Hatchet Jack

I half bridgeported an S4 engine with some itb's and Megasquirt II v3.57, and got it to run and idle for an initial break in as Mazda recommends. Now I'm moving on to tuning past idle.

I'm confused about spark timing. For bridged engines (full bridge, all out race I assume), Racing Beat recommends 20* max leading with a 7* split. With the capabilities of a 12x12 table of ignition timing MSII offers, how could I better translate this into a street machine?

For example: WOT at 7500 rpm, then the throttle is lifted. Should this high rpm, low load condition still be at 20* to keep from detonating, or can this low load be advanced quite a bit? I'm not going to run any fuel cut.


Sources:
http://www.racingbeat.com/manuals/timingchart.pdf
Spark Timing Myths Debunked - Spark Timing Myths Explained:: Application Notes
How To Megasquirt Your 2nd Gen RX-7: Programming The ECU (MS2)
- About 1/4 down the page
- Thanks for the writeup, Aaron!

Aaron Cake

iTrader: (1)

I run about 18 degrees at idle (1500 RPM) on my bridge, very quickly bringing timing up to 25 degrees by 2500 RPM. Cruise area of the map is 35 degrees. Obviously once in boost I taper quickly to about 15 degrees up to 8 or so PSI, then 10 degress after that.

Split is 15 degrees at idle, narrowing to 0 degrees at atmospheric is approached, back up to 15 degrees under boost.

When the throttle is lifted (we're talking the bottom range of bins on the map...high vacuum) I typically run about 38 - 40 degrees to help with afterburn. Note that this is at full decel, in fuel cut.

The timing map here is a good place to start:
How To Megasquirt Your 2nd Gen RX-7: Programming The ECU (MS2)

Hatchet Jack

Awesome. This gives me a lot to maul over. Without fuel cut, would you think that since there is an a/f mixture still being drawn in, timing should be more retarded to prevent detonation of the a/f mixture during throttle lift?

Aaron Cake

iTrader: (1)

Why wouldn't you use overrun fuel cut?

At light load during heavy decel you're going to tune very lean anyway so the car doesn't pop, buck and load up with fuel. There's no danger of detonation at zero load.

Hatchet Jack

Makes sense. About the overrun fuel cut, the thought of no lubrication during closed throttle at 7500 rpm frightens me. Also, extended engine braking (mountains) in general would make me nervous. May you explain why this shouldn't be an issue?

Aaron Cake

iTrader: (1)

The rotor housing chrome surface is porous.

j9fd3s

iTrader: (3)

for an application like this, where you're tuning from scratch, and the recommendation is from a locked distributor, i will start with the whole table set to that value, and then go and tune the fuel maps.

once the fuel map is at least roughed in, i will come back to the timing map and start working on it.

idle and high idle settings are very easy, as you can just adjust things until it idles the best, i've found that the rotary likes between -5 BTDC L and 10 BTDC L at an idle, you can run more but idle quality suffers.

WOT, you want to go to the dyno, with the RB numbers, or even possibly less timing, and try a few different ones. i would try 18, 20, and 25 degrees, and see what the curves look like. on an average engine i would expect a bigger power gain between 18 and 20, than between 20 and 25, and you want to settle on the least amount of timing to make power. so if you make 155hp @18 degrees, and 160hp at 20 degrees, but only 162hp @25 degrees, you want to run 20.

for just cruising down the freeway, i run about what Aaron runs, 30-35BTDC, and on deceleration the factory maps go to like 48 degrees, basically the less load the more timing it'll want.

Hatchet Jack

Fantastic information! Just what I needed to hear.

j9fd3s

iTrader: (3)

we usually talk about timing like more is better, but this is incorrect. its like goldilocks, too much is too much and too little is too little, so we're aiming for just right.

basically timing works like this graph

and to have some cushion for unforeseen things like cold days/plugged fuel filters/etc we want the timing to be slightly less than peak.

a rotary CAN knock, even NA, and we want a little margin.

if you want further reading google Minimum Best Torque

Hatchet Jack

This is new to me!

So, if your timing is, say 15 deg too retarded, you're looking at only 90% of the potential torque available, 145 ft-lb instead of 160 ft-lb? But even if 160 ft-lb were attainable at 24 deg, you'd want to take it back to around 21 deg or roughly 98% of the potential torque, 156 ft-lb instead of 160 ft-lb.

Is this correctly interpreting the graph?

toplessFC3Sman

iTrader: (1)

Yea, basically. The curve can be peakier or broader depending on the operating point & specific things about your engine, and you may not be able to achieve 98% of MBT timing because of knocking