THANKS for your reply

So for one chamber of pulses such as on picture of my O scope screen pulses per second times 60 equals rpm?

5.5 pulses in one second x60= 330 RPM?

I did the same thing with a video from my phone but was hard to get a accurate count of pulses per time period is why I went with teh O scope method but wondered if my formula was right or not

 

1 engine revolution = 1 compression event per rotor. So just count the spikes per some period and figure out what that makes a minute.

 

So compression pulse count per second times 60 equals engine RPM correct?

5.5 pulses per second time 60 = 330 RPM right??

This scope trace each horizontal block is 200 milliseconds (1/5 of a second)

 

I see 8 pulses in 2 seconds in that trace, so 240rpm

 

So 4 pulses in 1 second = 4 x 60 = 240 RPM? or 8 x 60/2 = 240 RPM ??

240 RPM is a more believable cranking RPM compared to all other figures I have seen but would put my compression in the extraordinary zone. I find this hard to believe for a rotary that has has 100,000 miles on it but it does 14.6 in the 1/4 which is new car spec so that makes it more believable but I still don't believe it. Vertical resolution is 2 volts per block, sensor used is 0-200 psi = 0-10 volts, so 1 volt equals 20 psi, 20 psi x 6 volts = 120 psi that would make my compression 120 psi which at 550 ft elevation normalizes to 129.6 psi, is this possible???

The sensor is accurate to +/- .5% of full scale so that's +/- 1 psi and I checked it against a normal gauge at 20, 40, 60, 80, 120 psi which is as much as my compressor can make and it was dead nut accurate.

I guess the only way to validate is a real compression test at the dealer??

 

You may want to measure the actual milliseconds between tops of peaks for an accurate number. Take at least 5 seconds of data and average the time between peaks then convert that into peaks per second. I was estimating with the 8 in 2 seconds.

I've done something similar but with a data logged so I can get manipulate the data after.

Also are you accounting for the sensor cavity volume? Adding the sensor + any adapter increases the combustion chamber volume.

 

I took cavity volume into consideration when making the probe and made it so it simulated a spark plug and added next to nothing to chnamber volume because I wanted as accurate of reading as possible. Sensor is fitted into the base of old spark plug that was gutted and threaded where the insulator once was. A small poly tube with only a 1/8" id was fitted inside it and exits right were electrode was then potted all around it with epoxy. The volume it adds to the combustion chamber area is next to nothing especially if compared to say even just a normal 1 ft compression tester hose is probably 20 times the volume of this device. Even if I were to increase the pulses in the screen shot to 10 10 in 2 seconds would be 300 RPM which would put my normalized compression to 113 which would still be a good number right?

I'm more worried of getting a accurate figure to find out if its low or not then getting a stellar figure to brag about. It is why I'm so concerned in what is the proper RPM calculation from the scope screen shot as if its 350 I have a compression problem, at 300 its good, at 250 or below its stellar.

 

This is rear chamber, about 6 volts there too, 120 psi, about same timing between pulses, I'm going to go with 270 RPM, 9 pulses in 2 seconds?

 

My god. You guys are still talking about this?

Just get a Bluetooth OBD2 adapter, measure the cranking RPMs with the Torque app and call it a day :P

 

[QUOTE=Nisaja;4808227]My god. You guys are still talking about this?

Just get a Bluetooth OBD2 adapter, measure the cranking RPMs with the Torque app and call it a day :P[/QUOTE

What magical OBD2 adapter can read cranking RPM when crank position sensor is disconnected for for the compression test ??

 

Why would you want to measure both at the same time? Measure the cranking rpm first, and then the compression. Your cranking rpm is not gonna change within a day, so long as the battery is good. Even if it does, you will hear it.

 

cranking RPM changes with plugs out, to get a accurate number of what true compression is the at the time of the test you need to knw what the engine was cranking at that time not earlier in the day. Following the factory procedure you remove a plug from each chamber and measure compression in each chamber individually. The mazda tester hooks up to the crank sensor to get RPM info from, I may have to make up a device to do that with too and have that input go into channel B of my O scope, or just be happy with what I got and just drive the thing LOL

 

Yeah man just be happy! A rough estimate is enough to give you an idea about the engine.

 

I don't get what you're trying to do. All the info you need is on the scope: milliseconds between compression peaks. Say it's 200ms (average it out, they will have 4-5ms variance) that makes 60000ms/200ms = 300 rpm.

 

Thats EXACTLY what i was looking for!!! THANKS!!

 

12/24 is normally used and semis and other big trucks for cranking aid

 

黙れ、初心者ちゃんが

It’s moments like this when I miss 9krpm ...