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Old 04-17-2017, 09:12 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, 10:35 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, 10:40 AM   #3
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Quote:
Originally Posted by Tango View Post
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, 01:06 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, 01:31 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, 01:51 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, 08:26 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, 08:35 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, 10:39 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, 10:41 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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