HI all, the heads on my ZZ4 lack the provision for the EGR on my 87. The port is just not there. I get a nasty ping at WOT. Anybody know how to compensate for this? Would setting the egr duty cycle to all 0's fix this or do I need to add more fuel to compensate? I have access to tunercat and a prom burner. Thanks

 

Normally the EGR isn't doing Anything at WOT - it ought to only be opening at low load/light throttle situations. Not at idle or coast, and not past a certain % TPS. Look at the constants table in the BIN file to see where the EGR enable/disable TPS %'s are.

Duty cycle doesn't matter if it isn't active, anyhow. One other thing - note that "100%" duty cycle actually means the EGR control solenoid is porting vacuum to the valve Half the time - on/off for an Effective "duty cycle" or "On time" of 50%. A vacuum gage on the line to the valve will show you this - it'll pulse. The responce time of the diaphram/spring in the valve itself is supposed to act to damp out the pulses of applied vacuum and "float" the valve plunger at some average opening amount.

Bottom line is that EGR isn't the reason or the cure for a WOT detonation. You need to know what your A/F ratio is when it's detonating, and how much timing you have in it. As long as the A/F is close (you can change the Target A/F ratio in WOT) all you can do is pull timing advance out until the detonation doesn't happen - or run higher octane fuel

 

OK, I understand that the EGR isn't causing the problem now. Thank you. That said, with the missing ports on the heads, is the egr valve actualy serving any purpose? Would it be just as well to install a block off plate? Also, is the egr tube on the back of the manifold serving any purpose? I wouldn't mind getting rid of them both as there is no smog in my area. Sorry about all the questions, but I have one more, can I use the BLM's to get an idia of what the A/F ratio's are? Thanks

 

EGR is from the right header to the manifold. Have you confused Headers with Heads?

BLM is numerical representation of the right mix - 14.7 to 1 under cruise conditions - which is lean enough to cause high combustion temps. EGR is used to cool things down and without it, you run the risk of detonation. At the least, the knock sensor will pull timing; at the worse, it'll burn valves and toast a piston. Assuming you transferred the stock set up to this ZZ4; ie, headers, intake and all the EGR plumbing, (and hopefully the knock sensor), this shouldn't be a problem. Post your scan data - tell us how the motor went together.

 

You know - I Missed that part in the original post.. The ports that are "missing" from the heads on the ZZ4 are exhaust crossover passages - Not called EGR, as such.

Exhaust crossover ports are a relic of carbureted motors - they allowed exhaust to cross over thru the intake manifold underneath the plenum (beneath the carb - helped out by the heat riser valve on one of the exhaust manifold outlets) to both warm the incoming air charge (promoting fuel vaporization) and helping to keep the carb from icing up in freezing weather. Sort of the same ideas that running coolant thru the throttle body does on a TPI. In the '70's, when EGR valves first appeared, it was simple to tap into that crossover passage in the intake manifold to make the plumbing work.

On the first couple of years of TPI the exhaust gas supply for the EGR was still taken from those crossover ports (the crossover passages are present in the manifold for that reason - I think only open to One of the heads..I'd have to look at my original '85 manifold again to refresh my memory).

That changed along the way (1987 I think) and those crossover ports in the manifold were deleted in favor of a threaded port at the right rear of the manifold to connect, via a pipe over the valve cover, to the exhaust manifold. With that there is no need for any exhaust passage between the Heads and the intake manifold. It took a few years for GM head castings to catch up with the idea that no crossover port was needed..heads didn't get used Just on TPI cars, afterall.

The EGR ports in all the TPI Manifold bases connect to the Plenum thru those small tubes on the runners. The EGR Valve just plugs up the porting and blocks exhaust gas (however it gets into the manifold) from seeing the plenum.

If you remove the EGR valve itself you therefore have both an exhaust leak and a vacuum leak. That's what the block-off plates stop. If you remove (and block off at the exhaust manifold) the supply tube over the valve cover you will Then not need to plug the Exhaust side of the EGR valve porting - but will still need to plug the Intake (vacuum) side. The EGR valve itself makes a cheap and effective "plug" - just unkook the vacuum line to it...

Question - what TPI base manifold are you using? Doesn't the ZZ4 use "normal" (as opposed to Vortec type) intake manifold bolting? With that your original '87 base manifold works - right? So you have the OEM "over the valve cover" EGR connection bolted back up? If so then you do still, in fact, have a functioning EGR system now (assuming it's all working..). You really won't gain anything by taking it Off, other than a cleaner Look.

BTW - the Original, and still today, reason that the factory uses EGR is to lower combustion temps (at Cruise, primarily), thereby lowering NOx emissions. Like I said before, EGR is Not used under load and therefore does Not effect WOT detonation.

I would be surprised if there was Any gain in exhaust valve (or piston)life due to EGR pulling down combustion temps at cruise - that's not where the highest temps are seen anyhow. Sure - you Can get into a lean condition there that might cause temps high enough to melt stuff, but however you managed to do it (get it That lean, and Keep it that lean in a closed loop system with an O2 sensor) EGR isn't going to save it from self destructing...

 

Thats good news, I thought without the port on the head, the EGR was dead. So, I am back to data logging and adjusting the computer to match the motor then. Just ordered the laptop, now I just need the ALDL cable and I should be able to find out whats really going on with the computer.

Thanks