Big Cam Pre Igniting My World

[JonezN]

Have a 2006 Silverado 4.8 that had a lifter going out. Replaced cam with a BTR Stage 4 Truck Cam Intake/Exhaust 0.553", 109* LSA, also installed long tube headers. After getting everything back together I started the truck and it had a tick/tap with perfect oil pressure. Ive swapped cams in the past and had the car run... ran like crap but would run on a base tune enough to limp it to the tuner or atleast on a trailer. Never had one sound this way and the sound wouldn't go away no matter what I tried from re torqueing the rockers and checking push rod lengths, to going back in the motor to check crank to cam timing which was dot to dot. So I went to replace my lifters again thinking this was the problem only to discover the the engine is now pre igniting/ detonating. Mostly in cylinder 7 and 8 with some signs in cylinder 5 and 6. When I checked around but most local tuners won't touch it due to liability and has forced me to try and get this thing going myself. So I bought HP Tuner and went in a raised my idle RPM and and Idle Airflow so that I could get the thing to stay running without my foot on the throttle. I am currently running a tune that a friend who "knows some things about tuning" helped me start that idles with little to no detonation until you give it throttle and some times when the engine is hot it has a light sound to it. It mostly has detonation between 900 and 1500 rpm and goes away above this rpm range in park but it is some what noticeable in most ranges in gear being less noticeable at higher RPMs. Sound tries to go away at idle but still can't quite figure this one out. Just started tuning this so bear with me as I am learning. Don't have a wideband yet but I am in the process of getting one soon. Attached is a stock tune, tune Im using, and the a log at idle and driving.

[5FDP]

This was kind of a terrible camshaft choice just a FYI, huge ass cam in a tiny engine with no high stall torque converter. It going to be slow as balls down low and will only sing between 4,500-6,800rpm.

Fueling at idle and light load doesn't look bad per what the fuel trims are saying, so it's not at fuel issue there. You won't known what fueling is like in open loop without a wideband o2 sensor.

Settle the timing at idle down. Aim for 18-22ish degrees of timing with the over/under spark corrections tables around 10-12+/- degrees.

The log is missing knock retard, burst knock, cylinder airmass and your injector pulse width avg's.

[JonezN]

Forgot to mention the 3200 stall

[JonezN]

So I tried lowering the timing a bit around idle and it helped some with the noise but it was still there a bit at idle. Also tried advancing it a bit and the sound increased a bit at idle and was about the same pattern of tapping from 900-1500 RPM and then settle out and disappear around 2k RPM. At this point I am going to open the engine back up and check over everything again and see if maybe I’m off a tooth or possibly re degree the cam to see if I messed up there.
Though it’s “a terrible camshaft choice” (which you’re right) the engine should still run without a tap with it. If nothing is found messed up mechanically then maybe adjusting my injectors pulse and retard the timing more might help? No clue at this point

[kingtal0n]

The motor probably doesnt have a timing pointer but if it did I would say first check your timing. Weird things can happen to engines at random, reluctor rings can break free and timing can move 40* one tooth or whatever.

The only thing I can recommend is to immediately pull every single spark plug and check for oil, debris, color, condition, and perform a compression test

At low load it is VERY difficult to detonate a gasoline engine. It would take ridiculous timing 40*+ and very high temperatures, along with oil contamination.
So if what you say is true the detonation is likely an oil related issue, assuming the fuel is good.

This is brand new high quality 93 octane, right? Not old fuel.

Again repeat: a gasoline engine should not detonate easily at low load.
Something drastic must be wrong for this to happen, we already discussed oil contamination but lets talk other options, what other drastic thing diagnostically speaking...

cam timing one tooth off could theoretically create obscene cylinder pressure condition. So this is why you will compression test ASAFP to verify the cranking compression at say 500rpm and 800rpm or something like that.

[JonezN]

Engine doesn’t have a timing pointer. I have removed the heads completely a week or so ago which is how I found out that the sound was not a lifter but pre ignition because of damage to the piston head and cylinder head. And I have since reinstalling the heads back on changed the spark plugs and wires with no change. I don’t think it’s oil or carbon build up causing the knock because the sound starts as soon as you fire it up cold and the piston heads had minimal carbon when I had the heads off. It does have new 93 octane in it. Tried to upload a picture but won’t let me
Just hoping to figure this out so I can replace the pistons and heads.

[JonezN]

Definitely want to check my crank cam timing now because if the cam is off a tooth that would throw off the time of for the reluctor on the cam gear. I will update what I find after I get the cover off

[kingtal0n]

I don't think the cam gear/teeth is used for engine ignition timing purposes

Cam gear being off a tooth can cause weird compression issues which can result with engine detonation maybe

[JonezN]

The cam gear is the cam reluctor that the cam sensor reads which is used by the PCM with the crank sensor to determine the ignition timing. So if my cam is off one tooth and I believe the gear has 44 teeth, that would mean 22 teeth for one crank revolution, so 360/22 is 16.36 degrees off or 32.72 degrees off for the entire 4 stroke cycle. And over advancing is a common issue for pre ignition which is why I am going to check again. I could be wrong but I am shooting in the dark at the moment

[kingtal0n]

The cam gear is the cam reluctor that the cam sensor reads which is used by the PCM with the crank sensor to determine the ignition timing. So if my cam is off one tooth and I believe the gear has 44 teeth, that would mean 22 teeth for one crank revolution, so 360/22 is 16.36 degrees off or 32.72 degrees off for the entire 4 stroke cycle. And over advancing is a common issue for pre ignition which is why I am going to check again. I could be wrong but I am shooting in the dark at the moment

The engine will run without the cam sensor plugged in. It uses the crankshaft to determine timing and firing order, the cam sensor just tells the computer which cylinder is on the compression/exhaust. If you unplug the cam sensor, the computer will make a guess on the first attempt to start the engine and you either get a clean start or a backfire.

[JonezN]

Ok I was under the impression that the cam reluctor influenced ignition timing more. Im going to check over all the mechanical stuff this weekend.

[JonezN]

Decided not to open the front cover after discussing on here. Dropped the transmission and pulled the flexplate. Looks like the holes the bolts go through are a bit ovaled so maybe there was play between flexplate and converter. Flexplate also looks like it might be slightly warped and possibly hitting starter. Ordered new flexplate, bolts, and starter. I will update once it’s back together. Hoping this is the problem and the detonation issue was short lived.

[JonezN]

Thought I would update with what I found. Number 8 piston head cracked. Looks like the detonation did more than I had thought. But from the looks of the heads and the pistons there was no new signs of detonation. So now it’s time to take the block to the machine shop and rebuild it.

[kingtal0n]

I would swap the engine. 4.8L is the cheapest version, you are going to pull it anyways

This will keep down time to a minimum, its faster than machine work
this should also reduce the cost, its cheaper than machine work

and it will save you from going through this all over again once the machine shop dicks you over which I can almost guarantee will happen.

Also did you say back to the machine shop? Because that would explain how you managed to break one of the most durable engines ever produced. The 4.8L supports around 800-1000rwhp with gen4 internals. The only real way to screw one up is to open it and let a machine shop do something to it.