Air/fuel Ratio's And Good Old Knock!

[williamsf1]

This is a convo I have just had with SK, I thought others might get something from it too

quote:-

"AFR's are an endless subject........

Before I start, I should mention we are talking about A/F ratios under boost, not idle or cruise A/F ratios where 14.7 (stoic) is considered rich these days for some engines.

Let start off with......lambda sensors are not accurate, even the most expensive Fast and Wide (F&W) sensors have an accuracy rating that is not 100%. Plus it depends on where you place them in the exhaust, shoving them up the rear muffler with a venturi isn't as bad as just sticking a raw lambda sensor at the pipe. The exhaust gets contaminated by ambient air that gets sucked in the exhaust, this is particularly a problem with huge diameter exhaust outlets at low rpms. It isn't as good as having a lambda sensor right at the turbo outlet, like the car manufacturers do. But that would mean having 2 F&W sensors in the case of a GTR. On the race cars we have a blanked off bung in the exhaust just behind the standard lambda sensors where we screw the dyno F&W lambda sensor in when we are tuning.

Lambda sensors are not all calibrated the same. They compare the exhaust with the ambient air and as you well know the ambient air changes a lot. So they have to be calibrated regularly, most dyno shops do it once a week if you are lucky. Some do it only when they get a new lambda sensor. They could do it every day, maybe even twice a day in changing conditions.

So we have differences in A/F ratios that may be simply the different calibration or placement of the lambda sensor. The actual A/F ratios themselves may be the same. Good dyno operators know about the variability of A/F ratios and allow some room for error. For example, the target A/F ratio might be 12 to 1 but they tune at 11.8 to 1 knowing that they have a 0.2 margin for lambda sensor inaccuracy. Some tuners have total faith in their lambda sensor readings and tune to exactly 12 to 1. Sometimes that blind faith is misplaced and they are actually tuning to an A/F ratio higher than 12 to 1.

Based on what we have seen, RB engines seem to make best power at 12 .5 to 1 and sometimes 12.8 to 1. This is OK for a race engine where we monitor the A/F ratios all the time and where every single horsepower is required. On a road car, that is tuning a bit too close for the changing conditions and unmonitored life that they experience. With the accuracy of ECU's these days and the consistency of fuel, most people generally accept that around 12 to 1 is reasonably safe. A few years ago 11.5 was considered risky, this has increased as F&W lambda sensors have become more accurate, easy to obtain and relatively cheap and tuners have more faith in the equipment and their ability.

Knock almost as endless a subject as lambda.............

Firstly, knock sensors are simply microphones, they are calibrated to hear upper cylinder pre-ignition frequencies and pretty much ignore all other noises. Unfortunately there are quite few noises in cars that have similar frequencies to pre-ignition and the knock sensor can't tell the difference. Being a microphone, they simply transfer the noise into electrical signals that the ECU receives and interprets. The displayed knock is simply a number, it's not knocks per minute/second/hour.....it's just a number. In the case of a Power FC (my terminology) 100 is really bad (stop right now and fix it), 80 is bad (stop soon and fix it), 60 is almost OK (get the tune checked as soon as you can), 40 is OK (maybe get the tune checked if you are worried), 20 is perfectly safe (press on) and 10 is too low (the engine is not tuned close enough).

On a Power FC there are 2 items you can change for knock with the lap top software (Apexi Excel or Datalogit). One is "threshold" (default is 60) that flashes the dash warning light when the knock exceeds 60. Sometimes we change that to 50 or even 40 if the engine is tuned close. The other is "setting" (default is 9) which I never change, I always assumed it meant the frequency of the dash warning flashes. Maybe it doesn't, I have been meaning to do a bit of research on that one, now is as good a time as any."

[Beer Baron]

interesting point about knock being too low under 20. my thing never reads a knock above 18 or so. it used to read around 25 after a good drive, but after the cam gear tuning it's much lower. maybe it's time i went for a more agressive tune, i know the car is running very fat up top, as per my request for safety margain at the track. it does occaisionaly see the rev limit too :oops:...

[lwells]

This is certainly an interesting subject given a number of recent dyno tunes done for forum members with AFRs in the 12:1 to 12.5:1 range. When I first started reading this forum, very few people were willing to venutre much beyond the 11.5:1 to 12:1 range.

I'd love to hear what some of the tuners have to say...

[williamsf1]

Yeah as a side note I am looking to buy an adjustable soft/hard cut rev limiter that will use ignition cut... this way I can keep my power FC and not have to worry about tagging the 8210 limit and leaning out!

now my car has been tuned with the cams, it revs agressively all the way to the limit, where as before it would ease off at about 6500 and made shifting there the done thing... now I have to catch it before it ramps all the way!!!

(only once have I tagged the limiter (first drive after tune!), and it did give an increase in knock my bad.... )

[Mik]

Yeah as a side note I am looking to buy an adjustable soft/hard cut rev limiter that will use ignition cut... this way I can keep my power FC and not have to worry about tagging the 8210 limit and leaning out!

now my car has been tuned with the cams, it revs agressively all the way to the limit, where as before it would ease off at about 6500 and made shifting there the done thing... now I have to catch it before it ramps all the way!!!

(only once have I tagged the limiter (first drive after tune!), and it did give an increase in knock   my bad.... )

<{POST_SNAPBACK}>

I'm still not convinced about the whole lean out on rev limit thing. If the power FC cuts fuel only on rev limit, then shouldn't their be no combustion as their is no fuel at all? The Power FC doesn't limit the fuel, it cuts the fuel.

Also, if the Power FC cuts fuel and not ignition, then why does a car running a power FC backfire and blow balck smoke when it bounces of the revlimiter??

[GunMetalR33]

I'm still not convinced about the whole lean out on rev limit thing. If the power FC cuts fuel only on rev limit, then shouldn't their be no combustion as their is no fuel at all? The Power FC doesn't limit the fuel, it cuts the fuel.

Also, if the Power FC cuts fuel and not ignition, then why does a car running a power FC backfire and blow balck smoke when it bounces of the revlimiter??

<{POST_SNAPBACK}>

That might be a PFC Pro. I have the Pro version and it uses ign cut for rev limit not fuel cut. I often hit the rev limiter (7000 rpm on rb25det) and have not noticed increased knock.

What do others run as there rev limit on an rb25det???

[williamsf1]

I can tell you for sure that the knock goes much higher when you hit the limiter in a normal FC.... the fuel cut in true terms is correct, but like most things it isnt an on / off thing... therefore at some stage the mixture is leaner than optimum and at very high rpm... I would prefer an ignition cut and spray fuel / fire out the back rather than banging my engine...

anyone with any good quality ignition cut rev limiters??

[Beer Baron]

Mik, makes a good point. I have hit the rev limit a fair few times. and when rotating the tyres for warmth have been on it for a few seconds, before i grab second and i never see an increase in knock during those occaisions.

[GunMetalR33]

anyone with any good quality ignition cut rev limiters??

<{POST_SNAPBACK}>

I DO! , I DO!

see post above yours, PFC Pro

[Mik]

I can tell you for sure that the knock goes much higher when you hit the limiter in a normal FC.... the fuel cut in true terms is correct, but like most things it isnt an on / off thing... therefore at some stage the mixture is leaner than optimum and at very high rpm... I would prefer an ignition cut and spray fuel / fire out the back rather than banging my engine...

anyone with any good quality ignition cut rev limiters??

<{POST_SNAPBACK}>

Yes, but is it picking up knock or noise / shock from the backfiring? I know that cars with faulty coils also have high knock readings when they miss and backfire.

[R31Nismoid]

my RB25 was 8000rpm

But yeah, the knock often spikes when you hit the limiter as its pretty saveage.

Also ive noticed the PFC sometimes isnt exactly on the cut either. I've seem and extra 150rpm in one instance looking back @ the peaks

[Amaru]

my RB25 was 8000rpm

But yeah, the knock often spikes when you hit the limiter as its pretty saveage.

Also ive noticed the PFC sometimes isnt exactly on the cut either. I've seem and extra 150rpm in one instance looking back @ the peaks

<{POST_SNAPBACK}>

True,

I have my rev limit at 8000rpm and have hit it a few times, then the peak rpm reading on the avc-r is ~8150rpm.

Im 99.9% sure I dont have a PFC Pro, yet when it has hit rev limit its spewed black smoke/popped/flamed out the exhaust, which would indicate ignition is being cut not fuel. How can I check if it is a PFC Pro?

[GunMetalR33]

True,

I have my rev limit at 8000rpm and have hit it a few times, then the peak rpm reading on the avc-r is ~8150rpm.

Im 99.9% sure I dont have a PFC Pro, yet when it has hit rev limit its spewed black smoke/popped/flamed out the exhaust, which would indicate ignition is being cut not fuel. How can I check if it is a PFC Pro?

<{POST_SNAPBACK}>

Do you have launch control? If no then you dont. Also there would be a different software version displayed on startup. I dont ever pay attention so I dont know what the version is though.

[Amaru]

How do I check for launch control?

[lwells]

The PFC Pro usually has a little silver sticker (with black writing) which states as much. I would assume you can probably also tell from the serial number.

[GunMetalR33]

go to the rev menu, the lauch control setting is the one under the rev limit setting. Set it and see if it works. If you dont have a pro then it might not even be there.

[Beer Baron]

mine is not a pro version, and like wise it spits fuel etc on the limiter, yes the limiter can be a little lazy too and overshoot by 100 rpm or so, but what do you expect at 8000rpm!!

i still say i've never seen a knock increase on the limiter.

but i also would like an ign cut limiter for peace of mind, and would like to be able to set a stationary limiter (launch control).

[williamsf1]

ok so does anyone have any bright idea's about getting say an ignition cut limiter with a bright LED to say "hey dummy change gears"

Im in the market.....

and no I dont want to buy a P FC Pro...

[Beer Baron]

no ideas, but that's what i want too. reckon iwould pay $100-$200 or so. otherwise i'd just buy the PFC pro.

[Amaru]

Jaycar have a kit?

EDIT: Indeed they do which cuts ignition (with another kit) - http://www.jaycar.com.au/productView.asp?I...x=&SUBCATID=347

So for $50 or so, you should have a ignition cut rev limiter

Edited September 21, 2005 by Amaru