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Old 03-28-2014, 03:54 PM   #1 (permalink)
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what gear for 7% grade

There's a 4 mile 7% grade where my civic gets down to 22 mph in 5th gear. This has only been an issue the last time when a truck was riding my bumper. This is a 2 lane road that only has 2 brief passing lanes going up. Previous trips I either had more momentum going up, and more space to anyone behind me, or else might have put the car briefly in 4th gear.

The car is probably fine getting up in 5th gear, but the low speed could be a hazard when there happens to be other vehicles going up quite a bit faster. The speed limit is 50mph in this part. Considering this, at what speed would you put the car in 4th gear?

I could probably get away with only using 4th gear for the worst parts.

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Old 03-28-2014, 04:01 PM   #2 (permalink)
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I think you already answered your question. 4th gear unless you are not holding up traffic. 5th at 22MPH can't be good for the engine. Once had a 63 Valient that would not climb a 7% grade in high gear, 3 speed manual. Downshift to second at 35, up shift to third at 50, gradually loose speed, rinse and repeat until I got to the top.

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Old 03-28-2014, 04:28 PM   #3 (permalink)
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The main thing is to keep your peak bsfc in mind. I have to guesstimate, but I would guess keep it at about 2500rpm with 4/5s throttle, whatever gear gets you closest to that.

Brake Specific Fuel Consumption (BSFC) Maps - EcoModder

Or install a mpg gauge and experiment with different gears and throttle settings.
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Old 03-28-2014, 08:32 PM   #4 (permalink)
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I hate guys like you because when you bring me your engine to rebuild all the cylinders are oval. Many cases I end up sleeving the thing as you lugged the engine and scraped the oil film off the side of your cylinders.

If I had to go up a 7% grade I would select 3rd gear and about 3500 rpms,but thats me. This way you get adequate cooling and lubrication flowing through the engine. Lugging an engine like that also created harmonics throughout the derivation and extra undo stress to the system.
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Old 03-28-2014, 09:32 PM   #5 (permalink)
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@ Old Mechanic, there are two places that I go down in 4th (from the other direction), probably the same good places to go up in 4th.

@ P-hack, I was using 4/5s throttle, but as you mentioned, the rpm's were too low. I will use 4th in those parts from now on.
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Old 03-28-2014, 09:44 PM   #6 (permalink)
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@ Cobb, why would going up in 3rd gear at 3500 rpm make it cooler?
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Old 03-29-2014, 02:30 AM   #7 (permalink)
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Quote:
Originally Posted by Cobb View Post
I hate guys like you because when you bring me your engine to rebuild all the cylinders are oval. Many cases I end up sleeving the thing as you lugged the engine and scraped the oil film off the side of your cylinders.

If I had to go up a 7% grade I would select 3rd gear and about 3500 rpms,but thats me. This way you get adequate cooling and lubrication flowing through the engine. Lugging an engine like that also created harmonics throughout the derivation and extra undo stress to the system.
But they got a better MPG number readout on a gauge before a $5000 rebuild.
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Old 03-29-2014, 03:00 AM   #8 (permalink)
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Quote:
Originally Posted by johnlvs2run View Post
@ Cobb, why would going up in 3rd gear at 3500 rpm make it cooler?
I'm not a mechanic, but my common sense tells me that running it like that puts the engine under tremendous stress, but only pumping enough fluid around to accommodate 2300rpm's (just as an example), but at a way higher engine load. I'm probably wrong but that would be my guess.
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Old 03-29-2014, 10:20 AM   #9 (permalink)
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Quote:
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...Many cases I end up sleeving the thing as you lugged the engine and scraped the oil film off the side of your cylinders...
2500 rpm is hardly "lugging". Do you have any actual data on this? We DO know it is how you get the most efficiency out of an engine. Folks been driving like this for decades and 100s of thousands of miles so I'm not sure you have the cause and effect completely sorted yet.
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Old 03-29-2014, 10:27 AM   #10 (permalink)
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My original 37 Ford, with no oil filter and a 6 volt system, with a starter that cranked at 100 RPM, produced 60 PSI of oil pressur in 5 seconds if you pushed the starter button with the ignition switch in the off position.

You can't put your car in gear fast enough to not have oil pressure after starting it up.
Lugging at 2500 is laughable. Why would a VX pull fine from 30 MPH at 1000 RPM and still have every valve lash in specs after 62k miles at the first adjustment.

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