Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: dOc !]
#997593
05/22/11 04:46 PM
05/22/11 04:46 PM
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Joined: Jan 2003
Posts: 1,619
MA
ragtop
OP
top fuel
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OP
top fuel
Joined: Jan 2003
Posts: 1,619
MA
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Quote:
IF you don't have a big-torque motor ... it could actually HELP mpg.
Would that be because of the increased intake velocity from the higher RPM?
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Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: 360view]
#997598
05/23/11 04:19 PM
05/23/11 04:19 PM
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Joined: Jan 2003
Posts: 1,619
MA
ragtop
OP
top fuel
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OP
top fuel
Joined: Jan 2003
Posts: 1,619
MA
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Quote:
When my pinion bearing began to howl at 46,000 miles,
I swapped in a $75 set of NOS Chrysler 3.21 diff gears in place of stock ratio 3.55 in a 1995 Ram 360 Magnum 2wd 518 auto where the truck weight was 5400 lbs.
Highway mileage at 70 mph
over the exact same I40 trip from
Durham to Wilmington NC and back
went up by
1 MPG,
Thanks. What was your hwy mileage, just to see what percent, your 1 MPG is?
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Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: ragtop]
#997599
05/23/11 04:37 PM
05/23/11 04:37 PM
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Joined: Jan 2003
Posts: 5,386
Pikes Peak Country
TC@HP2
master
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master
Joined: Jan 2003
Posts: 5,386
Pikes Peak Country
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Quote:
Or are there other factors that I am not aware of. Thanks.
Compression ratio and cam profile.
The volumetric efficiency point of an engine exactly matches the torque peak. If this torque peak occurs lower enough in the power band and you have a high enough compression ratio to squeeze it without detonation and this new higher rpm is closer to the torque peak, your mileage may go up.
eg; my 74 D200 gets better mileage at 75mph than at 55 mph. At 75 mph the cruise rpm is closer to the torque peak and it produces more useable power with less throttle, so the engine is working more efficiently.
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Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: TC@HP2]
#997600
05/23/11 04:47 PM
05/23/11 04:47 PM
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Joined: Jan 2003
Posts: 28,067
Irving, TX
feets
Senior Management
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Senior Management
Joined: Jan 2003
Posts: 28,067
Irving, TX
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I really wish people would forget they every heard about torque peak.
That is measured at wide open throttle. Who cruises on the highway at WOT?
Due to the laws of physics and fluid dynamics things change quite a bit at part throttle. Port velocity, vacuum signal to the carb, and a host of other things all change at part throttle.
What happens at full throttle is completely different.
We are brothers and sisters doing time on the planet for better or worse. I'll take the better, if you don't mind.
- Stu Harmon
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Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: feets]
#997601
05/23/11 10:24 PM
05/23/11 10:24 PM
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Joined: Nov 2006
Posts: 1,456
oklahoma
forphorty
pro stock
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pro stock
Joined: Nov 2006
Posts: 1,456
oklahoma
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Quote:
I really wish people would forget they every heard about torque peak.
That is measured at wide open throttle. Who cruises on the highway at WOT?
Due to the laws of physics and fluid dynamics things change quite a bit at part throttle. Port velocity, vacuum signal to the carb, and a host of other things all change at part throttle.
What happens at full throttle is completely different.
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Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: ragtop]
#997603
05/24/11 09:15 AM
05/24/11 09:15 AM
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Joined: Jan 2006
Posts: 8,162
USA
360view
Moparts resident spammer
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Moparts resident spammer
Joined: Jan 2006
Posts: 8,162
USA
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Quote:
Thanks. What was your hwy mileage, just to see what percent, your 1 MPG is?
Depending on wind and air temperature,
after the 3.21 diff gears were installed
MPG at a steady 70 mph over this route
varied from 18.7 to 19.2
This was with 87 octane Exxon gasoline,
which back then did not have 10% ethanol in it.
The 2wd truck also had a ARE brand tonneau on the 6.5 foot long cargo box.
With a girl friend working at the Blockade Runner Hotel, and me living in Durham and going to graduate school during the week,
I did this trip frequently,
and also tested
running with the 1995 MP PCM engine controller on 93 octane,
running without the factory front air dam,
running without a radiator fan,
running without the air intake tube that goes from passenger side fender to air cleaner box so that the throttle bodies got hotter air,
running with/without KN washable drop in air filter,
running with 180, 195 and 205 degree F thermostats,
running with/without AC on,
running with aerodynamic vortex generators magnetically attached to rear of roof,
and a few other mods.
First main thing I learned:
wind direction and speed has more effect than most vehicle owners realize,
and every test run needs to be adjusted to take that into account if you are testing with a single vehicle.
Second main thing I learned:
Claude Travis,
known as 'Mr MPG' in the 18 wheel truck testing world,
knows a lot about how to properly test trucks for modifications, and incorporated his wisdom into the TMC/SAE Type IV test procedure.
http://www.landlinemag.com/Archives/2004/May2004/Bottom_Line/Does_it_work.htm
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Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: feets]
#997604
05/24/11 12:12 PM
05/24/11 12:12 PM
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Joined: Jan 2003
Posts: 5,386
Pikes Peak Country
TC@HP2
master
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master
Joined: Jan 2003
Posts: 5,386
Pikes Peak Country
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Quote:
I really wish people would forget they every heard about torque peak.
That is measured at wide open throttle. Who cruises on the highway at WOT?
Due to the laws of physics and fluid dynamics things change quite a bit at part throttle. Port velocity, vacuum signal to the carb, and a host of other things all change at part throttle.
What happens at full throttle is completely different.
Yes, things change considerably, however, to totally dismiss the power curves of an engine is akin to just taking shots in the dark. The power curves at least give a frame of reference to start with. Time on a chassis dyno, or seat tie on extended drives, can then be used to refine the results, if desired. I also think that since the stock mopar big block heads were undersized to feed a 440 from the get go, stock heads are going to be producing high velocity at lower rpm levels than most aftermarket or other make heads.
Quote:
Torque curve and efficiency have little to do with it unless you're towing or driving in hilly condition. Why do you think today's cars have over drive transmissions? My late model van turns 2000 rpm on the freeway at 70 mph, no where near it peak torque.
My reply stated my experience was with a truck, and since I live in Colorado, all my usage with towing is in hilly conditions. Some obviously bigger than others. I also had the chance to test out this operation on a trip from Denver to Phoenix and back. Not exactly flat there either.
I also think we would all agree that factory epa mileage estimates, warranty longevity vs customer claims, and driver perceptions mean that the OEMs may very well set cars up to cruise in a method that is not entirely optimal. Does that mean cruise rpm should all be cranked back up to 3500rpm by the OEMs, no. But perhaps engines should be built to produce more torque lower in the power band. To the average car buyer, torque means nothing. Except for heavier duty truck marketing (3/4 ton+), nearly all advertisements talk horsepower and/or mileage.
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Re: From 3.23 to 3.55 Hwy mileage drop?
[Re: ragtop]
#997607
05/25/11 02:15 PM
05/25/11 02:15 PM
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Joined: Jan 2009
Posts: 903
Oregon
rtplumcrazy1
super stock
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super stock
Joined: Jan 2009
Posts: 903
Oregon
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back when i had my 340 Duster i swiched from 3:23 to 3:55-and noticed no diference in mileage-UNLESS i drove the car harder. Then the mileage dropped about 1.5 miles per gallon. My 340 engine was prety much stock-except for headers-carb-and intake manifold. If i had it to do again-i would not change from the 3:23 gears-car ran aout the same with both sets of gears. But when i went to 3:91 gears-big drop in mileage.
Put a big block 4 speed Scat Pack Dodge in your garage.
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