Some people have mentioned using negative split in order to have a smoother idle. This was in conjunction with not using the ISC to control idle and or with ported engines.

I do not use the ISC to control idle for two reasons:
(1) after so many mods, it would not work anymore.
(2) stock ISC timing is very inefficient and causes more carbon in the exhaust.

WO the ISC, you have two idle points: no load and full load.
Full load is the baseline minimum. No load and the rpms go up.
I previously tuned for smoothess full load (IGL, 3 split, fuel, air).
If the no load is enough cells apart, it can be seperately tuned (timing & fuel only).
Mine was like this but I did not really care for the 1300rpm no load idle compared to the 950 rpm full load idle. Plus the no load idle would not smooth out using normal tuning techniques.

Then I installed a custom solenoid to only add more air flow when the AC was on.
This helped by reducing the no load rpms down to the full load level. This also required going to one set of timing and fuel values since the same cell area was being used for both idles. My optimum load timing was 19,16 using the 3 split base. With this setup, no load was more erratic than before.

Thus I ventured into he world of negative split idle control. I ended up with (12 IGL,24 IGT). The results are that full load idle is the same, but no load idle is way smoother than before.

Plug type does affect idle smoothness as some idle better than others.

 

wow that is pretty cool. I need to bring my car down to Houston one of these days.

 

hey chuck, any plans on coming to the new jersey area for a tuning session anytime soon?

kevin.

 

No, I am retired and my tuning notes is just a simple sideline to help others.

 

Chuck, here is my idle while switching between my normal +3 split to -12. The EGT difference between rotors decreases, manifold pressure increases slightly and the engine runs smoothly.
Barry

 

i tried running the negative split and played with different values, and splits. It seemed like my car had a stronger idle with the +3 split, but it was hard to tell which i prefered. I just kept the +3 split. I might play with it again another day.

 

With high negative split, you have to decrease IGL to keep in the sweet spot of idling.

You have to play with both at the same time. Just like when doing non-ISC controled idle tuning, I have to adjust air, fuel, IGL, and IGT together.

 

Would the negative split be more of a "needed on a Power FC" type of thing then a rule of thumb for correcting low load/highload idle differences?

I'm just wondering if I should play with this on my TECgt....

 

On some of the older negative-split threads they spoke of it allowing leaner AFRs. I thind I wll try 14s instead of my normal high 12s around idle.
Barry

 

It will only work when not using the ISC to control idle. The PFC does not run the idle timing in the maps when the ISC controls idle. The stock idle timing is only for emissions control and is very far off optimum for best clean burning idle. Clean burning refers to the exhaust free from carbon not clean emissions which is different.

 

I have heard of other people running negative split on different engine management systems.

 

...sounds very interesting. As i don't have big experience with programming my pfc, is there a how-to on the web about changing to negative split?

 

i don't think there is. chucks notes cover it a little bit, but you don't want to run negative split anywhere else. The only way to run negative split is to run manual idle timing, which pretty much requires the datalogit. Though i suppose there would be some way to set it with the commander it would be more difficult. The pfc uses hidden ignition maps to control the idle. so even if you change the idle cells, without manual idle control the pfc will do whatever it wants with it.

 

...ok, good to know.

But i've heard of some people using negative split not just for idle...

But for me it's more important to get a DL fist...

 

Some tuners use negative split for cruise.
I am just slowly starting to test it, but it is hard tio get immediate results.
Idle tuning gives immediate results.

 

not sure how applicable this is to the REW, but the neat thing about the rx8 is that you can plug in the IDS and watch the timing in various situations.

at zero throttle, the Leading in the rx8 is always -5BTDC. its independent of rpm.

at idle the leading is -5BTDC, trailing can be 10-15BTDC.

the rx8 will cruise at really low rpms (2500ish) and load (constant speed) in negative split, but the negative split goes away really fast with load.

the timing advance curves are also very steep. cruising at 2500rpm, advance is in the 40's, full throttle goes to around 0, and then starts climbing with rpm.

rx8 is fully advanced by 6000rpm, @30btdc L, 20-25 trailing.

rx8 runs lots of timing, be careful =)

 

The RX8 is NA and thus can run more timing than a boosted engine.

 

^^ is there a chart of Rx-8 timing numbers floating around?

 

I hope to do some cruise logging and see if anything substanial shows up.
It will be around 65mph, with stock IGL, and testing at lease 3, -3, and -6 split.
Too bad I do not have access to a car with cruise control which should keep the speed more steady on the flat road I plan to drive on.

Any Houston voluntees?

 

i'm in brenham, but i come to houston from time to time. its about an hr and a half away from your area i think.

I have cruise control. I wouldnt be able to do any full throttle testing though as my car is still being broken in, but cruising and such is fine. also i'm running manual idle control already