Acceleration fueling
Discussion
stevieturbo said:
Just because you run a lean mixture, doesnt mean it will detonate. It sure wont help matters though.
Sorry Stevie, i've just reread what i typed, and i've realised it was slightly vauge.
what i meant was, with the timing set for peak power and the fueling set at say 12:1. If you then start running 14.7 AFR with the same timing, surely it will det badly?
vixpy1 said:
GreenV8S said:
Marquis_Rex said:
The last point is that the engine won't "burn out" or misfire if you run at stoich at WOT. I've seen this stated here and in another post by someone who runs a rolling road. All that will happen is that the car won't produce as much torque. Simple.
I thought that richening the mixture was often used to cool the combustion chamber, reducing the danger of detonation. In that case, removing the 'extra' fuel might lead you back into detonation?
I would have thought running an engine such as the Rover V8 at 14.7:1 on WOT would be det city..
Det is most likely to occur at low speeds and then mostly on engines that have very good volumetric efficiency at low speeds. This is because there is more time for it to occur at low speeds. Det is barely an issue on high revving formula 1 engnes.
Engines that have very good volumetric efficiency at low speeds (90% plus -below 2500 rpm) are ones with some sort of variable cam phasing or perhaps boosted engines. The other major factor to consider for det is the compression ratio specified (I'm going to be assuming 98 octane fuel is being used). A typical boggo Rover V8 is what? 8,5:1 CR may be 9.5:1 CR? Very low, and doesn't produce very high VE. It's combustion chamber design isn't the best in terms of detonation but the other factors predominate.
The irony of it is, that although det is more likely to occur at low speeds, it's when/if knock occurs at high speeds that engines get majorly destroyed.
As I said before fueling will effect the knock limit, but it makes no sense to control knock with fueling-it's not an elegant way to do it, it's WAY TO slow a stretegy- and if you have only one MAF sensor - how can you do cylinder selective knock control? The Rover V8 will have BIG cylinder to cylinder Volumetric Efficiency variation and there for AFR variation- especially on tuned versions with high overlap. This means that you're limited by your worst cylinder. So you build a trick Rover V8 running an 11.5:1 CR and ,say, cylinder 7 starts to knock like buggery because it has the highest Volumetric efficiency, is running the leanest and perhaps running the hotest, the other cylinders are fine: What are you going to do? Are you going to richen up the mixture on ALL cylinder because of this one cylinder and lower the overall power? Assuming you don't have one lamda sensor per cylinder, how will you know precisely how rich that cylinder is now running with poxy after market bits? An OEM would probably devise a strategy that could try to decifer which cylinder fired last and infer from that where the last slug of ai came from but then an OEM wouldn't try to use fueling as a strategy to limit knock.
vixpy1 said:
Sorry Stevie, i've just reread what i typed, and i've realised it was slightly vauge.
what i meant was, with the timing set for peak power and the fueling set at say 12:1. If you then start running 14.7 AFR with the same timing, surely it will det badly?
Charlie, it depends how knock limited that Rover V8 is at peak power. Unless it's a highley modified trick engine I doubt it's that knock limited. That means when the ignition is swept and torque monitored the torque value will reach a peak and if you sweep further the torque value will decrease. Even a Jaguar Superchared V8 isn't knock limited at peak power and neither is the new Aston AM V8 and they have high CRs and better breathing (hence higher cylinder pressures) than the RV8.
If the engine is knock limited at peak power (a good example is the E46 M3) then you'll never reach this optimum timing of best torque- the engine will knock before you do. The best you can do is map close to the detonation border limit with a safety factor.
I understand why you chose the RV8 as an example - becuase it's an older combustion chamber design with an off centre plug position, but I would probably still reckon the otehr factors predominate and it isn't knock limited at peak power. If this is the case, the engine probably wouldn't knock if you went to 14.7:1 (but you may burn out the cats!). It's not ideal becuase cylinder temps would probably be higher. If it was knock limited at peak power then for the ame ignition the lean mixture would make things worse, but my point was that ignition has a larger effect and if you were forced to run at at stoich for some reason ( I don't know why- some enviro-green reason) you could do it and retard the ignition to compensate. If you were forced to run 50 degs ignition at WOT and try to stop it from knocking via the mixture you could be running at rich as 8:1 with black smoke coming out of the tail pipes and I still think it would be detonating.
Edited by Marquis_Rex on Friday 14th July 15:42
Marquis_Rex said:
vixpy1 said:
Sorry Stevie, i've just reread what i typed, and i've realised it was slightly vauge.
what i meant was, with the timing set for peak power and the fueling set at say 12:1. If you then start running 14.7 AFR with the same timing, surely it will det badly?
Charlie, it depends how knock limited that Rover V8 is at peak power. Unless it's a highley modified trick engine I doubt it's that knock limited. That means when the ignition is swept and torque monitored the torque value will reach a peak and if you sweep further the torque value will decrease. Even a Jaguar Superchared V8 isn't knock limited at peak power and neither is the new Aston AM V8 and they have high CRs and better breathing (hence higher cylinder pressures) than the RV8.
If the engine is knock limited at peak power (a good example is the E46 M3) then you'll never reach this optimum timing of best torque- the engine will knock before you do. The best you can do is map close to the detonation border limit with a safety factor.
I understand why you chose the RV8 as an example - becuase it's an older combustion chamber design with an off centre plug position, but I would probably still reckon the otehr factors predominate and it isn't knock limited at peak power. If this is the case, the engine probably wouldn't knock if you went to 14.7:1 (but you may burn out the cats!). It's not ideal becuase cylinder temps would probably be higher. If it was knock limited at peak power then for the ame ignition the lean mixture would make things worse, but my point was that ignition has a larger effect and if you were forced to run at at stoich for some reason ( I don't know why- some enviro-green reason) you could do it and retard the ignition to compensate. If you were forced to run 50 degs ignition at WOT and try to stop it from knocking via the mixture you could be running at rich as 8:1 with black smoke coming out of the tail pipes and I still think it would be detonating.
Edited by Marquis_Rex on Friday 14th July 15:42
All very interesting mate, thanks for that 
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