OK, Most people that have installed an S-AFC in there car have noticed a discrepancy between the RPM readings on the S-AFC and on the factory tach. Usually to the tune of a few hundred rpms.
I know mine is out by maybe 100-200 rpms.
Also- I havn't checked to see if it's a ratio... eg, is it out by 100 rpms across the board, or by much more at high rpms than at low rpms? I should check....

Simple question:

Which one is right????

 

id like to know

 

My Idle displays a solid 750 while the S-AFC displays, very rapidly, any where form 739 - 760. Good question. I would trust a brand new rpm gauge (featured on the S-AFC) rather than a 14 yr old tach. Just my opinion.

 

I'm pretty sure the afc is right.

my gap gets bigger as the rpms go up... idle is both same. off by like 200 at 2000 and like 3-400 at 4000

but it's aroud the same at redline. well when I shift using the tach the AFC max is around the same

 

What does the SAFC run off of? The wire from the CAS going into the ECU? I have that wired into my turbo timer and there also is a discrepancy.

 

Actually, I'm pretty sure they're both right. The S-AFC gets it's RPM signal from the CAS. The tach get's it's signal from the trailing coil. Therefore, two different RPM sources, two different readings. The trailing coil signal is more accurate than the CAS, so take the tach as the "real" RPM.

Of course, I could be wrong. I'd check the manual (ECU wiring diagram) but I'm too lazy right now.

 

Aaron... I know you know your stuff... but them both being right is a physical and logical impossiblity if they are reading 400 rpms apart!!!!!!
I know what you are saying... the source is off, not the display.
I'd still like to have concrete evidence though.... anyone have a tach to hook up under the hood or something to compare??

A freind of mine works at a tool shop, and they have a snesor that detects RPM on any shaft... it just looks like a timing gun that you aim at the rotating target, and it will tell you it's rpm's!! Not sure what method it uses... radar, optical, infrared, who knows, but that sounds pretty concrete to me....


Anyone install an aftermarket tach and seen a difference with the factory one?

 

Yes i have two friends with autometer tachs and they read about 3-400 rpms off so there is a pattern with the stock tach and aftermarket

 

Which reiterates the question.... which is right?????

I stumbled across a strangely dark, yet very possible theory....
Does anyone think Mazda could have exaggerated the factory tachs for bragging rights, advertizing (8000 rpm redlines..) etc????
Not exactly unthinkable!!!

 

no factory tachometer is 100% accurate. the AFC is the one that is right. ive done a little work on piston engines, using an external tach under the hood and the stock tach is always off. oh and the external tach was calibrated. some cosmic tool that the shop had, mega expensive.

 

If the S-AFC gets Its signal from the Crank Angle Sensor and the Tachometer gets It's signal from the Trailing Coil Pack how are they both right? Additionally, If there Is two different RPM sources, two different readings, than which Is right? If the trailing coil signal is more accurate than the CAS, than why Is It suggested to take the tach as the "real" RPM? They both collaborate In unision. I would think something Is not displaying correctly.

 

cant believe this thread died. Mabey I can bring light to someting... At 8000rpms your sopose to be going 140mph I think according to a miata calculator. You can find any speed or gear ratio using it. 23inch tires 4.1 final and 2.00 fourth gear ratio you will be doing 140 or around thier at 8000 in fourth. My tacks rpm and speed are dead on but mabey their both related? anyways at 8000 rpm my S-AFC reads 7500 rpms.

 

it's just digital vs. analog. new tech vs old tech...

 

i thought i was the only one w/ this problem! the difference between the two in my car isn't always the same though.

 

Von... same for me... a negligable amount of difference in the low end, but around 500 rpms difference at redline!!
On the stock tach I don't get 140 mph untill about 8500 rpms.... next time I hit that speed I'll look at the S-AFC tach....

PS. Where is the miata calculator?

 

My aftermarket tach is slightly faster than my stock (300 or so rpms) and so is my turbo timer. I get one signal from the trailing coil, which is the same as the stock tach and one from the leading coils output. And both the aftermarket tachs are the same signal. This leads me to conclude that the stock tach is just slightly inaccurate.