Datsun Dreaming
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Joined: Jul 2004
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You're saturn explination works wonders when people are running stock tire sizes, but in most cases no one is. Going to a 4.36 can net you improved gas milage.
Comparing the amount of gas used between my car (4.08 & 255-0-17) and epsteins car (4.36 & 245-40-17) in july on our 2000+ mile deals gap trip we always put the same amount of gas in out cars. I checked how much gas all our cars where using at the time and we used up about the same amount every time.
Comparing the amount of gas used between my car (4.08 & 255-0-17) and epsteins car (4.36 & 245-40-17) in july on our 2000+ mile deals gap trip we always put the same amount of gas in out cars. I checked how much gas all our cars where using at the time and we used up about the same amount every time.
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Team White Car 2010
Team White Car 2010
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My brother and I met in Jacksonville once to swap cars. My brother's S13 with tuned SR and 4.3, and my completely stock S13 with fully maintenanced KA. Both cars went 250miles. We both filled up. I got 26mpg, which is what it's rated for. He got a hair over 30mpg.
On that Tennessee trip, I was averaging 25mpg in my full weight plus sub/amp S14, with 245's, 4.3, and stock KA. That's only 1 mpg less than my S13 would do with the same motor, but with a much heavier car, shorter gear, and higher rolling resistance tires.
I like that plot up there, greenman. I knew things like that existed, but never searched them out. Behind all that math, you should probably punch in (4.363-4.083)/4.083. Roughly 7%. Pick a spot on that graph. Now divide tq by 7% and multiply revs by 7%. Ain't exactly a big shift. Especially considering that the 2 islands that make up most of that plot only differ by 10%.
On that Tennessee trip, I was averaging 25mpg in my full weight plus sub/amp S14, with 245's, 4.3, and stock KA. That's only 1 mpg less than my S13 would do with the same motor, but with a much heavier car, shorter gear, and higher rolling resistance tires.
I like that plot up there, greenman. I knew things like that existed, but never searched them out. Behind all that math, you should probably punch in (4.363-4.083)/4.083. Roughly 7%. Pick a spot on that graph. Now divide tq by 7% and multiply revs by 7%. Ain't exactly a big shift. Especially considering that the 2 islands that make up most of that plot only differ by 10%.
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oh yeah, swapped out my motor mounts today. Pretty easy. Scoot the motor up a little with the jack, unbolt the crossmember and loosen the steering u-joint, remove.
So, the motor mounts that used to be in there were stock units that I completely sealed up (externally) with Windo-Weld. Put them in with the SR and thought, man, this engine shakes a whole lot for an SR. Then I drove my brother's identical car with Nismo mounts. Way less vibration. Apparently Windo-Weld'd mounts are stiffer than Nismos. Went back to stock for that awesome daily driver isolation.
So, the motor mounts that used to be in there were stock units that I completely sealed up (externally) with Windo-Weld. Put them in with the SR and thought, man, this engine shakes a whole lot for an SR. Then I drove my brother's identical car with Nismo mounts. Way less vibration. Apparently Windo-Weld'd mounts are stiffer than Nismos. Went back to stock for that awesome daily driver isolation.
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Joined: Feb 2005
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I didnt notice that big of a difference when normally driving with the diff in. Like some said a small drop in acceleration, but the gains from having no more one wheel peel are worth it.
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Gonna elaborate on this a bit.
Above is a BSFC curve for a DOHC 1.9L Saturn engine. Close enough to our KAs and out of boost SRs.
You want to minimize BSFC, as it is a measure of fuel consumed per unit of power. In this case, maximum efficiency is 2500RPM and 124N-m of torque. At this engine speed and power output, 250 grams per kw of fuel are consumed per hour.
For reference:
250 grams ~= 1/10 of a gallon
124n-m ~= 90lb-ft
1kw ~= 1.3hp
and, 90lb-ft at 2500RPM is about 45hp. Which is about where you'd require in cruise at 70mph. Funny how that works. Good job, auto engineers.
Anyway, moving on. Downshifting is like moving along a line parallel to the two i drew above. Those lines may not be exact, they're pretty arbitrary, and their actual slope is dependent on the transmission ratios.
Anyway. You'll see that downshifting ALWAYS puts you in a worse BSFC category, given that you're over like 2500rshared_pm.
FYI, part of why this is:
Gasoline engines thermodynamically are not very efficient at partial throttle. The engine experiences signifigant pumping losses from a partially closed throttle. Diesel engines don't have this problem, and so their torque peak is typically right at best BSFC. Which is why the euro guys have gotten such good city MPG out of diesels.
95% of TR probably doesn't care about anything I just said. Here's a sweet S13 dropped on its nuts:
Above is a BSFC curve for a DOHC 1.9L Saturn engine. Close enough to our KAs and out of boost SRs.
You want to minimize BSFC, as it is a measure of fuel consumed per unit of power. In this case, maximum efficiency is 2500RPM and 124N-m of torque. At this engine speed and power output, 250 grams per kw of fuel are consumed per hour.
For reference:
250 grams ~= 1/10 of a gallon
124n-m ~= 90lb-ft
1kw ~= 1.3hp
and, 90lb-ft at 2500RPM is about 45hp. Which is about where you'd require in cruise at 70mph. Funny how that works. Good job, auto engineers.
Anyway, moving on. Downshifting is like moving along a line parallel to the two i drew above. Those lines may not be exact, they're pretty arbitrary, and their actual slope is dependent on the transmission ratios.
Anyway. You'll see that downshifting ALWAYS puts you in a worse BSFC category, given that you're over like 2500rshared_pm.
FYI, part of why this is:
Gasoline engines thermodynamically are not very efficient at partial throttle. The engine experiences signifigant pumping losses from a partially closed throttle. Diesel engines don't have this problem, and so their torque peak is typically right at best BSFC. Which is why the euro guys have gotten such good city MPG out of diesels.
95% of TR probably doesn't care about anything I just said. Here's a sweet S13 dropped on its nuts:
Awesome post. I've always wanted to graph the relationship between speed and MPG so I know how fast I can go on the highway before the MPG takes a huge dive due to wind resistance and stuff.
My pre-calc professor once put up such a graph of a Ford taurus's MPG to show how it's a parabola (-X^2) with MPG as the depended variable (Y-axis) and speed the independent (X-axis), and everyone was literally falling asleep...
Suppose I'm in the minority here cause I love this stuff, and I regret not going on to Calc I.
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Decided to look into the 248/232 cam swap for ka's. Has anyone here ever done this? If so, i'd love to hear your opinions on it.
Also, I drove her all over the place.
Also, I drove her all over the place.
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1983 AE71 Corolla- 16v 4age... racecar 
1993 D21 Hardbody- a/c, music... grocery getter
- Garrison -
Just throwing it out there, by the looks o your car you can make anything work on it.
1993 D21 Hardbody- a/c, music... grocery getter
- Garrison -
