Engine note - what are the determining factors?

[renrut]

The sound an engine makes is something which can turn you on or off a car without even seeing it or driving it anywhere. Its also one of those things that journalists often use to criticise a car when there isn't anything else wrong with it but they dont like it, or to stick the boot in just a little bit more, or to redeem a car that is otherwise hopeless.

So what actually determines how the engine sounds? I'm aware that revs, valves timing, cylinders, intake and exhaust play a part in this but what plays what part? Is it analogous to a musical instrument? I remember the video of the renault F1 team playing tunes on their F1 engine so clearly its something that can be fine tuned to a great degree. If this is the case why do most turbo diesels still sound sh!t and not sound like a nice turbo petrol? Any rules of thumb for tuning for a good engine note?



And no I don't want to stick a big catering size bean tin on the end to make it sound like its bowels are loose.

[renrut]

Thanks for the reply, my topics usually seem to be ignored for some reason.

I was looking more for rules of thumb, e.g. increasing revs will give you a higher pitch. That seems a fairly obvious one to me but I have very little knowledge of acoustics.

[renrut]

Ah, I guess its obvious when you think about it. So the main distintive noise is entirely firing order timing and everything else follows as it sets the pulse spacing that hence why you can listen to an engine and immediately tell its a V8 e.g. the Rover lump. So if you have a cross plane crank V8 its something like 1-8-7-2-6-5-4-3 out of the exhaust pipes you get on one side 1 wait 7 wait wait 5 wait 3 while the other pipe is producing wait 8 wait 2 6 wait 4 wait.

For the flat plane its more like a pair of 4 cylinders twinned so something like 1+8, 3+6, 4+5, 2+7 which gives you out the pipes a continuous 1,3,4,2 and 8,6,5,7 all equally spaced.

I'm gonna have to do some more reading on this and how sound an musical notes are created. But that alone explains why V8s sound so much 'richer' than 4s and why most 4s have a plain dull sound to them.

[renrut]

Pumaracing said:

Avocet said:

A "conventional" V8 has crank pins at 90 degrees to each other. It has 2 power strokes per revolution but not a the same time. The "flat plane" crank used in the AJP 8 also has 2 power strokes per revolution but sounds like 2 four cylinder engines running at the same time, because that's (effectively) what it is!

Someone did explain it to me once but I'm not quite sure about the precise explanation - something to do with the 90 degree engine producing a "richer" sound because more of the harmonics of the "note" were present. I have to say I much preferred the Rover engine notes to the AJP ones, although the AJP did have a certain "something" at 7000RPM!

A cross plane crank in a conventional 90 degree V8 has firing intervals at the same 90 degrees as a flat plane crank but the firing order is different.

On a cross plane you get two consecutive firing strokes on the same bank followed by two consecutive ones on opposite banks repeated ad infinitum. On a flat plane the consecutive firing strokes are always on opposite banks. This leads to a different sequence of exhaust gases and pressures going down each manifold. For the most usual Chevy firing order where cylinders 1,3,5,7 are on the left hand side of the car and the even cylinders on the right the firing order is as follows.

...1
8
4
...3
6
...5
...7
2

So you get an exhaust sound (and god I'm going to feel like a tool for trying to put this into words) dum, dah dah, dum dum, dah dah, dum.

So there are two high pitched notes followed by two low pitched ones and then repeat. With a straight 4 or flat plane v8 it's the same pitch, and therefore a much less interesting sound, from every piston.

To answer the OP the main influences on engine note are firing order and exhaust system. The induction sound will only be a factor if it's not well silenced which it is on all production cars via the air filter. An open set of DCOEs can drown the exhaust sound quite easily though.

Dave Baker
Puma Race Engines

Edited by Pumaracing on Wednesday 24th September 13:41

Cool! Cheers mate, and thats explains the typical 4 pot sewing machine sound too. So would they all sound the same if fitted with a single final exhaust? Or is it more how the various collectors are arranged? i.e. 4-2-1 sounds noticably different to 4-1 even though they're both out of a single final pipe, or would it be quite a subtle difference?