Tach not matching engine speed
 

So to begin with, this is on a Automatic in manual mode. First noticed the issue after installing a 4.88 rear and getting Flex Fuel tuned. I know...broke a cardinal rule by doing 2 different mods at pretty much the same time....oh and lets go ahead and get the "you should have bought a manual" out of the way.

I've had the Edelbrock for over a year and never noticed this issue. The issue is that sometimes when I up shift with a decent amount of throttle, the tach responds to the up shift immediately by dropping about 1k but you can hear the engine RPM's stay the same and sometimes it's more noticeable than others. In this vid, around the 6 second mark is where it's the most obvious but it has been worse.

Has anyone encountered this before? Maybe @Lunatic since I know you're SC with the 4.88 rear.

https://www.youtube.com/watch?v=rm1I...ature=youtu.be

I see that all the time. The tack always responds a bit faster then actual engine noise. Not enough of a time lag for me to be concerned about, but shows tach is just for show and not directly coupled to actual rpm. That's electronics for you.

The tach is an "educated guess".

Yeah, well mine rode the short bus. There are times when it literally jumps out of an idle dip. I call bullshit.

It's an educated guess and tires to "get ahead". Try a WOT pull, and lifting at high rpm. You'll see it suddenly drop.

Oh, yeah. I've seen that too. Their look-ahead calculations in the control loop are a bit optimistic. :D

I just thought it was funny.

That's a polite way to put it.

Well that's good to hear that it's not something I did. I guess I'm just watching the tach a bit more closely now after the latest mods.

Are your shift times consistent across the board in manual mode? Normally the shift/tach response is pretty quick so I don't notice that lag however lately it seems there are times where the delay between when I tell it to shift and when it actually shifts are much longer (tach responds quickly to the shift input but trans does not)

So I guess my question is...Is this inconsistency normal?

The dash will likely be getting the rpm data off the can buss. you can log that with ecutek or oft or whatever.


The dash electronics likely "dampens" the movement of the tach (or it a limitation of the actual mechanism), just like it does for fuel temp gauges.

that "educated guess" due drive-by-wire tacho nature imho has one con vs mechanical tachometer. - it may hide some issues that could be diagnosed easier on directly linked tacho. For example mentioned in this thread rpm lag, or jump on full accel press on my MT that may look almost as if there had been clutch slippage, or uneven fluctuating idle rpms readings artificially flattened .. There are other lags that i dislike on dash too, eg. gear indicator lag, that shows number long after clutch was released. What's a use in gauges, if they lie vs real readings they should show? Tacho that is good only for seeing when closing redline to know when upshift might aswell be replaced with simple on-off light indicator.
It might be that i simply expect too much. If car has as good tight handling and lagless NA engine, rather nice brakes/gearbox for stockers (stock clutch aside), then i somewhat expect less lag/response everywhere else too including dash gauges, while i wouldn't care about such in some family saloon or SUV.

I figured that's where it was getting it's info from.

I guess my real concern is the inconsistency in time between when I tell it to shift and when it actually shifts. The ECU knows I want it to shift because the fake tach (hey new word...Fachometer) drops 1k on upshift input. Most times the tranny responds very quickly but sometimes there is a pretty "big" delay (relatively speaking), which you can see in the vid. I thought maybe I created this "problem" with the 4.88 diff or tune because I never noticed it before.

We have a tach? The only time I even look is when trying to figure out something for somebody here. Of my 30 or so MT cars only the last 4 even had one so I don't put a lot of trust in the things anyway.

LOL, I feel the same way about the speedometer!

The auto trans control unit and the engine ecu will exchange a lot of data and then it makes yp its mind if its going to shift. Its possible that the tune and thte addition oc the changed diff ratio, may effect the way the trans control unit makes the decisions on when to shift.

The wheel speed sensors provide speed data and it knows engine rpm, but because of diff ratio change the speed vs rpm will now be different, this may upset the auto transmission control unit calculations