Why is the hz on for example 10000 rpm only 80 and on like 5000 rpm it is 120 hz?
This would mean 5000 rpm is much higher rpm than 10000 rpm. How do i calculate engine rpm from hz only and find the true rpm reading from it?
Does distortion make the reading inaccurate?
Btw the engine is a 1.5 litre V-16 engine. 16 cylinders in a V configuration.
This would mean 5000 rpm is much higher rpm than 10000 rpm.
Or your measurement is bogus.
How many cylinders? If each one makes a detonation noise, you have to multiply the RPM times that number. The one pulse per revolution per cylinder only holds true if there’s no muffler. The muffler’s job is to even out the individual exhaust pulses. You know how someone with a bad (or no) muffler goes down the street you can clearly hear each bang of each cylinder but it’s much harder with a proper system.
Does the engine have tuned exhaust? Those are designed to reduce detonation noise to near zero at certain speeds. If you measure one of those, you’re just going to get everything but the RPM, and the effect will come and go with speed.
And all that is assuming you didn’t overload the microphone or measurement system. This might be a good place for a dynamic (moving coil) microphone rather than one of the other types. They have an almost unlimited loudness range. If you don’t overload the preamp, you should be good to go. Dynamic microphones do have a loudness range, but it tends to self-limit rather than shorting out, exploding, or destroying themselves. That’s the business of never blowing into a microphone. It might be the last thing that microphone ever does.
Rock band microphones are almost all dynamic/moving coil types. They’re not indestructible, but they’re close enough. I had one Shure SM58 come back from a gig in two parts. It was still working.
Koz
The engine is a 1.5 liter V16. So 16 cylinders in a V configuration.
I just used the pitch change plugin to see what the pitch is over a short or medium long part of the sound which is a interior recording from various rpms.
Change Pitch and Change Tempo are the two effects most likely to damage the sound. Change Speed has much less distortion. It changes everything at once and doesn’t try to rip the sound apart and put it back together.
interior recording
Interiors are designed to isolate you from the engine. Some of them are so well designed you can park a vehicle in a quiet lot and use it for a vocal recording studio.
Koz
You might be far better by taking careful measurements and use Analyze > Contrast to get the RMS or overall loudness measurement rather than trying to over-analyze it down to cylinder pulses. And even then, there are meters who do that for a living rather than forcing your computer to do it.
Some phones have an app that can do it, too.
One caution, there are two common measurements, A and C. C is relatively flat measuring all common sounds, A is the one that hears like your ear does, ignoring very high and very low pitch. Most hazardous sound laws are written in A.
Koz
Is your microphone directional? Most microphones are and that can have a big effect on your measurements.
Koz
It is a recording off youtube.
The interior recording starts at 5:51.
You might try looking at the [u]spectrum[/u].
Like all real world sounds, the exhaust sound is not a single-frequency and as Koz suggested the muffler & exhaust system act as a filter (or maybe like a tuned organ pipe*). And, the perception of pitch is complex. Musical instruments (and the human voice) create harmonics that make the note/pitch more defined than “random sounds”.
… It’s probably easier for a human to learn the correlation between the sound and RPM (for a particular engine) than for a computer to analyze it. 
If there were one “pulse” per ignition, you’d have 8 pulses per revolution (a 4-cycle, 16 cylinders) and you divide by 60 to get Hz (cycles per second). At 5000 RPM that’s 667 Hz. But, the original “explosion” inside the cylinder most-likely contains all frequencies and is non-pitched.
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“Frequency, is lower on higher engine rpm?”
Simply no, it’s higher on higher RPM.
I would globalize the sound of an entire revolution in one, since part of the sound may be affected by the structural behavior of the engine.
RPM=rev/min
Hz=rev/sec
Take in account that engines with more cylinders produce a higher tone because they rev up higher, but at same RPM the sound is not necessarily in higher tone, since it is not proportional to engine’s cylinders quantity.
It is a recording off youtube.
So it’s been through YouTube’s processing.
This isn’t scientific measurement, it’s entertainment media.
Good luck.
Koz
It is a open top open wheel racing car.
It is in my recording but i know it is supposed to be higher rpm = higher hz.
Which is your recording? Are you able to post short samples where the RPM is known?
Just some comments/observations.
A V8 has a deeper-lower pitch than a 4 cylinder even thought the V8 is firing at twice the frequency (at the same RPM). A Ferrari V12 “sings” at a higher pitch than a V8 (maybe because the v12 revs higher). And, I’m pretty sure there is some relationship to cylinder diameter.
In the video, I DO hear the pitch increase with the revs but it’s not that pronounced until the engine really winds-up and it doesn’t “feel” proportional to the RPM. There SEEMS to be a LOT of “noise” that’s not related to RPM.
And, I didn’t see a tachometer in the video. (I didn’t watch the whole thing.)
That reminds me that there is a game, called “BMW M3 Challenge” where engine sound is not made from just one sample at constant revs, but they split the revs range into three or four ranges, each with its own pitch.