Boost is to : Rattle

[FUBAR]

How much of a role does boost level play into rattle? I have tuned most of the 'rattle' out via reducing a couple degrees of main timing and fine tuning via pilot time reduction. Now I still have a hint under low boost conditions initially when hammering on it. I have a open volute turbine (hence the need for my question, not slow spool, but could be better) and will be changed swapped for a divided housing soon. But theory in tuning timing with regards to boost is what I'm interested in, not the solution to my hardware situation. I hope you don't mind Rich, but I dug a quote out of my notes where you were speaking on the topic. I am quoting this to hopefully get this thread started and if anyone wants to jump in and enlighten on anything, please do!

"It's a combination of timing advance, engine load, fuel qty and boost.

Obviously if the timing is advanced, you're going to dramatically increase cylinder pressure.

Compound that with getting on the throttle (adding fuel), with little or no boost and you're simply generating to much cylinder pressure....timing over advance."

So..under low boost conditions there is a lack of _______ to ________ that causes high cylinder pressure that produces the 'rattle' sound. Obviously rattle can be fundamentally broken down into timing to early trying to force the piston back down on the upstroke. But there are A LOT of variables (as Rich also touched on in his original postings) that dictates the sweet spot..not to mention the whole other can of worms...pilot injection. If the conversion should stray to variables other than boost itself..that's okay too. More information the better.

[2006Cummins]

Just out of curiosity; it's not pressure that you are hearing, is it? I have found that too much pressure before boost causes a noticeable rattle. I have never been able to run enough timing to actually cause a noticeable rattle, and have it respond favorably.

[FUBAR]

Pressure...one of the many viable variables. Too much pressure is pushing too much fuel at the given conditions..I.e. essentially advancing the timing without actually adding degrees of timing. That's not set in stone, just my way of thinking. I've got it really dialed in with pulling literally 1-2* of
Main timing and a few more pilot timing. I just really notice it when I go to accelerate fairly hard when the boost isn't built up..so...

[anarchydiesel]

Pressure can be a major component of rattle. What kind of injectors are you running now andrew?

[FUBAR]

Stock injectors and 45% over Extrude Honed Nozzles. Truck as it sits 87k mi on the clock, mechanical lift pump, Motorsport Diesel (Shane's) 392 pump, the said above nozzles. Valve lash is in spec so no much noise from that aspect. I can push fuel all day long at the wimp of a will, so I think I may follow the tree you may be barking up Cody..interested to hear your opinion to see if I'm correct.

[kaiserbailey]

Less boost should cause the fuel to burn slower and have less rattle. Too much pressure combined with too much timing would definitely cause rattle.

This is a good read, long, but good: http://killerbeeperformance.com/down...e%20Diesel.pdf

[cumminsDK]

Research "ignition delay" as well. This is what causes diesel rattle pilot injection reduces it, compared to older diesels. Any condition that increases ignition delay will cause it. Higher pressures usually reduce ignition delay as the fuel will ignite faster.

[kaiserbailey]

Research "ignition delay" as well. This is what causes diesel rattle pilot injection reduces it, compared to older diesels. Any condition that increases ignition delay will cause it. Higher pressures usually reduce ignition delay as the fuel will ignite faster.

Yes, but it also causes it to burn much faster. If timing is BTDC, and fuel pressure is excessive, the cylinder pressure will spike BTDC making a rattle.

[cumminsDK]

I would agree if we were talking gas engines where the flame must spread from spark across the whole fuel mixture but diesels ignite spontaneously from heat and compression remember as boost (and cylinder pressure) go up we can run more timing without rattle. Gas engines when you advance timing the mixture starts to burn and as the flame spreads across the mixture pressure builds and if it gets to high for the particular octane it spontaneously combusts or detonates (spark knock)

[FUBAR]

Interesting...definitely will also read that when I get a moment too.