5% increased gas mileage

NoDOHC · 01-08-2012, 05:42 PM

I don't share Ted's hate for wideband sensors, but I otherwise agree with his post above.
There is certainly a place for a wideband, however they certainly do not tell the whole story of the combustion process.

To idle at 200 RPM a stock 4-port rotary engine runs about -25 kPA manifold pressure. To 'idle' at 2500 RPM will run about -70 kPA manifold pressure. The 6-port is able to run at about -30 kPA at 200 RPM. Rotary engines don't run reliably below 200 RPM.

You can never access points on a MAP curve below the closed throttle MAP at each RPM. Mine cranks at 325 RPM and has -10 kPa Manifold pressure at that speed. I can't get a lower pressure (more vacuum) at 325 RPM without adjusting how low my car idles (closing the throttle plate further). Thus I only have access to 3 MAP points at 325 RPM (only 3 MAP points are valid operating parameters for my engine). My engine runs -90kPa at 6,000 RPM, closed throttle, thus I can access pretty much all the load points at that speed.

As Ted said, the BAC valve will not allow much vacuum below the idle setpoint. (Which is typically 750 rpm). The 4-port in my car has no BAC valve, thus it idles only at the speed set by the throttle plate. This amounts to about 550 rpm cold and 900 rpm warm, with a brief peak at 1,000 rpm as the oil gets warm but the mix is still rich.

To tune without a wideband, just lean the fuel up about 10% from stoic, (if you have a narrow-band sensor) or until the engine starts to surge (not really a mis-fire, but a slower torque variation, almost feels like driving into a headwind on a windy day).

If the weather is warm you should be able to break 30 MPG, I can barely get 30 in the winter, but it is usually well above that in the summer.

With vacuums deeper than about -55 kPA, the engine requires a richer mix (probably due to exhaust reversion). This is why most of us idle at -5 to 5 degrees BTDC to avoid running very rich at idle. This is unpleasant if you have to sit in traffic for very long. If you run more advanced timing, you will pull more vacuum, which will require a richer mix. My engine will run just fine at 16:1 at WOT at 1000 rpm (it is not tuned to do so, but it will), but would not even dream of idling at that AFR. You can idle at stoic if you run -5 deg BDTC, but you end up injecting more fuel than you do to idle at 14:1 at 5 deg BTDC.

RETed · 01-11-2012, 05:08 AM

Another area to look at if your ECU can do this is try negative ignition timing split - this is just a fancy way of saying: fire the trailings before the leadings.
There was this big thread on the Evil Forum (I think) talking about this...
I tried it with my Haltech E8, and it does allow you to run slightly leaner AFR's at idle and vacuum cruising.
I believe the numbers were you could gain another 5% - 10% in gas mileage doing this properly.

Now, the cavaet...
Although I was able to do this with my Haltech E8, it wasn't stable enough that I kept at it.
My set-up acted tempermental and sometimes felt like lean surge when cruising under heavy vacuum.
This annoyed me enough to give up trying to get that last few 1 - 2 mpg out of the engine.

-Ted

01-08-2012, 05:42 PM	#1
NoDOHC The quest for more torque Join Date: Sep 2008 Location: Sheboygan, Wisconsin Posts: 855 Rep Power: 17	I don't share Ted's hate for wideband sensors, but I otherwise agree with his post above. There is certainly a place for a wideband, however they certainly do not tell the whole story of the combustion process. To idle at 200 RPM a stock 4-port rotary engine runs about -25 kPA manifold pressure. To 'idle' at 2500 RPM will run about -70 kPA manifold pressure. The 6-port is able to run at about -30 kPA at 200 RPM. Rotary engines don't run reliably below 200 RPM. You can never access points on a MAP curve below the closed throttle MAP at each RPM. Mine cranks at 325 RPM and has -10 kPa Manifold pressure at that speed. I can't get a lower pressure (more vacuum) at 325 RPM without adjusting how low my car idles (closing the throttle plate further). Thus I only have access to 3 MAP points at 325 RPM (only 3 MAP points are valid operating parameters for my engine). My engine runs -90kPa at 6,000 RPM, closed throttle, thus I can access pretty much all the load points at that speed. As Ted said, the BAC valve will not allow much vacuum below the idle setpoint. (Which is typically 750 rpm). The 4-port in my car has no BAC valve, thus it idles only at the speed set by the throttle plate. This amounts to about 550 rpm cold and 900 rpm warm, with a brief peak at 1,000 rpm as the oil gets warm but the mix is still rich. To tune without a wideband, just lean the fuel up about 10% from stoic, (if you have a narrow-band sensor) or until the engine starts to surge (not really a mis-fire, but a slower torque variation, almost feels like driving into a headwind on a windy day). If the weather is warm you should be able to break 30 MPG, I can barely get 30 in the winter, but it is usually well above that in the summer. With vacuums deeper than about -55 kPA, the engine requires a richer mix (probably due to exhaust reversion). This is why most of us idle at -5 to 5 degrees BTDC to avoid running very rich at idle. This is unpleasant if you have to sit in traffic for very long. If you run more advanced timing, you will pull more vacuum, which will require a richer mix. My engine will run just fine at 16:1 at WOT at 1000 rpm (it is not tuned to do so, but it will), but would not even dream of idling at that AFR. You can idle at stoic if you run -5 deg BDTC, but you end up injecting more fuel than you do to idle at 14:1 at 5 deg BTDC. __________________ 1986 GXL ('87 4-port NA - Haltech E8, LS2 Coils. Defined Autoworks Headers, Dual 2.5" Exhaust (Dual Superflow, dBX mufflers) 1991 Coupe (KYB AGX Shocks, Eibach lowering springs, RB exhaust, Stock and Automatic) Last edited by NoDOHC; 01-08-2012 at 05:48 PM.