Knocking sound at stock boost in 5th gear?
 

My FD idles, runs and boosts very well but makes what sounds like detonation under full boost, but only in 5th gear. The boost is the stock 10psi and the motor pulls smooth in every other gear but for some reason when I'm cruising in 5th at say 3000 rpms the noise becomes apparent under the hood when I hit full boost. I've heard detonation before and it does sound like BB's in a metal coffee can. There is no check engine light and the fuel filter has just been replaced. I was thinking that the timing might be off but I looked though the FSM and it said there is no adjustment for the ignition timing which is controlled entirely by the ECU.

My flow mods are just a downpipe, rx7store high flow cat w/ air pump connected, RB catback, and a drop in K&N filter. The car also still has the stock intercooler duct and undertray.

I'm assuming if this is in fact a knocking issue that it would be related to the increased load in 5th gear but at stock boost? :scratch:

I'm still new to RX-7's but not new to working on cars. I've never ran into an issue like this but maybe some of you guys with more rx7 knowledge can enlighten me on some possible causes to this issue.:icon_tup:

Get a boost gauge, if you haven't already, to keep an eye on the boost. Good HF cats flow almost as well as Mid-Pipes and on a stock ecu that is not a good idea. Have your compressions check too.

I have an electric prosport boost gauge.

I know freeing up the restriction in the exhaust on a stock ecu isn't a good idea because it can cause boost spikes/creep but I adjusted the turbo precontrol rod back a bit to lower the boost back to 10psi. The thing that throws me off is that 3rd and 4th is smooth as butter all the way to redline. It's just when I shift into 5th and floor it at low rpms it makes this sound. You think it would be the other way around since the engine needs more fuel at higher rpms.

Unlikely that it is detonation. Does the car "buck" when you do this? What about 5th gear at a little higher rpms?

nope it feels smooth and normal just makes that distinct "ticking" noise. I haven't tried it in higher rpms, just at around 3000 rpms at 80 mph.

That ticking noise may be the boost control solenoids , opening and shutting the waste gate .

But I'm just making a random guess.

It really cant be detonation. Your ECU can supply plenty of fuel at that rpm. You don't get the same noise in 4th gear at 3K?

At that rpm, the pinging (if it is that) wouldn't be enough to cause damage but maybe you should look at the exhaust to see if anything is loose or in contact with another object.

No there's no noise in the other gears. Last night I hopped on the freeway and did a full boost run in 3rd-4th-5th shifting at about 5500. As soon as I hit full boost in 5th I heard the ticking sounds my buttcheeks clenched and I immediately backed off the throttle.

I want to think it's something else but I want to be sure. I just bought a mechanical defi boost gauge so I will verify the boost with that...maybe the sending unit for the prosport is giving incorrect boost readings. I'm also going to hook up my defi din gauge to the fuel line to check the pressure under boost.

You maybe have a transmission grind in 5 gear!

hm the 5th gear syncho is pretty shot..

My RX-7 has the same set up...down pipe, airfilter, but i'm running stock cat... had similar issues when I first bought it 3 years ago. Come to find out the previous owner had a trailing spark plug in the leading position...My engine would knock under high load situations. I was never able to lay into the throttle too much... Ne ways took it to Mazdatrix for some new plugs...Problem solved... NO MORE KNOCKING. BOOM. :) hope you find out what's causing your problem

7's on the bottom and 9's on the top right? They were good but caked with carbon. They must have been super old.

Unless you have emissions you can try all 9's as they burn colder and thus less knock