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Old 10-05-2010, 01:32 AM   #1 (permalink)
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Too much of a good thing --15 minute gravity drop

I guess I'm lucky with the start of my commute, in that I drop from 7400' in elevation to 5800' in my first 9.5 miles every day. Right now on an average day I can touch the gas once if no one is around, just enough to get from a rolling stop to 30mph on one flat spot after a stop sign. If there's traffic, I usually have to use gas to get from 30-45 for a merge (mountain highway with a 45mph speed limit), but without traffic, I can roll up to merge speed fairly easily, just late in the ramp.

My question is just how much should I tempt fate with this gravity drop. It takes a good 15 minutes and that's a lot of time to waste with an idling engine. I want to start playing with EOC, but obviously with that big of a drop, I'm on and off the brakes for short pulses several times a minute with the twisty mountain roads. I saw the thread on adding a PVC vacuum reservior from 2008 and that's very tempting, but even if I extended my number of brake applications from 2 to 6 or 7, I'd have to recharge my vacuum every 1-2 minutes to maintain safe, solid brakes. Is it worth it? Are there any other alternatives, like maybe figuring out some kind of 12v vacuum pump? I just hate the idea of not taking advantage of this tremendous geographic advantage since I'm definitely paying for every drop of gas to get to the top of this mountain.

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Old 10-05-2010, 02:48 AM   #2 (permalink)
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Seems like you could take advantage of DFCO, where you get the benefit of both the vacuum being generated for the brakes and zero fuel use of EOC... At least what I'm understanding is that the route is steep enough to require brake use, so taking advantage of engine braking too would (slightly) reduce the need for brake use.
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Old 10-05-2010, 05:42 AM   #3 (permalink)
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Your vacuum reserve will last a lot better if you don't lift your foot off the brake any more than is necessary. Also, just get used to the non-boosted effort. Graham Hill used to set his cars up to require 180 lbs on the pedal to skid, to gain sensitivity.
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Old 10-05-2010, 06:32 AM   #4 (permalink)
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Quote:
Originally Posted by NachtRitter View Post
Seems like you could take advantage of DFCO, where you get the benefit of both the vacuum being generated for the brakes and zero fuel use of EOC... At least what I'm understanding is that the route is steep enough to require brake use, so taking advantage of engine braking too would (slightly) reduce the need for brake use.
i'd agree with this.

Currently i have a 2 mile section on my commute where i can p&g between 60-50mph by doing the following.
Use slope to accel car up to 60 in IDLE
Use dfco in 4th to engine brake to 50

Th slope is such that the car will often maintain above 50 for quite some time in dfco.

I also use dfco for slowing whilst driving in 'accordian' traffic
helps me keep my distance and keeps brakes avail if required in addition.
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Old 10-05-2010, 08:11 AM   #5 (permalink)
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I'd probably try to find myself a 12V vacuum pump. Kicking the engine over for DFCO just to refill your vacuum seems like a hassle to me.
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Old 10-05-2010, 09:28 AM   #6 (permalink)
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Have you tried your brakes without vacuum assist?
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Old 10-05-2010, 10:31 AM   #7 (permalink)
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Quote:
Originally Posted by Daox View Post
I'd probably try to find myself a 12V vacuum pump. Kicking the engine over for DFCO just to refill your vacuum seems like a hassle to me.
i'm using eo(n)c as i'm not a fan of the concept of engine off cruising regarding my car..especially in nose to tail 40mph+ driving

Therefore the engine is on anyway, so using dfco is easy..
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Old 10-05-2010, 12:34 PM   #8 (permalink)
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If you have to brake, keep a steady brake foot. Apply once, release once. You can get lots of braking out of a single, shallow displacement of the brake pedal, which uses very little vacuum.
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Old 10-05-2010, 02:43 PM   #9 (permalink)
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Quote:
Originally Posted by endurance View Post
My question is just how much should I tempt fate with this gravity drop.
Does it maintain speed when you keep it in gear ?
Then that'd be an option.

Another option is to coast down, then shove it in top gear and bump-start it when speed builds up, slow down using engine braking, then start coasting again.
Probably won't work using an automatic transmission - but I just saw you have a manual transmission.


Before you go coasting without the engine running, check out how much longer the brakes will work !

Even in low-speed stop-and-go traffic on a slight downhill, my car ran out of braking power after only 2 stops, and the brake pedal is very hard without power assist - so engine off coasting is out of the question for me.
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Old 10-05-2010, 03:09 PM   #10 (permalink)
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I installed a kill switch (toggle, not momentary). On long mountain descents I'll leave the car in 5th gear, flip the switch to kill the engine, and alternate between clutch in (EOC) to speed up, and clutch out for engine braking & to keep the vacuum booster charged. Less heat into the brakes that way, which is nice because my rotors warp easily :/

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