Pittnathan
Well-Known Member
Hi. I love the sound/look of my cobra slip ons. However I've been told they're costing me a good 15+ hp due to lack of back pressure. Is it possible to stuff my own baffles?
Can I creat backpressure on cobra straight throughs?
- Thread starter Pittnathan
- Start date
Help Support VMAX Forum:
They are costing you power due to the loss in scavenging NOT backpressure. No way to recover that lost power without shit canning them!
Sean
Sean
LongNap
Active Member
Interesting... I took mine off a couple of days ago and I thought I felt a increase in power just going back to stock.
BTW, I have a set for sale
Pittnathan
Well-Known Member
I think I've got a stock set kicking around I can put on. See if its worth changing my cobras or if it's not really a big difference stick with em. Could save me ?'s!
You'll gain 10-15 HP by putting the stock back on!
According to the people who have dynotested them, they are for looks and sound, not for performance. Why would anyone want to reduce power from stock, well, that's their loss & business.
Rusty McNeil
Well-Known Member
Harley guys do it all the time, but not as much as you used to see. There are still some guys out there that think if it's got a crossover it's not a true straight pipe, and "straight pipe" is what they hear about and want. Not even understanding the effects.
LongNap
Active Member
I like the stock much more then the Cobras!
I didn't realize the power gains before putting them back on and just thought the jetting was that far off. Makes sense now that I think about it, but I think people do buy them for looks and sound (not giving much thought to power) or they think more flow = more power?
I would put the stock back on... pretty easy to do and the bike runs so much better! IMHO
Now off to find my oil issue!
mgosset1
Well-Known Member
long nap is there cheap enough i might take em.....hell at my age if i lose 15 hp. don't bother me that much.....i got no business going that fast at my age......i would like to find a set for my bike, like the way they look....
Back pressure is a very mis understood concept. Sean hit it on the head, your losing scavaging effect. This is due to loss of velocity. Simple way to put, if you at the beach the breeze is lightly blowing 3-4 mph because it doesn't have to be any where and can take its time getting no where due to the wide open spaces. Go a block in from the water where there are tall building and that exact same wind is not going 10-12 mph because there is only so much space for it to flow through it has to "fight" and speed up to get into that space. Thats where your velocity and scavaging comes from. If you exhaust is too free flowing, the spent gasses dont' have anything to do but hang out and party and barf back into the cylinder, where there is proper sized pipe they party harder and are in a hurry to get to the other party at the end of the street and they all want to get there first because the toilet is clean and insted of puking in the nasty dirty toilet(cylinder) they have a nice free open place to puke when they get out of the end of the pipe. Make sense?
The best way to create backpressure in those Cobra Pipes is to use one of those 'Big Rigs' in that thread to drive-over them.
Marks uses a crossover pipe. The OEM also has a cross over built into the muffler set as well as front pipes.
Sean
Sean
tugla
Well-Known Member
kinda what i was thinking about, never got around to doing it, more as a test to see.
thor_eyerdhal
Well-Known Member
Back pressure is ALWAYS bad (except when dynamically controlled, see bottom example)!
If increasing back pressure add HP, the headers primaries are not matched to the engine.. ( probably too big in diameter).
Primaries diameter must prevent velocity loss.
Primaries and secondaries length will affect power band (scavenging effect boosted by a strong backpulse created at their end ). A short length will boost the top end power, a long length will boost the midrange or low end power.
When using full straigthru without X or H pipe, the secondaries length will be equal to the total exhaust line, it will boost low-midrange power, not topend.
When using X or H pipe, a back pulse is created shorter in the exhaust line, it will be better for high and top end power. And you can tune the peak boost by moving the crossover location.
When using stock Vmax mufflers, a backpulse is created in the first chamber => sooner than in some straigthru aftermaket exhausts. That's why max power can be better with stock exhaust !! (but low-mid power can be lower)
Big diameter primaries will kill power everywhere, except (may be) at high rpms, backpulses created by secondaries won't correct the loss and creating back pressure will be the only way to counterbalance this. So this is just a bandaid...
Exup system are made to use big diameter primaries (Vmax 1700). Top end power is very good, because big diameter primaries are used, low-mid power is preserved by backpressure (Valve partially closed)
If increasing back pressure add HP, the headers primaries are not matched to the engine.. ( probably too big in diameter).
Primaries diameter must prevent velocity loss.
Primaries and secondaries length will affect power band (scavenging effect boosted by a strong backpulse created at their end ). A short length will boost the top end power, a long length will boost the midrange or low end power.
When using full straigthru without X or H pipe, the secondaries length will be equal to the total exhaust line, it will boost low-midrange power, not topend.
When using X or H pipe, a back pulse is created shorter in the exhaust line, it will be better for high and top end power. And you can tune the peak boost by moving the crossover location.
When using stock Vmax mufflers, a backpulse is created in the first chamber => sooner than in some straigthru aftermaket exhausts. That's why max power can be better with stock exhaust !! (but low-mid power can be lower)
Big diameter primaries will kill power everywhere, except (may be) at high rpms, backpulses created by secondaries won't correct the loss and creating back pressure will be the only way to counterbalance this. So this is just a bandaid...
Exup system are made to use big diameter primaries (Vmax 1700). Top end power is very good, because big diameter primaries are used, low-mid power is preserved by backpressure (Valve partially closed)
Similar threads
