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NVSmith 04-26-2012 06:28 AM

Timing by Vacuum
-This may be old news to many of you and maybe won't work with most electronically controlled cars unless you have a computer hookup, but...
-Way, way, back in the days of distributors & coils & carburetors and such a mechanic taught me how to adjust the timing of a car by using a vacuum gauge. He pointed out that a timing light actually timed only one cylinder since it was hooked to just one plug wire.
-He ran a vacuum guage from the intake manifold, ran the engine at about 2k rpm and then adjusted the distributor until it read max vacuum. He would then back off about 2" Hg on the guage and lock down the distributor and remove the guage. For kicks, he would again hook up the timing light and show how far off factory specs he was.
-Now this trick was particularly useful since we lived at about 5k ft elevation.
-In the days of JC Whitney tools that allowed one to adjust the distributor while driving, on my trips from Reno to Las Vegas (450 miles; two stop signs and four traffic lights, no speed limit) I hooked a vacuum guage to the dashboard and played with various settings close to max vacuum. This leaned the engine out and my Dodge Slant Six gave fantastic mileage. One just had to remember to back off a couple of inches when slowing down or entering town.
-Of course I wasn't driving at 110 mph either. 65-70 or so was good enough.
-Four stroke reciprocating engine airplane pilots know about this and they also know that they have to watch EGT (exhaust gas temperature) lest they start burning valves.
-I havent the faintest idea how to go about doing something similar with an ECU but imagine that the O2 sensor(s) would play a major part.

gone-ot 04-26-2012 01:29 PM

Today's ECUs, once in "closed-loop" operation, keep the engine at stoichiometric 14.7:1 A/F all the time, which is NOT what you (and aircraft pilots) do when you manually adjust the timing for "maximum best vacuum" or best fuel economy (which is quite 'lean' operation).

user removed 04-26-2012 03:03 PM

When you go that lean on a modern engine your NOX skyrockets. It's the same reason Honda stopped making the VX. Sadly when the govt makes absolute levels of emissions that can not be individually violated by even a very small percentage, then many innovations are never implemented. Had govt made emissions a combination of the individual components with variations that might individually exceed desired levels, then many improvements would have been allowed to continue development and the progress of economy and fuel delivery strategies could have taken a radically different course.


NVSmith 04-26-2012 03:42 PM

-To both "Old Tele man" and "Old Mechanic": I agree with both of you.
-Note that I said this was something we used to do and included the caveat on burning valves.
-It was just something we did, such as using a vacuum connection to the intake manifold to "inject" a water/alcohol mixture.
-I note that some diesel engines are using a similar philosophy and replacing up to 20% of the diesel fuel with propane/CNG for increased power and reduced emissions.

-Thanks for your comments; they are appreciated.

gone-ot 04-27-2012 03:52 PM

...FWIW, the Flight Engineers in our EC-121 "Warning Star" radar planes would do exactly what you described with the Wright R-3350 "turbo-compound" engines to achieve maximum economy, BUT they also had cylinder-head temperature gauges to monitor how HOT the cylinder heads were getting, something not available in today's automotive engines, which only monitor coolant temperature, not actual cylinder-head temps.

NVSmith 04-27-2012 09:26 PM

-If memory serves, the R-3350s, at least the early ones, had a really nasty reputation for overheating anyway. Maybe by the time of the Connie and EC-121 they had it cured, but it was a problem with the early B-29s.
-Overheating is most definitely one of the major problems of a lean burn.
-I think the water/alcohol injection was to help cool the cylinder and prevent detonation, but that's really out of my field.

user removed 04-27-2012 09:57 PM

If by R3350s you are referring to the B29 engines, the problem was solved by adopting fuel injection. Poor fuel distribution caused the rear cylinders to get leaner mixtures. By solved I mean the reliability of the engines grew exponentially when FI was adopted.


user removed 04-27-2012 10:02 PM

I remember a friend of mine who used to race go carts telling me about the hopped up Briggs&Stratton engines running on alcohol. I think they dynoed at 18 HP. He said he could put his hand on the cylinder head and it would not burn, but the exhaust would burn him badly. Funny how the heat from combustion was concentrated in the exhaust.

One reason why you can get so much more power out of most engines wit ha significant amount of alcohol for fuel.


Christ 04-27-2012 10:31 PM


Originally Posted by Old Mechanic (Post 303635)
One reason why you can get so much more power out of most engines wit ha significant amount of alcohol for fuel.


Because there's less heat loss to the chamber? Doesn't that have to do with the faster burn rate of the fuel?

NVSmith 04-28-2012 09:13 AM

R-3350 hear & using alky
-I did a little Googling on the 3350s, but not enough to realize they were fuel injected. I'm impressed at that: during WW2 only the Germans managed to mass produce fuel injected engines; I didn't realize we ever did. One thing that did pop up was an offshoot of the R-2800 development. It turns out that some bright machinists found a way to cut superfine cooling fins that allowed better air circulation. There were still a depressing number of B-29 crashes due to engine fires. Well, we had a war to fight. I wonder how many hours total & B-29 specific pilots & copilots had before shipping out.
-Yes, alky fueled engines have some advantages. I wonder what compression ratio he used? One problem, up to recently, was that the alky just ate up the rubber & neoprene fuel lines. That's why car manufacturers were/are against even 10/15% alky content in gasoline. The stuff is also hygroscopic. The super serious downside to alky is that, in general, one cannot see an alcohol fire. I worked in disaster management (what a euphemism) for a few years and the two biggest industrial problems we faced were chlorine leaks and alcohol filled tanker cars. Unless someone saws the grass burning under a tanker "we" had no idea there was a problem. One of our projects was to get temperature sensors mounted on tankers.
-RE: compression ratios. Boy, are we getting way off the original topic, whatever it was. One of the reasons that the so called dual-fuel (gas & propane/CNG) cars don't do so well on straight natural gas is that the comprssion ratio is too low. The natural gas has fewer BTUs per unit of energy (that didn't come out right, mea culpa) but the users don't want to go to high compression heads because it affects resale value. Dummies.
-Good info, Old Mech. I'm learning new stuff!! Thanks.

Christ, I simply don't know about the heat loss to the chamber. I know that water/methanol injection was used to temporarily cool cylinders and prevent detonation, but I don't know about the heat of a pure alky engine. One thing I did find out is that ethanol & methanol have only about 2/3 the BTUs per volume of gasoline, so it stands to reason that alky would burn cooler and the cylinder head would only produce (max) 2/3 the power but the engine had been designed to dissipate the heat from burning gasoline. On the other hand, I may have just opened my mouth & firmly inserted a 10 1/2 E foot.

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