Strange feeling when going downhill

[bhad]

Hi

Just bought a z4m coupe.

Question. When driving around 70 mph downhill, I let go of accelerator. There is like a weird noise. Sort of engine braking noise. Is that normal?

[Finnegan]

Can you post a sound clip? It's pretty hard to diagnose what a noise is without actually hearing it. If it's a decent hill, there's going to be some change in sound when you do that. What's "weird" or not depends on what it actually sounds like. IDK, someone else can try here, but I'd need to hear it.

[bhad]

Sounds as though it's severe engine braking. Same sound as when one would disengage the cruise control when going downhill. Still new to this car.

[Kgolf31]

Are you slowing down at a good rate of speed?

[bhad]

Just letting go of accelerator at around 70 mph

[bhad]

Feels as though car does not enjoy coasting. Is that normal?

[Kgolf31]

Originally Posted by bhad

Feels as though car does not enjoy coasting. Is that normal?

This might be a stupid question, but is this your first manual car?

I've learned on a manual and been driving one the whole time I had my license (a big 5 years...whoooo).

Anyways, manual cars (from my experience), don't coast well at all. Because you're always in gear, the car is naturally going to slow down and engine brake. The higher RPM you are in a gear, the more engine braking you will get.

Automatic is completely different, because the car is taken out of gear and then the engine idles as you coast. In a manual, when the gas is completely out of the equation, not being used and you're driving in gear...the engine actually shuts off, and the gears keep the engine moving. That's where the engine braking comes from.

If you go into first, go up to around 4k and let go of the gas, you will slow down dramatically, it's all engine braking.

Correct me if I'm wrong, but that's the way I've learned it.

[Finnegan]

^ Wouldn't have thought of that Kgolf. Good questions!

[bhad]

Owned plenty of manual cars but not as powerful as this one

[Kgolf31]

Originally Posted by bhad

Owned plenty of manual cars but not as powerful as this one

Okay, it may just be the different compression ratios in the engines that you have owned.

My 2002 325xi never slowed down as much as this car does...but it was still present in my old car and never thought it was something wrong

[Flying Scotsman]

Originally Posted by Kgolf31

This might be a stupid question, but is this your first manual car?

I've learned on a manual and been driving one the whole time I had my license (a big 5 years...whoooo).

Anyways, manual cars (from my experience), don't coast well at all. Because you're always in gear, the car is naturally going to slow down and engine brake. The higher RPM you are in a gear, the more engine braking you will get.

Automatic is completely different, because the car is taken out of gear and then the engine idles as you coast. In a manual, when the gas is completely out of the equation, not being used and you're driving in gear...the engine actually shuts off, and the gears keep the engine moving. That's where the engine braking comes from.

If you go into first, go up to around 4k and let go of the gas, you will slow down dramatically, it's all engine braking.

Correct me if I'm wrong, but that's the way I've learned it.

Yeah, your wrong lol

Auto's engine brake as well, not as much as a manual does
Auto's are still in gear as they slow down
Engines do not shut off when coasting (manual or auto)

[esqu1re]

Originally Posted by Flying Scotsman

Yeah, your wrong lol

Auto's engine brake as well, not as much as a manual does
Auto's are still in gear as they slow down
Engines do not shut off when coasting (manual or auto)

I agree that the engine does not shut off, but this conversation reminds me of Top Gear, where Jeremy Clarkson was talking about how taking it out of gear while you come to a stop actually uses more gas than if you were to just coast down to a stop while in gear.

Now, it's Clarkson, so lots of things are wrong, but I always wondered how that logic worked, if at all.

[Kgolf31]

Well you're using less gas. I know on my E46 when you coast MPG needle goes to 0. When you kick in the clutch and throw it in neutral, you spike to 20 then level around 40 and stay there.

[gcochard]

Originally Posted by esqu1re

I agree that the engine does not shut off, but this conversation reminds me of Top Gear, where Jeremy Clarkson was talking about how taking it out of gear while you come to a stop actually uses more gas than if you were to just coast down to a stop while in gear.

Now, it's Clarkson, so lots of things are wrong, but I always wondered how that logic worked, if at all.

You use more gas during idling than you do when compression braking? I'm assuming this is because the ECU shuts off the injectors if it can. Definitely uses more brakes to slow out of gear...

[gcochard]

Does it sound something like this?
4 seconds and 14 seconds...

[ZeD4Mr]

This might be a stupid question. What gear are you in at 70? One would assume at 6th, which is around 2800 rpm....
My V8 SUV is at 1500 rpm at 70 mph. These cars are a bit different.

[bhad]

Cliffhopper-yeah kinda sounds similar to that

Yup in 6th gear

[Flying Scotsman]

Originally Posted by Kgolf31

Well you're using less gas. I know on my E46 when you coast MPG needle goes to 0. When you kick in the clutch and throw it in neutral, you spike to 20 then level around 40 and stay there.

Would say somethings wrong with your gauge if it goes to zero, on mine it would swing to infinity and if i have the mpg on the display it would show 99+ mpg on coast.

[Kgolf31]

Whoops, sorry. I haven't driven an E46 in awhile.

I was thinking 0, as infinity. The needle would swing far left, not right.

Either way, injectors or something has to be shutting off to save fuel and affect the needle. If you were coasting and injectors were fighting, you wouldn't go to infinity.

[Flying Scotsman]

Originally Posted by Kgolf31

Whoops, sorry. I haven't driven an E46 in awhile.

I was thinking 0, as infinity. The needle would swing far left, not right.

Either way, injectors or something has to be shutting off to save fuel and affect the needle. If you were coasting and injectors were fighting, you wouldn't go to infinity.

On coast the engine isn't under load, just enough fuel to keep in running.

[esqu1re]

I see now. I didn't know injectors shut off when coasting in gear. I always thought the engine was running, because I still heard the exhaust when coasting.

[gcochard]

Originally Posted by bhad

Cliffhopper-yeah kinda sounds similar to that

Yup in 6th gear

I think it's the diff or transmission whining. Mine usually only happens out of gear or right at the no-load point though.