Does anyone know how to measure tachometer accuracy?

[hrach1]

Does anyone know how to measure tachometer accuracy? I know my tachometer is not accurate because I did a engine swap and I installed 6 cylinder tach chip in the instrument cluster but I know my tach is not accurate. I am contemplating on changing my diff from 3.25 to 2.93 for smoother freeway cruising but I want to know my current rpm and see if it is really necessary or not.
Are there any auxiliary digital tachometers that I can install on the car easily and see my real rom while driving?
I know there are some laser non contact style rpm gauges but I want to be able to see the rpm while driving.
Any help would be greatly appreciated! :-)

[Kershaw]

You can buy an aftermarket (mechanical or electric) tachometer. But why not just buy the right cluster?

[hrach1]

Well, whatever I buy must be temporary just to find out how much error I have in the tachometer but what do you mean buy the right cluster? I have a S50 in an E30. which cluster should I buy to keep the OEM look of the E30 M3 cluster but get the right rpm reading?

[Kershaw]

I bet that question has been answered in the 24v swap forums.

[Joester]

all the ones i've dealt with have been homemade with a laptop, a breadboard, and a data-aq system, but maybe they sell some sort of inductance sensor that you can wrap around one of the spark plug wires to read true RPM? pretty much just like a timing light, but instead of flashing a light it tells you a number.

i don't know if such a thing is marketed commercially but both my brother and my roommate have made on in our garage, so surely someone out there manufactures it.

[jlevie]

Why would you think the tachometer would be inaccurate if you have the coding plug for a six cylinder engine in the cluster?

[hrach1]

all the ones i've dealt with have been homemade with a laptop, a breadboard, and a data-aq system, but maybe they sell some sort of inductance sensor that you can wrap around one of the spark plug wires to read true RPM? pretty much just like a timing light, but instead of flashing a light it tells you a number.

i don't know if such a thing is marketed commercially but both my brother and my roommate have made on in our garage, so surely someone out there manufactures it.

Thanks for your reply. Yes, that is what I was hoping for. I know at least smog shops have it. They attach a sensor to one of the spark plus wires and read the rpm on the computer screen. I was hoping I can find a stand alone version.

[hrach1]

Why would you think the tachometer would be inaccurate if you have the coding plug for a six cylinder engine in the cluster?

Because I cruise at 4000 rpm yet it does not feel like it. I have been driving manual cars all my life and I can tell if the rpm is accurate or not.

[jlevie]

What transmission is in the car? If a G260 4000rpm would be what you'd see if the coding plug was for a four cylinder engine.

[Joester]

ok so i just talked to my boss who is also a car guy, and he has an inductive pickup timing light that also displays RPM. might want to look into buying one of those!

its expensive, but this is only the first one i found. It displays RPM. I'm sure there's less expensive options that also have an RPM display

http://www.jegs.com/i/Equus/390/5568...FctcMgodJE0AeQ



EDIT: here's one for $75 http://www.wayfair.com/Equus-Product...FcxcMgod23EAQA




another possibility is to just do the math. You know your tire size, diff ratio, and whatever-gear-youre-in-ratio, and bam, you got engine RPM right there.

If you tell me the following, i will do the math for you right now.

-cruising speed
-cruising gear
-what transmission you have
-what ratio differential you have
-real tire diameter (go measure it right meow.)

[hrach1]

What transmission is in the car? If a G260 4000rpm would be what you'd see if the coding plug was for a four cylinder engine.

I have a S50, mated with 1995 M3 ZF transmission and 3.25 LSD diff. Coding plug was from a 6 cylinder cluster according to Bill Arnold who gave it to me.

[hrach1]

ok so i just talked to my boss who is also a car guy, and he has an inductive pickup timing light that also displays RPM. might want to look into buying one of those!

its expensive, but this is only the first one i found. It displays RPM. I'm sure there's less expensive options that also have an RPM display

http://www.jegs.com/i/Equus/390/5568/10002/-1?sendroicid=bbef8492-2d00-41d6-b158-38db0de2603c&sendroikwd={keyword}&gclid=CL-RheSM8LMCFctcMgodJE0AeQ

EDIT: here's one for $75 http://www.wayfair.com/Equus-Product...FcxcMgod23EAQA




another possibility is to just do the math. You know your tire size, diff ratio, and whatever-gear-youre-in-ratio, and bam, you got engine RPM right there.

If you tell me the following, i will do the math for you right now.

-cruising speed
-cruising gear
-what transmission you have
-what ratio differential you have
-real tire diameter (go measure it right meow.)

Thanks joester
Yes, I may need to get one of those and check the rpm. The RPM should be the same standing or driving right? I am thinking about getting one of these timing scanners and rev up the car to 1000 rpm and see what is the actually reading.

Anyway, here is what I have in my car.

S50 engine
1995 M3 ZF transmission
3.25 LSD diff
Cruising speed 70 mph
Cruising great 5th
Rear tire diameter 245/35/18


Thanks in advance

[Joester]

no, i said the REAL tire diameter, not just the specs. Go measure it form the ground to the top of the tire and ill tell you what RPMs you should be at.

[hrach1]

no, i said the REAL tire diameter, not just the specs. Go measure it form the ground to the top of the tire and ill tell you what RPMs you should be at.

Sorry, I misunderstood. I am not driving the car today so I can't actualy measure it but I found out the diameter from Khumo'a website.
It is 24.8".

Thanks again

[jlevie]

I have a S50, mated with 1995 M3 ZF transmission and 3.25 LSD diff. Coding plug was from a 6 cylinder cluster according to Bill Arnold who gave it to me.

The M3 transmission has a 1:1 5th gear, so 4k could be realistic at about 80mph. But check the numbers on the coding plug against realoem.com.