Fuelling on the overrun
Discussion
Just wondering what peoples thoughts were on this subject. I know folk occasionally like pops and bangs, but curious how this is achieved. My main reason for adjusting the fuelling is to smooth out any slight judders during deceleration in a gear and just using engine breaking. I have the canems system so its easy to modify the fuel table but I noticed its only the bottom part of the fuel table that gets used during this state, i.e high vacuum. I know Dave will have some views on this but he's running LPG!
Any thoughts appreciated.
Any thoughts appreciated.
My car has an Emerald ECU and has map 2 set as a duplicate of my map 1, but with poops and bangs.
This is achieved by setting the fuelling on the over-run extremely lean.
My AFR gauge shows the air fuel ratio is in the 12-14 zone in normal driving, 10-11 under choke on cold start up (and idle raised by 200 rpm), but goes off the top of the scale on the over-run, ie over 20, and gives the crowd pleasing pops and bangs.
On the normal map, no pops and bangs, AFR stays around 13-14 on the over-run and there are no extraneous noises.
Hope this helps
This is achieved by setting the fuelling on the over-run extremely lean.
My AFR gauge shows the air fuel ratio is in the 12-14 zone in normal driving, 10-11 under choke on cold start up (and idle raised by 200 rpm), but goes off the top of the scale on the over-run, ie over 20, and gives the crowd pleasing pops and bangs.
On the normal map, no pops and bangs, AFR stays around 13-14 on the over-run and there are no extraneous noises.
Hope this helps
Thanks QBEE. To be honest I'm not too fussed about the poops and bangs or pops and bangs but more focused on getting the car super smooth on deceleration. I suspect the problem is not necessarily fuelling. This is a good article:
http://www.vintagemodelairplane.com/pages/Rover_Te...
ummm
http://www.vintagemodelairplane.com/pages/Rover_Te...
ummm
QBee said:
My car has an Emerald ECU and has map 2 set as a duplicate of my map 1, but with poops and bangs.
This is achieved by setting the fuelling on the over-run extremely lean.
My AFR gauge shows the air fuel ratio is in the 12-14 zone in normal driving, 10-11 under choke on cold start up (and idle raised by 200 rpm), but goes off the top of the scale on the over-run, ie over 20, and gives the crowd pleasing pops and bangs.
On the normal map, no pops and bangs, AFR stays around 13-14 on the over-run and there are no extraneous noises.
Hope this helps
I always thought it was overfuelling until someone like Simon, Mark or was it Joolz This is achieved by setting the fuelling on the over-run extremely lean.
My AFR gauge shows the air fuel ratio is in the 12-14 zone in normal driving, 10-11 under choke on cold start up (and idle raised by 200 rpm), but goes off the top of the scale on the over-run, ie over 20, and gives the crowd pleasing pops and bangs.
On the normal map, no pops and bangs, AFR stays around 13-14 on the over-run and there are no extraneous noises.
Hope this helps
pointed out it was actually leaning it off that creates the pops and bangs when shutting off the throttle. Which makes perfect sense when you think about it, extra heat in the exhaust system. I don’t really have many pops and bangs,, I wonder if that’s why my exhausts never seem as hot as they used to, could be my imagination though!
Jason sort of tuned my pops and bangs so I have to use a tad of throttle if I want it but it doesn’t pop otherwise which I really like,,, it just sort of burbles instead
Edited by Classic Chim on Tuesday 7th May 13:37
Edited by Classic Chim on Tuesday 7th May 13:38
t loads of ign retard on the over-run will create crackles and pops not sure why you would want this however 
rather than making them faster 
Owen, to make your decel smoother you'll likely benefit from more fuel and moderate timing numbers.
Run excessively advanced timing and the engine will fight itself causing juddering, if the mixture is lit too early maximum cylinder pressures will occur before the piston has reached and indeed passed TDC where best use of maximum cylinder pressure can be exploited, too retarded and it'll pop and crackle like a bowl of rice crispies. Everyone wants something different but trial and error will get your decel events as you like them but always keep in mind AFR is intrinsically linked to timing and timing is intrinsically linked to AFR, the two work hand in glove so change one and the other will be affected.
Also consider engine speed and road speed are massive influencers when it comes to deceleration smoothness, you can run AFRs as lean as you like above 2,000rpm but below this and especially during slower road speeds the same lean AFRs will promote trouble for sure. Other critical factors are the type of transmission deployed and the weight of the vehicle, for example a Range Rover for which the 14CUX was designed weighs almost two tons which means massive inertia compared with a light weight TVR and make no mistake this inertia helps a lot to smooth out deceleration events. The lean emissions friendly 14CUX deceleration AFRs that work so well on a two ton Rangie with it's slush box do not work nearly so well on a 1060kg TVR with it's manual transmission, but adding fuel to the TVR's decel diet can really help here
Nearly all Range Rovers come with an auto box which shouldn't be ignored, an auto effectively acts as a giant hydraulic cush drive or slipper cluch if you're into your V twin motorcycles, so you can soon see why tuning the lightweight manual transmission TVR presents a far bigger challenge, this is especially true during deceleration events as this is where poor mapping will soon be exposed. The motorcycle analogy is a very accurate one because motorcycles carry fek all weight so have begger all inertia, it's also highly undesirable to have even a momentary lock up of the rear wheel especially when cornering.
This is why you often find a vulcanised rubber cush drive system in the rear hub/sprocket of a motorcycle, it's there to limit snatchy deceleration events, this gets worse the fewer cylinders you run so a big thumper (a big capacity single) can end up completely unrideable without such help. Modern large capacity singles and especially high performance V twins where violent down shifts are to be expected take this a stage further with a system known as a slipper clutch, as you go down the box entering or mid bend the point of least resistance becomes the clutch not the rear tyre, this is a good idea as a locked rear tyre mid bend on a performance motorcycle seldom ends well.
A TVR is not only a light car it's light at the rear end too, add to this a big V8 that gives immense levels of engine braking and in extreme conditions such as an excessively early down shift rear wheel lock ups can easily occur. During less aggressive driving you may not get into a lock up situation but the snatchy deceleration event especially a lower road speeds is very common indeed, the right ignition timing is critical to make things smooth in the lightweight TVR during deceleration and just one degree can make a big difference here.
As discussed lean mixtures (AFRs) do not help either, the age old cure all of 'Fix It Wit Fuel' is never more relevant than when it comes to dialing in the smoothest possible deceleration. Do also consider that a vacuum leak of any sort will defy all attempts to tune for smooth deceleration and remember an idle air control valve of any sort is just a giant vacuum leak, if the 14CUX stepper motor or even your Canems PWM idle valve is letting too much air in under deceleration you'll never achieve a smooth off throttle situation.
I eliminated my Canems PWM idle valve as I couldn't trust what it was doing or even what the software was telling me it was doing, as soon as I replaced it with my manual fast idle system I saw immediate benefits especially on decel. When I push my simple choke knob system fully home I know without any doubt it's 100% closed allowing no air in whatsoever, this has delivered noticeable drivability improvements everywhere but especially during deceleration events.
Hope this helps?
Dave.
Run excessively advanced timing and the engine will fight itself causing juddering, if the mixture is lit too early maximum cylinder pressures will occur before the piston has reached and indeed passed TDC where best use of maximum cylinder pressure can be exploited, too retarded and it'll pop and crackle like a bowl of rice crispies. Everyone wants something different but trial and error will get your decel events as you like them but always keep in mind AFR is intrinsically linked to timing and timing is intrinsically linked to AFR, the two work hand in glove so change one and the other will be affected.
Also consider engine speed and road speed are massive influencers when it comes to deceleration smoothness, you can run AFRs as lean as you like above 2,000rpm but below this and especially during slower road speeds the same lean AFRs will promote trouble for sure. Other critical factors are the type of transmission deployed and the weight of the vehicle, for example a Range Rover for which the 14CUX was designed weighs almost two tons which means massive inertia compared with a light weight TVR and make no mistake this inertia helps a lot to smooth out deceleration events. The lean emissions friendly 14CUX deceleration AFRs that work so well on a two ton Rangie with it's slush box do not work nearly so well on a 1060kg TVR with it's manual transmission, but adding fuel to the TVR's decel diet can really help here
Nearly all Range Rovers come with an auto box which shouldn't be ignored, an auto effectively acts as a giant hydraulic cush drive or slipper cluch if you're into your V twin motorcycles, so you can soon see why tuning the lightweight manual transmission TVR presents a far bigger challenge, this is especially true during deceleration events as this is where poor mapping will soon be exposed. The motorcycle analogy is a very accurate one because motorcycles carry fek all weight so have begger all inertia, it's also highly undesirable to have even a momentary lock up of the rear wheel especially when cornering.
This is why you often find a vulcanised rubber cush drive system in the rear hub/sprocket of a motorcycle, it's there to limit snatchy deceleration events, this gets worse the fewer cylinders you run so a big thumper (a big capacity single) can end up completely unrideable without such help. Modern large capacity singles and especially high performance V twins where violent down shifts are to be expected take this a stage further with a system known as a slipper clutch, as you go down the box entering or mid bend the point of least resistance becomes the clutch not the rear tyre, this is a good idea as a locked rear tyre mid bend on a performance motorcycle seldom ends well.
A TVR is not only a light car it's light at the rear end too, add to this a big V8 that gives immense levels of engine braking and in extreme conditions such as an excessively early down shift rear wheel lock ups can easily occur. During less aggressive driving you may not get into a lock up situation but the snatchy deceleration event especially a lower road speeds is very common indeed, the right ignition timing is critical to make things smooth in the lightweight TVR during deceleration and just one degree can make a big difference here.
As discussed lean mixtures (AFRs) do not help either, the age old cure all of 'Fix It Wit Fuel' is never more relevant than when it comes to dialing in the smoothest possible deceleration. Do also consider that a vacuum leak of any sort will defy all attempts to tune for smooth deceleration and remember an idle air control valve of any sort is just a giant vacuum leak, if the 14CUX stepper motor or even your Canems PWM idle valve is letting too much air in under deceleration you'll never achieve a smooth off throttle situation.
I eliminated my Canems PWM idle valve as I couldn't trust what it was doing or even what the software was telling me it was doing, as soon as I replaced it with my manual fast idle system I saw immediate benefits especially on decel. When I push my simple choke knob system fully home I know without any doubt it's 100% closed allowing no air in whatsoever, this has delivered noticeable drivability improvements everywhere but especially during deceleration events.
Hope this helps?
Dave.
Thanks Dave, excellent piece of information. I’ll give it go at the weekend. If only there was a self mapping/closed loop feature on the canems system. It’s a shame the aftermarket ecu guys don’t develop a similar system to GEMS. Would make an excellent university project. Knock sensors, accelerometers, vibration sensors could all be used to give the smoothest possible performance.
lancelin said:
Thanks Dave, excellent piece of information. I’ll give it go at the weekend. If only there was a self mapping/closed loop feature on the canems system. It’s a shame the aftermarket ecu guys don’t develop a similar system to GEMS. Would make an excellent university project. Knock sensors, accelerometers, vibration sensors could all be used to give the smoothest possible performance.
OEM systems like the GEMS are way way more sophisticated than the typical aftermarket ECUs used on our cars, fact!You can run closed loop with all the usual suspect after market ECUs and the Canems system is no exception, closed loop lambda feedback being the only active fuel correction feature available in our case. I use it but don't expect miracles from closed loop lambda feedback, combustion on a Rover V8 especially at idle is very inefficient and the wilder the cam profile the worse this gets as more valve overlap is the enemy of a smooth idle and good low speed drivability.
Even if you are considering using closed loop the car should always be mapped in open loop mode first, only when the car is performing perfectly should you turn closed loop back on and even then the amount of correction should be kept to an absolute minimum or the feature can give more trouble than it's worth.
Closed loop is really just there to fine trim your perfected open loop map and the less correction needed the better, my maps also automatically come out of closed loop above a certain RPM/load threshold and the feature does not become active until a fixed coolant temp figure is reached. I built a target AFR table based on what I felt were the right figures then proceeded to map the car based on the AFRs it actually really wanted, there was a big difference in the numbers I ended up with. Then before turning closed loop on I rebuilt the target AFR table to match these 'what the engine wants' figures, once closed loop was back on this ensured the engine was always getting what it wanted with minimal correction from lambda feedback.
To some this is a back to front way of doing things but in truth it's the best way to ensure the engine gets the AFRs it wants, if you shoot for AFRs you think the engine needs not what it tells you it wants you will never get smooth results. Back in the day when I was carb tuning this was all done by feel, sound and smell, ignoring AFRs and returning to my carb tuning training has yielded by far the best results. You can get really hung up on AFRs but in many respects they are just an unhelpful distraction in the pursuit of a smooth drive, it was always drummed into me to listen to the engine and the engine will tell you what it wants.
It matters not if you're running carb or injection an internal combustion engine is still an internal combustion engine so the rules are exactly the same, if you fall into the trap of tuning to AFRs you may feel stoich (14.7:1) should be your target below 2,000rpm, but ignore the AFRs for a minute and just give the engine what it wants and you'll likely end up with figures between 13.0:1 to 13.5:1. Now consider the fuel economy difference between 14.7:1 and AFRs in the thirteens is the best part of fek all so in the pursuit of best drivability the slightly richer AFRs become an absolute no-brainer!
However while fuel economy is hardly impacted at all the engine will be way more polluting at 13.0:1 than it is at stoich and the life of your catalytic converters will be significantly shortened, this is why the 14CUX system tries to pin AFRs at 14.7:1. I recommend ignoring 14.7:1 and all the emissions bollix, ditch your cats then just go old school and give the engine what it wants
Why would you not shut fuel off completely on the overrun if you're not worried about pops and bangs..?
This thread a year ago discussed this on the 14CUX..:
https://www.pistonheads.com/gassing/topic.asp?h=0&...
This thread a year ago discussed this on the 14CUX..:
https://www.pistonheads.com/gassing/topic.asp?h=0&...
Dominic TVRetto said:
Why would you not shut fuel off completely on the overrun if you're not worried about pops and bangs..?
This thread a year ago discussed this on the 14CUX..:
https://www.pistonheads.com/gassing/topic.asp?h=0&...
I found things much smoother when getting back on the power with the fuel supply maintained on the over run. Not really an issue at higher rpm and higher load but noticed the difference on light throttle low speed.This thread a year ago discussed this on the 14CUX..:
https://www.pistonheads.com/gassing/topic.asp?h=0&...
carsy said:
Dominic TVRetto said:
Why would you not shut fuel off completely on the overrun if you're not worried about pops and bangs..?
This thread a year ago discussed this on the 14CUX..:
https://www.pistonheads.com/gassing/topic.asp?h=0&...
I found things much smoother when getting back on the power with the fuel supply maintained on the over run. Not really an issue at higher rpm and higher load but noticed the difference on light throttle low speed.This thread a year ago discussed this on the 14CUX..:
https://www.pistonheads.com/gassing/topic.asp?h=0&...
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