More ignition timing = less flat spots?!?!

[Cubes]

Hello ppls,

I've recently got my RB30DET up and running.

To my suprise the AFR's were fine with the RB20t ecu so it was run in on the stock ECU. The base ignition timing was set to 15degree's.

I had to wire open the wastegate as the transition on to boost would cause the car to surge (go a little dead then lunge forward).

On the dyno using a Diag tool that plugs in under the steering coloulm it was showing that as the car would go dead it would dump a heap of fuel in and drop the ignition timing from 22degree's to 8degree's. Then the car would lunge forward.

It appears as if the ECU is picking up a little detonation that cannot be heard and reacting.

I was sitting there thinking about the car when I remembered some thing a tuner Tim Possingham once told me. At the time we were talking about tuning, flame fronts, detonation, ignition timing and the affect of reversion.

He mentioned during the conversation some thing about *sometimes* when you get a certian type of detonation its best to advance the ignition timing as it actually helps prevent detonation in that situation. It was something about the early flame front helps keep out exhaust gases while the exhaust valves are still slightly open or something.

I can't remember for the life of me the exact theory behind what he was saying but just for a test I pumped around 25degree's ignition timing in to the RB30DET and took it for a spin.

To my suprise the lunge and flat spot as basically gone.

The car now pulls constantly as it comes on to boost and when on boost.

The car is running a lower static comp as to what the RB20t was (8.2:1 vs 8.5:1).

Its going in for the tune on Friday.

Hopefully the SAFC will do the job for a few months until I get that AP PowerFC.

Has any one had any experience with a new motor that runs a lower static comp and how it reacts with the stocker ecu maps?

[sweetr33]

we had sorta similar drama in a mates rb25det cefiro, was set on 17degrees but had flat spot upt it slighty to around 20 and the flat spot was gone, although i dont know about the early flame font bull shit, any sort of uncontrolled burning is considered pre-ignition or detonation so i dunno what he is on

[Cubes]

I think what he was getting at is that with an earlier burn it prevents hot exhaust gas being sucked back in to the combustion chamber. Hence Reversion.

It may also affect my RB30DET more so as its running stock RB25DE N/A cams, due to the cams being NA they have a different lobe center than the turbo spec cams. NA generally runs more overlap.

The more I think about it the more I remember what he said and how it is beginning to make sense in my case.

[The Dan]

What you have said about the early flame front is absolutely true. and reversion does exist. But too much timing will actually cause this. So it's not really a matter of giving it more timing but giving it the correct timing. You can even go to the extent of making sure all your plugs face the opposite way to your inlet and you can help prevent this also. I think the cams will have a lot to do with it also but it is just a matter of tuning the ecu(Power FC) to match your specs. You will see the difference and feel it too.

[dangerous_daveo]

I have actually head about that with the spark plugs from one of my mates who was in gemini racing, it was so bloody tight they did anything to get the slightest advantage.

[Cubes]

What you have said about the early flame front is absolutely true. and reversion does exist. But too much timing will actually cause this. So it's not really a matter of giving it more timing but giving it the correct timing. You can even go to the extent of making sure all your plugs face the opposite way to your inlet and you can help prevent this also. I think the cams will have a lot to do with it also but it is just a matter of tuning the ecu(Power FC) to match your specs. You will see the difference and feel it too.

Excellent.. So i'm on the right track

Thats what it comes down to.. It needs a decent tune as running slightly different cam specs, lower static comp, bigger motor, bigger valves all attributes to my problem.

BUT on that note I'm pretty damn impressed with the rb20t ecu being able to handle such a change to the whole engine setup.

[Cubes]

A quick explaination of exhaust reversion.

Valve overlap. In a worst-case scenario the gas flow will reverse as cylinder and exhaust gas following its downward pressure gradient enters the intake tract.

One way to envision this is the following: If the cylinder has mostly emptied, both valves are open, intake pressure is 15 PSI and exhaust pressure is 30 PSI what is the direction of flow? The inertial momentum of the flowing intake air can and does partially offset the reversed pressure gradient, however at street engine speeds the effect of manifold pressures can predominate.

This reversion phenomenon not only reduces cylinder filling but also can increase the likelihood of detonation because the cylinder retains hot exhaust gases with reactive combustion end products.

[Cubes]

The more I think about it the more I realise I am going to get no where with simply an SAFC or a PowerFC for that matter.

It will be only a bandaid.

I need to reduce the exhaust back pressure before the turbo (slap on a larger turbo) or get my self an adjustable cam gear to close the exhaust valve later...... I think its later so it also opens later.....

Fiddling with ignition timing will bandaid it but it is not optimal.

[Bl4cK32]

yer Joel ...bigger turb

In first gear it flicks to 4000rpm so so damn quick with such little throttle.

It feels a little too touchy to drive around me thinkx.. Definately a bigger turbo will be needed to reduce the response

:bahaha:

[ace_nz]

how does starting the burn sooner affect what happens on the next stroke, there is no flame front on the overlap stroke. Also if your only running 25 degrees of advance at 3000 rpm your missing out on alot off off boost response.

[Cubes]

I'm really not sure...

But it works, I did notice a drop in boost response.

[raist60]

You sure you werent just reading the timing wrong? I know on RB25s that its actually running 15o when the light and balancer says im running about 25-30o (with a normal non adjustable dwell timing light)

And if i run 15o according to the balancer and light then im actually running a whole heap less (thus more flame down exhaust, more spool but a shitty running car)

[Cubes]

Mine has that crappy double pulse ignition timing issue also.

On the harmonic balancer it was reading 20-25degree's when on the Diag tool that plugs under the dash it was reading 15degree's.

[Busky2k]

When I first bought my car I noticed it had a flat spot around 5500rpm, and thats the RPM where the VCT changes the cam timing. I bumped the timing up by 3 degrees, and the flatspot dissapeared. Dunno how to explain it tho..

[Cubes]

Its going in for a tune on Friday.

It hauls arse now. I've sorted out the ignition timing. There is no pinging but it is advanced quite a bit. I figure its fairly safe due to the lower static comp.

Stirred up a late model STI today...

I left him playing catch up. Mind you if he launched with high rev's I would have no chance due to good ol' RWD.