Incessant ping-like sound

[ktm]

Have you tested the injector clip and connection at #4?

[X64v]

Yup, the contacts are all good, nothing intermittent. I checked the plugs today, the lean plug followed the fuel injector. The #3 plug is now white and #4 is dark. I don't like having a 10% difference in flow rates, and I want bigger injectors anyways (these 338's won't cut it soon) so I'm going to just order some bigger ones that are better flow matched with an o-ring rail and go from there. My flow test was only at 100% duty cycle, perhaps that injector is taking longer to open than the rest, making it flow even less fuel (which would be why adding 12% to my fuel table didn't work).

[X64v]

Alright, this weekend I swapped in a set of 400cc injectors that are known to be good. Ping is still there, not changed at all. I'm 100% out of ideas.

 

In the interest of full disclosure, I've had to re-tune with the new injectors (switched from low-z to high-z which messed with everything just a tad), so my tune isn't dead-nuts on like it was before, BUT the leanest AFR I see at WOT from onset of boost is 12.0:1, which quickly tapers down to the low 11s, so that should not be responsible for the pinging at all.

 

Any last ideas?

[Clifton]

Did you get it worked out?

[ktm]

Well, I think I've confirmed that it really is ping. On 97 octane, the ping sound was totally gone in 1st and 2nd gear, and was greatly reduced in 3rd and 4th.

 

Now, I have two more questions. First, why would this engine setup be pinging now, and so badly? Everything's identical to my previous engine set up with the exception of the bore/stroke size and the compression ratio. And second, why isn't this ping blowing my head gasket? I blew a head gasket a few weeks ago from fuel starvation on a hard left-hander (the light ping turned to heavy knock), but otherwise I can hammer on the motor all day with this light ping and it doesn't seem to do anything.

 

Ok, first of all in response to you last post, you know its knock. As Clifton stated, adding fuel does little for knock resistance. You need to pull timing to mitigate knock. If 97 octane eliminated the ping in 1st and 2nd, then pull timing! I know you said you pulled timing, but maybe you did not pull enough.

 

You may not be blowing your HG because your boost pressures are low. I kept popping mine at 15 to 17 psi with detonation. It never popped due to detonation alone. It was always coupled with a high boost event.

[X64v]

I've got it close. I backed off the timing a few more degrees (loaded up the spark map I was running with the L24), and switched back to a 160* thermostat. Switching the thermostat helped the most. Now it will ping a single time in first (just once, every time), and then three or four times in second. I can't get out of second in town so I don't know what it will do in the higher gears yet, but so far it's much better.

 

I still don't understand why this motor with lower compression is pinging more than the higher comp motor with all the same settings. Perhaps the E88 head just doesn't like the 86mm bore for some reason?

 

I'm just about to call it good and throw on a meth injection kit I've been putting together.

 

Edit in response to KTM's post: Yeah, timing is now back to ~24* at 7psi. When I originally started to try to deal with this, it would ping all the way back to 20*, where I had zero power, so pulling timing 'til the pinging stopped was not an option. And I did blow a head gasket when it knocked majorly, the light ping just won't do it on its own.

[X64v]

As a final follow-up post, it seems to have been the e88 cylinder head that was the root cause. I switched to a p79 and am now running 24* of timing at 12psi with no pinging whatsoever (unintercooled, BPR7ES plugs, 160* thermostat, 100*+ ambient temps).

[Six_Shooter]

I was just about to suggest that quench may be the problem.

 

I'm not familar with the pistons you are using or the combustion chamber shape in the head, but I know that if the quench is good, then fuel can get down around the side of the piston above the top ring, and once ignited cause the piston to slam against the side of the cylinder.

 

*edit*

 

Do you have a knock sensor on your engine?

[X64v]

Well I'm running dished pistons, so no quench, but the shape of the entire combustion chamber is different, going from a lemon-like shape to...I guess a lemon with a bite out of it.

 

And no knock sensor. I was detecting it audibly, with it being confirmed by blown head gaskets if it got too bad. I had to pull the head recently for something else after running this p79 at 12psi for a few weeks, and the fire rings were still perfectly round, confirming the absence of pinging.

 

Edit: I also performed cylinder head cooling re-routing modifications, detailed here, though I have absolutely no proof as to whether they helped or not.

[Six_Shooter]

Yeah I'm debating on whether I should drill and tap my head or not, I basically have to decide tonight, as I hope to have the intake bolted on by tomorrow night at the lastest, if not later tonight.

 

You still have quench with a dished piston, it happens with the edge of the piston (the raised area) against the face of the head, that is why the piston is dished and not just shorter or lower in the bore to retain good quench.

 

I'll have to look for some pictures to see the differences, I'm not visualizing this lemon shapped combustion chamber.

[Zmanco]

I'll have to look for some pictures to see the differences, I'm not visualizing this lemon shapped combustion chamber.

From a P90 head (same chamber as P79)...

[Six_Shooter]

From a P90 head (same chamber as P79)...

 

I call that a "heartshaped" combustion chamber, it's the Chevy guy in me. That shape has been found to work well, for efficiancy, power and resisting detonation, more specifically pre-ignition.. With the swirl created by the shape it is found that the air/fuel mixture stays in suspension longer than a cylinder that has less swirl.

[X64v]

I meant looking at it from the side, like so:

 

E88 with P90 pistons:

 

Kinda looks like a lemon.

 

P79 with P90 pistons:

 

No longer as open, but there is definitely no quench there. That ring you talk about running around the outside of the piston that's at full height isn't enough to do much, and plus with a stock head gasket (.049" thick) it's out of quench range anyway (from what I read, it must be <.040").

[Six_Shooter]

Quench does have a lot to do with the raised area at the outside of the piston, that's why it's there. That's where you will measure that "0.040" quench" with a dished piston, heck even with a flat top, since the combustion chambers in the heads are almost always a dome shape or some variation of, like your two paint pictures there. Even dome pistons wouldn't come that close to the head nearer the center of the piston, or more specifically in the area of the valves, since most engine builders with cut the pistons to have at least 0.100" clearance between the valves and piston through it's cycle, both as the piston comes up to TDC and as it moves away from TDC.

[X64v]

Quench does have a lot to do with the raised area at the outside of the piston, that's why it's there. That's where you will measure that "0.040" quench" with a dished piston

 

Yes, that is where I'm measuring. It's .049" (the edge of the piston comes even with the block, .000" deck height, and then the gasket is .049" thick). Plus, even if it were within quench range, that amount of quench is inconsequential compared to a piston actually set up for quench (a flat top with the dish cut out exactly under the combustion chamber to achieve the desired SCR).

 

There is absolutely zero quench with the P79 head/P90 pistons, unless you use a thinner gasket, in which case the quench is still very minimal.

[Six_Shooter]

Yes, that is where I'm measuring. It's .049" (the edge of the piston comes even with the block, .000" deck height, and then the gasket is .049" thick). Plus, even if it were within quench range, that amount of quench is inconsequential compared to a piston actually set up for quench (a flat top with the dish cut out exactly under the combustion chamber to achieve the desired SCR).

 

There is absolutely zero quench with the P79 head/P90 pistons.

 

 

Listen I'm not going to argue with you about this, I have been building engines for many years, quench is a term that describes how close the piston comes to the head, being it 0.020" or 0.090" or any measurement of proximity of piston to head at TDC, in your case you have 0.049" quench, if indeed your gasket is 0.049" compressed thickness, and your piston come to zero deck and not proud.

[X64v]

Alright, if that's your definition of quench, then what I mean is the quench area is negligible.

[Daeron]

All important semantic points, and a GREAT anecdotal thread.... For posterity's sake, is there any chance you could snap a photo of the combustion chamber on the E88 head you were using, and also possibly get a photo of the old piston vs new piston up here?

 

This thread is VERY close to an ideal tool demonstrating something that can happen when parts get mis-matched, in a marginal case. In other words, it is a good illustration of an "outside chance." The "common wisdom" here is pretty much that the combination you put together may work, or may not.. so regardless of all that, the more photo examples of what was being used here, the more useful this thread is in the future. I am certainly bookmarking this to refer back to later.

[X64v]

Daeron, that is a good idea indeed, and I happen to already have all the pictures necessary. To recap:

 

I used a 1973 E88 (One year only, the 'submodel' is N33, it's got 240z valves but a large 47.8cc chamber) with unknown 83mm flat-top pistons (small valve reliefs no where close to where they should be) that popped out of the deck .030" to give a static compression ratio of 8.9:1 on my L24. I ran this set up for a few months at 7psi and 24* of ignition timing with no detonation. I feel that that was a reasonable amount of timing, and so in my opinion this head/bore/piston combo 'worked'. This was unintercooled, with bp6es plugs, a 160* thermostat, and 110*+ ambient temps.

 

The E88:

 

The unknown 83mm flat-top pistons:

 

I then rebuilt a stock l28et bottom end. Standard rod and crank journal sizes, stock 86mm bore, stock P90 turbo pistons. I re-used the E88 pictured above, giving me an SCR of 7.9:1. This is the set-up I had problems with, which is why this thread was created in the first place. After all my fiddling, I was able to run no more than 23* of timing at 7psi, and even then it was pinging residually. I feel that I should have been able to run much more ignition timing, so in my opinion this head/bore/piston combo did not work. This was unintercooled, with bpr7es plugs, a 160* thermostat, and 80* ambient temps.

 

Same head, but these 86mm stock turbo P90 dished pistons:

 

As a last ditch effort (and as a bit of an upgrade), I made some small modifications to a P79 (which can be seen in my "members' projects" thread), then had it rebuilt at a machine shop. Using this on the dished pistons shown above, I have an SCR of 7.35:1 (stock l28et ratio), and am able to run 29* of timing at 7psi. I'm currently running at 12psi with 24* of timing with no detectable detonation whatsoever (had the head off for an unrelated reason, and the fire rings were perfectly round). I feel that this is plenty of timing, and so in my opinion this head/bore/piston combo works (as did Nissan). This is unintercooled, with bpr7es plugs, a 160* thermostat, and 100* ambient temps.

 

The P79 chamber:

 

I guess the point here is that the E88 liked the flattop pistons, but would not work with the dished pistons for reasons unbeknownst to me. The P79, with negligible quench area gain (IMHO), likes the dished pistons just fine.

 

Anything I missed?

[Daeron]

Quite the contrary, that is EXACTLY what I had in mind, most excellent!! I especially love the photo of the "late E88" head, and the juxtaposition of the two combustion chambers is perfect to compare the two. I would suggest posting the E88 photo to the L6 heads and descriptions page; I have been halfway thinking about going through that thread, and my own collection of head photos, and assembling one single "master" post that has descriptions, sources, and photos of every head yet posted. "One of these days" at the very least, I want to do that as much for my own abundance of knowledge as for the value of it for the forum.

 

I think it is worth pointing out a few things.. you mentioned that the quench area seemed somewhat negligible, but from my understanding alot of the function of the quench area is to essentially squish a certain space to push the mixture in towards the flame kernel as it expands. The thinking is to provide a pad where a flat top piston pushes nearly everything out of one section, and that movement inside the dynamically combusting chamber assists proper ignition and helps prevent detonation. Poor explanation, but what I am grasping at is that "quench" is a very subtle thing, caused by nearly totally squeezing a section of the piston face dry against the firing deck of the head. In actuality, the stock turbo pistons, with their dish, pretty much by definition kill the quench effect of this head (almost?) entirely. TRUE utilization of quench pads only occurs with well matched flat top pistons that nearly impact the head at TDC, and if a relief is needed for compression then the piston relief mirrors the CC in the head as much as possible.

 

So, overall, what I am saying is that a small quench pad is still a quench pad... IF you are using flat tops, which a stock 280ZXT engine does not. I cannot say why this combination works as well as it does, but I am speaking about "quench theory" if you will.