kcobean

iTrader: (2)

So Sharif recently spurred me to do some reading about engine operation, and specifically, the combustion cycle, detonation, pre-ignition, etc.

I found a REALLY good article that I think many of you that have detonation concerns can learn something from. I know I learned a TON. It's a lengthy read (8 pages) but worth every word. Enjoy.

http://www.streetrodstuff.com/Articl...tion/index.php

bknezevic

Wow, thanks for the link. GREAT read.

Sharif@Forged

iTrader: (92)

Nice article. I guess I was close. Mean Peak cylinder pressure was at 14 degrees ATDC, rather than the 15 degrees I quoted in the other thread.

bullseye

One of my co-workers was uncharacteristically moping around the office the other day. He finally admitted suffering a case of pre-ignition on a recent date. An ignition event occurred *way before* TDC, despite his valiant efforts to delay it. The results, he reported, were immediate and catastrophic. It seems his piston will probably not be going anywhere near that cylinder again... Sorry, guys. I couldn't resist.

kcobean

iTrader: (2)

You were close enough to prove that you really know your stuff. Thanks Sharif!

350zWarz

good article Kelly, i see you're getting ready for your kit....i might be too soon

Sharif@Forged

iTrader: (92)

hehe....funny stuff.

bacalhau16

iTrader: (32)

that was definitly one of the most imformative articles i've ever read. this should be sent to every newby that joins any car forum looking to modify their car. thanks a bunch!

gersteinp

iTrader: (1)

The article was really excellent and explains why VDC-induced fuel cut at WOT under boost can lead to detonation and catastrophic engine failure. Quoting:

"In the same vein, an engine running at full throttle may be happy due to a rich WOT air/fuel ratio. Throttling back to part throttle the mixture may be leaner and detonation may now occur. Bingo, the piston overheats and scuffs, the engine fails but the postmortem doesn't consider detonation because the the failure didn't happen at WOT."

joshbrittain

great read, I definitely learned a lot.

Sharif@Forged

iTrader: (92)

I still maintain that fuel cut is a complete absence of fuel, not a reduction in fuel. So if the injector in a given cylinder doesnt open, no burn results, and no damage to the engine can occur.

So even if the throttle plate closes, and reduced airflow, there is no fuel in that given cylinder. It would be a really bad engineering design if it just reduced fuel to induce an RPM limit.

gersteinp

iTrader: (1)

If fuel is cut when the engine is turning over at 5-6K at WOT under heavy boost, do you really think that's a safe situation? The cylinders are still hot, the spark plugs still firing, and residual gases still present in the cylinder. It's just crazy to turn on VDC if an FI engine is being pushed hard. Actually, this is very similar to the situation that the article warned about--pedal lift-off.

I know that driving aggressively is the very time when you might want VDC, but, unfortunately, it's then a trade off between engine safety and total vehicle and driver safety. I'm somewhat embarrassed to say that I would pick the former.

kcobean

iTrader: (2)

I tend to think along the same lines as Sharif. After the exhaust and intake strokes occur, the amount of residual gases from the previous combustion cycle can't possibly be significant enough to cause an unsafe increase in chamber pressure (if there were even enough to be ignited by the spark or heat/pressure to begin with). Most of those residual gases (per the article) also layer the top piston and thus are quite far from the spark plug.

IF the ECU doesn't cut spark as well as fuel, a firing spark plug in a chamber of compressed air should be relatively harmless regardless of how hot the cylinder is. The most wear would probably be to the plug itself.

Pedal lift-off is an entirely different situation because in that case, fuel, air and spark are all still present and subject to "mis-management".

If you've ever pushed the car hard enough to engage VDC, you know how violent the deceleration is. I think a lean condition caused by a short cycle on the injectors would be more subtle. This really feels like the engine has just shut off altogether for that brief second.

Anyway, glad you guys are liking that article.