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Rear Axle ratios, what can i use?


Guest Mr. Norberg
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Guest Mr. Norberg

Gentlemen.

As an Swedish owner of an -48 Buick Roadmaster Convertible, and with fuelprices that since long have become ridiculous (2 dollars for a quarter gallon) its always on my mind how to make my beloved Buick to suck less fuel.

Today it is as thirsty as the spaceferry 2 seconds after ignition.

After some digging i have come to the following conclusion;

-I can use any differential from any Buick up to 1955 and it will fit directly without any hassle in my rear axle. Correct??

-The only thing to take in to consideration is the fact that the length of the driveshaft between the gearbox and rear axle differs depending on if its an 3-shift or an Dynaflow. Correct??

The thing is, i do want to keep the original Dynaflow that already sits in my car, since its the ancestor of all modern automatic gearboxes and deserves to stay in service. But, if i tuck in an lower ratio diff in my rear axle.. as low as possible; how does my original Dynaflow deal with the higher torque?.. i mean for a given speed of the car, the rpm in the dynaflow is going to be lower and the torque and stress is going to be higher... like driving uphill all the time.. kind of..

Anyone with experience in this matter?

My Buick at my brothers wedding :-)

BrllopSEP2011Buick.jpg

Edited by Mr. Norberg (see edit history)
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I don't know what the original ratio for your Roadmaster with Dynaflow is, but I am guessing it is around 3.9 to 1. A differentail with a ratio of 3.6 to 1 should give you better gas mileage and somewhat higher road speed with a little less pickup from a standstill. If the 3.9 to 1 ratio is correct for the original, then the 3.6 to 1 is about an 8% difference. If your original ratio is 4.1 to 1 them the 3.6 to 1 is about a 12% difference. I don't think either of these will cause serious problems but I must say that my only experience is with a rear end swap on a standard transmission car.

The change is done by replacing the complete pumpkin and might require you to use your original drive shaft and torque tube.

Joe BCA 33493

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" i mean for a given speed of the car, the rpm in the dynaflow is going to be lower and the torque and stress is going to be higher... like driving uphill all the time.. kind of..

Anyone with experience in this matter? "

I have no related experience with a 48 Dynaflow but I would think you'll not find appreciable milage gain unless you are driving long distances at highway speeds. on Local runs I would think you'd stand a better chance of higher gas milage if the vehicle can power up to speed quicker with the higher rear axle ratio. Too bad with the original closed rear axle set up, an overdrive unit is practically impossible.

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It might be well to get the "higher" and "lower" orientations a little better defined.

A "lower" gear ratio would be like going from 3.6 to 3.9.

A "higher" gear ratio would be like going from 3.9 to 3.6.

A "higher" gear is numerically lower than a "lower" gear, which is numerically higher in number.

. . . now that that's done . . .

In reality, unless there's a mechanical issue with your current rear axle, you can spend LOTS of money and not gain any significant fuel savings--period.

As for the functioning of the DynaFlow . . . the "coupling" (I'm not sure if I should call it "fluid coupling" or "torque converter" for this trans, but I suspect "torque converter") is where the inefficiencies are. How soon the driving and driven segments of the coupling are running at pretty much the same rpm will determine how soon you might reach "cruise" economy levels. With the added mechanical leverage of the 3.9 (for example) rear axle ratio, it should get to a given road speed sooner and with less ultimate throttle input than if it had the 3.6 (for example) ratio. The other fact, unfortunately, is that DynaFlows were not known for their fuel efficiency or efficient power flow (compared to a manual transmission vehicle).

Even if you did find a good rear axle with the later "higher" ratio, you'll probably notice fuel economy improvements (at cruise) of about 5% at best. Dropping the cruise rpm a few hundred rpm might make things smoother and quieter, but not cost effective with respect to gasoline mileage.

You funds can be better spent by getting the fuel mixture curve of the carburetor(s) changed somewhat. Putting the car on a chassis dyno and simulating "road load at cruise" would tell you where the mixture ratio is for the gasoline you have available. This will give you the information to start seeing what can be done to improve things. Cruise fuel/air ratio should be about 14.8 to 1. Idle mixture can be down into the lower to middle 13.0s, possibly, but being a little leaner is better than not, for idle.

Sometimes, if the fuel octane will allow it, you might advance the base ignition timing (AFTER setting the ignition points to spec) a few degrees. This might also slightly compensate for a richer mixture, leaning it slightly.

Otherwise, an exhaust system with larger diameter pipes and a lower-restriction (but NOT louder) muffler might let it run easier at highway speeds and with less throttle. Key thing is "less throttle" as that will generally mean that the engine is not working as hard. Higher resultant manifold vacuum usually means more fuel efficient, but that might remain to be seen, in this case.

In the realm of spark plugs, many of the modern platinum and iridium plugs now use a .060" center electrode and a narrower ground electrode. Or they use a combined design to have the spark's "kernel" not be as shrouded by the ground electrode, which means more exposure to the fuel/air charge in the combustion chamber.

One of the old tricks was to shorten the ground electrode (what you bend to set the plug's gap) to where it just covers the center electrode about 1/2 way. You can first use a pair of diagonal cutters to snip it and then use an ignition point file to dress it square again. NGK, DENSO, and a few others build plugs of this nature, but they might not have plugs for your Buick Straight 8 (I like NGKs better, with their "V-Groove" plug being close to what I'm advocating you do). The other similar trick, with no cutting required, is to take a pair of pliers and gently and carefully bend the ground electrode (rotating it as you do) sideways to uncover 1/2 of the center electrode. This might not work as well, but it's easier to do.

I ran across this trick, first, in an old Petersen Publications (the parent company owner of "Motor Trend", in the earlier times) book on vehicular ignitions. Seems they called it "J-Gap" and Champion built some racing plugs this way. I spent some hours, back then, J-gapping the Champion plugs in our '66 Chrysler, plug making sure the gaps were "just right".

In more recent times, I discovered the NGK V-Groove plug in an NGK spark plug brochure at a Toyota dealer. Seems that some OEM Toyota plugs were this way. NGK sells these plugs under "V-Power" with a different heat range nomenclature than what the OEM-spec Toyota plugs use. I discovered that one heat range of the OEM Toyota plugs crossed into the same heat range and thread spec for my (then) recently-purchased '80 Chrysler Newport with the 360 V-8. So I got some. The Motorcrafts that were in the car were burning perfectly, but when I put the NGKs in it, I could tell an immediate difference in the sound of the engine and the way it responded to throttle input. I've NEVER had something like that happen! It didn't seem to get significantly better fuel economy, but the sound and eagerness made up for that.

I started using these NGKs in my other cars, too. In my '77 Camaro 305, they didn't make the same immediate improvement . . . BUT when I first started it in the morning, if it hinted at trying to die on a cold start, just a slight pat of the accelerator pedal (for a short shot of accel pump fuel) would bring it back immediately. In my '70 Skylark 350 V-8, it ran a little better, or seemed to. So, if the combustion chamber is more open and less active (turbulence-wise), the greater difference in driveability they seemed to make.

So, tweaking the spark plugs in this manner might help. With greater flame kernel exposure, it SHOULD be a better burn, no matter what. Ensuring the center and ground electrodes have sharp edges on them, that should also help decrease the voltage requirement for the spark to jump between them.

As for the exhaust system, I suspect the factory pipe is about 2.0" in internal diameter, all the way to the back. Upsizing that to about 2.25" pipe, with a lower-restriction muffler (like an OEM-spec Walker exhaust muffler for that pipe size or one of their DynoMax mufflers might be an option. Get the system bent to STOCK CONFIGURATION, rather than how many muffler shops (in the USA, at least) like to do them, a muffler of the same approx length as stock, and it should look "like it came that way", until people start looking closer. Or somebody hears it sound different (hopefully better).

To me, these "tweaking" and "fine tuning" operations would be more cost effective and make the vehicle more fun to drive. Fuel economy increases should be more forthcoming as less throttle would be needed (keeping the "power system" of the carburetor "closed" and not needed as much).

The other issue could be driving style. I suspect driving a DynaFlow could by similar to what I encountered when I change the trans in my Camaro to a later Z-28 version, with a looser torque converter . . . but with the existing highway gears (2.56 in this case). Until the torque converter coupling "locks up", or gets to its most efficient power transmission capability, "slip" will happen in the torque converter, especially with a torque converter with a higher stall speed. So, a steady throttle at cruise is highly important! Not making the converter slip unnecessarily, basically. In my case, the stall speed was about 2000 rpm, with 60mph being about 2100rpm. But "stall speed" is at WOT rather than part-throttle (as cruise is), but it was still a little looser than normal, especially for that "highway gear", so a steady throttle (and paying attention to such) is very necessary.

So, I'd recommend you find a speed shop with a chassis dyno to do some "road load fuel curve" tests . . . NOT power checks at full throttle per se. At this point, you're just looking to see where the fuel mixture is as a baseline of sorts. Under "road load", you can also determine at what throttle input the carburetor's power mixture kicks in. You might have the car tuned "to factory specs", but the fuels they had back then were NOTHING like what we've got now, which could mean some carburetor re-jetting might be needed . . . larger or otherwise . . . for best and most fuel efficient performance.

Just some thoughts . . .

NTX5467

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It might be well to get the "higher" and "lower" orientations a little better defined.

A "lower" gear ratio would be like going from 3.6 to 3.9.

A "higher" gear ratio would be like going from 3.9 to 3.6.

A "higher" gear is numerically lower than a "lower" gear, which is numerically higher in number. NTX5467

Thanks NTX. I think you're right about the orientations. I have to adjust to this as I always called em the opposite.

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Guest Mr. Norberg

NTX5467, Many thanks for your extensive awnser. Obviously you have given this a lot of thought.

After reading your awnser i think im going to stick to getting the engine running as efficiently as possible, as you suggested. For now.

-Having the Cylinder head reworked is my first step and its abbout be by finished now. The valve seats looked terrible.. Interesting this with the sparkplugs and how the spark as such are exposed.. or not.. i would'nt have given that a second thought.. The carb, well.. is cleaned out with new seals and yes.. maybe new jets would contribute some as well..

I have also since the day i first saw it, always suspected that all the 90° angles in both the inlet and exhaust manifold is causing the engine to suck a lot more fuel as well.. but to change that would be to visible.. hmm

-how to make a change and still make it look vintage.. a row of giant bike carbs from the 50´s ? :-)

Edited by Mr. Norberg (see edit history)
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