I've got a S5 NA RX-7 with a header and full custom exhaust, running an RTek 2.1 ECU. I can adjust L/T timing and split as a function of load and RPM. Is it worth it to use some of my dyno time to play with ignition advance? I've got three hours scheduled and I think the fuel tables are pretty spot on from the street tuning I've done, so I imagine I'll have time to play with other parameters.

Some have suggested advancing the timing a couple degrees from 3,000 RPM up and some people have said it's not worth the effort. Thoughts?

Thanks,

John

 

There is a lot of power to be found through timing. Well worth it.

 

Any suggestions on where to start?

 

retard 3 degrees, then advance 3 degrees over stock. see where stock timing is ok, and see where it gains/looses.

 

Both leading and trailing? Should I play with the split as well?

What you described was the approach I was going to take with AFR. I have my WOT tuning at 13.2 from 3,000 RPM to redline right now and was going to go a little richer and a little leaner to see how it responded, then move along to timing.

Obviously I am new to this tuning thing so I'm hoping to learn a lot.

Thanks,

JV

 

exactly, i'd start with stock timing, and do the 3% fuel and see what it likes.

then do the same with the timing. i would leave the split alone first.

then you'll be able to look at the runs you played with fuel, and dial that, then timing. (then you could do fuel again...)

then maybe mess with the split.

http://www.daytona-sensors.com/tech_tuning.html a little down they have a chart of timing vs power

"Ignition Timing

We often receive inquiries about optimizing ignition timing during dyno tuning. Very sensitive and precise dyno tests are required to determine MBT timing. The chart below shows why (ref: The Internal Combustion Engine in Theory and Practice by C. F. Taylor, pp. 443). The engine torque curve is very flat near the MBT timing value. A useful rule of thumb is that advancing or retarding the timing 5 degrees from the MBT value reduces torque about 1%. You cannot reliably measure a 1% torque change on a chassis dyno. We have witnessed many dyno test sessions where attempts to optimize ignition timing generated strange results that were probably caused by measurement error.

In most engines (assuming compression ratio and other factors, such as air fuel/ratio, are within reasonable limits), the MBT timing value is a few degrees below the detonation limit. If you select a wide open throttle (WOT) timing advance curve that is retarded about 3 degrees from the point where detonation is detected, you should be close to MBT ignition timing.


so if you increase timing, and don't see a gain, put it back, you want to be on the safe (retarded) side

 

Great. Thanks for the tips. Tomorrow is the day.

 

actually one more tip.

i find dynoing is usually hot, smelly, loud etc etc. its not the best place to make rational choices

its much better if you have a plan, or series of if/then statements.

that way you just follow instructions.

like yours could be something like

1. baseline pull

2. 3% leaner

3. 3% richer

4 look at all three runs, pick the rpm bands each mixture makes best power.

5. dyno results of #4

etc etc

reality may not match the plan, so its best if its not set in stone.

 

I'm a radar engineer by trade and am reasonably decent at making rational decisions so I think I'll be okay, but good advice nonetheless.

 

our old dyno haunt was ATP's old shop, but it was a little cinderblock room, it was basically enclosed too.

my 71 mercedes was ok, but the ITS car was like being hit by a bomb.... 110DB + fumes for a couple hours and we were done

 

I saw an increase of about 10whp moving from 20* BTDC to 27*BTDC.

 

No fumes, and I brought good ear protection so it was a non issue.

Unfortunately, it was a wasted day. My 6 ports were not actuating so my numbers were pathetically low. Converted to Dynojet numbers (his dyno reads 12-15% low compared to a 'jet), it put down 145 to the wheels. I fixed the aux ports and the RTek is showing a lot more airflow above 4500 RPM, but I haven't been back to the dyno.

I did play with timing and saw with 3 degrees more advance from 3k RPM up I picked up 8hp. I advanced 2 degrees more and didn't see any more gain so I backed it off. I'll probably have to repeat this exercise at the next dyno session.

 

another thing you should remember is that when you start to considerably increase timing, you might have to play with fuel a bit to start to see more gain. also you should try reducing the split in a tapered manner down to about 2 degree split.

 

Two questions spring to mind:

1. That's ALL?

2. What fuel?

 

87 chevron w/ chevron 2stroke 1 oz per gallon.

 

I think he meant to ask at what afr...

 

No, I meant what fuel. I have to run at least 89 with my timing at ~25 degrees or it breaks up at high RPM.

Maybe I'll try 29 degrees and 92.

BTW - I run *slightly* more premix than 1:128. I think it's closer to 1:70. I'm not running Mazda apex seals.

 

well I put 16oz in at fill up, so it's slightly more than 1:128 but it's just easier to say 1oz per gallon.
I have only once seen break up at high rpms and it was on the dyno with old thin oil so i took it off changed the oil and it went away.

 

what is a good base spilt value 15* at full load ?

 

Hey,

Umm, no point trying better fuel. Your 13B rotors are MDR type and burn the fuel slower than LDR 12A rotors, and you are reducing combustion speed increasing octane. AFR in range 12.8 is best for peak power with timing around 24, more does nothing... split of 5 is fine.

Performance increases will be made by moving the air in and out quicker, ie porting wide for mid range, tall for top end. Tuning intake and exhaust length and making them as free flowing as possible

 

You cant generalize more timing does nothing. I made more WHP at 27* than I did at 25*. Period end of argument. Every car and setup is different. You need to try and see what works for your car.

 

Why not?

Slower burning chambers, generally speaking, are more detonation prone. The key to eliminating detonation is to burn the fuel before it compresses/heats the end-gases to the autoignition point.

That's a myth. Detonation resistance and flame speed are not related. Some methods of increasing octane do reduce flame speed, but others increase it. Good race fuel will burn a hell of a lot faster than pump gas!

...meanwhile, others have found that power keeps increasing all the way to a little leaner than 14:1. I've certainly found that 13.5:1 makes more power than 12.5 or even 13:1.

Some racers are running timing in the 30-33 degree range, on race fuel. No word on air/fuel ratios other than being shockingly lean...

What is this "split" of which you speak?

 

+1

Trailing is so overrated A good, leading wasted spark ignition solution
and no need for trailing anymore.

 

I take it you two haven't seen the plot of RPM vs. stationary gear and bearing load measured in kg vs. engine RPM with and without trailing ignition?

I will have to school myself more on flame speed and fueling it seems. RE leaning, my engine when I had staged throttles would not even cruise leaner than 14:1 and it was borderline misfire the entire time. Are you Americans measuring AFR with a WB02 sensor downstream of the catalyst or thermal reactor or using an airpump and trying to compare AFR's to 'race cars' lol without any of those systems?

You can see pictures of said engines porting in my display picture. Exhaust ports are at least the size of TII exhaust ports maybe bigger.

 

Most of us Americans don't run cats or thermal reactors. I use wide band and made peak power around 13.5 but EGTs were really high. So I run richer.