Confirming Some Assumptions About Back-Pressure

[raysigh]

Hey guys,

I know if you run an aftermarket exhaust, you need to get it tuned for a proper AF ratio. 

However, is my assumption correct, that I can baffle my open exhaust to a point that won't be damaging my valves because I've made the mixture rich enough? 

I understand this would almost certainly NOT give me any performance, in fact would probably reduce my performance, as this is not a tuned procedure with any kind of data. This is more a question on whether or not you can prevent the excess heat and possible damage from an overly lean mixture JUST by baffling. (This includes the options of adding multi-stage baffles to increase that back-pressure.

[WeTrolling]

12 minutes ago, raysigh said:

Hey guys,

I know if you run an aftermarket exhaust, you need to get it tuned for a proper AF ratio. 

However, is my assumption correct, that I can baffle my open exhaust to a point that won't be damaging my valves because I've made the mixture rich enough? 

I understand this would almost certainly NOT give me any performance, in fact would probably reduce my performance, as this is not a tuned procedure with any kind of data. This is more a question on whether or not you can prevent the excess heat and possible damage from an overly lean mixture JUST by baffling. (This includes the options of adding multi-stage baffles to increase that back-pressure.

I have the Project SC CRT without a tune and it runs like ASS. Run a tune no matter what. 

[raysigh]

2 minutes ago, WeTrolling said:

I have the Project SC CRT without a tune and it runs like ASS. Run a tune no matter what. 

Yes, as I mentioned that is the understanding. My question is whether or not damage can be prevented (ie. rich enough not to overheat) just by baffling.

[raysigh]

It will DEFINITELY run like ass. But again, not the question.

[WeTrolling]

When I talked to 2wdyno they said I could run it temporarily but would not recommend it over time. From experience my bike def runs hotter.

[klx678]

21 hours ago, raysigh said:

Hey guys,

I know if you run an aftermarket exhaust, you need to get it tuned for a proper AF ratio. 

However, is my assumption correct, that I can baffle my open exhaust to a point that won't be damaging my valves because I've made the mixture rich enough? 

I understand this would almost certainly NOT give me any performance, in fact would probably reduce my performance, as this is not a tuned procedure with any kind of data. This is more a question on whether or not you can prevent the excess heat and possible damage from an overly lean mixture JUST by baffling. (This includes the options of adding multi-stage baffles to increase that back-pressure.

From what I have learned over the years, valve issues will have nothing to do with backpressure (reflected waves back to the exhaust), but rather leanness.   

If you stuff up anything enough it will run rich, unplug it too much and it will run lean, but that leanness can be compensated for through jetting with carbs and the ECU with EFI bikes.    You just don't want to run too lean or you will burn exhaust valves for sure if they get hot enough.

Backpressure is actually reflected waves in the exhaust system.  The exhaust system is designed to reflect waves back, pushing back against the exhaust flow.   The idea of a proper tuned exhaust is that it will reflect a pressure wave back up the exhaust hitting the valve just in time to push the escaping fuel charge back into the cylinder.  Miss-timing of said waves will allow unburned fuel to escape and less mix in the cylinder.  When correctly timed over a certain range it actually can have a bit of a supercharging effect, ramming unburned fuel mix back in the cylinder.    Two strokes are really sensitive to backpressure, the designs of expansion chambers time and magnify the pressure waves.  Reverse cone megaphone exhausts have similar effects on four strokes.  Even the MotoGP bikes have mini megs on the exhausts.    They are somewhat tuned in design to reflect back at proper timing.  Same with tuned exhaust lengths. 

[raysigh]

19 hours ago, WeTrolling said:

When I talked to 2wdyno they said I could run it temporarily but would not recommend it over time. From experience my bike def runs hotter.

Was this with a baffle or without? 

[raysigh]

2 hours ago, klx678 said:

From what I have learned over the years, valve issues will have nothing to do with backpressure (reflected waves back to the exhaust), but rather leanness.   

If you stuff up anything enough it will run rich, unplug it too much and it will run lean, but that leanness can be compensated for through jetting with carbs and the ECU with EFI bikes.    You just don't want to run too lean or you will burn exhaust valves for sure if they get hot enough.

Backpressure is actually reflected waves in the exhaust system.  The exhaust system is designed to reflect waves back, pushing back against the exhaust flow.   The idea of a proper tuned exhaust is that it will reflect a pressure wave back up the exhaust hitting the valve just in time to push the escaping fuel charge back into the cylinder.  Miss-timing of said waves will allow unburned fuel to escape and less mix in the cylinder.  When correctly timed over a certain range it actually can have a bit of a supercharging effect, ramming unburned fuel mix back in the cylinder.    Two strokes are really sensitive to backpressure, the designs of expansion chambers time and magnify the pressure waves.  Reverse cone megaphone exhausts have similar effects on four strokes.  Even the MotoGP bikes have mini megs on the exhausts.    They are somewhat tuned in design to reflect back at proper timing.  Same with tuned exhaust lengths. 

Alright then I guess my assumption is OVERALL correct, although the caveat is dependent on the timing of the reflected pressure wave - which can only be determined (I assume), by a dyno run, or if it has been completed before like an akropovic for example. 

However, it also does sound like it is a SAFER bet to plug it vs letting it run more open - until time that I can get my ECU flashed. Based on the shape, I assume a straight pipe design would have the least pressure wave, and would require more plugging. I currently have 3 baffles in my DanMoto XG-1 (1 in each of the two tips, and 1 in the midpipe). 

I kind of ghetto adjusted the baffles by wrapping exhaust wrap up to different lengths, starting with the most aggressive baffling (leaving 1 set of holes at the tip of the baffle). I rode my friends stock FZ07 back to back to compare, noticed a low-mid range power decrease, so I opened up the next set of holes to which point the performance "felt" the same.

If my previous assumptions are correct, I am running this exhaust on the RICH side as opposed to the lean side. 

Was considering the FTecu tune that JakeTheSnake used in the future.

[klx678]

First understand my comment is only to get the whole "backpressure is needed" thing out of the picture.   Sure it is part of the situation, but the actual problem is the fuel mix, not the exhaust system.   If not, then even with a reflash the exhaust would have to be "plugged up" to get the higher backpressure or should we say the proper wave reflection for proper operation.   That isn't the case though.  The ECU is the problem after installing the exhaust.  That was the only reason I posted what I did.   There's a lot of misconceptions about exhausts and the "back pressure" thing is one of them.  Believe it or not even a straight pipe has the reflected wave patterns based on length of primary and secondary tubes.  But I will also say I am far from all-knowing in the area of exhausts.   

I would say you may be doing the right thing putting in the baffle, it probably is a bit better running with the baffle to bring it closer to the OEM level of flow.  One other thing would be to contact the manufacturer and ask if there need be a reflash of the ECU to run a different mix.  If they say no, it isn't needed or no it isn't needed if running the baffle, you're good to go.

I picked up a Delkvec exhaust with the longer 18" cannister, specifically because they stated it could be run without a reflash.   I am doing the exhaust for aesthetic reasons, not performance.   Looks and sound, hopefully not too loud of sound, but I did pick up the "quieter baffle" as well as the  one in the pipe.  I don't really want to have to reflash the ECU and I don't really like the look of the stock end cover on the XSR.  It is about what I want to see and hear.  If I gain a bit of performance, fine, but that wasn't my reason.

[raysigh]

4 hours ago, klx678 said:

First understand my comment is only to get the whole "backpressure is needed" thing out of the picture.   Sure it is part of the situation, but the actual problem is the fuel mix, not the exhaust system.   If not, then even with a reflash the exhaust would have to be "plugged up" to get the higher backpressure or should we say the proper wave reflection for proper operation.   That isn't the case though.  The ECU is the problem after installing the exhaust.  That was the only reason I posted what I did.   There's a lot of misconceptions about exhausts and the "back pressure" thing is one of them.  Believe it or not even a straight pipe has the reflected wave patterns based on length of primary and secondary tubes.  But I will also say I am far from all-knowing in the area of exhausts.   

I would say you may be doing the right thing putting in the baffle, it probably is a bit better running with the baffle to bring it closer to the OEM level of flow.  One other thing would be to contact the manufacturer and ask if there need be a reflash of the ECU to run a different mix.  If they say no, it isn't needed or no it isn't needed if running the baffle, you're good to go.

I picked up a Delkvec exhaust with the longer 18" cannister, specifically because they stated it could be run without a reflash.   I am doing the exhaust for aesthetic reasons, not performance.   Looks and sound, hopefully not too loud of sound, but I did pick up the "quieter baffle" as well as the  one in the pipe.  I don't really want to have to reflash the ECU and I don't really like the look of the stock end cover on the XSR.  It is about what I want to see and hear.  If I gain a bit of performance, fine, but that wasn't my reason.

Ohh ok I see where you're coming ok. I doubt DanMoto 

I doubt DanMoto has tuning info but I guess I could give it a shot. But also, the baffles I'm using are not factory supplied (their baffles are barely anything). It'd be worthless at this point.

Been trying to message this guy for some info with no luck haha

[shinyribs]

20,000 miles and counting with an Arrow system and a stock ECU.  No issues. With the baffle out it's a straight thru pipe. Nothing could be less restrictive. 

 

None of the aftermarket exhausts give any appreciable power gains, therefore we can determine that the stock system was not "choking the engine" as we might assume. 

No extra power gains = no extra air flow = no lean mixture. 

 

[mjh937]

31 minutes ago, shinyribs said:

No extra power gains = no extra air flow = no lean mixture. 

 

I am not sure that statement is correct.  If the stock tuning is on the lean side (which I believe it is) then running it even leaner would reduce power.

[shinyribs]

The stock tuning is fine. The bikes run cool and reliably for lots of miles with no issues. They are clean, efficient engines. That doesn't mean they are lean. 

What I said is aftermarket exhausts give practically no horsepower gains, which points to no real air flow gains. If the flow hasn't increased, why do you need more fuel? 

The stock exhaust doesn't restrict the engine at all, it's just whisper quiet. A testament to the engineers involved. 

 

Edited November 19, 2020 by shinyribs

[raysigh]

4 minutes ago, shinyribs said:

The stock tuning is fine. The bikes run cool and reliably for lots of miles with no issues. They are clean, efficient engines. That doesn't mean they are lean. 

What I said is aftermarket exhausts give practically no horsepower gains, which points to no real air flow gains. If the flow hasn't increased, why do you need more fuel? 

The stock exhaust doesn't restrict the engine at all, it's just whisper quiet. 

 

That... is a good point? Are you saying that any adjustments are negligible to any detrimental effects to the engine? 

[shinyribs]

13 hours ago, raysigh said:

That... is a good point? Are you saying that any adjustments are negligible to any detrimental effects to the engine? 

I believe a stock bike running a stock tune is perfectly safe. According to manufacturers who sell and dyno aftermarket exhausts, they seem to be worth 2-3hp on average. There's that much fluctuation between dyno pulls without making any changes. Since there's no real performance to be gained with aftermarket pipes it seems likely to me that there are no real changes to the amount of total air flow through the engine. 

 I'm not anti- aftermarket exhaust. I have one simply because I liked the looks and sound of it. I'm not guaranteeing everyone can run any aftermarket exhaust without  looking at the tune. Judging from the conversations regarding exhausts available for this particular bike, the guys with tuned ecu's typically take a big hit on fuel mileage, and the numbers show there just isn't any extra power to be had.

It's hard for me to preach that folks MUST flash ecu's to go along with their exhausts because of that. That's all I'm saying. Every system can differ and needs to be looked at on a case by case basis. There may be a system out there that really pumps up the power and really needs more fuel to accommodate it. I haven't seen that, but that doesn't mean it can't exist.