transmission swapping for MPG
 

okay, lots of talk about manual transmission swaps going on lately.

can some of you who are doing this give examples of the ratio difference you'll be achieving, before & after?

am i correct in assuming you're doing the swap to lower your engine RPM for all given road speed (that's "final drive", correct?), or are you just aiming for a taller top gear?

is there a risk of dropping too far down the engine's torque curve, and you end up using the lower gears longer?

feel free to speak about this in terms a 5-year-old would understand, for my benefit and the benefit of all the other 5 year-olds visiting the site. :)

I'll leave the tough
 

I'll leave the tough questions for the people who know what they're talking about, which isn't me. Maybe DaX can chime in.

I'm swapping the transmission from a 1990 CRX HF into my 1989 Civic Sedan. The sedan weighs 2200lbs, and the 90-91 CRX HF weighs 1967lbs. As you can see, the difference isn't that great, so the transmission will be able to handle the extra weight.

The 90-91 CRX HF also had a small 60hp 8-valve engine. My car (right now, at least) has a 92 HP 16 valve engine. My future engine will be 92HP at 4500RPM, but will probably be around 70 HP most of the time (Vtec-e)... but I digress.

The point of this is that the transmission itself will hold it's own. The gearing (and final drive) are designed for fuel economy. They are much taller gears, which hinders acceleration, but promotes higher top speeds and higher fuel economy. I'm even willing to bet that I could probably break 45mpg (maybe even 50mpg) with the transmission swap alone.

And the beauty of Honda is that all of their transmissions are interchangable (within about a 10 year range).

I'm unsure if you have a XFi or not, but I bet that the transmission for the XFi is actually different than your transmission, and swapping them would in fact give you at least a few better mpg. We can all be on the lookout for one if you want.

Re: transmission swapping for MPG
 

The HF, CX, VX, HX trannys are desireable for thier taller gearing. The 88-91 CRX HF had a 2.95:1 final drive ratio while the 92+ CX, VX, HX have 3.25:1. Compare that with my del Sol with a 4.06 and vtec versions at 4.25. Now you can see what a huge drop in rpms that would give you at highway speeds. As long as the engine has enough torque (not under high load) at the cruise rpm, you should see an increase in MPG. At least that is my understanding.

do you happen to know of
 

do you happen to know of anyone who's done this and documented (properly) the effects on mileage?

you're right, the XFi is a 3.79 and mine is a 4.39. final drive (?) i think is what that refers to. but the xfi also had 12in rims (mine has 13s), so the effective difference between the two isn't as great as it appears.

i have read that the 4cyl Metros used the same transmission as the XFi. a 4-cyl would be easier for me to find, because believe it or not, the XFi was never sold in canada (despite being manufactured in ontario).

correct me if i'm wrong:

it takes the same amount of power to move the car down the road at a given speed, regardless of gear ratio. so the beneficial effects of taller gearing at a given road speed then are:

- reduced internal engine drag from lower rpm?
- a wider throttle opening required to develop that same amount of power (so reduced throttling losses)?

Quote:As long as the engine
 

ah, ok. so my "wider throttle opening" consideration doesn't apply here, as it would indicate a higher load?

would this shift a car's "sweet spot" (the speed at which it gets the best steady-state fuel economy)?

The main idea, I believe is
 

The main idea, I believe is to lower rpms. If you're at the same load but only spinning two thirds the times you're squirting one third less gas into the engine, :)

Auto to manual + hf tranny = huge difference for me

I'm so excited, cruising around town at 1k rpms, :)

Quote:I'm so excited,
 

My Saturn has enough torque so that I can cruise as low as 20-25 mph in 5th gear. This is right around 800-850 rpms. It idles at 750. Of course, if you try to speed up, you have to downshift to at least 4th.

I remember when i could do
 

I remember when i could do that with my old Sl1. but now i have the auto. Auto trannyies are no fun.

Re: I remember when i could do
 

Roger that -- I miss running through the gears. When the tranny craps-out on the Teg (I'm not trying -- that's too expensive), but I might consider a swap. At that point it might be more of a rally or SCCA Solo I race car than a commuter car - so roll cage and the whole works -- it all depends. This car will probably go for years, so I probably have to wait a while. (knock) That was me knocking on wood.

-RH77

Honda auto trannies
 

Honda auto trannies generally don't die unless they're severely abused, so I think you've got a while to wait on it, :p

Just buy a crx!

Re: Honda auto trannies
 

I'd love to buy a fun car -- it's too bad Honda didn't come out with a convertible until the S2000 (barring of course the very early ones). The goal is to get a fun convertible for the weekends, but also good on mileage. The Del Sol might be an option, but the max budget is 2-grand, so that includes an old Mazda MX-5, or even an ancient MG-B. I'm not a mechanical wizard, so I may just save my money and make the 'Teg more economical and sell the TL to get an '06 Civic.

RH77

Re: Honda auto trannies
 

https://images6.theimagehosting.com/bimg.th.jpg

Convertible CRX

Ho-ly Crap
 

Sign me the F- up. My local fire department will loan me the jaws-of-life tool, so I can cut the roof off. After that, well...

No seriously, very cool. Too bad it wasn't production. That Honda Beat that MetroMPG was talking about sounds cool -- mid-engine, RWD roadster -- but impossible to license in the U.S. Oh, what to do. The Integra and me will probably share a bond for a few years. I just need to get it up to decent marks in the fuel econ category.

Honestly, I'd be happy to have my '99 Si back. The same guy who had the CRX that blew up, just bought a black Civic Si just like my old one. Lucky dog. I could rig up a selectable cold/hot air intake to get power when "necessary" and econ when I don't. I averaged 25 mpg, which wasn't bad for a pocket rocket. So much fun on the autocross circuit -- some guy in an MX-3 V-6 would always beat the crap out of me in my class, but I didn't care. It was fun, I learned a lot, and besides I couldn't afford race tires. Good times...

RH77

Re: The main idea, I believe is
 

SVO, are you through with your tranny swap?!

I'll be able to document, as I will be going from auto to manual HF within the next few months. I'm starting the rebuild on the tranny tonight...plenty of pictures to come.

Quote:SVO, are you through
 

I still can't afford a clutch, I have no job, but I might be getting one for free depending on whether or not I can trade some turbo oil lines to this guy who's getting one from a client of his. Everything else I have besides nuts and bolts.

20-30 mpg tranny
 

Salt Lake inventor has this:

https://deseretnews.com/dn/view/1,1249,635179379,00.html

Gogins said the transmission will change the world because it would increase mileage in a car from 20 mpg to 30 mpg. "We wouldn't even need a teacup of oil from the Middle East," Gogins said.

from
 

from https://www.greencarcongress.com/2006/02/vw_introduces_6.html

the revised VW Polo reduces fuel consumption by 11% (0.5 liters/100km) to 3.9 liters/100km (60 mpg US) compared to the previous model.

i suspect the changes listed above are in order of contribution to the FE improvement.

of course being a diesel, they can use even lower ratios than gas cars, since there's more torque low down in the rev range.

note, see new "experiment"
 

note, see new related "experiment" thread:

rpm/gear choice VS MPG at constant speed

I've really got to stop
 

I've really got to stop coming by this site so much, all you guys have me thinking about lately is how to increase my MPG. =)

As far as taller gearing increasing MPG, I would think the greatest advantage would be in reducing the mechanical losses, mostly from the lowered RPM of the engine AND transmission (well at least the input shaft half). The ECU should also be able to better meter the fuel as there are fewer combustion cycles between O2 samplings (or more sampling per cycle, depends on the speed of the processor in your ECU).

Also, engines are normally most efficient at there torque peak (which is a good indicator of the engines peak VE). If you look at a CRX Hf, you'll see that the peak torque is about 2,000 RPM, right about cruising RPM.

Re: I've really got to stop
 

Huh? What else is there? Fuel economy is the ultimate measure of performance. :-D

Re: I've really got to stop
 

I'll try that line on some girls next saturday night and let you know how it goes over. =)

Re: I've really got to stop
 

Hey baby... My car gets 50mpg... How you doin'?

Re: I've really got to stop
 

It would totally work. The first thing a woman thinks when you say this is "Hmmm...less money spent on gas means more money spent on me..."