TUNING FOR MILEAGE
By Pete Snidal, Cheapskate Mechanic (C)2000
Having been in the business of trying to get people interested in some products which really do increase mileage in every case it has yet been my privilege to examine, I have found a startling fact: Many People Don't Know How To Calculate Their Fuel Mileage.
Now I'm not interested in being judgemental about this - a lot of us have been out of school for some time, and a lot more of us never learned much while we were in there, so I've prepared a file you may want to click on: How To Calculate Your Fuel Mileage
Once we're past that, let's discuss a few of the parameters that affect fuel mileage....
Checking Engine Condition
If you are to get good mileage and power, your engine must be in good condition. Valves and piston rings must be sealing well - leaky valves allow expensive fuel to wheeze out before it's burnt, and bad ring sealing will do the same. So you should check your engine's condition by doing a compression test, using a reliable compression gauge. If your engine's in poor condition, some of the fixes described here may help, but none as much as doing the rebuild that is really necessary. And while you're at it, be sure to change the camshaft and lifters - a worn camshaft will do awful things to power and mileage, and most factory-stock cams are bad comprimises, trading things like low emissions at idle speeds for economy at running speeds, stuff like that. Look around the aftermarket parts sources for a camshaft that is more tailored to your specific needs.
CHEAP FIXES - THE EASY ONES
1. Ignition Timing - The Flame Front
The Flame Front is the term used to describe the beginning of the cloud of rapidly expanding gasses that is the basis of the internal combustion engine. When the fuel mixture is ignited by the spark plug, or in the case of the diesel, by the introduction of fuel into the superheated compressed air in the combustion chamber, at the top of the compression stroke, the burning occurs outwards, in this case towards the piston, and it is the combustion of which this flame front is the beginning that makes the power to get us from A to B. Obviously, we want this combustion to be as efficient as possible - to give us the most bang for the buck, the most piston movement and power for the least amount of fuel.
The basic idea is that the flame front has to meet the piston at exactly the right place. If the piston's still on its way up, it's obviously going to be an ugly situation - flywheel inertia is forcing piston up, expanding gasses from burning fuel are trying to push it down. It gets over the top, but accompanied by pinging (spark knock), power loss, and pretty soon serious engine damage (holes in pistons, for instance - the piston crown gets so hot after enough of this madness that it semi-melts and gets a hole blown in it. Ugly. So excess spark advance - firing too soon - is to be avoided at all costs.
On the other hand, , retarded spark is when the piston has gotten over the top, and the flame front actually has to chase it down the "power" stroke, shortening the actual length of time the piston is being driven by the combustion. Needless to say, you're not going to be developing the best power this way, and the incomplete combustion results in crappy mileage, exhaust valve and general overheating - from fuel burning where it shouldn't, because it didn't burn well enough where it should. So iginition timing is all-important.
The best way to control timing used to be left to the driver. Manufacturers assumed s/he had the situation in hand. Since most don't, manual - driver-controlled - spark timing hasn't been an option snce the days of the Model T and A - that wasn't a turn signal lever there under the steering wheel. Being a mileage freak myself, I was partway to building a driver- controlled timing arrangement on my schoolie. Got as far as mounting a cable and control in the dash, routed to the distributor. Intended to fab a new distributor clamp arrangement so I could turn the dist while driving - when conditions change, such as hitting a hill, the driver can advance the spark till he hears ping, then retard just enough to stop the pinging - the maximum advance possible will always give you the best power and mileage. But overdoing it will blow your engine. (This is why the advance specs set by manufacturers will always be conservative.) And of course, when you get to flat ground again, the engine, being under less load, will happily carry more advance, so a power change will require another advance setting.
In the days of Model T's and A's, and even motorcycles of the '50's, manual advance was the order of the day. - the left handlebar of Harleys and Indians until the mid-50's had a twist grip just like the throttle on the right for tuning the advance as you rode - changes in power setting require changes in advance.
How did we handle it after that? Centrifugal and vacuum advance controls. These basically allow a mechanic in a shop to set the timing on an idling engine and get it "in the ball park." When the engine rpm increases, it generally needs more advance. So we have a centrifugal weight arrangement inside the distributor that does this. When the load increases, we generally need some retard. So we have a vaccum pot on the distributor, which relaxes on low vacuum (high throttle opening) and lets another spring retard the timing. Crude, but ball-park effective. All distributors have an "advance curve" which is set by such things as the strengths of the springs in there, the size of the weights, the size of the vacuum diaphragm, etc. If your mileage is bad, it could be your distributor's curve has gone to hell - this can be tested on a special distributor machine which any reputable tune-up place has. A blown vacuum diaphragm, or stuck advance weights could allow your timing to look great with the light in the shop, but leave you 'way retarded on the road. So you might want to get your curve checked as a first step.
Ping Timing
Ping is the name given to the sound made by pre-ignition, or detonation. It is also called spark knock, among other things, and sounds like marbles rattling around in your engine. It appears, if at all, when you open the throttle, and disappears when you back off. It is a sound of major danger, but it is can also be the sound you need to tell you that your ignition is just about right.
Personally, I have never relied on timing lights. So, how DO I set timing? Well, I want that top advance under actual driving conditions, so I set the timing first of all in the shop for fastest idle, then retard it just a touch. Then I take it out for a test drive, wrench in hand, and adjust the timing on the drive. I'm looking for just enough advance to make it ping when I'm lugging it - putting on more load at less rpm than I would normally drive it. I punch it harder than I normally do on acceleration, or leave it in a higher gear for longer than I normally would, and I expect to hear just a little pinging. Then, I back off on the throttle, or drop it down a gear, the ping goes away, and I know I'm right on the edge of pingland - that's where we want to be. Then I tighten down the distributor, and occasionally lug my engine a bit, listening for that tell-tale ping to let me know my timing's as advanced as it can be.
Addendum: Mar, 2000:At least, that's how I used to do it! I've since been contacted by an earlier reader of this article, Michael Starkey, who told me of his experiences at a time when he was fortunate enough to be performance-tuning on a chassis dynamometer. He originally tried for max advance, too, but tells me he found that the best power (and therefore, presumably, highest efficiency) is at a point a bit more than just under the ping level - so now I go back a bit more retarded once I've stopped the ping. Thanks, Mike! 444
One last thing - NEVER drive on ping. Sounds like marbles rattling around in the engine at low rpm/heavy load; goes away the instant you back off on the throttle - you can "play" it with the gas pedal. It'll do major engine damage in a hurry. It's caused by too much ignition advance, or too lean a mixture - lean mix pings too, and that's why, if you're experimenting with carburetor jetting, you need to be sure you're not running to lean by doing a plug check. Not in the shop, but on the road, pulling halfway hard, where it counts.
2.) Exhaust Tuning
Most race engine builders have discovered the importance of Exhaust Tuning. Your exhaust system has a resonant frequency, just like a radio antenna or a trombone, and there is a proper length for your exhaust system for any given rpm, or pulsation frequency. There are formulae to determine this, which factor in pipe diameter and length, rpm, no. of cylinders on the pipe, and other factors. I've always been a "thumbnail tuner," myself, and I found what's best for the average V8 is dual pipes with a cross-over, and a pipe length of 11 to 12 feet. My 391 powered Ford schoolie, weighing in at a conservative 28,000 lbs., running a Holley 4 bbl carb and Allison 4-speed automatic, delivered a consistent 7 mpg running up and down the west coast between B.C. and BC. This is with pipes that were not crossovered, and at a length of 12 feet. - That may not sound like much, but in our travels we met a number of incredulous owners of Ford 3/4 tons, carrying campers, and powered by the very similar Ford 390, who seemedto have a lot of trouble believing me. "That's more than I get! they'd say!
So much for the "cheap" stuff - after all , you need an engine in good condition, timed one way or another, and an exhaust system in any event. Now, let's look at some mileage "frills" - things you can get by without, but maybe, in the interests of good economy, you shouldn't.
A good place to start is your ignition system. If you're still running points in your distributor, you should look for an alternative high-energy electronic ignition system. Since these have been stock equipment on just about every engine made since the early '70's, auto wrecker parts can do this for you. If you insist, you can also buy after-market ignition systems with many claims for better mileage, but, like most other things for sale, this is a good time for caveat emptor - buyer beware. I just checked out an ad for an ignition system that comes with a 30-day guarantee that it'll give me 15% better mileage - but the price is over $500.00 US! Wheeee! (Using Pro-Ma in a healthy engine show me 25% or more in every case. Price to try: under $20.00!)
An Extremely Good Idea in ignition systems comes on many new vehicles, but the aftermarket version is, so far, something I've heard about but never yet seen. I heard that Carter made it, or even maybe still do. It's for electronic ignitions only, and it's basically a black box that fits into your ignition system, and electronically lags your timing JUST ENOUGH to stop the ping. It's input is in the form of a sensor or two that bolts onto your cylinder head(s). These sensors sense ping, send the info back to the box, which lags the spark just enough to stop the ping. In the installation, you set the distributor 'way too advanced, and the box makes sure the ping doesn't happen. Ideal, and I've been looking for one. All I've been able to find is that Bosch supply such a system stock on later VW's, Saabs, and Volvo's, but I don't think it'll bolt on to my Ford 391 and 460's.
There was an ad on the Art Bell show a few years ago which
sounded like it, but by then I wasn't into RV's, and my cars and bike do
very well anyway, just keeping them properly tuned, and of course using
my favourite fuel and
oil additives. But I'll be trying to track this thing down for
my Flxi for sure - rear engine, no possiblitly of hearing ping, so I'll
have to set my timing conservatively or find one of these gadgets. The
price I heard was around $200.00. If you run into one, please let me know.
Click Here To Mail Me (Editing note:
I finally tracked it down - it's the $500.00 one. Later.)
Camshaft
Ignition timing is not the only timing variable in an engine. There is also camshaft timing - the opening time, closing time, duration, and lift of the valves can make huge differences in the performance characteristics of an engine. Such characteristics as power band - the range of rpm in which good power is developed, maximum horsepower - generally, you trade big power for a narrow power band, wider band gives a torquier, lower-rpm engine with greater range of roadspeeds in any given gear, and yes, mileage; all these can be traded around for one another by choice of camshaft.
Different manufacturers, like Iskenderian, Crane, etc. make after-market cams, ground off the normal range, for specialist apps like high-end power (race cars) mid-range torque (us) and torque/mileage. the latter are often referred to as "RV cams." RV doesn't necessarily mean just for RV's, by the way - we used one in a 460 Ford-powered dirt track stock car for years - but it does mean wide mid-range, and hopefully better mileage. Although I've tried one in a race car - we wanted wide-range torque and low rpm with tall gearing for motor life, and it worked well against all those wheezy little small-block Chevys - I have yet to put one in an RV engine. Next time, for sure. If my 391 isn't satisfactory in my new Flxi, I'll build a 460 with RV cam for certain.
Changing cams does cost money. Not a lot extra if you're rebuilding the engine, but it's a bit of a hassle to do midway through the engine's life, unless you're got it apart for something like a top end or freshening. Definitely a thing to consider in these cases particularly, since a worn cam will give you really weird timing, with definitely detrimental results to power and mileage. I understand that smallblock Chevvies respond particularly well to cam replacement - they're only $35.00 or so, so you should always replace them when rebuilding.
Additives, You Say?
Finally, you can spend just a little money trying additives. There are thousands of them on the market, and you can spend a lot of time trying different ones until you find one that works. I did this for quite a while, and as you know, found one that works very well. I use Pro-Ma oil and gas additives in everything I own; wouldn't dream of doing otherwise, and it saves me a bundle. You can read all about that elsewhere. By clicking on the link, for instance.)
Don't be a Hick! - Click the pic!
