the low (30 mph) and high speed (up to about 70 mph) cruise. If you are not familiar with your carbs circuitry then hit CarTech’s website http://www.cartechbooks.com for book titles that will inform you of such.

Mixture Optimization.

First the mixture. During any mileage optimization the mixture should be optimized as near as possible first – not the timing. However you will need to ascertain, if a vacuum advance system is already used, whether it is manifold vacuum or ported vacuum. Manifold vacuum is seen when the tapping that ultimately leads to the distributor vacuum can and the vacuum gauge comes direct from beneath the carb butterflies. In other words it reads whatever condition exists within the intake manifold. A ported vacuum source is one that originates from a port that enters the carb body just above the position the butterflies occupy at idle. In this situation the distributor see’s no vacuum until the butterfly is opened past the vacuum port. This was done as a means to cut emissions. With no vacuum advance at idle it was necessary to open the throttle blades more to achieve idle speed. This meant that more air had to enter the engine and as such there would be less residual exhaust gases than with the butterfly’s more closed and with more ignition advance for the same idle speed. The wider throttle opening made for less parts per million of un-burned hydrocarbons. What we want here is manifold vacuum as it will use less fuel to idle the motor. As for the possibility of increase parts per million hydrocarbons we find that our fine tuning will, for the most part, compensate. So if you want the lowest fuel consumption at idle use manifold vacuum for the distributor vacuum advance. There is usually two ports on carbs typically used for V8’s so find the relevant one and hook up the distributor accordingly. The next step before starting any tuning is to make sure your engine has a decent set of spark plugs suitably gapped.

At this point we are ready to start tweaking the mixture for better fuel economy. First you need to explore the idle mixture possibilities for your engine. For these setting to be in the ball park you will need to make sure the idle speed timing is at least somewhere near where it will be when the tuning is finalized. A good starting figure with the vacuum advance hooked up is 30 -35 degrees at 800 or so rpm.

Your first move at this point is to stop the engine and turn each idle screw right in then back them out exactly two turns. Now restart the engine and turn the idle speed up to 1000 rpm. At this point you can adjust each idle screw the same amount until a desirable mixture ratio is seen on the O2 meter. Just what that will be depends on the lean limit miss-fire capability of your engine combination. Typically a factory set spec engine will be around 13-13.5/1. This satisfies catalytic converter requirements but is not the most miserly with fuel at idle. A typical V8 with a typical street performance cam will usually idle just fine at 14/1. However with an ignition that delivers an aggressive spark to a big gap the mixture can often be leaned out to about 15/1. Here you need to reference the vacuum gauge. Within reason the lower the vacuum is the leaner the mixture the engine is running on. Keep leaning out the mixture until the idle gets unstable and then return the idle screws to the last point at which a stable no-stall idle was seen.

Our next move is to see what sort of advance is optimal at the idle we intend to use. This is where the vacuum gauge truly comes into it’s own. What you will do here is to time the distributor and at the same time adjust the idle speed at the carb until the highest vacuum is seen at whatever idle speed you have determined is needed. Now make a note of the vacuum, advance and idle speed. This is what you will want your distributor to deliver under these idle conditions.

OK from here we go to the low and high speed cruise settings. Just take you vehicle out on the road and check the mixture at say 30 or so mph in high gear then at 50 and at 70. Under these conditions most engines can run at 16/1 but if everything is on the money even more economical fuel air ratios can b e used. A small block Chevy with a good ignition system on can cruise at 17/1 but whether you can use that depends on how well the carb transitions from cruise to a wider open throttle. Running this lean at cruise can cause some drivability problems that are not easily calibrated out. Here you will find a two plane intake to be more forgiving than a big plenum single plane. Depending on the frontal area and drag co-efficient a V8 powering say a truck could well be tipping into the main jet circuit. To lean this out and retain power you will need to reduce the size of the main jet and increase the size of the Power Valve Restriction Channel to compensate.

Another factor that the O2 mixture analyzer helps out on is the accelerator pump and jet calibration. Because it acts so fast you can see pretty much what is going on with the pump jet actuation and calibration. Here you need to calibrate the system until it uses the minimum of fuel enrichment to cover the momentary lean spot caused by opening the throttle.

At the end of the day a wide band O2 analyzer will tell you which way you need to go with your adjustments. There is however one aspect you always need to keep in mind. An O2 mixture analyzer reads looks at the amount of oxygen there is in the exhaust to determine what the air/fuel ratio is. If there is a misfire we find that oxygen will, instead of being consumed in the burning of the fuel, will show up as unused oxygen in the exhaust. This will make the O2 sensor think that the mixture has gone a lot leaner than it really is. So the golden rule here is make sure you are not measuring mixture and going by the results indicated when the engine has any form of misfire present.

Timing.

We all pretty much know how to get the best timing for power after the mixture has been optimized – run down the strip and test until you get the best time. But for mileage things are somewhat different. Fortunately the vacuum gauge makes life very simple here. What you do is adjust the timing so that at the test speed the highest vacuum is shown. Yes it is that simple to set it – now you need to determine what that setting is because riding down the freeway sitting under the hood with a timing light is not a practicality. I went through this scenario about 25 years ago with my Chevy powered (CA Spec) Pontiac TransAm. Here is how I did the timing to get a vacuum can just right for the job.

This is my 1G cornering Pontiac TA about 1982. It was tested in the above form by an English auto magazine editor. The published figures for acceleration were 0-100 in 8.8 and 0-120 in 13.0. Not bad for a street driver that my mother used to get the groceries in! Best ¼ mile in the form you see it above (street tires) was 12.38 seconds at 118 mph. As for mileage at 65mph cruise this went from 13 at 65 mph to 20.2 at that same speed. A higher rear end ratio would have been a good idea but I never got around to it.

To get some sort of an idea as to the vacuum ‘curve’ required I did tests at 30, 50, 70 mph. At each of these speeds the rpm and vacuum gauge reading where noted. After finding what was optimum at each I reproduced the highway advance in the workshop as follows. Connect the distributor vacuum advance to a Mighty Vac hand vacuum pump (most parts stores have or can get them). Rev the engine to the rpm for the speed involved then pump the Mighty Vac to the vacuum seen on the in vehicle vacuum gauge. What ever the advance reads at the damper is now what is needed at that rpm and speed. Note it for each. At this point I called Kelly Davis (Steve’s dad of late) at Performance Distributors and he made me up a HEI distributor unit that combined the vacuum and mechanical advance curves required.

Sounds like a lot of trouble to go to but getting all this right was really worth while. Freeway mileage at 65 mph went up from just over 13 mpg to 20.2. As impressive as this is that’s only half the story. Driving around town with the dyno distributor which had no vacuum advance I was lucky to see much over 9 mpg. After the time spent on finding out what was needed and having Kelly build it for me I got right on 15 mpg! And just for the record wide band O2 mixture analyzers did not exist in those days. However a friend of mine who has since passed on was an electronics engineer and built several systems for my project cars using the regular O2 sensors used back in the day. He also built an 8 cylinder unit for the dyno. What a deal this was – beats cylinder temps by a country mile. From the best we could see by mass flow of fuel and air plus exhaust temperatures the first hour with O2 sensors on one of our race motors resulted in a 16 hp difference in output over the best we could see without such. The big deal though was that all this was done by stagger jetting. The average air/fuel ratio remained almost unchanged!

Conclusions.

From the forgoing you can see that mixture ratio’s and timing are very important aspects of achieving economy as well as cutting raw pollutants. But when you are done setting up, the vacuum gauge goes on paying for itself for as long as you own the vehicle. If you drive to keep the vacuum high you will find that you get way more mpg than if you ignore it and get on the gas anyway. Observing that vacuum gauge is a great way to teach yourself economy driving. You will see results the first tank full of gas – and that’s a promise.

David Vizard

[old blue 75]

David a couple of questions. With the timing set up for mileage
are you still getting your total WOT timing correct?
I also have a Chevy truck that needs mileage tuning. My question is, after you tune
for mileage and have your leanest settings achieved, when you use the truck to pull
your race car to the track do you risk burning pistons because the mixture is to lean?
Or are you far enough into the throttle to bring in more fuel?


Thanks old blue 75

[rookie]

Old Blue,
I'm not sure about towing, I would think that would take a totally diff. setting than what’s needed with no load, may be you could use a elec. valve that stops the vac. adv. from coming in conjunction with the thing used on carbureted cars with a/c that bumps up the throttle setting when the a/c is tuned on????

Hook it to a switch labeled, Tow/Haul mode.

The way I did my timing on my 350 with iron heads, 0 deck, flat tops, 262 extreme energy cam pumping 200 psi cranking pressure on 92 oct.

I found that 1 soft spring 1 medium spring worked best.
I started by setting total timing to 36* without the vac. adv. Hooked up, the I hooked up the vac. Adv. As David describes and dialed it in to give me 30* at idle this gave me 40 at part throttle ant more and it would ping going up hill.

This is mostly a trail and error process but well worth it.

Setting my timing this way allowed me to close the throttle blades about 3 turns over previous settings it idles lower smoother and has far better part throttle response.

Thanks DV. Wish I had known this 10 years ago.

I also use small pieces of wire in the idle fuel feed jets on my holly to lean out the idle and part throttle.

[Brian P]

@ old blue,

If you have the correct amount of vacuum advance (i.e. the difference in ignition timing between no vacuum (engine at full load) and cruise vacuum), the towing situation will sort itself out. The extra load when towing will reduce the vacuum, causing less advance.

The tricky bit might be that the vacuum advance parts that you can actually get, might not allow this to be done without causing other problems, hence why everyone has gone to mapped electronic-control ignition timing nowadays.

[rookie]

Also keep in mind that there are different vac. cans some soft for low vac motors i.e.: big cams, big heads, race motors.

Also different cans pull different amounts of vac. 10* 15* 20* and some are not adjustable.

I had a complete Pertronics system.

May be Steve Davis of Performance Distributors will chime in on this or you could contact him through his article.
Distributor Curve Science Simplified

He seems to be very quick at responding to questions and very helpful.

[DavidVizard-GFN]

Originally Posted by old blue 75

David a couple of questions. With the timing set up for mileage
are you still getting your total WOT timing correct?
I also have a Chevy truck that needs mileage tuning. My question is, after you tune
for mileage and have your leanest settings achieved, when you use the truck to pull
your race car to the track do you risk burning pistons because the mixture is to lean?
Or are you far enough into the throttle to bring in more fuel?


Thanks old blue 75

Here is how this all shakes down. First as the vcuum goes away so we are left only with the mechanical advance. This is exacly as required when the throttle is at, or near WOT. In that respect the engine runs as if there were no vacuum advance.

When towing the engine is at wider open throttle to go at cruise speed (say 65 mph) than when not towing. This results in less vacuum adfvance coming in so hopefully we once again have the correct timing for the circumstances.

As for mixture I leaned only the part throttle mixture not the WOT mix. This left the WOT situation unchanged so no piston burning from that aspect.

DV

[MAP]

Greetings David,

In another thread I asked Steve Davis about the efficacy of a purely mechanical/acoustical method of controlling spark advance, versus an electronic means using sensors for manifold pressure, rpm, and possibly throttle position. And come to think of it, we need an intake temperature sensor too, since temperature strongly affects permissible advance as well. A purely mechanical/acoustical system misses this entirely...

It would seem that an electronic system ought to be able to optimize spark advance with greater precision, repeatability, and range, than a purely mechanical/acoustical one.

But then again, when electronics die or malfunction, it usually means a really long walk home or a tow to the repair shop.* A system for this application not based on electronics OTOH, would probably fail gradually and "gracefully." It would probably a lot simpler and cheaper than the electronic one too.

Can you gauge how these strongly opposing factors would play out in general favor of one or the other approach?

Thanks,
Mark

*Most to the point, electronics don't like heat. In Yuma, AZ where I live, that's a huge consideration, which probably explains why GM is building a hot-weather test facility here.

[DavidVizard-GFN]

Mark,

All the factors you mention do play into the game here but they are falling into an area of deminishing returns.

The temperature and humidity of the air do play a role in determining the optimal spark advancve but to a far lesser degree than the vacuum. If we get the centrigufagal and vacuum advance right then we are probably 95% of the way there in terms of timing optimization for 95% of the ocassions.

All this of course points to the advantage of a fully programmable EFI system.
DV

[Nick]

David (or anyone else that might know these answers),

Can running leaner than stoich AFR's at cruise conditions/light loads cause any of the following problems over a long period of time (assuming a properly functioning engine):

Burned exhaust valves
Burned intake valves
Increased engine wear (especially cylinder bores)

I've read these things on other car-related sites from time to time, but they may very well be myths. I tend to think they're not true, otherwise I suppose cars like the old lean-burn honda Civics (the VX I think, which came with a WB O2) would've been very problematic, but I don't know for sure.

Thanks,
Nick

[MAP]

Greetings,

I hope answering here won't short-circuit the effect of the last post, but I just wanted to comment quickly that I'm much relieved to hear, David, that addressing the factors you discussed would get us "95%" of the way there.

MSD, to the best of my knowledge, still sells their advance-control kit (the same one you alluded to about halfway through your article) which allows you to adjust timing even while driving. So when the weather's cold, we add a few degrees of ignition advance, and conversely, when hot, we retard a few degrees.

In sum, maybe this is all that would be needed to go from 95% to 99(+)%? If so, the entire system would still be remarkably simple, cheap, and probably quite reliable.

Best,
Mark