Matt22

iTrader: (10)

how important is a good quaility timing gun when tuning your cars?
I mean should i get a 65 dollar MSD timing gun? or spend way less and get a cheap one at the local parts store? Thanks Matt22

13btnos

iTrader: (1)

Get a regular timing light all those fancy timing advance ones don't work on rotary. Let me say they do work but you have to zero it out and can't use the advance degree wheel, so why pay for a feature that your not gonna use.

Matt22

iTrader: (10)

sounds good. I will have to pick up a regular one then.. Thanks matt22

dbgeek

The advance light does work, but you have to do simple math:
4 stroke motor: 1 spark per 2 revs
2 stroke motor: 1 spark per 1 rev
rotary motor: 2 sparks per crank rev (on 2nd gen 13b, primary spark & waste spark - waste spark occurs with opposing rotor's primary spark, at crank angle 180* on that rotor)

That means that the timing events (spark on Leading plug) is happenning 4 times more than on the equivalent 4 stroke engine. Therefor you have to divide the number on the "advance" by 4 to get actual crank advance.

To test this theory, the crank pulley has timing marks for Leading (20*) and Trailing (5*). Put the light at 0* (you should start here every time no matter how it is timed), and look at the marks. If you are running stock timing, the marker pin should be right on the Leading mark. Turn the advance ****, while still holding the trigger, until the marker pin is on the Trailing mark. The advance **** should read 60*. Divide this by 4 for actual = 15*, which is the crank degrees between the leading and trailing spark events.

To measure timing advance - set the advance **** to 0*, press trigger, turn **** until pin is over the corresponding timing mark, look at **** value, divide by 4. That is your advance beyond stock.

Pete

rotaryrudy

iTrader: (2)

I would jjust get a nowmal one because that sounds to confusing.

dbgeek

too hard to divide by 4?
ok....

Most of that was the proof. Besides, there's a good chance that someday, he'll own a PISTON (yes, i said that word) powered vehicle that he'll want to tune, in which case, he won't even have to do basic elementary math!

BMS2004

ok so for us first gens with no wasted spark we just divide by 2 since we have 1 spark per rev. in other words 40 degrees = 20 degrees advance? is this correct?

dbgeek

yep, just like a regular 2-stroke engine. I've tested the 60* light = 15* crank with the above method. If you have leading and trailing marks on your crank, or have some way of marking crank degrees, just test it. Hook up the light, and set the advance on the light to about 10*, and verify against marks on the crank.

Since I don't know anything about FB ignition, I'll add that you may want to be careful. There may be a vacuum or centrifugal advance acting on the distributor that WILL affect the timing, depending on vac or RPM. Many tuners tune the total advance (~4000 rpm or so) rather than just the idle timing. Racers lock the distributor (FB) so they can dial in the advance they want to run.

On the FC, advancing the timing moves the power band lower, but may disrupt peak power on top. Remember that work = area under the curve, not how high you peak; this mean that you may improve acceleration times by moving the power band down somewhat int he curve - it all depends on the application. I've run between 0 and 10 degrees advance, with the result being smoother power earlier. Be careful about going too far, as NA rotaries are resistant to knock, but boosted ones are not. Advancing the timing is like playing with fire - if you start doing funky things, yer gonna get burned. I've read on several forums that people seem to like running about 6* static advance on NA FCs. This sounds like a good compromise to me between engine longevity, usable powerband, and AFR (advance = leaner exhaust due to longer burn time).