steep downhill coasting
 

There's a long downhill that I do periodically, with 4 miles of it approaching 7 percent grade, including two sweeping 180 degree downhill curves. Before any mods I did all of the downhill in 5th gear. This transitioned to 4th gear after grill blocks and wheel covers, which may or may not have included some touching of the brakes.

The last trip with the new bumper cover the car was flying down at high revs in 3rd gear (approaching 55-60), and lightly touching the brakes didn't seem to do any good, which freaked me out a bit. Is it possible the car could rev too high in 3rd gear? What if the car is still going too fast, just ride it out, use the brakes more, or start out in 2nd at the crest?

Whats the speed limit there? I had some nice down grades in WV, but the tacoma I drove had a terminal velocity of about 55 mph in neutral.

The speed limit is 55, and beyond that gets quite dangerous on the curves.

Actually it might be 45 on the curves.

I've been in the 80's in neutral going down much lesser grades.

Is this an auto trans or manual? Sounds like a manual.

Yes, a 5 speed manual.

I'm new to this so bear with me. Why not coast down the hill in neutral or with the clutch in?

The car would go too fast in neutral.

I'm keeping the car in gear to slow down and keep from going too fast.

I can easily coast down the entire 10 miles without using any gas.

Isn't that what brakes are for?:D

Back about 20yrs ago when I still rode motorcycles on the street I used to ride with some guys up to Prescot, AZ. On the way back we would descend Yarnell Hill, nearly a 5000ft drop in 8 miles. We would race down the entire thing engine off and coasting. He who used the brakes the least won (there was drafting too). Good times.:thumbup:

Video of the road:
https://www.youtube.com/watch?v=ZxLjqw3DzuA

What truck drivers do is say a safe speed is 65. In gear press the brake to slow down to 45 keep it in gear to keep the speed down and you will speed up and repeat

The object is to not use the brakes, and to not wear them out. :cool:

I've been down AZ89 from Prescott. According to the comments there's a 1355 drop (4400 to 3045 feet) in 8 miles, an average 3 percent drop, which I wouldn't compare to 7 percent around two 180 degree curves, and then a sharp 90 degree curve at the end of the second one. There is no way you'd be going 90 on those, not twice anyway.

Basically I'd like to know if letting the engine rev up in 3rd gear is okay.

How many rpm in third?

I'm not sure exactly but usually shift out of 3rd at 20 mph, which is 1500 to 2000 rpm, so maybe 4200 to 5500 at 55.

Brake pads are cheap.;)

I can't remember how much elevation it was (I'm not sure I ever knew really). In the description section it says 5000 to 235ft. It could be wrong.

[EDIT] I just looked it up. It drops 1735ft. We were hitting speeds well over 100mph coming down there. Couldn't tell you how fast in the middle of the corners, I was a little busy looking where I was going. Just for reference this is me riding:turtle::

http://i767.photobucket.com/albums/x.../1002x1000.jpg
http://i767.photobucket.com/albums/x.../1001x1000.jpg
http://i767.photobucket.com/albums/x.../1000x1000.jpg

No problem, that thing could spin at 3k for years.

Does your motor go full lean while coasting or is it still pumping fuel? If it is still pumping fuel maybe you can figure out what is less expensive, the pumped fuel, or brake pads.:)

No gauge?

Good point^^^^

If you don't have a gauge I can probably look up all the info needed to calculate it. I will just need the year, make , model, engine, trans and tire make, model and size.

The stats are in my signature. I was watching the road, not my gauges, but 55 is a good estimate of what the top speed was on the curves.

Found the gearing info. What tires do you have?

1st 3.250
2nd 1.909
3rd 1.250
4th 0.909
5th 0.702

Final drive
4.058 sedan
4.250 coupe

I'm just guessing here because I don't know your tire size. I guessed it at 25".

But 3rd gear at 55mph would have you at 3745rpm

It's a coupe. Altimax 175/70 R14 84 T

Thank you! That doesn't seem very high.

I guessed high on that tire. It is 23.65".

So in 3rd @ 55mph you are at 4,150rpm

4th @ 55mph you are at 3,025rpm

I highly doubt you will see any ill effects from compression braking at 4,100rpm. But depending on how your ECU is set up you may be using more fuel at 4,100 than 3,000 so running it in 4th and using the brakes a little could be a better method. That all depends on your "tune" though and I wouldn't know where to find that info at.

Are you saying intermittent braking at 4150 down a 7% grade should not cause any undue heating or wear on the brakes? If not, then that's good to know.

The car should be in dfco when coasting in gear, so hopefully any fuel use would be minimal.

Power or manual windows? Open them up for extra drag when needed. If power,that's even better since you can easily lower&raise them all.

They will wear, but the wear ever time you use them. I highly doubt they will over heat and cause any issues. If that is a worry you could always add brake cooling ducts, they are very effective.

The curves you speak of, are those before, after or during your decent? Because if after, I advise caution using the brakes... Its better to heat the tranny a little using it as a brake, than superheating your actual brakes, making them either fail or to hot to work properly, and you going off a cliff.

I do feel bad for you though, when I drove 38 miles to work, there was a spot of about ~2 miles of 6% grade that I didn't have to brake on. I remember going 120+ a couple of times in neutral with the engine off and a tailwind. I only did that with no traffic around though.


I used my brakes a couple of times when there was traffic and I had to keep my speed around 75. By the time I reached the bottom, they were so soft I couldn't make my car stop in a safe distance. If OP uses his brakes, I recommend you never let off once you apply brakes, superheating (leading to instant failure) of the brakes is when you release, and allow oxygen to make contact with the metal of the brakes. My dad did this once in a semi, and it almost killed him...

If he is "superheating" his brakes he has a brake problem. Not a hill problem.

I've gone down some of the longest/steepest descents in the US in 7000# trucks towing 12000# trailers and effectively no compression braking without over heating the brakes (truck or trailer). That little tiny car won't have an issue at all.

MetroMPG, thanks for that suggestion about windows. The windows are power, but the only one that works is the driver side front, I don't know why, and the moon roof on top.

Baltothewolf, the curves are the steepest part of the descent of the hill, followed by a relatively straight stretch where I get up to 80 very easily just coasting in neutral. I guess having the car in 3rd helps a lot more than it seems, and I appreciate your warning on the brakes.

Sqidd, don't trucks have a different type of brakes? I think there's a truck speed limit of 15 mph down the hill, but it's strange, I mostly encounter trucks coming back up from the other direction.

F-350's have the same kind of brakes that your car does. Just bigger.

I don't know what the limits are in CA but in every other state I haul at 75-80mph. Close to 100mph down hill.

update
 

My top speed on the downhill yesterday was 49.1 mph in 3rd gear, at approximately 4050 rpm (console) and not using any brakes, which was plenty fast enough with the curves.

I'm curious if the rpms holding the car back puts any undue load on the engine, and what the rpms might be if I went down in second.

No problem.

Probably lower, at a much lower speed. :)

Be careful not to downshift into wrong gear. My Audi mechanic friend was rebuilding a transmission from a wrong downshift, rev limiter only works on the throttle rpms.

I love a good downhill (rolls down all windows) when I'm sweating in my car. I've even stuck my arm/hand out to slow down an extra mph when needed! Brakes are for emergency use only!

For those who said "why not neutral" because keeping your car in gear is infinity MPG and coasting in neutral at hwy speeds is "only" 200-400 mpg. That and most hills in my parts would put me to 100+mph in neutral.

If you have not done this before, try downshifting one gear at a time, until the tachometer gets too close to the redline. If you don't have a tach, then go by ear, I guess?

Can you post a video of a trip down and up this hill? I would love to see it.

You yanks really don't do manuals do you lol


The revs will be the same going downhill, on the flat or uphill in each gear. So if you want to know what revs are safe at each speed try accelerating in each gear first to see what speed each gear achieves to the rev limiter.

Above that speed for each gear you can/will grenade your engine in one quick lift of the clutch. If that sounds far too complicated, don't argue just stick to autos.