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Old 09-07-2014, 01:32 AM   #21 (permalink)
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It's a coupe. Altimax 175/70 R14 84 T
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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.

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Old 09-07-2014, 01:41 AM   #22 (permalink)
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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.
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Old 09-07-2014, 07:19 AM   #23 (permalink)
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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.
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Old 09-07-2014, 07:33 AM   #24 (permalink)
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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.
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.
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Old 09-07-2014, 07:33 AM   #25 (permalink)
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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.


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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.
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...
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Old 09-07-2014, 07:38 AM   #26 (permalink)
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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.
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.
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Old 09-07-2014, 10:47 AM   #27 (permalink)
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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.
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Old 09-07-2014, 12:34 PM   #28 (permalink)
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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.
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Old 10-15-2014, 03:26 PM   #29 (permalink)
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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.
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Old 10-15-2014, 04:02 PM   #30 (permalink)
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I'm curious if the rpms holding the car back puts any undue load on the engine
No problem.

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and what the rpms might be if I went down in second.
Probably lower, at a much lower speed.

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