I need to think out loud some more

And you get to read it, lucky you! This is one of the ways I solve problems, but I usually don’t keep the process after I’m done, but I’m thinking since this isn’t how to make a man-portable nuke, or how to shut down the stock market, or any other destructive thing that could cause chaos and destruction if I let it out in the wild, this one I will let people see the process.

The problem needing solution tonight is the question of will using a Powerglide transmission and a quick-change rear axle be better for the autocross and still usable for just driving? From known data the QC has about 10 pounds less mass than the 9″ Ford rear end or is way cheaper than an equal weight fabricated aluminum housing 9″ Ford, going back to the Eternal Triangle: Light, Cheap, Strong, pick any two.

The other thing is you can’t street drive with the PG without changing the final drive ratio from the race setup, the ratio is Too Low for any highway driving. The final drive for the autocross setup is top of the RPM band in first gear at 40 MPH. For an LS engine that would be 6500 RPM. Now because the PG comes in two low gear ratios, 1.78 and 1.82 you can see the problem. Even going from the 23″ tall race tires to the 27″ tall street tires that would make the freeway RPM an unbearable 3110 or 3042 depending on which ratio transmission I get. Either one would result in horrible mileage, horrible engine noise and added wear and tear on the engine. What I’m looking for is a highway cruise of 1900 RPM or less, so you can see what the problem is.

Now the reason I want to run the PG is weight, both absolute and rotating mass. My other options are the 4l60 and variants, or the 4l80, the first weighing in at almost 200 pounds with fluids, the second is 240± with fluids. A fully race prepped PG is [drum roll] 96 pounds with fluids so 100 to 140 pounds less weight on a car that would weigh 1800± pounds with the 4l80. And do I really need to show how much 140 pounds off an 1800 pound car is as a percentage? Plus I don’t have the actual rotating mass for all 3 transmissions but I know the 4l80 is the highest and the PG is way lower and the 4l60 is somewhere in the middle but closer to the 4l80. And going back to Commonly Held Beliefs About Rotating Mass For Hot Rodders every pound of mass rotating at engine speed is equivalent to 5 HP, so going from the 4l80 to a PG not only takes 140 pounds off the static mass and sprung weight, but it takes a Large Amount off the rotating weight. As an added bonus the PG is one of the strongest automatic transmissions you can buy for normal car engines. Of my two choices the 4l80 has the best power handling but slightly worse ratios which is why I was looking so hard at the 4l60 based transmission. But neither of them can hold a candle to the PG in torque capacity. The PG is pretty much the standard transmission for a Monster Truck with 1800-2000 HP alcohol-fueled big-block engines, so strong, and light, and low rotating mass.

But to use it with the Sprint-T I need a way to easily change the final drive, or invest in a truck and a trailer to haul it between races and pretty much resign myself to only driving to my local grocery which is close enough to not drive me crazy with the RPM and noise from the engine. That’s where the QC axle comes in, it takes about 15 minutes and about $70 for a different set of spur gears to set the highway cruise to 1900 RPM. Now the QC cost is about $1k over the Ford 9″ unless you try to get the Ford as light as the QC and carry two center sections to have the race and highway ratios because part most of the higher cost of the QC over the 9″ is magnesium and aluminum EVERYTHING. Also changing the final drive on the 9″ requires hours of work setting the lash and engagement depth on the gears, or carrying around a spare centersection to swap from one to the other, and also about an hour of laying on my back at the race track going from one to the other. The QC requires an extra set of spur gears at $70/set and about 15 minutes unbolting the rear cover, swapping the spur gears, and bolting the cover back on and putting the gear oil back in the housing. That last bit is very important if I want to keep driving more than a few miles from the track.

Now I have been thinking about it and I can live with a PG and highway gears without much problem beyond the one they made fun of back when the PG was a production transmission installed on common road cars, driving 70-80 MPH still in 1st gear, as there are only two plus reverse. Actually if I have my sums right the shift from 1st to high under full throttle would be at 131.5 MPH. Which is even funnier than the vaunted 70 MPH shift from 1st to high ridiculed in the magazines of the times. The engine would remain below 3500 RPM all the time and would rely on low RPM torque and the torque converter to run without stalling. Which is almost the same as the speed the engine would be forced to spin at 60 MPH with the race gear all the time, so from one extreme to another in engine speed. Engine speed in 1st gear at 40 MPH with the highway gear would be 1976. That is a swap I can live with. Especially with the benefit of 100 pounds less empty weight and a similar but lower reduction in rotating weight for racing the autocross. But if I decided I needed a higher engine speed for around town but not making long trips on a freeway, all it takes is consulting a chart, picking a gear set, plonking down another $70, and spending another 15 minutes under the back of the car changing the spur gears. Or maybe just swapping the gears top for bottom on the race gears, because that would be a thing that was possible because the spur gears for the race gear would be a reduction set to get the RPM that high for that slow is WOW! The race gear needs to be 6.11:1, and the highway gear needs to be 2.54:1 and there is no way to get from one to the other without using different spur gears. If I get the low-inertia (rotating mass) 4.12 ring and pinion I can get close at 6.08 and 2.79 with only one set, giving me 2083 RPM at 60 MPH, but if I get the more common and slightly cheaper 4.86 the 6.12 spur gears give me 3.85 swapped top for bottom, which would be a decent setup for around town but loud and drony 2875 RPM for highway use. The bad thing about the 4.86 is the tallest final drive I can get is 2.58 which is Really Close resulting in 1926 RPM at 60 instead of 1900.

I shall have to let this one percolate through the grey matter for a while, comparing the costs of the 4l60 variant that will support the engine I get with a 9″ rear housing to fit the Sprint-T, and the costs of a PG and QC to fit the Sprint-T including 2 sets of spur gears, or a 9″ to fit and another center section. And looking at the assembled and ready to go 9″ center sections the cheap ones with a spool are $620 plus tax, for a race-only application. The ones with the highway gearing higher than 3.00:1 are scarce and expensive because most vehicles are equipped with overdrive transmissions to bypass the need for such tall gears. They used to be very common in the ’70s before overdrive transmissions were common, but I’m only finding used gears above 3.0 and even those are way expensive, so I might have to run the QC if I choose to run a PG. Or it just might not be economical to run a PG because of how expensive the support equipment required to run it on the street especially when a 4l60 can get 6500 RPM at 40 MPH with a 3.62 rear end ratio on the 23″ race tires, and get 1900 RPM at 60 MPH with the same rear end ratio and the 27″ tall street tires. Now that’s not a standard ratio but the common 3.50 is super cheap (for a 9″ rear end ring and pinion) and gets the race ratio close (41 MPH) and is just a tiny bit tall for the highway ratio resulting in 1830 RPM instead of 1900. Even closer is the Ford 8.8″ rear axle which (some of them) came with a 3.55 and a limited slip. Much more thinking is required, and as you can see there was already much thinking and consulting of texts and web pages done to get this far.

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