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Old 02-09-2006, 12:55 PM   #1
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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.
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Old 02-09-2006, 01:11 PM   #2
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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.
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Old 02-09-2006, 01:49 PM   #3
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Re: transmission swapping for MPG

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Originally Posted by MetroMPG
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.
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.

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Old 02-09-2006, 01:50 PM   #4
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do you happen to know of

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

Quote:
I bet that the transmission for the XFi is actually different than your transmission
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)?
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Old 02-09-2006, 03:00 PM   #5
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Quote:As long as the engine

Quote:
As long as the engine has enough torque (not under high load) at the cruise rpm, you should see an increase in MPG.
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)?
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Old 02-09-2006, 04:29 PM   #6
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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,
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Old 02-09-2006, 04:33 PM   #7
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Quote:I'm so excited,

Quote:
I'm so excited, cruising around town at 1k rpms,
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.
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Old 02-09-2006, 05:24 PM   #8
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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.
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Old 02-09-2006, 05:53 PM   #9
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Re: I remember when i could do

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I remember when i could do that with my old Sl1. but now i have the auto. Auto trannyies are no fun.
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
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Old 02-09-2006, 06:01 PM   #10
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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,

Just buy a crx!
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