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Exhaust "Pops & Bangs"

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Hello Everyone!

I am new to EFI tuning and have never posted in the forums before, but I am extremely interested in learning as much as I can from the community. I've been rolling over the thought in my head of how to tune in "pops and bangs" into the exhaust when someone revs the engine or switches gears. I think I may have figured it out, but wanted to consult everyone here.

I know that the general concept of how to do this is to disable the deceleration fuel cut off at the RPM limit you want the exhaust bangs to start, and then to also retard the timing so that the combustion happens while the exhaust valve is open. The part I was trying to figure out is where/how to retard timing without sacrificing performance.. but I think I may have figured it out! I should retard timing in the high RPM + low load areas of the maps, correct? This part of the map should really only be accessible when you let off the throttle after revving, or gear shifts, right? Hopefully I can get some insight from you all, and hopefully I'm not too far off :).

Thanks!

Hi,

yes you got the right idea, its usually around 0-20kpa area of the map, you are not gonna sacrifice any performance doing this, if you remove timing around your cruising area your egt will be higher and your fuel economy will be worse

Ah thank you Fahad! Glad I am on the right track. Thanks for the note about egt; I knew this would be an issue, especially if the exhaust still has cats. Do you know how much timing I should try removing? Or just start removing timing until it happens?

it depends on the engine and how aggressive you want it, you can start from 20 degree to -20 degree maybe -30 if you are pushing your luck, at -5 its when it really start to bang and that is too aggressive for a daily driver on the street, my sweet spot is between 20 and 5 degrees.

afr richer than 12 makes it pop better but it will read false lean anyway so dont add too much fuel you might flood the engine.

I can't thank you enough Fahad, this is perfect information! Exactly what I was looking for, thank you for your input.

I've been thinking about this some more, and wouldn't I want to give a greater spark timing number not smaller? A smaller number has the combustion happening sooner, which is further away from the exhaust stroke. Wouldn't I want larger timing numbers in the high rpm + low load areas in order to have the combustion happen close to when the exhaust valve starts opening?

you got this one backward i used to think the same, too much advance (larger value) will backfire in the intake, too retard (smaller number) will back fire in the exhaust.

when you add timing (advance) you are making combustion earlier you give it time to combust inside the the engine which increase cylinder pressure when you reduce timing you are making the combustion at a later point (retard) maybe even with the exhaust valve open which decrease cylinder pressure.

keep in mind there is two different terms before top dead center (BTDC) and after top dead center (ATDC) this is what got you confused.

Could you possibly take a look at the spark table I have attached? Wondering how you would alter the numbers in this table in order to achieve what we are talking about. I'm hoping maybe then I will better understand what you're describing. Thanks again!

Attached Files

i have highlighted the cells that might help, you can try making it 0 for start and interpolate around it to smooth the timing change.

another thing datalog the spark airmass on deceleration to find the right row as the first two rows might not be enough

Attached Files

Okay yes that is the area that I thought the changes should happen, so that's good. I agree that it might take another row or two, but we will see with the datalog where the air mass goes.

I think maybe the disconnect I'm having is that maybe your numbers previously stated are referring to BDC? My spark table shows negative numbers for BTDC, and positive numbers for ATDC. So if I make the numbers smaller, that would increase my cylinder pressure and maybe cause some knock to occur. But if we are talking BDC, then the smaller number would make sense as it is closer to the exhaust valve opening.

Sorry for all the questions Fahad! I think we have the same concept in our heads, just are saying it different maybe.

Attached Files
  • Valve-overlapping-in-term-of-piston-position.png
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