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Discussion Starter #1
Prologue

When Honda designed the Insight, they went all out in the name of efficiency. Honda geared the Insight’s transmission to provide maximum fuel economy in lean burn, at 60-65 miles per hour. When I’ve driven my Insight in the flatter, low-elevation parts of the U.S., all the crazy things Honda did to make the Insight light weight and aerodynamic, come together in a glorious union of engineering excellence. I can easily purr along at 65 MPH and get over 70 MPG in the process.

For example, in August of 2016 I went on a road trip from Lewiston ID, to Indianapolis IN, then to Augusta GA and back home. All said and done, I drove 5,846 miles, and averaged 78.0 MPG for the entire trip.

The essence of my enjoyment driving the Honda Insight largely boils down to being able to achieve in excess of 70 MPG on road trips. The principal way I achieve this is by driving at a speed of 60 to 65 miles per hour on the highway and by staying in learn burn as much as possible.

For the uninitiated, lean burn is when the engine is operating at an air-fuel ratio greater than stoichiometric. For gasoline powered cars, stoichiometric is considered to be an air-fuel ratio of 14.7:1. In lean burn, the Insight’s engine will typically operate at air-fuel ratios around 20:1 to 22:1. The upshot is that lean burn allows the engine to extract more power from the same volume of fuel, thus improving fuel economy.

However, it’s not quite all sunshine and rainbows. The region in which the engine can operate stably in lean burn is relatively narrow. As you climb in altitude, the progressively thinner air causes this region to get smaller and smaller. Somewhere between 2,500-3,500 feet in elevation, the lean burn region shrinks to the point that it is no longer possible to reliably maintain 60+ mph on level ground in 5th gear. From an economy standpoint, this renders 5th gear useless.

Where I live in Northern Idaho, much of my driving is at an elevation of 2,000-5,500 feet. Above 4,000 feet, I find myself never shifting into 5th. This also segues into another shortcoming of the Insight’s transmission. Steep grades. Specifically, those 6% and greater.

In their quest to make the Insight the ultimate fuel-efficient commuter car, the Honda engineers had to make some compromises with the transmission gearing. The spacing between 1st and 2nd, as well as 2nd to 3rd, quite large. The 1-2 shift comes with a 98% RPM drop and the 2-3 shift is about 60%

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These large gaps between gears isn’t a serious problem on mostly level terrain. However, on a 7% grade, you can rev it up to 5,000 RPM in first gear, shift into second and then suddenly realize there’s nobody home at 2,500 RPM. While the extra torque from the IMA motor will supplement the engine’s power deficit, better gearing would be helpful.


Shortcomings in Summation

In an Insight’s transmission, the gap between gears 1, 2, and 3 are quite large. This is a pain point when accelerating on steep grades. Additionally, fifth gear is generally unusable in conjunction with lean burn at higher elevations.
 

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Discussion Starter #2 (Edited)
...Continued.

Two Steps Sideways

In March of 2019, I performed a Honda Civic Hybrid (HCH) transmission swap on my 2006 Honda Insight. After 5,000 miles, I concluded that the HCH’s 12.2% lower 5th gear was too low for my taste.

In my HCH Transmission Swap Anthology, I concluded with, “It would seem that something between the Insight’s 5th and the HCH’s 5th might be in order…but that’s a story for another time.


That time has arrived.

Thanks in part to Sigma Projects, I knew that the 2007-2008 Honda Fit 5 speed manual transmission shares the same basic architecture as the one in the Honda Civic Hybrid. In addition, as part of doing this Frankentrans project, I was able to determine that the Insight's manual transmission is also in that same family.

This is important because it means that the 3rd, 4th, and 5th gearsets, and the ring and pinion, are all interchangeable. Truth be told, I didn’t know any of this for 100% certain when I started. However, that didn’t stop me from spending many hours theory crafting in my spreadsheet with the different available ratios gleaned from poring over numerous Honda specification charts.

My goals were:
  • Use only Honda gear ratios to keep costs under control. Custom cut gears are expensive.
  • Achieve a 6% lower geared 5th compared to stock.
  • More even gear spacing, if possible.
  • Eventually, I settled on a combination that best achieved these goals.
What I did was to take a Honda Civic Hybrid transmission and replace the HCH’s final drive with an Insight ring and pinion. Then I replaced the HCH’s 0.711 5th gearset with a 0.757 5th from a 2007/8 Honda Fit.

Note: The primary reason for using an HCH transmission as opposed to an Insight transmission is that I wanted to have a 1.87 ratio second gear instead of the 1.75 ratio in the Insight. Second gear on these transmissions is not easily changeable because second gear is machined directly onto the input shaft and the input shafts do not interchange.​
Lastly, using the HCH transmission case means all the minor fitment issues I talked about in my HCH Transmission Anthology post, also applies here. This was a non-issue for me as I had already solved these fitment issues back in March when I did the HCH swap.​

All this adds up to the one thing, the Frankentrans!

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Compared a stock Insight transmission, the Frankentrans’ 1st gear is the same, 2nd is 6.4% lower, 3rd is 11.6% lower, 4th is 6.0% lower and 5th is 6.1% lower.
 

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Discussion Starter #3 (Edited)
...Continued.

The graphs below help illustrate the differences. Clicking/tapping on the images below will bring up a larger version of them.

83658


Utilizing the more evenly spaced HCH 2nd, 3rd, and 4th ratios, really helps smooth out the ratio spread between gears.

Here is an overlay of the above two graphs that should help illustrate the differences in each gear.

83660


Everything is a Compromise

The truth of the matter is that there is no perfect answer to gearing. With only five ratios to choose from, compromises have to be made somewhere. In the case of the Frankentrans, I chose to compromise a small bit of top-end fuel economy for extra drivability.

One thing I couldn’t modify was first gear. This is an unfortunate, but necessary compromise of using only off the shelf parts. The only realistic way to change first gear is to modify the final drive ratio. For that, there are no viable options from Honda. The ideal solution would be a 3.4 ratio final drive, but custom machined gears are expensive, and that would lead to a further compromised wallet.

Having an extra gear would be nice too…
 

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Discussion Starter #4 (Edited)
...Continued.

Modification Time

Having done all the research I could, I worked out a time with Scott (KLR3CYL) to once again travel 1,100 miles down to Jue Motors, in Covina, CA so that we could accomplish the re-gearing.

Scott and I are good friends. Scott works for and is long-time friends with Jimmy Jue, the owner of Jue Motors. Jimmy is a terrific guy and I am incredibly grateful to him for allowing Scott and I to work together on projects like this one in his shop. Jue Motors is an excellent independent automotive repair shop with a sterling reputation.

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This is an HCH differential with an Insight ring gear bolted to it.

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My HCH Transmission Frankentrans is back together and ready for reinstallation.


Over the course of a few days in late July 2019, Scott and I performed the aforementioned modifications to the transmission. After reassembling everything and reinstalling the transmission into my 2006 Insight, it was finally time drive the car and see if it was worth all the effort.


First Impressions

The Frankentrans doesn’t immediately feel radically different compared a regular Insight transmission. And that is by design. What I’ve done is smooth out some of the rough edges and improve drivability while sacrificing a minimal amount on the fuel economy front.

And that is where the Frankentrans shines. The 6.4% lower second gear means accelerating from 10-11 MPH in second is easier and less likely to necessitate a downshift into first. Third gear is 11.6% lower which makes accelerating in third easier. Fifth gear is 6.1% lower, dramatically improving drivability in lean burn.

After the first 80 miles driving with my Frankentrans, the thought occurred to me, “This is how Honda should have geared the transmission.” In my humble opinion, I have put together the best combination of off the shelf parts that both maintains the fuel-efficient spirit of the Insight and improves drivability.
 

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Wow! Great work. Everything you described are gripes I have about my Insight... in Idaho, to boot! (Southern, though... but I know the altitude struggle well). I'm without IMA, so that makes it that much worse as well.

I usually find the edge of lean burn to be 55-60mph for me currently (unless it's really cold out, like right now, then it's definitely lower). What did your modded trans come out as for this? Any MPG change data?
 

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Discussion Starter #9
I haven't yet collected some good A/B fuel economy data. While I have gotten slightly worse fuel economy overall with the Frankentrans vs when the stock transmission was in, it's more likely that is contributable to external factors like traveling with 150 pounds of extra stuff in the car, driving a bit faster, torrential rain, new tires, ect.

If you are running IMA-less, I would consider an HCH swap. More even ratios, and a 12.2% reduction in overall gearing really help perk the car up without IMA.

As far as costs, I have done a terrible job of keeping track. Between buying the HCH Trans in Feb 2019, rebuilding it with all new bearings, tearing it apart again and making the modifications in July, I'm in about $1000 on parts and another $150 on some R&D expenditures. Also, keep in mind that I've done all my own labor.

That being said, as a general call to action:
If you like what I've done here and you really want your own Frankentrans, you should contact Scott (KLR3CYL) or myself. I would budget $2,000 for the modification. It'll might be less than that in actuality, but keep in mind that this is a custom, one-off style of project and the exact costs will vary depending on a dizzying variety of external factors.
 

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Very innovative and clever, but won't this screw up the critical up/down shift arrows :LOL:
 

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...Continued.

The graphs below help illustrate the differences. Clicking/tapping on the images below will bring up a larger version of them.

View attachment 83658

Utilizing the more evenly spaced HCH 2nd, 3rd, and 4th ratios, really helps smooth out the ratio spread between gears.

Here is an overlay of the above two graphs that should help illustrate the differences in each gear.

View attachment 83660

Everything is a Compromise

The truth of the matter is that there is no perfect answer to gearing. With only five ratios to choose from, compromises have to be made somewhere. In the case of the Frankentrans, I chose to compromise a small bit of top-end fuel economy for extra drivability.

One thing I couldn’t modify was first gear. This is an unfortunate, but necessary compromise of using only off the shelf parts. The only realistic way to change first gear is to modify the final drive ratio. For that, there are no viable options from Honda. The ideal solution would be a 3.4 ratio final drive, but custom machined gears are expensive, and that would lead to a further compromised wallet.

Having an extra gear would be nice too…


I would really like to significantly lower ALL the gears, especially first. Instead of the stock 3.2 final drive, how about 3.88? I see it was used in several Civic models, would it fit in the Insight transaxle? It would be about 20% lower if I am calculating this correctly. The gap in the gears wouldn't matter so much if they were all lower. That would still give a quite relaxed 25 mph/1000 rpm, about what 4th gear is now. So the 2000 to 2600 rpm range that is good for "lean burn" would then occur at about 50 to 65 mph instead of 60 to 75, at lower speeds I drive anyway. I am not using the IMA, and see this as a better alternative to the bigger engines or turbos that some people are installing. I think it would feel more powerful, be more fun to drive, and still give excellent gas mileage. One problem would be the speedo/odometer would be off, is there any way to adjust the gauge readings to match the different final drive ratio?
 

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Please include your location in your Profile, as ALL G1 Insighters have done.
Thank You.
Willie
 

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Discussion Starter #17
As I wrote you in response to the PM you sent me:

"Yes, changing the final drive affect all gears. However, I'm not sure where you are planning on getting a 3.88 final drive. If you are driving IMA-less I would consider an HCH Swap, which will get you a 12.2% lower 3.6 Final drive."

Are you looking at the 3.842 final drive that was used in the 7th gen Civic HX? If that's the case, it's important to understand that the 7th gen Civic HX came equip with a "D" series engine. Honda's D series engine spins in the opposite direction as compared to the Insight's engine as well as the "L" series engines used in the HCH and Fit.

While it's more than likely that the ring and pinion would physically fit, the helical gears are cut in the opposite direction and that is going to alter the thrust loads on the bearings inside the transmission in a way Honda never intended. Without further analysis, it's hard to say what the repercussions of that would be.

Non-sequitur: I need to update my HCH Transmission Swap Anthology thread with a nice comparison graph like the one I did in this thread.
 

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My fathers micro van came with 155/65r13’s stock

if you wantrubber burning power mount these, loose a little LRR and gain a small amount of weight
 

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Wrong post for this thread. Please relocate!
 
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