We’re saying that the noise is resulting from the increased combustion chamber pressure due to the change in throttle position. We’re specifically NOT attributing it to RPM change.
Think of it like this: on the engines of the 60’s and 70’s (and earlier), especially with manual transmissions, it was possible to have the transmission in high gear, push down the throttle and lug the engine badly enough to make it ping. Let up on the throttle and the ping stops. No difference in RPM, only in combustion chamber pressure.
The same thing is happening here; except the timing is controlled so the engine won’t ping, but the pressure still rises enough to affect the plugs and wires.
At the risk of hijacking this thread (but I want to learn more) …
I understand your explanation of why the volume of the RFI noise would increase with throttle position, even if RPM does not change, but I do not understand why the frequency would increase. The firing rate varies directly with RPM, so if RPM does not change then frequency of the interference noise/buzz should not change. (I’m presuming that the RFI is the RF components of the discharge or leakage from the plugs or wires, modulated at teh firing rate. The frequency you hear in the radio’s audio is the modulation freq (and its harmonics), just like with an early 20th Century spark gap radio transmitter.)
Most problems like this are due to the antenna having a poor ground connection to the body of the car, which acts as part of the antenna system or, a bad center coax connection to the radio antenna input. I recommend you check for a problem there first. If that doesn’t help then the noise may be getting in through the power supply connections, as ki4amd suggested. If that is the case then you may be able to block the noise by placing a .1 microfarad capacitor across the power lead of the radio and ground. This will bypass high frequency noise to ground.
This kind of problem is often due to either a bad antenna ground or bad connection of the coaxial cable center pin to the antenna input of the radio. I suggest you check those areas first for the trouble. If you don’t find any problem in those areas then the RFI may be getting into the radio though the power supply lead, as ki4amd suggested. If that is the case you may be able to bypass the noise by placing a .1 microfarad capacitor across the power connection to the radio and ground. This should bypass most of the RFI signal to ground and stop it from getting into the radio.
Edit- Sorry for the double post. The first one didn’t show up until I sent another one.
You could try making an RF choke for the power wire. Wind several turns of number 12 wire around a piece of ferrite rod, or through an Amidon ferrite core. Tape it in place and wire in series with the radio power lead. This did work for me in suppressing vehicle noise on the AM band, though it didn’t completely eliminate it.