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Old 04-12-2008, 06:06 PM   #1
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Default FYI: NB Tach Response

So I've had this eerie feeling when something happens in my car on the MS, and I look at the logs, that I wasn't quite at the RPM I thought I was at judging from the tach. Something at "5400 or 5500" on the tach was at 5100 in the logs, etc.


This was taken at "7000" RPM, but it was recieving a signal of only 6790



In fact, at ~1k, the tach is dead on. Here's how I put it elsewhere:

I fed the tach known spaced tach pulses, and found that the tach is accurate at low RPM (1000), and from ~1500-~4000 is picks up linearly about 175 rpm of offset (it reads high). Then it stays there until ~7200 (reading ~7375) and by 7500 it's reading correct again, which it retains through 8000 rpm, which is as high as I tested.

So, just an FYI....

More pics at 1000 rpm steps here:
http://abefm.smugmug.com/gallery/470...78447143_jQFm9
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Old 04-12-2008, 06:19 PM   #2
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Good info. Now build something to correct it.
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Old 04-12-2008, 07:05 PM   #3
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My tach is way off just like yours. I have my hard cut rev limiter setup for 7200 RPM but the tach indicates something like 7500-7600 when i hit the limiter.
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Old 04-12-2008, 07:08 PM   #4
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I believe a similar phenomenon exists in the NA. Well, at least mine. Fairly accurate at low rpm, but not even close (couple hundred off) up top.
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Old 04-12-2008, 07:53 PM   #5
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That oil pressure gauge is scary looking.
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Old 04-12-2008, 07:57 PM   #6
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Yea Abe, ya *might* wanna check your oil level one day.
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Old 04-12-2008, 09:49 PM   #7
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my tach is extreamly slow.... i posted up about it a while ago. just makes you feel better bout redlining it
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Old 04-12-2008, 10:03 PM   #8
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Quote:
Originally Posted by patsmx5 View Post
Good info. Now build something to correct it.

I lovingly fashioned a post to the guys who write the MS-II extra code, thinking this handy bit of artisandry would be what it would take to fix the issue, asking for a scaling factor on the RPM out pulse train.

However, it seems even that wouldn't work, since the response varies across the board.

Quite annoying.

Oh, and I run synthetic oil, so I only build 3 psi of pressure. That's all it needs, I could run it there all day, and it was smooth as silk in that picture, too.
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Old 04-13-2008, 12:35 AM   #9
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I'm 500 rpm off at redline. indicated 7500 but the dyno and datalogs show 7000.
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Old 04-13-2008, 03:56 AM   #10
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Quote:
Originally Posted by y8s View Post
I'm 500 rpm off at redline. indicated 7500 but the dyno and datalogs show 7000.
That's really interesting! I believe I checked in the past with my scope and my jimstim is pretty decently accurate, but I was definately ~200 rpm over at an indicated 7000, and I think I was pretty much right on at 7500.

I've got a 2000... Yours is 99? Maybe it's just in the tolerance, but wow...
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Old 04-13-2008, 05:29 AM   #11
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The NB revs ~6700 standard, tach usually shows ~7200 then.
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Old 04-13-2008, 10:50 AM   #12
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Quote:
Originally Posted by AbeFM View Post
That's really interesting! I believe I checked in the past with my scope and my jimstim is pretty decently accurate, but I was definately ~200 rpm over at an indicated 7000, and I think I was pretty much right on at 7500.

I've got a 2000... Yours is 99? Maybe it's just in the tolerance, but wow...
01 hydra. interesting aside, after letting off the gas on the dynojet, the rpm went up to 7550 on the datalog.
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Old 04-13-2008, 12:46 PM   #13
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I don't have a Miata yet, so forgive my ignorance...

Are you saying the problem lies in the dashboard tachometer, or are there any boxes that the tach signal pass through that might influence the results?

Is this something we can fix, or something we have to band-aid?
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Old 04-14-2008, 05:52 PM   #14
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I'd say the 'band aid' is just knowing it's off.

Hmmm, another fine dynojet 'feature' it sounds like to me. I'll trust my NIST traceable oscilliscope before something that can't tell if RPM are increasing or decreasing.

Quote:
Originally Posted by Sentic View Post
The NB revs ~6700 standard, tach usually shows ~7200 then.
Redline is quoted as being 7200 - and from my recent testing, I would say the offset is smaller than that - though it sounds like yours agrees with Y8ts...
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Old 04-14-2008, 08:12 PM   #15
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I have seen this on my '99 with MS as well. A couple of times I thought the rev limiter was not working correctly but confirmed afterwards it is working just fine. As long as the MS is correct the dash tach error is not a big deal for me.
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