Shorter gears give better acceleration, but is there a way to estimate the results of a gear change? I've seen calculators on line but they ask for rpm through the lights and other data points I don't have.
Simply put, if I change a 2.73 to a 3.55 what sort of result can one expect if everything else is the same? Lets assume a 3 speed automatic and 300hp in a 3500lb car.
Gearheadotaku (Forum Supporter) said:Shorter gears give better acceleration, but is there a way to estimate the results of a gear change? I've seen calculators on line but they ask for rpm through the lights and other data points I don't have.
Simply put, if I change a 2.73 to a 3.55 what sort of result can one expect if everything else is the same? Lets assume a 3 speed automatic and 300hp in a 3500lb car.
The reason those web calculators are asking for that data is that it's required to answer the question. The width of the power band is a huge factor, as are the gear ratios of that 3 speed. Tire size, transmission gear ratios, and redline are all also relevant.
I don't have an answer for you but you might find this video helpful, it's time stamped to the part you might be interested in but you should check out the full thing/channel if aren't following already.
All I did with the corolla was figure out what gear ratio would give me a trap RPM close to where I think the engine's peak power might be. Even then, all those calculators can't tell you how the car will hook. They're theoretical optimums. My car changed from 4200 to 6200 by going from 2.73 to 3.73 gears. We'll go with the 3.73 even though that RPM is too high, but the car won't run to that theoretical ET anyway.
This is one of those "how long is a piece of string" questions, because the engine's ability to accelerate plays a large role. A 300hp Caddy engine will be a lot different from a 300hp rotary, for extremes.
The closer your first gear acceleration time is to the engine's ability to free-rev, the less benefit you will see. I had a car where the lowest gear was so low that 1st gear didn't accelerate as hard as 2nd. (I use the term "hard" purely academically, an 80hp Subaru does not "accelerate hard" as much as it "gathers momentum") All the same, this is a lot of why Powerglides were popular.
Unless you have a Diesel or a 1700lb car or both, you'll see a benefit from going from 2.73 to 3.55. Going up to like a 4.56 is probably too far.
Part of the reason for asking for rpm through the traps is to know if the change in gearing will require an additional shift or result in rpm limited speed.
On my 95 T-Bird LX, swapping from 3.27s w/an open diff to 3.73s w/a Trac-Lok only gained me .15 second in the quarter mile.
I was hoping for more, but that car didn't really have enough power to spin the tires even with an open diff.
If you can deal with the limitation on top speed that is imposed by a high numerical diff ratio, the effect of using a different ratio can be profound.
I raced an old MG at various tracks in the Pacific Northwest that ranged from almost flat wide open tracks to almost hill climb conditions. I used a banjo rear end which meant that the gears are carried on a removable third member that you can change out in 30 min. or so. I had ratios of 3.9, 4.1, 4.3, 4.55, 4.88 and 5.125. I also ran a close ratio trans with an OD which I only used in top gear, so had plenty of scope to tailor the car to a particular track or use.
I ran one hill climb using the 5.125 and the change was huge - twin strips of rubber from the 7.5" wide slicks and I still had a top end of c. 90 mph. Definitely a PITA to change very often even with the removable 3rd member, though.
The answer to the original question would require a day at the track and a bunch of diffs to play with.
Years ago, '90s, I believe David Freiberger (sp?) did a quasi scientific test on this very subject in Hot Rod Magazine. The interesting thing was that they proved that a higher stall torque converter was just as advantageous as a pretty steep gear swap. At least on their particular set up. It made me go out and put a 10" converter in my junk. Then I learned that those "street fighter" converters were not a good match for taller gears (I had 3.08) due to old man heat. This was before lock-up converters were everywhere and I'd leave home with 2000 rpm being 60 mph. Before I got to work it'd take 2500 to run 60.
A 401 CJ said:Years ago, '90s, I believe David Freiberger (sp?) did a quasi scientific test on this very subject in Hot Rod Magazine.
I have an issue of Hot Rod from the 1950s where Roger Huntington did it. I forget what he used for a test car, I think a Chrysler product. In his case there was little difference in the quarter mile with any rear end ratio, but that was with a car heavy on torque at low rpm.
In reply to stuart in mn :
The tires of the era also basically sucked, and the engines didn't accelerate well. Heavy rods, heavy pistons. That's a lot of why you'd see them smoking the slicks off the line. The tires made more traction when slipping that much and the engine didn't need to accelerate as much.
Heck, for street tires, there was a drag test about ten-fifteen years ago using Coker bias ply repops and they found that they posted quicker 60' times by just launching at 4000rpm and burning the tires. I believe the test car was a 455ci 442.
With all that in mind, taller gears kind of make sense.
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