Large Baffles

[glider.]

In these HD engines, you need some back pressure for the engine to run right. Changing to a baffle that is larger than the header pipe (1 3/4") will reduce back pressure and create running problems like exhaust reversion.

Reversion is when a sound wave travels back up the pipe and hinders the exhaust flow and can cause fuel stand off which is fuel spraying back out the intake.
When you go larger than the 1 3/4" baffles in any muffler, you will also sacrifice bottom end torque too besides the other problems mentioned above.

 

[Sunnysideup]

The Samson dealer rep recommended their 1 3/4" baffle if I was to keep the stock air cleaner. This tells me your probably on the right track Mr. Data.

 

[PaPa T]

I have the rush 2" baffles in o9 Heritage. Have Tfi and Big sucker cari. So your saying I should ruduce the baffles to 1 3/4 for better performance and lest wear on engine?

 

[glider.]

I have the rush 2" baffles in o9 Heritage. Have Tfi and Big sucker cari. So your saying I should ruduce the baffles to 1 3/4 for better performance and lest wear on engine?

Not so much the wear on the engine but you need some backpressure for the engine to run properly. 1 3/4" pipes into 2 " baffles reduce the back pressure significantly and the low end torque will suffer as a result.

 

[PaPa T]

Mr. Data I ride like an old man. How can I tell if it is not running correctly? I thought it was running just fine. I do not get on it hard I just like to cruise.

 

[glider.]

It's not a HUGE difference but if you are "tuned in " to a bike or engine, you will see the difference.

It's basically the bottom end will be a bit "soft" and not "crisp" with throttle response when you have the larger baffles.

 

[rancid]

well this just grinds my gears, back pressure is a restriction to flow which = loss of power. if the ID of the baffles and header are the same size there is no restriction to cause backpressure, add to that a good 2-1 is about the best streetable pipe cos they create great bottom mid range and with time and testing good top end. now how does a 2-1 work? by directing gas flow across the path of the opposed cylinder it creates scavenge, which is??? ding ding ding... you guessed it.. vaccume (think painters spray gun as air moves across the face of the paint pick up, it sucks paint up.) the last engine i built had the current touring system with removed CAT and a smoother 2-1-2 and screamin eagle mufflers, it made 106ft lbs and 60 hp at 3000rpm and a peak of 119hp, then removed the screamin eagle mufflers which have flow restricting, backpressure creating baffles and fitted a set of S&S straight through 2" core mufflers, ran 110ft lbs and 62 hp at 3000 and 124 hp peak at 6300, engine temp was within 10 degrees run to run and correction was readjusted for atmospheric changes run to run. i sure know i want the system with less restriction.

 

[glider.]

ran 110ft lbs and 62 hp at 3000 and 124 hp peak at 6300

Using your own numbers here is the point that I have made a few times in regards to back pressure.
Lower back pressure is more effective at higher RPM's or race type applications but for a street bike that doesn't see those type of RPM's, the restriction is needed for other reasons.

Explain for me what reversion is caused by and the effects it has on an engine and if they are beneficial at all.

Thanks

 

[rancid]

Using your own numbers here is the point that I have made a few times in regards to back pressure.
Lower back pressure is more effective at higher RPM's or race type applications but for a street bike that doesn't see those type of RPM's, the restriction is needed for other reasons.

Explain for me what reversion is caused by and the effects it has on an engine and if they are beneficial at all.

Thanks

the figures at 3000rpm are low to mid rpm range, and the dyno run started at 2500, the less restrictive mufflers and header helped from right off the bottom, reversion is only really an issue on huge "show pipes" that have stupid shapes, on fancy choppers, aswell as on close of throttle it can cause popping. a reversion wave has huge benefit, if the exhaust is designed right the reversion sound wave (of no mass and not a high pressure wave) will block the exhaust port stopping charge loss. think about a 2 stroke, they live and die on timing the reversion wave, big fat expansion chamber high top end, long narrower expansion chamber = strong low mid range. no back pressure just good design, thats all you need.

 

[wilks3]

You never want backpressure, why would you? What you want is the exhaust sucking out the spent gases and drawing in the fresh gas/air mixture. But this takes "tuned exhaust", the correct cam overlap and right sized carb/TBI.
Real world, the EPA DOES NOT want ANY unburn't fuel leaving the combustion chamber.
So you get cams with little overlap and restrictive exhaust to keep some exhaust in the CC.
Its easy to slap some pipes on, RE-cal fuel and go. But, you still need the correct cam overlap, correct exhaust scavaging and right sized carb/TBI to be right.
As far as reversion goes, theres pages upon pages on this. Best reading is from Buddy Reher of Reher-Morrison Pro Stock fame. Their engines actually get 105-110% V.E.
wilks3
:USA