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Old 11-19-2008, 11:02 PM   #1 (permalink)
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Can swapping in taller gearing result in worse fuel economy?

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NOTE from Darin: This thread calved off from http://ecomodder.com/forum/showthrea...hape-5924.html
---

Keep in mind that reducing gearing is not always the answer either. I can personally vouch for this.

I once mated a CRX HF transmission to a slightly modded 1.5 liter engine... I LOST even baseline efficiency, to the extent of going from 40MPG down to 21 MPG.

I started researching how this could happen... to find that:

the CRX HF gets its' max torque at something like 1800-2000 RPM... meaning that doing 70 in 5th gear puts it in it's max efficiency area (area of peak torque)

Putting that same transmission against an engine that makes considerably less torque in that area will not gain net efficiency... you will lose it.

Please keep that in mind when choosing gearing.

This is the reason that most Honda transmissions place cruising speed at close to 3k RPM... it's where max torque usually is in those engines. It's also key for efficiency.

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Old 11-20-2008, 11:16 AM   #2 (permalink)
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Quote:
Originally Posted by Christ View Post
Putting that same transmission against an engine that makes considerably less torque in that area will not gain net efficiency... you will lose it.

Please keep that in mind when choosing gearing.
This is OT - but that did not happen with my car, despite moving further away from the torque peak RPM/best BSFC island at cruise with the taller final drive installed. Best BSFC <> lowest overall fuel consumption. It only means lowest fuel consumption per unit of power produced.
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Old 11-20-2008, 01:39 PM   #3 (permalink)
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Originally Posted by MetroMPG View Post
This is OT - but that did not happen with my car, despite moving further away from the torque peak RPM/best BSFC island at cruise with the taller final drive installed.
Ditto for my previous CRX Si. I went from about 30 MPG with the regular short-geared Si transmission, to 35+ with the taller-geared DX transmission...

-soD
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Old 11-20-2008, 01:45 PM   #4 (permalink)
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Stop for a minute and think about the ratios you're going to/from...

If you had changed your Si to a HF tranny, you'd have been dogging from day one.

The HF has more than 2x the gearing of a Si. Hence the reason your Si was at 3500-4000 RPM @ 70mph, whereas the HF is only at 1800-2000.

I'm not saying that changing gears won't improve your efficiency, I'm just saying that straying too far from your max torque will.

The Si could benefit from taller gears, it's a sports car... put efficiency car gears in it, and obviously enough, it gets better... the HF was a complete redesign.

All I'm asking is that people think before saying "use longer gears" b/c it's not always the case.
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Old 11-21-2008, 06:03 PM   #5 (permalink)
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Quote:
Originally Posted by Christ View Post
I'm not saying that changing gears won't improve your efficiency, I'm just saying that straying too far from your max torque will.
There's obviously a point past which you are gearing "too tall" for best FE, but I don't think that the RPM you make max torque at is really a determining factor.

Figure that a given car needs 10 HP to go at cruising speed on the freeway. To get the best FE, you need to get the engine to make 10 HP as efficiently as it can, and select the gears to match that speed with whichever RPM is the most efficient for 10 HP. I'm pretty sure that is significantly less than where the torque peak is in almost all applications...

Of course, being able to make 10 HP ultra-efficiently won't help a lot if we can't even accelerate to cruising speed, so we need the flexibility to generate more power in a pretty efficient way, with the appropriate gearing to get us up to speed. Thankfully we have more than one gear in the transmission...


Now, according to experienced people who know how to use ScanGauges, the best way to accelerate to a given (steady state) speed is low and slow, and to up-shift somewhere around 2000 RPM. That number is reasonably close in most cars, from Accords to Yarises to Fits to Camrys, and yes Civics too. I doubt that most of those make peak torque that close to 2,000 RPM...

What I'm trying to say is that, while where the peak torque occurs does affect what kind of gearing you want for maximum FE, there are other factors as well. And I think the peak torque RPM is probably not the largest contributor...

-soD

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