Highway cruising gear

[Akari]

Originally Posted by subiestyle

hmmm. can you explain this further? first time i've heard this.

I've always heard that when you coast to a stop in gear, the transmission is turning the engine, so the engine is not struggling to keep itself running until you get really slow and the rpms drop below 1500. but at that point, I just disengage the clutch and stop, which turns on the injectors to keep the engine idle.

Yes, we're both talking about the same process here but now you're talking about coasting to a stop when earlier you were talking about coasting in gear while maintaining speed. There are only two ways you're able to coast in gear and maintain speed: Large hills where gravity is the force moving your vehicle, and when your vehicles goes into a stall prevention mode turning your injectors back active and allowing air past the throttlebody.

[DC2R]

Originally Posted by subiestyle

hmmm. can you explain this further? first time i've heard this.

I've always heard that when you coast to a stop in gear, the transmission is turning the engine, so the engine is not struggling to keep itself running until you get really slow and the rpms drop below 1500. but at that point, I just disengage the clutch and stop, which turns on the injectors to keep the engine idle.

Sounds like you guys are talking about the same thing. Basically in practice you would pulled it out of gear prior to running out of sufficient momentum.

Sent from my SM-N900T using Tapatalk

EDIT: didn't see response. Personally I pulse and glide when cruising. I only leave it in gear when I need to slow down.

[anthros]

Originally Posted by Darkrune

A lot of people don't take into consideration engine load vs. gear ratio and engine tolerances. Going WOT in 6th gear makes your load tables very high, which put stress on connecting parts in the engine. Save your rods and don't go WOT in 6th gear unless you are at a rpm appropriate level. This is especially true for people who are boosted.

This is interesting. Why do you think WOT in 6th gear at 3000 RPM is any harder on your rods than WOT in 2nd gear at 3000 RPM? Boost or no, your engine makes the same amount of torque at 3K regardless of gear ratio. Your engine doesn't care what gear you're in; the stresses on the rods are the same.


Cheers,

Jason

[serialk11r]

Uh, lugging? Have you guys ever seen an automatic transmission? They run the engine down to 1100rpm, and you can put high load on the engine at 1500rpm. Oh right, that's intentionally done by automakers to cause you to have to service your car more or something

[subiestyle]

Originally Posted by Akari

Yes, we're both talking about the same process here but now you're talking about coasting to a stop when earlier you were talking about coasting in gear while maintaining speed. There are only two ways you're able to coast in gear and maintain speed: Large hills where gravity is the force moving your vehicle, and when your vehicles goes into a stall prevention mode turning your injectors back active and allowing air past the throttlebody.

no, earlier i was talking about coasting with no throttle. i mentioned coasting to a stop to emphasize the point of no throttle.

but my point is you shouldn't be giving any gas in 6th gear at 45mph

[Poodles]

Originally Posted by SirBrass

*facepalm*

Uh, you don't know much about how gear ratios are even listed, do you:

Here's what the brz 6mt has:

1st: 3.626
2nd: 2.188
3rd: 1.541
4th: 1.213
5th: 1.000
6th: 0.767
Final Drive: 4.100

An aside: AFAIK, those ratios are the ratios between flywheel speed and the final drive ratio. You then take that speed, apply the FD and you then get your axle rotation. Apply equations for what that speed with a wheel of diameter matching that of your tires is, and you get your road speed.

So, percentage listings between gears is actually useless. /aside

That's actually quite a long gear for 6th if mapped out for rev range (y-axis) vs. speed (x-axis).

See how it suddenly goes long for 6th, whereas 1st-5th progress steadily up, till 6th just nearly flattens out? That's your economy gear. Much like 5th gear in the WRX:

http://www.ft86club.com/forums/showp...4&postcount=14

Correct. Also keep in mind the old 'vette and f-body T56 is a double overdrive transmission and I think the new 'vette transmission is triple overdrive. The twins have a single overdrive like an old 5-speed, it's got an extra gear before it's 1:1 ratio. This is the same as the Miata and it DOES NOT give you a higher top end or lower cruising RPM like a double overdrive, it's simply to have a closer ratio transmission to keep you in the power band.

Originally Posted by Darkrune

I am going to say thos again. Never floor your car in final gear ever. Bad ending.

Sent from my SCH-I435 using Tapatalk

You're entirely wrong on a single overdrive transmission such as this.

Originally Posted by anthros

You say that flooring the gas at, say 3000 RPM "won't severely lug" the engine; I'd say it's not lugging the engine at all. And because these are gasoline engines, the throttle valve restricts flow and increases pumping losses unless it is fully open. These pumping losses increase with the square of engine speed.

So if it's acceleration you're after, downshift by all means. But you won't hurt your engine if you go WOT at 2500 RPM. In fact, you'll save a little gas (at the expense of acceleration) compared to downshifting.

Here's a helpful column on the subject...it may clear some things up for the newer drivers on the board:

http://www.caranddriver.com/columns/...-lug-and-coast

Yep, it's fine.

Originally Posted by Darkrune

A lot of people don't take into consideration engine load vs. gear ratio and engine tolerances. Going WOT in 6th gear makes your load tables very high, which put stress on connecting parts in the engine. Save your rods and don't go WOT in 6th gear unless you are at a rpm appropriate level. This is especially true for people who are boosted.

No, as I've already said, it's a single overdrive transmission. Also, it's not bad on the engine on a multiple overdrive transmission at all, it's bad for the TRANSMISSION (seen T56's grenade from full throttle in 6th on cars with power adders)

Originally Posted by ntron1

6th gear is for 100 mph+.....

Um... no, not on a single overdrive transmission with such a high final drive.

Originally Posted by serialk11r

Uh, lugging? Have you guys ever seen an automatic transmission? They run the engine down to 1100rpm, and you can put high load on the engine at 1500rpm. Oh right, that's intentionally done by automakers to cause you to have to service your car more or something

I know you're being sarcastic, but automatics have a torque converter which multiplies torque allowing such things.

[Ganthrithor]

On the highway I usually turn on the instantaneous MPG readout and pick a gear and throttle position to minimize fuel burn for whatever my desired speed is. How else am I supposed to make highway driving challenging? For these purposes the highest gear isn't necessarily always the best one.

On a related note, I got about 30mpg on a road trip the other day doing an average of about 75mph, and that included a speedy run up and over a ~1200-foot mountain pass at the end. These cars really are pretty good on gas for being as fun as they are!

[serialk11r]

Originally Posted by Poodles

I know you're being sarcastic, but automatics have a torque converter which multiplies torque allowing such things.

First of all torque converters don't change the torque output of the engine, only the torque output to the shaft. The engine makes the same amount of torque as before.

Second of all newer cars lock their torque converters most of the time but that's not relevant as I said. 1200rpm is 1200rpm, and the engine is making some amount of torque regardless of what it's turning. Obviously you don't want to floor it at 1000rpm, even if it doesn't harm the mechanical components (on a modern wideband O2 + throttle by wire engine it shouldn't cause pinging or anything like that at least) the engine has horrible efficiency, but there's nothing wrong with rolling along a flat road at 1200rpm, which only uses like 10% of the engine's available torque.

If the manufacturer's programming is any indication of what speed it's safe to run the engine at, then 1500rpm seems like it would be perfectly fine for applying heavy throttle since it allows you to do that in manual mode, and I don't think anyone has blown their 6th gear. On this size engine 1500rpm is not efficient for producing maximum torque but it should not hurt the engine. The only difference that an auto could make is the auto can unlock the torque converter (though this isn't normal on modern cars) and that would give you a lot more shock protection for the rest of the driveline, but like I said I don't think the gearbox is really an issue.

[Darkrune]

Originally Posted by anthros

This is interesting. Why do you think WOT in 6th gear at 3000 RPM is any harder on your rods than WOT in 2nd gear at 3000 RPM? Boost or no, your engine makes the same amount of torque at 3K regardless of gear ratio. Your engine doesn't care what gear you're in; the stresses on the rods are the same.


Cheers,

Jason

Engine load is the answer to this question. Take a log of your engine loads at those gears and and rpm's. See what the differences are. Load is what bends rods. Your logic, therefore, is flawed.

[Akari]

Originally Posted by subiestyle

no, earlier i was talking about coasting with no throttle. i mentioned coasting to a stop to emphasize the point of no throttle.

but my point is you shouldn't be giving any gas in 6th gear at 45mph

But again that's not true at all. Giving 5-10% throttle to stay moving at in 6th gear at 45rpm results in very low engine load, low wear/tear on the engine and the highest fuel economy. Why wouldn't you want to do this?

To accelerate you'd want to drop down a gear or two but shifting isn't an issue.

[anthros]

N.B.: this was in reply to Darkrune's post above, not Akari's. Akari makes an excellent point.

Unfortunately, "engine load" doesn't mean anything in this context. What bends rods is esentially Euler buckling. The important factor in such cases is the axial force on the column (rod) which correlates directly with torque. There is no dependence on gear ratio.

You seem to be trying to understand this through injection and boost maps. Even if they were relevant—and they're not especially so—they don't depend on gear ratio either.

Cheers,

Jason

[Poodles]

Originally Posted by serialk11r

First of all torque converters don't change the torque output of the engine, only the torque output to the shaft. The engine makes the same amount of torque as before.

Second of all newer cars lock their torque converters most of the time but that's not relevant as I said. 1200rpm is 1200rpm, and the engine is making some amount of torque regardless of what it's turning. Obviously you don't want to floor it at 1000rpm, even if it doesn't harm the mechanical components (on a modern wideband O2 + throttle by wire engine it shouldn't cause pinging or anything like that at least) the engine has horrible efficiency, but there's nothing wrong with rolling along a flat road at 1200rpm, which only uses like 10% of the engine's available torque.

If the manufacturer's programming is any indication of what speed it's safe to run the engine at, then 1500rpm seems like it would be perfectly fine for applying heavy throttle since it allows you to do that in manual mode, and I don't think anyone has blown their 6th gear. On this size engine 1500rpm is not efficient for producing maximum torque but it should not hurt the engine. The only difference that an auto could make is the auto can unlock the torque converter (though this isn't normal on modern cars) and that would give you a lot more shock protection for the rest of the driveline, but like I said I don't think the gearbox is really an issue.

Um, no, torque converter is a torque multiplier and will make more torque. It also will only lock up at cruising speeds for better mileage (or in manual mode). This is why when you dyno an auto car, it should be forced into lockup to bypass the multiplication ratio of the torque converter so you get a real number.