Manual v. Auto and Decelaration (education requested)

[noahrob]

Hi,

Can someone explain this to me, or tell me where I can located the answer?

It seems to me that when I had an automatic transmission and let my foot of the gas from say 2500rpm, the truck did not do a nose dive. Nowadays, with the 465, when I let me foot of the gas it does a nose dive. Also, if I lug it in the wrong gear, which happens sometimes going around a corner, the truck starts to lurch.

Is the torque converter responsible for picking up a lot of this slack so that a manual transmission is much harder on, well, everything in the drive train? Or perhaps there is something else? Thank you for your time.

 

[colbystephens]

i believe it is the torque converter that makes it so much smoother.

that said, i hate driving automatics now that i've got the manual. i hate using brakes, i'd much rather use the engine compression to slow down. i don't think it's harder on the drivetrain, just operates differently than the auto.

 

[original balzer]

Think of a ratchet or the freewheel bycicles most automatics are like that. Whereas a standard (manual) trans is like the bikes that dont freewheel so if your pedaling like hell and get tired and stop pedaling the pedals will throw your ass off the bike. Same applies to the manual transmission only the pedals are attached to the engine directly and the engine slows the pedals down.

The imput shaft on the transmission is turning the same speed as the engine RPM on a manual transmission. the clutch disconects the engine from the transmission.

An automatic transmission has the tourque converter. Witch is actualy a simple hydrolic pump that serves to transmit the power to the transmission in replace of the clutch. Think of the tourque converter as a liquid clutch.

The lurching or surging your talking about it basicaly the engine being slowed down too far to the point where it dies then instantly starts again. The fix is simply slow down as you approch the turn, push in the clutch as you go around the turn, go to the next lower gear and relese the clutch as you exit the turn.

practice makes perfect

Balzer

 

[noahrob]

Perfect explanation. Now I wonder if I haven't answered my own quesiton in this process...with Fuel Injection, specifically TPI adn no VSS, how would the CPU know to keep sending fuel to the engine when I take my foot off the pedal? In other words, the fuel probably just cuts out instatnly which is part of the problem since the only one tha tknows the truck is still moving is my...not the injectors

 

[4X4HIGH]

The ECM uses TPS and also VSS to determine what is going on and when to cut fuel. If you don't have a VSS then the ECM has no idea how fast you're moving but it does see that the TPS switch is closed and makes an adjustment from that alone. The IAC motor is also involved to allow the engine to idle correctly when you come to a stop but once again without a VSS the ECM doesn't know what needs to be done. Why not just install a VSS so the whole system will work correctly?

EDIT: What you are now experiencing with the manual is called compression braking. If you were running an automatic with a doubler you could feel it as well if you were in low range and let off the gas.

 

[BKinzey]

What he said


Dat's Da Fact Jack!

 

[noahrob]

I'll hook up the VSS this weekend...as it stands, my truck reminds me of my '78 SR 500...a 13.5:1 single cylinder motorcycles...when you let off the throttle you damn near flew over the handlebars

 

[roadnotca]

Hell yes. Add lift/flex and it exagerates nose dive.
Torque convertors work the same as paddeling a boat, push and pull.
Clutch drive train works like a river stick.

 

[aeronautica86]

Compression braking can cause nose dive when letting off the throttle...

I also have to comment on these two statements

that said, i hate driving automatics now that i've got the manual. i hate using brakes, i'd much rather use the engine compression to slow down. i don't think it's harder on the drivetrain, just operates differently than the auto.

Your transmission was designed to make your car go forward and your brakes were designed to make your car stop. Using your transmission to slow down will cause extra wear. Use the system designed for the task at hand. What would you rather replace, your brake pads or your transmission?

The fix is simply slow down as you approch the turn, push in the clutch as you go around the turn, go to the next lower gear and relese the clutch as you exit the turn.

I guess this procedure isn't bad on the street, but years of track driving have taught me to brake and select the proper gear before entering a turn...like I said, I guess it doesn't matter too much on the street, its just a habit that keeps the car settled through the turn and allows you to control the cars line with both throttle and steering wheel

 

[loafer]

The amount of fuel is primarily determined by engine load (MAP sensor) and RPM. TPS, coolant temp, O2 sensor only modify the base fueling level, or injector pulse width. Weather or not the truck is moving doesn't really matter.

Perfect explanation. Now I wonder if I haven't answered my own quesiton in this process...with Fuel Injection, specifically TPI adn no VSS, how would the CPU know to keep sending fuel to the engine when I take my foot off the pedal? In other words, the fuel probably just cuts out instatnly which is part of the problem since the only one tha tknows the truck is still moving is my...not the injectors

 

[colbystephens]

Your transmission was designed to make your car go forward and your brakes were designed to make your car stop. Using your transmission to slow down will cause extra wear. Use the system designed for the task at hand. What would you rather replace, your brake pads or your transmission?

not trying to start an argument, but i'm going to disagree with this statement.

 

[original balzer]

Compression braking can cause nose dive when letting off the throttle...

I also have to comment on these two statements



Your transmission was designed to make your car go forward and your brakes were designed to make your car stop. Using your transmission to slow down will cause extra wear. Use the system designed for the task at hand. What would you rather replace, your brake pads or your transmission?



I guess this procedure isn't bad on the street, but years of track driving have taught me to brake and select the proper gear before entering a turn...like I said, I guess it doesn't matter too much on the street, its just a habit that keeps the car settled through the turn and allows you to control the cars line with both throttle and steering wheel

LMAO i dont know how you drive in the street, id agree in a race your dead on correct. But i havent tried to do a 1-g turn on the street since i was a kid and didnt know better. then again most the time on the street i drive like an old lady. My trucks usualy attract enough attention without agressive driveing, its hard to stay under the radar when your trucks 10ft tall.

Also this is how i would sugest to deal with his truck not knowing how fast its going until its fixed correctly. Even then, after its fixed, with a gas engine he still wont be able to go 10mph makeing a corner in 4th gear with out it surging and lurching. Its called lugging the engine

 

[Russell]

TPI definitely relies on the speed sensor for fuelling during deceleration etc. Its designed to literally cut the fuel right off when you let off the throttle and start rapidly slowing down, so if you're going 0 mph, and go from open throttle to totally shut, the ECM is probally cutting your fuel off.

Install the VSS, and a betcha things start working a bit better. Have you got a prom installed for a standard transmission vehicle? There are differences in how the IAC responds under different situations, when the ECM cuts the fuel, and how high it idles, along with a couple other things as compared to an automatic prom.

As far as aeronautica ideas go, I whole-heartedly agree with the downshifting deal. Its far easier to replace disk pads and drum shoes than it is to replace a clutch. You will wear a clutch out WAY faster by downshifting all the time, cause you always slip it a bunch to make it a smooth transition into the lower gear, regardless of if you rev the engine up to match the RPM you'd be at in the next gear.

That, and other drivers get kinda spooked out when you suddenly start rapidly slowing down without any tail lights coming on

As far as shifting around corners go, I usually have the truck in the next gear down before entering the turn, and engage the engine half way though the turn as mentioned above

 

[aeronautica86]

LMAO i dont know how you drive in the street, id agree in a race your dead on correct. But i havent tried to do a 1-g turn on the street since i was a kid and didnt know better. then again most the time on the street i drive like an old lady. My trucks usualy attract enough attention without agressive driveing, its hard to stay under the radar when your trucks 10ft tall.

Also this is how i would sugest to deal with his truck not knowing how fast its going until its fixed correctly. Even then, after its fixed, with a gas engine he still wont be able to go 10mph makeing a corner in 4th gear with out it surging and lurching. Its called lugging the engine

haha, well yeah as I mentioned it really doesn't matter on the street, especially for trucks; I'm used to driving a 944 turbo, so I'm new to trucks, and when I'm in an SUV, I drive like an old lady too.

yeah, he shouldn't be trying to drive 10mph in 4th

 

[loafer]

If the speed sensor is reading 0 MPH it will not cut fuel. Otherwise the engine would stall if you reved it in neutral while not moving.

TPI definitely relies on the speed sensor for fuelling during deceleration etc. Its designed to literally cut the fuel right off when you let off the throttle and start rapidly slowing down, so if you're going 0 mph, and go from open throttle to totally shut, the ECM is probally cutting your fuel off.

Install the VSS, and a betcha things start working a bit better. Have you got a prom installed for a standard transmission vehicle? There are differences in how the IAC responds under different situations, when the ECM cuts the fuel, and how high it idles, along with a couple other things as compared to an automatic prom.

As far as aeronautica ideas go, I whole-heartedly agree with the downshifting deal. Its far easier to replace disk pads and drum shoes than it is to replace a clutch. You will wear a clutch out WAY faster by downshifting all the time, cause you always slip it a bunch to make it a smooth transition into the lower gear, regardless of if you rev the engine up to match the RPM you'd be at in the next gear.

That, and other drivers get kinda spooked out when you suddenly start rapidly slowing down without any tail lights coming on

As far as shifting around corners go, I usually have the truck in the next gear down before entering the turn, and engage the engine half way though the turn as mentioned above

 

[4X4HIGH]

How wrong you are. There are other variables to let the ECM know what's going on under other conditions. The TPS switch, MAP sensor also come into play.

 

[Russell]

Yep, its a multi-variable deal, not just speed and TPS. However, the other stuff will automatically be sending the proper signals to tell the engine to cut the fuel just cause of the way an engine works while decelerating (high vaccum, low TPS, IAC position, speed, neutral safety switch set for drive, etc)

 

[loafer]

I'm aware of that. That was in my first post. I was trying to point out that the vss reading 0mph really is not going to screw up the ECM too much.

Engine load (MAP or MAF) and RPM determine the base fueling level. coolant temp, TPS position and rate-of-change, O2, etc. are used to modify and trim the base fuel level.

How wrong you are. There are other variables to let the ECM know what's going on under other conditions. The TPS switch, MAP sensor also come into play.

____

 

[dyeager535]

If that's the case, why do some TBI/TPI setups stall during decel with the "fix" of grounding the park/neutral switch?

IAC is handled differently during decel due to VSS.

With something forcing the engine to run on decel (manual trans in gear for instance) it would likely be less noticeable that the IAC was incorrectly working due to load/speed differences.

 

[loafer]

I'm not saying VSS is not used by the ECM (in my case it is not used, nor is the park/neuatral switch) it will fuction with an open circuit on the VSS though with out much trouble.

Unplug the VSS and go for a ride. The TC not locking up and lack of cruise will probably be the only thing you notice w/o it.