Altho by no means the only two, two of the first subjects to pop up regarding mileage are overdrive and rear end ratios. Apparently the purpose of these is to slow the engine down, perhaps to put the engine in a good part of the power curve and/or just to keep it from blowing as much air and gas through it.

How far do you want to slow it down? Assumedly one could get it wrong. I suspect 1000 RPM and 5000 RPM are both out. But I don't see people talking about RPM much. What is the engines "comfort zone" in this respect? If I want an interstate machine, how fast do I want it to be spinning half a day at a time? What am I aiming for other than just swapping in a piece of magic hardware?

I might want to cruise 65 most of the time or 85 most of the time. What engine speed should I prefer for whatever I choose as my primary "rubber on the road" speed?

I have watched the tach on my 86 Dodge van (225), my 84 Ford van (351), and my Mom's 90 Buick Riviera (3.8 litre). The slant six typically operates in the 1500-3500 RPM range, the 351 is the same, but the Buick has overdrive and it operates most often in the 1200-3000 RPM range but mostly in the 1800-2500 area.

My theory to good gas mileage is to build a motor for low RPM torque and horsepower then gear the vehicle appropriately. This is why I am so interested in running an overdrive with a low rear axle gear ratio.

The fewer RPMS the motor turns the less gas it will use, therfore the more miles per gallon. THe RPM your motor will turn to maintain any given speed will depend on the gearing (rear axle, transmission, torque converter) as well as the internals of the motor (camshaft, pistons, valves) and the carb size.

Ross,

Most 225 powered passenger cars will be top speed limited by drag with 2.76 gears. Put another way you run out of horsepower before you run into red line which is only about 4000 RPM for the average 1bbl equipped 225. With an overall ratio lower than 2.76 you can get better economy at 55 MPH as the horsepower required to go 55 is a lot less than 85 MPH, but you may not have the power to drive 85 MPH in high gear.

At 60 MPH a 2.76 geared vehicle with 195/75R14 tires will be running right about 2200 RPM. At 85 it's 3100 RPM. You need the higher RPM at the higher speed to make enough power to overcome drag. You can't just say I want to run 2000 RPM at 85 MPH as you won't make enough power at that engine speed to do so.

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Joshua

No, I specifically and deliberately eliminate all those variables and turn them into constants by specifying a stock, off the showroom floor, let's say 1966 Valiant 100 with no power or AC or anything. It's just an ordinary /6 225 and an old a-body. An ordinary but well maintained 225 engine without anything special done to it. A run of the mill /6. I rule out gimicks and hypos and specialties so I can zero in on one thing. I specify as average as one can imagine, with no replacement rear ends to worry my brain, no replacement ignitions or disc brakes instead of drums - zip - nada. I'm not talking about any of these things. Variables have all been nipped in the bud. Variables have become the worlds ultimately boring constants!!!

Aha! Here we go!

An experienced /6 mechanic is traveling with a friend to a meet at the other end of the country in his friends stock 1964 Dart driver with a 225. They're running late and fast. At some point the mechanic says, "You're pushing it!"

Now, the simplest of simple questions. At what point would he NOT be pushing it?

Thanks.

I would guess, and it is just a gues since I know next to nothing about 18 wheelers or even diesel engines, the the RPM range the truckers pick for their motors is the RPM where the motor will make the most torque and horsepower and therefor produce the most fuel efficiency.

18 wheelers also have 12, 14, 16, 18 , etc.. gears to choose from, so if they need to maintain a certain RPM they have far more gearing choices than you would in a three speed valiant.

Think of it like building a 1/4 mile car- you know the motor will be turning max RPM the whole time it is running, so you can build the engine and select gears and components for that RPM range (typically 4500+).

A street driven car with a three speed transmission has an engine that will need to operate efficiently across a much wider RPM range than a 18 wheeler. Its not like you can wind the motor in your valiant up to the top of the power band then just grab the next gear.

As far as the answer to your question about pushing it, a stock slant would most likely run out of breath at 3500-4000 RPM, so once the car reached about 75 it would be turning near that speed. In that sence, the car would top out in speed and any attempt to make it go faster would be "pushing it." The redline max RPM of a slant is much higher than 4000 RPM, so you wouldn't be risking engine damage turning that RPM, but you would be pushing the boundaries of the one barrel carb. A bigger carb would flow more air and therefore turn more RPMs, as would a biger cam or bigger valves.

This is what I was talking about above when I said that the best RPM for your car will depend on a large number of variables.

Ross,

What is it that you want to know? I used to run my wife's stock 225, A904, 2.76 gear, disc brake '72 Duster up to 90 without worry. We even pulled a 5x8' U-Haul from Mississippi to California with it. 3.23 gears would have sure be welcome for that trip!

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Joshua

Aside from that I think I understand you to say if it can go any faster than it is, you're OK.

Thanks

When is an ordinary 225 happiest?

Not really. What I am saying is that when it comes to gas mileage there are too many variables to say what RPM will give the best gas mileage. A stock 170 from a 62 Lancer would get really crappy gas mileage if i put it in my van.

The best answer you are going to get without giving more specifics or defiing the parameters more is that a 225 will be happy in th 1500-3000 RPM range.

A related problem is engine vacuum.....

At lower engine speeds there's less vacuum and there's more likelyhood that you may engage the power circuit of the carb; really killing your mileage.... So you would be better off with more rpm to avoid the rich mixture.

A fuel injected engine can probably bypass the vacuum problem by only enriching the mixture after a certain percentage of throttle opening (i.e. staying in closed loop mode), and thus lower rpm would be beneficial.


Note: If you have a vacuum gauge you can test by watching gauge and notice the reading by keep a constant speed and changing gears...........

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

What I'm looking for is related to mileage only indirectly. That is, the mid-point of the primary effective operating range of the basic engine. Or to use a previous corny device, the point at which the engine is "Happiest" as opposed to "Pretty good,OK, or I went back on my depression meds!" A baseline to which one can relate all tweaks, variables, additions, replacements, improvements etc., etc.. A point of departure as opposed to a waypoint to be reached or stumbled onto. Every engine design has one.

A 225 is happy under load, for the most part, between 1500 and 3500 RPM. Yes, it will run outside this range, but the mid point for 1000-4000 RPM is the same. The mid-point is 2500 RPM which is just over 65 MPH in a non lock-up automatic equipped car with 195/75R14 tires and 2.76 gears.

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Joshua

Thanks

Just out of curiosity, how much does a lock-up automatic improve things over a non-lock-up?

Depends. In a relatively aerodynamic car with 2.76 or numerically higher gears it doesn't get you much. The less road load and the higher the gears (numerically) the less torque it takes to move the car and therefore the converter slips less. The pay back would likely not be reached in your lifetime assuming you had to pay to have the lockup trans rebuilt. But if you do foolish things like the factory did with late 318 cars and 2.45 gears the lockup helps more as the car is heavier, less aerodynamic and has less torque multiplication in the rear axle.

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Joshua