Auto trans vs manual

[wtreat]

It has been 25 yrs since I owned a Datsun 620 and wanted some input on what is a better transmission for fuel mileage. A 4 speed, 5 speed or a Automatic. Thanks in advance.

[KC Phirus]

It has been 25 yrs since I owned a Datsun 620 and wanted some input on what is a better transmission for fuel mileage. A 4 speed, 5 speed or a Automatic. Thanks in advance.

 

I've been told the automatic gets better MPG (probably because of the way most people drive a 5 speed). Even my grandfather said the automatic got better mpg. 5 speed will last you a lot longer and can be rebuilt. With an L20B and 5 speed you're looking at anywhere between 16 and 30mpg. It's all in the driving.

 

From personal experience, the automatic had AWESOME acceleration and power. Never calculated the mpg from it or the 5 speed I have now.

[Xander_42]

Theoretically the five speed will get better fuel economy because it is more efficient than an automatic. But, Phirus is also right, if you drive like me ("hard" acceleration, high rpm) an auto may appear to have better fuel economy.

[RedBanner]

Wait one minute, you couldnt rebuild an auto trans?

[KC Phirus]

Wait one minute, you couldnt rebuild an auto trans?

 

 

 

 

Would you want to?

[RedBanner]

Would you want to?

Would I WANT to rebuild a 5 speed?

Datsun autos are rebuildable too, and its not hard, I know becouse i made randy build me one and I watched :lol: he showed me how suple it was if done in the proper order, wich was laid out nicely in the fsm.

[KC Phirus]

Would I WANT to rebuild a 5 speed?

Datsun autos are rebuildable too, and its not hard, I know becouse i made randy build me one and I watched :lol: he showed me how suple it was if done in the proper order, wich was laid out nicely in the fsm.

 

I'm sure it's very simple compared to most.

I have to pull apart a 95 olds hydromatic to replace a sensor sorta thingy, someday.

eventually....

[mellotrongirl]

I love the 5-speed dogleg...get about 32 mpg; goes a little faster. Really handy along the freeway in hilly terrain, plus, for me in slippery winter weather driving with the poor traction 620's have over the rear wheels, a manual means more control

 

and if your battery goes dead, you can always push start it. For me it's just more fun/sporty manually shifting

 

As KC says--it's all in the driving. You should get slightly better mileage with the 5-speed over anything else if you're conscious of your RPM's, driving conditions, and you're not a leadfoot.

[albyneau]

Would you want to?

 

Sure~ bring it on! Autos had a reputation of earning the name "slush-box" in eras gone by. Any newer models (without the occasional design flaw) are quite efficient~ especially those with overdrives. Some Earlier overdrives were little more than a locking converter- which is still not a bad thing. Most automatics have a rather deep overdrive ratio, in the neighborhood of .7/1~ one that's too low most driving scenarios other than lond distance cruising or open-road commuting. The worst offender (to my knowledge) is the only known L-series fitting OD~ the L4N71B used in early 80's Maximas~ it's final drive ration's .686/1. That might, be fine for a salesman trekking the west coast's I-5, but you put that in a pickup you'll be sadly disappointed. If there's too much spread between the gears~ there's too much spread between operating rpm's at your chosen cruizing speeds. I say speedS because even tho it's an automatic, you still need to be mindful of engine operating rpm's. You don't want to chug-n-lug around in OD at 45 or 50~ us car guys will drop it a gear to keep that motor in a better rpm range for power/economy. Peak torque (and economy) occurs at peak volumetric efficiency~ but that's a discussion for another time, another thread. Our smaller, lighter vehicles are a bit more forgiving when lugging around in a light car/truck~ but that's of small benefit.

 

Standard transmissions have the benefit of zero slippage (theoretically), and while driving them they have indeed cornered the market on the "fun-factor". Economically speaking what they gain in that zero slippage they lose right back goin on-n-off the gas while shifting. Options for OD ratios in standard transmission in general are somewhat less restricted, but there's still not a lot of wiggle room.

 

Serious thought should be given to several things when choosing your transmission~ the most important of which is "how long will I keep my vehicle?" It might be fun right now to have a 5 speed with a clutch that locks up tighter than a pitbull's jaw~ but do you still want to mash that pedal 5-10 years from now? If you're flipping it, what's the best market value? Lots to consider....

 

Smaller engines and tires (like our Ratsuns) will enjoy a final transmission ratio of around .85 to .8/1. This will keep the operating rpm's nearly the same in 4th @55mph, or OD @65mph. Other variables will be determined by your desired build outcome: engine performance, axle ratio, and tire size. I'm pretty sure nobody here will be puttin 30" tires on their 510, so if you match your engine rpm to your axle ratio the OD ratios above will serve you well. All L-series 5-speed ratios fall in that range. If you've got a torquier motor you can opt for a lower OD. I've got one in mind~ but that's for another thread....

 

Peace~

Scott

[Laecaon]

I hated my auto, It seemed to just rob power from the motor.

 

For a long time til the mid 90's? or so many cars where said to get better mileage from the factory with a manual, weather that be by comparing a 4speed auto to a 5 speed manual, or being able to control your shift points.

 

But most importantly, if you often need to drive on the freeway, 5 speed is the way to go. It just helps make things quieter :)

[Z-train]

4 speed shouldn't even be in the conversation.If shifting doesn't bother you,then the 5-speed is superior in every measurable way.

[ggzilla]

5-speed will ALWAYS get better fuel economy on the highway than a 3-speed automatic. The overdrive gear gets up to 20% better economy than even a 4-speed.

[RedBanner]

Idk, if im rockin a manual of course I want a 5 speed.

But my new datto is an auto and im realy likeing the idea of just chilling in traffic on the way home. Stupid portland traffic.

[datzenmike]

In descending order:

5 speed

4 speed

3 speed automatic.

 

In town 4 and 5 speeds are the same.

 

On highway 5 speed has better mileage.

 

Automatics are just slightly under the 4 speeds.

 

The L4N71B has and insane overdrive because it isn't intended for an L4 cylinder but much larger and powerful VG and KA motors.

 

The 3N71B is a well designed and tough little mutherfucker. I have one in my 710 L18 sedan and I am quite impressed with it.. It's no fun to drive (as far as changing gears) but that's about it. Put in gear and forget.

[KC Phirus]

No idea what auto was in my 75, but it definitely wasn't robbing power.

Figure it had a good torque converter cause it would coast 45 for a good bit.

[ggzilla]

it definitely wasn't robbing power

You just didn't know it. It still had good power, but the torque converter loses about 5% power when cruising. Also a bit was robbed as the automatic significantly heavier. Not robbing a lot, but definitely some.

 

Still, an automatic 3-speed does perform well with the larger engines like the L20B.

 

If you retro-fit an Automatic 4-speed from an 80s Datsun or Nissan (the one with a lockup torque convertor) fuel economy will be nearly the same as a 4-speed.

[KC Phirus]

You just didn't know it. It still had good power, but the torque converter loses about 5% power when cruising. Also a bit was robbed as the automatic significantly heavier. Not robbing a lot, but definitely some.

 

Still, an automatic 3-speed does perform well with the larger engines like the L20B.

 

If you retro-fit an Automatic 4-speed from an 80s Datsun or Nissan (the one with a lockup torque convertor) fuel economy will be nearly the same as a 4-speed.

 

Definitely bigger and heavier. It was on an L20B and would light the tires lol.

[mellotrongirl]

That's why I haven't taken the rustbucket '79 kingcab I have to the scrapyard, cuz the 5-speed still worked like a champ when the engine blew up and the rest of the truck wasn't salvageable beyond the knick knacks I was uble to unscrew from it. I figure the trans is worth about $300

[Tice]

I'd take a Manual over an automatic anyday every simply because it's funner, there's more Driver-Car relationship, more control and (in the case of the Ratsun) it just seems right... But I'm 21 years old... If you're a little older, Manual may become cumbersome. really, it's down to the driver, and the intent of the car, as Scott said.

 

I drove a 91 Firebird from FL to AZ and back in October of 09 and it was a 5 speed. I did it becasue it was fun. I could've taken my dads Auto, but I didnt becasue I wanted the Manual... again, I'm young. haha

[DAT510]

I hate automatic. People argue that there is faster shifting which would be faster acceleration.... But I'd rather look cool than go fast :lol:

[ggzilla]

All the new high peformance cars are automatics. Specifically, an automatically-shifted manual transmission. Like Ferrari, BMW, Porsche and almost all the others.

 

So even they prefer manuals.

[Laecaon]

All the new high peformance cars are automatics. Specifically, an automatically-shifted manual transmission. Like Ferrari, BMW, Porsche and almost all the others.

 

So even they prefer manuals.

 

Well if you mean Manually shifted automatics, that would probably be speed.

 

If you mean Manual transmissions that are shifted automatically, that is probably so they dont get sued about the cost of a new transmission.

[DanielC]

I would consider a manual transmission a transmission that has cluster gear, and parts to connect various gears to the main, or output shaft. The shifting is done by moving syncros, shift forks, and other parts to accomplish this.

 

An automatic transmission has planetary gear sets, hydraulically operated clutches, and bands to connect various parts of planetary gear sets together.

An automatic transmission has a torque converter, and more importantly, an oil pump, that is always driven by the engine that supplies hydraulic pressure to operate the various clutch packs, servos, and valves to make the transmission work, and lubricate it. An automatic transmission may be controlled by simple mechanical devices, centrifugal governors, vacuum modulators, kickdown levers, and amanual shift selector, or it may be controlled by a computer, that has some sort of switch that send a request to the computer, that then tells the transmission to shift a gear, but it is still an automatic.

 

I would suggest that to get the same amount of power to the rear wheels, a true manual transmission would have less parasitic losses. However, if you compare an old manual transmission with only three speeds, to a modern four or more speed automatic, the automatic may gain an advantage, by being able to keep the engine in a more efficient operating range.

 

One advantage an automatic transmission has over a manual transmission is the torque converter. It is able to actually have more torque on the output of the torque converter, than the engine puts in the torque converter, especially at stall speeds, before the car starts moving.

[ggzilla]

I meant "an automatically-shifted manual transmission". That is the *peformance* choice of all automakers now - they shift faster than a regular automatic transmission (the kind with a torque converter and planetary gears).

 

They call these new types "automatics", but they are more like a manual transmission. That's why i call them "automatically-shifted manual transmission".

[DanielC]

I believe "an automatically-shifted manual transmission". uses planetary gearsets, torque converters, and clutch packs.