Can an exhaust restriction affect detonation resistance?
 

I touched up the tune on my car this evening after swapping the half-clogged 3" metal-core Magnaflow with an Enthuza test pipe and porting the wastegate.

The spark map I started the evening with:
Attachment 209211

The spark map I ended with
Attachment 209212
13-14psi

AFRs were a little leaner than I normally see, high 11s to 12.0:1. No knock in 3rd or 4th, but if I smack the throttle in 5th I can see a bit of knock after the accel enrichments die down. Nothing I can hear, just what the Knocksense is picking up, so it's really light stuff. What really impressed me was how much timing it took to make the car do that, though. I bumped it all up from the 8-10 degree combo I had to 11 degrees, and didn't get any knock at all, so I got aggressive and added 2 more degrees and got that tiny little bit. I will make a few pulls tomorrow afternoon to see if I can make it knock.

The other thing I am doing tomorrow is switching to a dualfeed rail. With AFRs in the very high 11s, and a TINY bit of knock, it will be interesting to see if that mod will affect either of those. If I can go from a tiny bit of knock to no knock with the dual feed, or even add some timing, it will be interesting.

Si

If "No knock in 3rd or 4th, but if I smack the throttle in 5th I can see a bit of knock after the accel enrichments die down. ", then dial in a bit more fuel.

Your timing is knock limited at full boost?

On pump gas, I am betting it is. I haven't dynoed to find out, but I know the car is definitely peppier at 14 degrees than it is at 10-11.

of course it is :P

I remember seeing 18hp when i bumped the timing up 7 (19° total advance) from 12° @ 10psi.

http://boostedmiata.com/timing/timingdyno.jpg

I think that first 10HP/10TQ increase was just a few degree of timing. I wish i still had all the logs :(

With an exhaust restriction, you're probably seeing considerably higher EGT's at high load, which would give you some detonation potential.

There is a 2nd effect too, which is more restriction lower VE. With a speed-density system it'll lean out with less restriction.

Jason, I thought I had noticed a little lean out, mabye 2-3 tenths, although I will never be sure since my fuel pump just bit the bullet (or so I think).

I dug up this thread because of an experience I just had.
I have the FM Turbo Dual exhaust with a catless midpipe. Basically the muffler is like a dual outlet muffler used backwards, where one of the outlets is an inlet, the inlet is the pass side outlet, and the remaining outlet is the drivers side outlet. This means that MOST of the exhaust wants to bypass the muffler part and come right out the drivers side.

Because my old job involved visiting businesses and I wanted to drive my miata, I cut a slot in the 'loud' pipe on the drivers side. I then made an aluminum disc with a flange on it that I could slide into the slot and secure with a hose clamp. This made it as quiet as stock for work. It only took 30sec and a screwdriver to open it back up later.

I lost the little block-off plate about a year ago, and have been tuning the car without it around. I found it while cleaning out the garage and put it in tonight.

I was driving along at about 4K in 3rd gear and punched it when my J&S lit up like a Christmas tree, full alert. It pulled my timing, took me back to wastegate pressure, and dumped in all kinds of fuel. That was strange. I was able to repeat the reaction twice before I got to my destination. Before I left for home I removed the restriction, and I was not able to reproduce the problem anymore.

Can a restrictive exhaust REALLY cause knock that badly?