2quick4u

[quote]Originally posted by ChrisB:
<strong>
We know an EGR works (to reduce detonation) by injecting spent end gasses into the combustion chamber - these gasses are straight out of the exhaust and as hot as they get - if injecting these gasses into the combustion chamber doesn't induce, but rather reatrds pre-ignition/detonation, then I don't think heat from the same gasses building up outside the port would do it.

The only way I can see cat's actually causing pinging problems is if you have too much overlap (for the rpm band you are getting pinging in).


Chris</strong><hr></blockquote>

Chris,

Exhaust gas can be as high as 1500 deg F when it exits the exhaust port. Believe me, it is nowhere near this hot when it gets back into your engine via the EGR system. First, it is nowhere near this temp by the time it gets to the inlet on the plastic intake manifold, if it did, the nylon 66 manifold would melt. Second by the time it gets mixed in the intake manifold with with the incoming engine air (5% EGR, 95% fresh air) the temperature of the inert exhaust gasses is the same as the intake air temp when it reaches the cylinder.

Secondly, I have experienced this on engines with stock cams, so no problem with excessive overlap.

As I said before, cylinder head temps are hotter on engines run with more back pressure. I feel that this causes the pinging.

[ November 15, 2001: Message edited by: 2quick4u ]</p>

ChrisB

It's just a question of balances I guess - yep, heat leakes at as the gasses go through the EGR - but it also leaks out big time in your exhaust gasses themselves (the hotter gasses will also be more energetic and more likely to evacuate - "cooling"), and the header tube is really no insulator.


I think both obersvations and theories are valid, it depends on which one takes precedence. I know on my car on the dyno going from an exhaust to open header I had to cut timing and increase fuel to keep from pinging - and a silverpig99 had to go to non-rfg tuning with his cutout open.


I guess it really depends on the magnitude of the back pressure as it balances out against the EGR process, and the net heat differential.

Chris

racer7088

In general when you remove back pressure you will increase cylinder filling somewhat or at least move it further up the rpm range. Sometimes the increased VE can start occuring low enough to cause detonation if you were close already and sometimes it helps proportionately more in the top end where VE is already dropping anyway. If it adds in too much cylinder pressure where you're already close to the detonation limit without much egr effect than you can start detonating earlier or more. On the dyno usually running better exhaust means more jet and maybe a slightly slower timing curve to avoid excess early pressure and possible detonation. It all depends on the breathing characteristics and compression of the engine your doing this to.

[ November 16, 2001: Message edited by: racer7088 ]</p>