Detonation on a Richer mix???

[HOLDAFLAT]

Hey all,

this is my first post and i'm hoping someone here can help me.
I am tuning a Nissan SR20DET (2.0l turbo petrol) with a Microtech ECU.
I run BP Ultimate (98 pump octane) and my intake temps range from 5 to 25 deg(C).
I have a map compensating for air temp at 1.4% fuel per 5deg(C) with the pivot point at 10deg.
My issue is that at or above 14psi boost at higher RPM (over 5000rpm) it is detonating.
I tried richening the mix (from around 12:1) to about 11:1 and it seemed to detonate more!
At the engine speed in question base timing is 24deg BTDC and then the boost map pulls 14deg out for 14psi, 18deg for 16psi and 22deg for 18psi.
I didnt think 10degrees of advance was excessive for 14psi - am I wrong?
Also why would a leaner mix be less prone to detonation?

I have researched as much as I can bear and the only two reasons i have come up with are as follows;
1) A richer mixture takes longer to burn and there is a phenomenon which basically says; the slower the mix burns, the longer the unburnt mix at the bottom has to heat up and pressurize, and then ignite by itself ie. detonate. or:
2) I'm a twit and its not detonation at all and just flames in the exhaust/turbo making crackly sounds.

I would like to run the engine richer as at the leaner ratios it felt too 'loose' - like the engine didn't have any 'bite'. It revved up quickly but gave the impression that if you were to load it up it would underperform...
Oh, and if you are wondering by now i don't have access to a dyno. All my tuning is done by road testing.

It had been working great until winter came along.

Any input would be awesome.
Thanks.

 

[JSteve2]

Detonation is caused by the mixture reaching combustion temperature before the spark. Since all you are doing is increasing the fuel but not reducing the amount of air, then the temperature before the spark will be about the same rich and lean in your scenario and there is no reason to think going rich will stop the detonation. Therefore, there are two possibilities here - 1) the rich mixture has a slightly lower combustion initiation temperature, or 2) the detonation is occurring both rich and lean, but the greater amount of fuel rich is making it more noticeable.

I can't tell you which of those scenarios is occurring without hearing it for myself. Not sure about your 10 degrees of advance question, since clearly when you have detonation the spark timing doesn't matter since the spark is not causing the combustion event.

Because it started in winter, I'm going to take a wild guess that you are just at the limit for this setup. The higher air density in the winter puts more material in the cylinder. Try nudging your boost down a shade, or use a higher octane fuel.

 

[BrianPetersen]

Note regarding above post: "detonation" is not the same as "pre-ignition".

With detonation, the combustion is initiated by the spark, but the end gas ignites before the flame front burns it normally, and ignition timing should have a strong influence. With pre-ignition, ignition is occurring without the spark. Normally if actual pre-ignition occurs in a running engine, it will very quickly destroy stuff.

Are you saying that at 5000 rpm, your base map calls for 24 degrees BTDC and the boost map is pulling out 22 degrees so you are only running 2 degrees BTDC under load?? something is drastically wrong, either with what you've told us or with your set-up of the engine.

 

[JSteve2]

Brian,
I assumed he really meant pre-ignition due to the "ignite by itself ie. detonate" and the nature of the problem. Of course, you know what you get when you assume.

 

[JSteve2]

PS - read it again and I see now. However, it still that a rich mixture would knock harder than a lean. Not sure about the timing issue though.

 

[vagman2]

I think you could hear the combustion process in the exaust pipes. That is because with richening the mixture you put the peak cyl pressure and burning far late after the TDC. EGT temps are probably higher now and power is less. I'd try with adding advance but it's difficult without dyno. Can you find and instal a knock monitor and EGT probe? It's always good advice to check the real timing with a timing light.

 

[HOLDAFLAT]

I'll try to clarify some things.
I did mean detonation, not pre-ignition.
Brian, that's correct. At 18psi and 5000rpm the advance is at 2 degrees. By 7000rpm it has dropped to around 15psi due to the turbo being too small. But I'm not too worried as I have a spare and want to see the limits of this system. The intercooler is large enough to rip the heat out of the intake charge.
As for the detonation it sounds akin to the crackling of a log in a fire. Could it just be mix burning in the exhaust?
When I hear it there is a very small degree of power loss. Should I substantially feel it?
One more thing. It only does it once the engine has fully warmed up (84deg coolant) when it is a tad on the cold side it won't do it (75deg).

Thanks!

 

[BrianGar]

have you got a lambda display fitted? And also do you know what the static and dynamic compression ratio is?

 

[HOLDAFLAT]

Yes I do run a wbo2 branded lambda which I log during runs against the microtech engine data.
As for the compression ratio. It's around 8.1 to 8.5 I can't exactly remember right now. But it is low 8s.
Dynamic compression ratio? No idea. Stock cams so I'd imagine it is not going to drop off too much at all.

 

[HOLDAFLAT]

just had a thought and went back through some datalogs.
i dont as a rule graph timing as i dont often change it. anyway, as it turns out the microtech ecu can only retard timing back to static.
This means as my static timing is set to 10deg BTDC that my minimum advance is also 10deg BTDC.
Damn ECU trickery!
this means at 14psi im running 14deg BTDC, at 16psi im running 10BTDC and at 18psi i am also running 10BTDC!

Do you think i need to lower my static timing or is 10deg retarded enough as some other online sources say.

I imagine anything under 5degrees advanced couldnt possibly have time to burn properly... though i could very well be wrong...

thanks a bunch for your input so far - keep it coming!!

 

[HOLDAFLAT]

Hey everyone,

I'm going to look really silly now. But i got hold of a dial-in timing light and as it so happens the timing was out by about 6 degrees.
This meant that at 18psi i was running 16deg of advance.

Anyway I have corrected that now and the detonation seems to have gone.
I have set the timing at 18psi to 13 degrees BTDC.

Thanks everyone for all your help.
I'll assume for now the rich mix wasnt helping due to the intake charge being to cold for vaporisation to have a substantial effect.

Thanks again.

 

[BrianPetersen]

Hahaha ... but getting the problem sorted (and not blowing up the engine) is more important than looking silly!

 

[HOLDAFLAT]

How true. Lol

I would have checked it earlier but the ignition system is coil on plug. Which sounds really romantic until you want to check timing using a conventional timing light lol. So I clip this inductive pickup WHERE now?

In case you are interested I lost 5hp according to my g-tech pro.

 

[patprimmer]

I don't know any serious tuner who trusts a dial in advance type timing light.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[HOLDAFLAT]

You know what pat, I completley agree.
I ended up timing it set to zero and using the balancer marks and then double checked it using the dial and TDC and I did notice discrepencies.
With the timing set at 15deg using the light in the old fashioned
manner, when I dialled it up on the timing light it thought I was pulling 14 degrees... Not good.
Mind you if anything it would make you set it too far retarded but yeah. I agree it's not accurate enough.

This was a Repco brand one though. A better brand may prove more accurate.

 

[HOLDAFLAT]

Sorry. I meant at 15degrees the dial thought I was running 19 degrees. I was thinking 4 difference and my brain wrote the wrong thing lol.