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Old 04-17-2017, 10:12 AM   #1
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5.9 Mechanical rpm?

Hello. I drove my bus back from iowa about 230 miles i ran rpm's of 2500-2800 rpm and top speed i estimated at 78 mph. On saturday i drove south on I-49 for about 60 miles each way and i saw my rpm's at 70mph @1600 Rpm. Previously the bus had been driven about 1-2 times per month to exercise it and give the kids a ride. I read that if they sit that the turbo will run better after about 200 miles. This is my first time back on the freeway since i got it here and with about 20 miles local street driving. Has the Turbo found a sweet spot?:
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Old 04-17-2017, 11:35 AM   #2
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70 mph @ 1600 RPM...???

That sounds like like something wacky with the tach to me. The first set of numbers seem reasonable but unless you switched to 54" tires in between drives , I can't see how 1600 at that speed is possible. Do you know your rear gear ratio?

Anybody else?
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Old 04-17-2017, 11:40 AM   #3
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70 mph @ 1600 RPM...???

That sounds like like something wacky with the tach to me. The first set of numbers seem reasonable but unless you switched to 54" tires in between drives , I can't see how 1600 at that speed is possible. Do you know your rear gear ratio?

Anybody else?
No i don't know the ratio. The tag is on the left front of the diff up high and i haven't squeezed up in to there.

I forgot to say that i think the head has been off it because i see fairly new looking Grade #8 exhaust manifold bolts. Why that expense. Tach seems to be in good shape with not being a jiggly needle.
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Old 04-17-2017, 02:06 PM   #4
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Where does the tach attach to the engine? I will check the wires if possible.
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Old 04-17-2017, 02:31 PM   #5
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I am guessing that something about the tach, including sender/wiring, etc., is giving a false reading. 70 mph at 1600 RPM makes no sense unless there are some VERY unusual gearing/tire size considerations. The previous RPM reading seemed pretty consistent with reality, but the 2nd reading appears way off.

BTW...
there is an outfit called "Tiny Tach" that makes a unique but quite accurate diesel system. Instead of needing gears or a magnet, it reads the fuel pulses going through the injector line (one pulse= one revolution) and reads it out on a small LED display. At under a hundred bucks using a super simple installation method, they can cure some otherwise expensive ills.
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Old 04-17-2017, 02:51 PM   #6
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Thanks. I will check them out.

Just did. Looks great and easy to install. What size injector line do i need?
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Old 04-27-2017, 09:26 AM   #7
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I just noticed that my tach stays a 500 rpm with engine and key off.
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Old 04-27-2017, 09:35 AM   #8
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So you're turning 500 rpm slower than what your tach's showing. That means you're actually only turning 1,100 rpm at 70 mph.

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Old 04-27-2017, 11:39 AM   #9
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I think I'll try that on my bathroom scale..........I feel lighter already!
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Old 04-27-2017, 11:41 AM   #10
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Line size does not matter. Tiny Tach just reads the pulse of fuel going through any given injector line which is equal to one RPM. Pretty ingenious actually.
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Old 04-27-2017, 11:31 PM   #11
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Quote:
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Line size does not matter. Tiny Tach just reads the pulse of fuel going through any given injector line which is equal to one RPM. Pretty ingenious actually.
Equal to two revolutions.

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Old 04-27-2017, 11:32 PM   #12
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Wouldn't that be a two-stroke?
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Old 04-27-2017, 11:35 PM   #13
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An injector pulse equaling one revolution would be a two stroke.

But we're in the Cummins forum, so one pulse equals two revolutions. Unless your injectors are firing at the top of your exhaust stroke for some reason.
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Old 04-29-2017, 09:25 AM   #14
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So you're turning 500 rpm slower than what your tach's showing. That means you're actually only turning 1,100 rpm at 70 mph.

Looks like i found the sweet spot!

That FIGURES OUT TO ABOUT 26MPG @70MPH!
I also have some lake lot's For Sale!
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