Just wanted to let you guys in on something I have found out. After the disappointments of last weekends races, I have been going over my car thoroughly. The datalogging was showing that I was shifting at between 7,000 and 7,500 rpm even though the tach indicated well over 8,000.

So I went out and bought a new digital tach and shift light to see if my datalogging was wrong or my tach was off. WOW the stock Mazda tach was optimistic by about 15%. An indicated 3,000 rpm on the stock tach was actually 2,450 rpm. An indicated 5,000 was actually around 4,200 rpm. The digital tach and the datalogging agreed! The stock tach was wrong.

This means that when I thought I was shifting my car at 8,000 it was actually shifting at about 6,800 rpm!!!!! No wonder I was slow in 4th, the car was just lugging below the porting power band all the time.

It was never a problem before because I used to shift at the rev limiter anyways, LOL....

Watch out as the stock tach really sucks.

Eric

 

When I idle at 1500RPM my Autometer Tach reads 1500RPM off my MSD. The one in my dash reads Zero. At 3000RPM. The Dash reads 1000RPM.

My old tach read 2200RPM when the car Idled at 1500RPM.






So yeah, stock tachs are unreliable.

 

When my dash tach reads 7500RPM, the megasquirt states 7100-7200rpm, so mine aint that bad
Oh, but at idle (around 700 - 800 rpm) my dash tach spikes on and off (so drop to 0 spike back up 700, drop back down to 0, while the megasquirt states its just fine, and the car runs fine)
Also when its hot out, the dash tach seems to get sticky, like moves up slowly, and sometimes gets stuck at certain points suring decel lol. MS reads fine still!!

 

Hmmm, is the SAFC a reliable reading? My SAFC and stock tach are in complete agreement.

 

On my SAFC, it reads 500 rpm lower than the stock tach at around 8000rpm.

I wonder which is correct.

 

yah my safc reeads about 400-500 rpm lower aswell

 

This is all true. The stock tach will almost always tend to be 300 to 500 RPM off.

 

this throws a Wrench at "the Stock idle is 750 rpm "..if the Stock Tach is showing it's Bang On,Then just what the Heck is the car actually Running at?..(I don't want to Know honestly ..It "sounds" like Idle to me!)

 

the idle reads the same as the safc it just tend to be 400rpm off by redline, it slowly starts to wander off as you rev it up

 

This is normal. I also see about 500rpm difference close to redline. My SAFC reads the same as my wideband datalog. Who knows if it is intended design or just some measurment error inherent in any analog device, maybe a bit of both.
Trust the SAFC/digital reading first. It is simply reading the injection firing events and doing a simple calculation, not having to represent that into a needle moving.
you could just get a shift light for $30 and ignore the tach altogether

 

my tach is pretty accurate actually up to 7000RPM.

beyond 7000 it's useless. Anything past 7800 reads as 7800

the tach in my FD is crappy as well, it's OK in the upper gears but it's so buffered that if you blip the throttle in neutral you can hear the engine revving down as the tach is still climbing!

 

reading this is just make me more tempted to remove the dash and instruments infavor of aftermarket, my stock tach is 4% optimistic over my SAFCii

 

Well, actually, I had a closer look and my tach is once again innacurate. I swapped gauge clusters late last year to fix a gauge issue and just went back to my old one, and this tach is off. In a TII (assuming GXL may be similar) every 1000RPM is 40km/h in 5th gear. Exactly.

so if your tach reads 2500 your speedo should say 100km/h dead on, if it doesn't, the tach or speedo is out.

Mine currently says about 2800 at 100, lappy says that we're doing 2500 exactly. At least I know the speedometer is accurate!

 

well, actually both the tach and speedo are a little inaccurate. speedo sometimes can just be inaccurate due to tire size.

 

Most speedo's read off from the factory.

When It reads 110 in my Miata, Im really doing 102. Its off by 8.

When Im going 110 in the RX-7, It reads 0, because my speedo doesnt work.

So I just do 2500RPM.

 

u know i've never really thought about it, but now that i know i might just mount my SAFC in front of my tach.