Greetings,

Some might recall I'm in the process of building a low compression, 500cid twin turbo stroker, with GM EFI installed.

Yeah, I offer no brand loyalty

Anyway, I'm at the point where I'm starting to fangle up the wiring harness and some of the intake bits, and I'm curious about EGR.

I'd rather not wire up and plumb EGR if I don't have to, just because its a PITA and I have enough work to complete this project. But at the same time, if it increases mileage, performance, etc, I wouldn't hesitate for a second splicing it all up electronically and mechanically.

I've done a lot of reading before I posted this, and was led to believe (even though the viewpoints I've read sometimes are opposite, go figure) that EGR increase gas mileage by diluting the incoming air with "spent" gasses, thus lowering the temperature in the combustion chamber a bit, as well as reducing the oxygenation capability of the incoming air. Maybe I just said the same thing twice? Anyway, this forces the driver to increase throttle position, which in turn lowers vaccum, which somehow decreases pumping losses.

My (mis)understanding of pumping losses has to do with the underside of the pistons - meaning the crankcase. I can appreciate better sealing by having a slight vaccum in the crankcase, but I don't quite understand how EGR which is topside of the pistons, achieves that.

Or am I way off, and should read 10 more books?

Also, I'm not sure how EGR would play into a lower compression, heavily boosted engine, either. I'm going to have a final c/r of about 7:1 to 7.2:1, with about 18-20lbs of boost on top of that, making the power with the turbos. I've done motors both ways (higher compression, less boost versus lower compression, radical boost) and the higher boost wins every time as measured on seat pressure on my fanny. Does the C/R and boost impact how EGR should work? Having access to all the code in the ECM (which is why I'm using GM ECM stuff), I can make the EGR open and close in any manner I want... jsut trying to understand this a bit better.

Thanks for any comments, thoughts, and ridicule

Frederic

 

I like thinking out of the box. Skip the egr. Negative crankcase pressure is good but that can be accomplished with a crankcase evacuation system without contaminating the intake charge. Egr doesn't lower the temp in the combustion chamber, but as you mentioned it does contaminate it. This contaminent effect does lean out the mixture, which does create better mileage....but the mixture can be leaned out simply enough in the fi loop without leaning due to contamination which suffers hp loss through the compromised intake charge.

 

Proliator,

Thank you for the reply! You're right, I can make things as lean or rich as I want (within reason), since the code and tables for this particular ECM are all over the net.

Crankcase Evac... I've seen such things on puller trucks before, where tubes from the oil pan go into the exhaust and act as a venturi, more or less. Is this what you were thinking? Or possibly a pump of some kind?

I'd be very happy not to contaminate the intake charge... going to have some turbos and intercoolers I'd like to keep crystal clean if I can

 

Look at Summitracing part number MOR-25900 $59.88 http://store.summitracing.com/produc...earchtype=ecat ....

or Moroso page:
http://www.moroso.com/catalog/catego...?CatCode=13023

Reduces crankcase pressure throughout entire rpm range for increased piston ring seal, reduced intake charge contamination, and fewer oil leaks.
Track and dyno tests prove significant increases in engine performance.
Includes two each: Oil Separators/Breathers, Breather/Filler Cap Grommets, One-Way Check Valve and Weld-In Nipple for header collector.

This goes from collector to valve cover, not oil pan to collector.

 

that entirely works for me, thank you!

And here is the start of the EFI wiring around the intake:

http://midimonkey.dyndns.org/intake1.jpg

 

I think you are very close. The EGR uses the exhaust as inert gas and a thermal load. It uses this exhaust to absorb heat from the combustion process. The exhaust is relatively 'cool' compared to combustion temps. Limiting the combustion chamber temperature limits the production of oxides of nitrogen (NOx). Which helps the auto manufacturer meet emissions requirements.

BTW you also get some detonation resistance from this effect as well.

This process is not the same as leaning the A/F ratio. In fact you can lower the NOx emmissions by richening the mixture. Unfortunaty you then get higher hydrocarbon emissions.

The decreased pumping losses comes from decreasing the vaccum in the intake. The backside of the piston has a bit less than atmospheric pressure. The top of the piston on the intake stroke has much less than atmospheric pressure. Especially at part throttle. If we decrease then intake vaccum (increase the pressure) by adding exhaust gas, we decrease the work the engine has to do to pull the piston down on the intake stroke. Less work pulling down pisons means greater efficiency.

Some folks on this forum have reported 1-2 MPG improvement by re-installing there EGR system on a 460.

The more turbo boost you run at highway crusing speed the less you will see any improvement from an EGR system.

Just my opinion...
-john