I picked up a FD about 6 months ago. It has a rebuilt engine with 800 miles on it. I just noticed knocking/detonation(?) at 3k rpm. It only happens when the car is under this scenario: (I'll be approaching a red light and slowing down. Light suddenly turns green. I'm going to fast to shift it back into 1st gear so i go into 2nd gear and step on gas. As it reaches 3k rpm, I hear the spray paint can rattle).

I've searched and checked up FAQ page. Nothing really comes up for knocking or detonation. Where do I start with figure this thing out? Any help is appreciated. thanks.

 

What ECU is in the car?
What octane gas?
How much boost?

3,000rpm is indeed the knock zone of the 13B as identified by Mazda. Stock ECU knock control would be in effect in the scenario you describe.

No aftermarket ECU incorporates knock control though- instead must tune and use apropriate octane to avoid knock.

 

Apexi Power FC
91 octane
10 psi

 

As suspected, no knock control with that ECU.
You have to check the tune that is in it or reload in a new base map for the car to make sure the ignition timing and air/fuel ratio are good.
In the mean time you can quickly verify the spark plug wires go from the coils to the correct plugs and check the intake tract to make sure you dont have a bunch of oil in there. Next, check intake temps- air filters drawing from hot engine bay in conjunction with a boost leak will get you sky high intake air temps leading to knock.

 

OP...have you checked the sensor screen on the PFC commander to see what your knock values are? Did this seem to come on suddenly or, since the car is new to you, did you just notice? Can you find higher octane and watch those values to see if it improves?

Blue....could it also just be something in his drive-train? The engine is under load, but with worn differential bushings, engine mounts etc. you can sometimes get some 'lugging' drive-train noise under those same conditions. Annoying but relatively harmless. ???

 

Good point!
Didnt stop and think OP might not be familiar with sound of knock on a rotary.

Could be a loose heatshield ringing when the engine shudders.

To me rotary light knocking sounds more crunchy than ringing. Kind of like footsteps in pea gravel.

By the time knock is heavy enough to sound like a hammer pinging on metal in the rotary it is usually acompanied by hesitation and broken parts.

 

Thanks. I'll check these.

 

It did seem to come on suddenly to me but i am new rotaries. Initially i thought it might be the hatch rattling then i was able to re-create while driving a smooth surface. 91 octane is the highest here in CA.. I can buy octane booster and put it in but i just filled up the tank.. I noticed sound when i was on my way to fill the gas tank.

I got time to check the car this weekend and next weekend. I'll check the stuff you and blue listed and report back. Thanks.

 

Blue mentioned heat shields which I forgot. That’s probably where I’d start. They flex and fatigue a little over time or just get bent a little. When that happens they can rattle with just the right resonance from the engine. And it seems like that resonance occurs when lugging as you described. You can get under the car with a rubber mallet and just start tapping on stuff and listen for the noise.

Don’t waist your money on octane booster.