what did chrysler do, to get away with the EGR

Hi Guys;
I am planning on swapping a HO head, with the newer style intake manifold(99 and above, have the manifold already). Now, the 91 and up cherokee's do not have an EGR setup, i wanted to know how they accomplisehd it as i have heard that my jeep will run hotter if i get rid of it and not to mention the emissions, any input or suggestions is appreciated.

Thanks;
Ravi

There was a cam grind change coincident with the deletion of the EGR valve in MY1991, which probably has something to do with the alteration of exhaust gas scavenging in the cylinder being delayed slightly or left incompleat.

If the EGR fails, it stays closed. If it stays closed, your HC and CO will drop like rocks, and there is little change in your engine operating temperature (mine failed for about a year - that's how I know.) Only reason I ended up changing it was because I failed smog when the "tech" tested it directly (after I told him specifically to keep his fingers off my engine. Why can't people follow instructions?)

Beyond that, it's not a problem to not have it. Deletion of the EGR valve directly will slightly improve engine efficiency, as evidenced by reduced HC/CO emissions (both of which are products of incompleat combustion of aromatic hydrocarbons - like octane and other compounds found in commercial gasoline...)

5-90

thanks that helps

ravi

There was a major camshaft change that offered way more exh valve overlap. This with a sufficiently restrictive exhaust system assured more 'polluting' of the intake charge to supress detonation and NOX as an emission. It definitely did eliminate the trouble prone normal EGR system.

I changed out the entire exh on my '98 with a Leach header and a full 2.5" system with a Flowmaster muffler. This has enabled the engine to rev seemingly without limit. It unfortunately has necessitated med to prem gas in some parts of the country because the backpressre has largely been eliminated, therefore much of the EGR effect.

Looking at the stock system this vehicle had, i would bet you the engineers had designed a full 2.25" or 2.5" originally from the stock header-like manifold. With this arrangement had too much flow and therefore detonation. I say this because it had that 2.5" pipe that suddenly grew out of that way smaller 2" section with the 1.5" 'clearance' crimp- like had been added as a band-aid.

I still do not regret the mods to the exh because i love the revability, and also how it and a few other mods have helped me to obtain the 26 to 28 mpg i now get.

Pray, how does increased scavenging increase detonation when the compression ratio remains below 9.0:1? Is your timing correct, or has it advanced on you?

I ask mainly because spark knock is caused by heat due to compression (see the Ideal Gas Law, IIRC) and the allowance of "hot spots" in the combustion chamber - typically caused either by poor circulation at the periphery of the chamber or pronounced sharp areas or ridges in the castings. Sharp edges will heat up quicker and retain heat longer than great flat areas, and you can easily get conflicting flame fronts as a result.

I'm concerned simply because I've built engines that run around 10:1 without incident, and able to run fine on 89 or 92 octane without any issues (usually 89) either with exhaust in place or running on an open pipe.

Generally, allowing overlap will increase intake flow partly due to the volume available from the escaping exhaust gas, and partly from the suction caused by the departure of the exhaust gas in addition to the suction caused by the dropping piston. More overlap is generally a good thing, as it will increase VE.

The larger amount of atomised fuel is also a good thing, as it flashes from a liquid state to a vapour within the chamber and lowers the overall heat content of the combustion chamber. This is part of the reason production engines are leaving the factory running slightly "rich" compared to a true stoichiometric ratio of approximately 14.7:1 (I believe production A/F is about 13.75-14:1,) which is further accounted for by the presence of the catalytic converter; itself intended to compleat the combustion process begun in the chamber and to reduce the overall presence of unburned hydrobarbons (HC) and carbon monoxide (CO) left over.

The EGR depresses Oxides of Nitrogen (NOx) emissions by "polluting" the intake air stream with a metered amount of exhaust gas. The presence of the EGR system contributes to the incerase in CO and HC by inhibiting the combustion process (lowering the temperature or "heat available,") which further necessitates the catalytic converter. It is entirely possible, with a little tuning, to delete the catalytic converter AND the EGR valve entirely and reduce overall emissions significantly, and one of the key issues is the overlap of the exhaust and intake valve events due to the cam grind.

Increasing the overlap has a point of diminishing returns, but more is generally better - both for power and for volumetric efficiency.

5-90

There are three kinds of knock, this is not spark knock i am refering to. It is detonation. It is usually the worst kind. Increased scavanging means more of the exhaust gases are removed from the combustion chamber. The amount is greater if less is able to exit, ie., more backpressure, therefore more EGR effect. As you know, EGR uses the principle of essentially 'polluting' the incoming fuel mixture with exhaust gases. This combination does indeed slow the burning process, however not necessarily making it more poluting. Is the tightrope the engineers walk when designing a system. This helps the poor gasolene we are forced to use to burn more slowly and not explode, or detonate. The overlap the engineers are using in this case is a little different than normal scavenging overlap used to create a 'pulse' to assist with intake charge.

Detonation also can result in too high a combustion temperature. This causes the development of NOX where O2 found in the atmosphere is made to split and combine with the Nitrogen, also in the atmosphere, in varying amts....therefore the 'X'. A four gas analyzer looks for this.

With the sensitivity these engines appear to detonation, i would think they do not have the best combustion chamber design. Something is not quite right- i would bet the quench is not particularly good. Other engines have had quench problems- Pontiac big block is one good example. Just make sure to set that to no more than .060, and more toward .040 by decking, and the insidence is greatly reduced. I'd bet that'd help here too.

I was warned by some the less restrictive exh system would result in something like this, but did not stop me. Fortunately is something that can be handled by a 'better' gasoline grade.

Gotta go-