How do you find accurate engine RPM?

[dcoplan]

I've got a Bugeye Sprite. The tach is working in that the needle appears to respond appropriately to the engine revs but I can tell it's way off - the needle needs to be pulled off, rotated, and put back on the spindle in the appropriate spot.

How can I figure this out? I'm guessing there's some sort of measure I can use for example...

Stock 948 Bugeye Sprite traveling at 30mph in 2nd gear should read [fill in the blank] on the tach. Or something like that.

Along these lines there's a seemingly nifty iPhone app called "Engine RPM" that analyzes the sound the engine makes to determine RPM's but I'd like to do it the old fashioned way.

Dan

 

[JPSmit]

I have two old tach/ dwell meters - also called engine analyzers. Have rpm too. One was free other $25. Like this

https://losangeles.craigslist.org/lac/ele/3844755124.html

https://losangeles.craigslist.org/sfv/tls/3872023497.html

 

[BlueMax]

This may be helpful for you....



If you want to find vehicle speed:

D x RPM
________ = S
337 x GR



To find the gear ratio:

D x RPM
_________ = GR
337 x S



To find tire diameter:

337 x GR x S
__________ = D
RPM



To find engine RPM:

337 x GR x S
____________ = RPM
D


To calculate a gear ratio from a pair of sprockets, take the rear sprocket tooth count and divide by the engine sprocket tooth count. If you have a jack shaft, start with the wheel sprocket, divide by the sprocket that drives it, then multiply by the sprocket driven by the engine, and finally divide by the engine sprocket. This will give you a number which you substitute for GR in these equations.

For example, 11 teeth on the engine and 60 on the wheel will give you a ratio of 5.45 : 1, so you would use the number 5.45 to replace GR in the equations.

10 teeth on the engine, 17 on the jack shaft input sprocket, 9 for the jack shaft output, and 54 on the wheel would give you the equation GR = 54 / 9 x 17 / 10. This comes out to a 10.2 : 1 ratio, so GR = 10.2

Here's the same thing formed into a handy equation to find what sprockets to use on your jack shaft. E is the engine sprocket, W is the wheel sprocket, JI is the jack shaft input sprocket, and JO is jack shaft output.


W x JI
________ = GR
E x JO


HTH

 

[dklawson]

Cable tach or electric tach?

On electronic tachs the easiest way for most of us to do this is using the dwell/tach meters JP mentioned. Basically hook up the meter and compare its tach reading to that of the gauge in your dash. If you set the meter for 8-cylinders instead of 4, you can double the reading on the meter to effectively double the range it will read. Of course, you can use this technique as a reference for the cable tach as well.

With cable tachs there is typically a number printed on the face (something like 3.8:1). Your actual numbers will be different. That's the ratio of cable turns to one increment on the display. For example 1000 engine RPM would be 1000/3.8 = 263 cable RPM (using the bogus numbers I listed above... remember, your numbers will be different). If you know that number, you might be able to use something like a "relatively" accurate electric drill or motor to run the tach and set the needle. At home I have a small lathe with a variable DC drive and built in tach that I use as a calibration source.

EDIT: Also... these are typically 40+ year old Smiths tachs. Accuracy is a relative term.

 

[nomad]

As Doug said, these are old Smiths tachometer's. Accuracy is a relative term!:glee:

All you should need is the diff ratio and tire diameter plus an accurate speedometer. I would recommend a GPS or you can do a fair speedo check by comparing the mile count on your odometer to highway mile markers.

Kurt.

 

[aeronca65t]

I'm cheap and good at math so I'd probably use the math formulas above (but with a GPS for speedo as Kurt suggested).

But another option is ~One Of These~

 

[nomad]

Wow! Hows that work Nial??

To elaborate on my above suggestion:
60MPH =1M/minute=5280ft/minute.
5280 ft/Tire circumference [diameter X pi]=tire rev's /min
tire rev's /min X differential ratio= engine RPM
Accurate method of measuring tire travel/rev is to put a pice of tape on the tire and mark the pavement next to the tape then roll the car forward, mark again and measure. This won't be 100% accurate since rolling diameter at 60mph is probably larger but it should be close.

Kurt.

 

[Gundy]

Knowing your various ratios etc to plug in this gem will generate some close WAGs.
The rear, if stock, would be 4.22. Final drive is 1 to 1. Plug in your tire size.
Charts. It should get you close.
I love my old dwell/tack meter. Love the GPS too.
https://www.mintylamb.co.uk/gearspeed/

 

[aeronca65t]

Wow! Hows that work Nial??

Kurt.

Works great! I use one all the time at work to check machine RPMs.

As I'm thinking about this, I'd bet there's an I-Phone "app" that will do this too, but I haven't checked.
The laser-tach is easy and cheap.

 

[David_Doan]

I'm cheap and good at math so I'd probably use the math formulas above (but with a GPS for speedo as Kurt suggested).

But another option is ~One Of These~

I have one of those and used it to calibrate my tach and speedo. It works best when the gauges are out of the car. I used a variable speed drill with a square screwdriver bit to drive them. Here's the chart I came up with:

The values in the blue boxes create the calibration factor. Those are correct for a bugeye. The full spreadsheet can be downloaded here if you want to fiddle with it.

Here are my results - as Doug said accuracy is relative. The top box is the Tach. Since these old gauges are non-linear, I set it to be most accurate between 4000 and 6000 rpm. The lower box is the speedo. It performed better, Best accuracy at 60 MPH

 

[dklawson]

Nial, WOW! GREAT price on that tach. Those used to cost hundreds.

Kurt, was your question "how does a thing like that work?" or was it "is that thing any good?" Functionally you clean a spot on a rotating part and either paint it white or apply a reflective tape. The tach shoots an infrared or laser at the target and counts the reflected pulses. I have used them in labs for years but didn't realize how inexpensive they have become. I cannot comment on whether that particular brand of tach is good but for $13... how bad can it be?

 

[nomad]

Actually Doug I was interested in both. For what they cost I was thinking of just giving it a try and finding out for myself.
The info is appreciated!

Kurt.

 

[bug_sixty]

Love those police speed display radar "slow down" trailers parked around town here...go up and down the block late at night. One City has a permanent one installed. Not very convienent. Rather than doing what's being done above... I sent mine to Deluxe Speedo in Denver after giving them the exact five mile difference in the odometer. For $150... I was done with that project.. I just had to many variables...Datsun tranny, tire size deal, 3.7 rear..... Now, I'm serious about the GPS deal... when this system goes south..

 

[dklawson]

The items above were for confirming the calibration of a tach. You certainly can send the tach off for calibration like you did with your speedo. These were just DIY methods of determining how far out of calibration a tach is.

 

[Jim_Gruber]

And for < $50 with all needed parts and pieces you can install a digital speedo using a Sigma Bicycle Cyclometer that reads 0-180 mph sitting right up on top of your dash in direct sight line. Did it for Bugsy and it works great. Button Magnet JB welded to the outside of the rear drum, a better magnetic reed switch from local electronics surplus, a couple of homemade brackets and some wiring and you're good to go. Ask me and I can send pics.