Air/Fuel gauges

[benracin]

So now when people ask me if I have an EGT gauge I can finally say yes. Oredered mine today. The next gauge I have been asked a lot about is the air/fuel gauge. I've been doing a little reading on these and it sounds like there's a lot of gauge types and what seems to be a lot of bad ones. So with our wacky engines, what kind of gauge to yall like, and how do you hook it up? I'm pretty in the dark about this but as I get the car sorted out I'm getting more interested in making the engine really work well.

Thanks again folks!

Ben
ITA Rx7

[bldn10]

My previous cars had EGT and I was constantly fretting over the numbers (and replacing not-cheap probes) while other drivers were just happily motoring on in ignorant bliss. Before you spend any more money on gauges, answer this question: What adjustments/alterations are you going to make based on either EGT or A-F readings? Unless you have a MoTeC or other programable engine management system inside your ECU box, there's not a whole lot you can do w/ these cars in the short term. At least not that I know of - but I would be happy to learn otherwise.

[C. Ludwig]

Since we're using an effectively narrow window of the rev range of a given rotary adjustments to the A/F ratio can be made with an adjustable fuel pressure regualtor. I've had varying luck on the dyno with fuel pressure adjustments and the A/F changes that go with it.

The cheap A/F, narrow band gauges (like an Autometer) are a complete waste of money. The lights will tell you only that you are above or below 14.7:1. Not how much, which is what you really need to know. I bought a PLX Devices wideband setup about a year ago and have used it to tune three cars so far. Two turbo road cars with stand alone EMSs and also playing with the RX-7. The PLX is one of the least expensive wideband setups on the market but has proven to be quite accurate when compared to the high-dollar Motec setup my local dyno operator uses. www.plxdevices.com (http://www.plxdevices.com)

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Chris Ludwig
08 ITS RX7 CenDiv

[noresull]

How useful are these on carbureted engines? I was under the impression these did not do much good on anything but EFI.

[Jim Susko]

The EGT is very helpful. Mount it about a food from the exhaust flange, then go to a DynoJet Dynamometer that has A/F monitoring and get your engine HP/Torque overlaid with A/F. The ideal A/F is 13:1. Drill or change jets until you get those numbers, then note what the EGT readings are for your engine. If you see this change at the track because of combinations of different temperatures, barometric pressures, or humidity, change jets either higher or lower until you get the original EGT reading.

The primary main jet effects low rpm, the secondary high rpm. These engines tend to richen up at the top end, so you have to compromise until you have the best overall setting.

Unless, of course, you've figured out how to get a flat A/F curve as I have. But that's a secret!

Jim Susko

[bldn10]

"adjustments to the A/F ratio can be made with an adjustable fuel pressure regualtor"

Chris, FWIW I have always heard that the ECU detects the change in fuel pressure and alters injector duty cycles to compensate - putting you right back where you started.

BTW I noticed the last time I raced that my fuel pressure was under 30. I'm going to replace the fuel filter - think that's it?

[C. Ludwig]

Sometimes yes, sometimes no.


The only way an ECU would detect the change is with the O2 sensor when running in closed loop mode. Anytime you're under WOT the ECU defaults to open loop anyway and runs off a predetermined map based on input from the airflow, MAP, and temps sensors. In open loop the O2 sensor input value is disregarded by the ECU. So in that case the fuel pressure adjustments would indeed hold true. Now when the O2 sensor is completely removed, unplugged, thrown in the trash, the ECU will default to open loop mode at all times. In this situation the fuel pressure adjustments hold true to all throttle positions, load variables and RPM range. The problem that comes in to play is that high RPM, high load maps are generally very rich from the factory. So that when we lean those maps down to an acceptable level the off throttle, low load, low RPM maps go lean.

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Chris Ludwig
08 ITS RX7 CenDiv

[This message has been edited by C. Ludwig (edited August 16, 2004).]