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Old 11-01-2020, 06:19 AM   #41 (permalink)
2007 Honda Jazz 1.3 i-dsi
 
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Quote:
Originally Posted by MetroMPG View Post
Welcome to the forum, Michaelflat1!

What size engine is in your Jazz?



How did you determine the fuel cut-off point (RPM) - does the car's gauge show instant fuel consumption?
1.3 (marketed as 1.4) i-dsi engine. The fuelcut points i can feel sometimes (I think my car has a sticky EGR valve/throttle body/somethings wrong, it can be jerky/very noticeable coming out of fuelcut)

Also i monitor fuel flow using OBD2 bluetooth adapter, and TorquePro. (only have avg MPG on the car's trip computer)

My jazz, has electronic throttlebody too, and you can see it slowly closing the throttle as you slow down (presumably to increase engine braking at low speeds, giving it a nice coastdown)

14% is around idle throttle and in DFCO it'll go from 20% to 15% throttle as engine RPM decreases (20% from around 3500rpm, 15% at around 1300rpm). Dependant on driving style too, if you mash the gas a lot, the gas pedal becomes more responsive, and the throttlebody doesn't open as much when off gas (to reduce rev hang). Consistent gentle driving is very beneficial in this car.


I find the close ratios useful, to keep engine rpms low. I love driving manual however, and find it quite fun to change gear a lot (i change gear sooo much, town driving is variable speed and i try to use DFCO as much as i can, smoothly).

I can see holding 2nd gear not being a disaster in regards to fuel economy, but only if you use close to WOT, keep that engine loaded, don't leave it crying out for a gear change come 40kmh.

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Old 11-05-2020, 03:32 PM   #42 (permalink)
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3-speed

Quote:
Originally Posted by Ecky View Post
I'll admit I'm skeptical of less gears = better, as evidenced by manufacturers adding more and more gears to chase economy, but those are more than decent numbers for a car of that era.

~

Today I found myself using 2nd to 6th a couple of times. 2nd gear will do approximately 78mph but the engine has gobs of torque, so I took 2nd from a stop out to around 35 and then straight into 6th. I'm certain this wasn't best for economy, but it was certainly lazy, and I got around 47mpg on my return commute, all city driving in the rain with the outside temperature in the 30's.
Ecky, with a V8 built for low RPM torque, I never lacked for power to shift with no problems at any speed. The heavy flywheel matched to the low RPM ability to pull without lugging made it a great car to drive at any speed.
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Old 11-06-2020, 01:14 AM   #43 (permalink)
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Originally Posted by Charlie Cheap View Post
with a V8 built for low RPM torque, I never lacked for power to shift with no problems at any speed
Americans addiction to the V8 and low-end torque begun when automatics were nearly science-fiction, as it would allow the driver to keep the high gear all the time if there is not any hill on the way... And it applied to engines with fewer cylinders too, such as the Chrysler flathead-six and the Chevrolet OHV straight-6 from the 3rd and 4th generations.
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Old 11-06-2020, 08:14 AM   #44 (permalink)
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My thought is that if it can stay in high gear all the time, and doesn't need middle gears for normal driving, such a car has too many low or middle gears and not enough high gears. A taller top gear will always improve economy, up to the point you can't stay in it.

I could use a 7th or even an 8th gear in my car, spaced as far apart as 6th is from 5th. 2nd accelerates at the limit of traction up to around 80mph, 3rd drops into the middle of the power band for passing at highway speeds (if I wanted to do such a thing). 6th is very tall compared with most 4 cylinder vehicles, but I find I don't need to downshift from 6th even climbing steep grades with a trailer full of concrete and construction debris behind my car. It will "high idle" the car up to 50mph on a cold morning in 6th.

So maybe the issue is too much engine, rather than gearing.
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Old 11-06-2020, 10:13 AM   #45 (permalink)
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Ecky and Rooster, even as a Hot Rodder I always built for TORQUE rather than Horsepower to get the most out of the engine at driving speeds. Torque is for economy and ease of driving while Horsepower is for top end racing...at least to my thinking. My 40 Ford coupe was very aerodynamic with the only flat material on the car being the glass. All else is curved surfaces, and with attention paid to economy right after the 1973 gas crunch, I was pleasantly surprised at the 24-25 MPG at highway speeds using the AC. Using the big car flywheel that was about 10 lbs. heavier than the Mustang, and matching gears, tire size, and engine mods, it just worked. When I built my 6-cylinder Mustang I used the same thinking, and it worked again. Now I am using that same build thinking on my 4-cylinder, which makes good torque for a 4. I'll need it with a .75 overdrive and tall (relatively) rear tires. My cam is a Melling stock replacement with slightly better torque, so I hope I am in the ball park with my math.
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Old 11-07-2020, 12:40 AM   #46 (permalink)
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Quote:
Originally Posted by Charlie Cheap View Post
even as a Hot Rodder I always built for TORQUE rather than Horsepower to get the most out of the engine at driving speeds
The average Joe tends to underestimate torque, while getting too impressed by the power figures.


Quote:
Torque is for economy and ease of driving while Horsepower is for top end racing...at least to my thinking
Carroll Shelby used to claim torque was actually more important than power, even for racing.
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Old 11-07-2020, 05:36 AM   #47 (permalink)
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With a higher torque you'll get more HP too, it's hard to seperate them. It's diificult to say you feel either the torque or the horses working.
My tiny 1.2 TDI has been chipped lately and has a lot more torque in the middle section, but also a lot more horsepower at a lower engine speed. For a quick insert or overtake I had to go up to 4000 rpm for getting that maximum HP. Now I already get that same value at 2300 rpm. That's a huge difference. Certainly for FE.

With such torque figures there is no need to skip gears, a lot of torque and power at low rpm gives the possibility to stay low in the revs in every gear, using the engine's sweet spot and still accelerate quickly. With a very short ratio gearbox this could be different. That's not the case with my car, the drop in engine speed is significant at every gear change.
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Old 11-08-2020, 04:21 PM   #48 (permalink)
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With a higher torque you'll get more HP too, it's hard to seperate them.
Depends on the displacement and RPM range too. When the Ford F-350 was phased out in Brazil on late 2011, it had a 3.9L engine with 120hp, and a higher torque peaking at a lower RPM than the 163hp 2.8L engine which was fitted to it from 2014 to 2019.

It's also worth to take a look at a 1.0L car engine and a motorcycle engine within the same displacement range. Even though the specific torque might be the same, the higher peak torque RPM might lead to the motorcycle engine be perceived as if it had a higher torque if geared lower enough to match the speed on the transmission output.

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