Yes...it's freakin loud. And does little or nothing for power.
Reverse cones are great if the meg is designed around them...if not then the reverse cone alters the way the meg functions quite a bit. In other words...a meg that is tuned nicely at 24" might be tuned correctly at 20" with a reverse cone.
I'm not sure of the relationship, but I know the reverse cone affects the overal dimensions of the meg, and can enhance or destroy hp at certain rpm ranges.
It's a very subtle science, I've noticed that a lot of modern GP bikes have a very subtle reverse cone...only a tad smaller than the largest OD of the meg.
But anyway...when I ran one I was over the sound limit at NHIS.
PS...a reverse cone still does it's thing even with trapp disks mounted to the rear of the cone.
I just reviewed some of my books on exhaust system design. Although there are formulas for just about every aspect of exhaust system design...that tell you how gas behaves in the system...you would be amazed how often these books, with all their science tell you that a truely effective design is often a matter of trial and error. There are a few hard and fast rules concerning how primary pipe ID and length effect power (torque) production at a given rpm range. But when it comes to hard and fast rules beyond the primary ( or straight diameter pipe) they just don't seem to exist. The theory behind megaphones is they basically slow the pressure wave, which prolongs the scavenging effect...picture it this way...if you can slow the wave and fit more waves heading out the pipe into the meg then their tendency to pull more gas out from the primary pipe is increased. However the down side is more surface area of gas to pipe...which produces friction and causes the waves to lose energy. All testing has shown that a properly designed megaphone will increase engine efficiency over a straight pipe...but a poorly designed meg will hurt power and produce less than a straight pipe. Ideally a meg causes the exhaust to give up most of it's energy trying to pull gasses out behind it...meaning a perfectly designed meg would probably have a fairly soft even flow of gas out the back rather than strong, pulses that knock your hat off ten feet away. Remember...the energy to produce those hair fluffing pulses ten feet from the meg has to come from somewhere...it comes from the engine...power that you'd rather put to use scavenging exhaust rather than blowing peoples hair around.
The final opinion of a book full of formulas is that megaphones are for the most part best designed by trail and error (with reasonable starting points of course). And when it comes to reverse cones...it says the resulting gas flow becomes so comples that there is no formula or theory that seems to encompass the wave dynamics produced and that there is no evidence that it produces more power than a properly proportioned megaphone. It may however correct and issue with a improporly proportioned meg.
In general the same old guidelines apply...short primary pipes and short megs enhance high rpm power ( because the higher frequency pulses need less length of pipe to fit in) and longer primary pipes and megs enhance lower rpm power (less frequent pulses need a longer pipe to fit the same number in).
But then...what are you designing for? A dyno test or a race? A short system may provide higher peak hp numbers at the upper range of rpm...but produce poor power at low rpm. So for instance...take a high reving 200cc engine....it has very little power at lower rpm ranges....so, dispite the fact that it's a high revving engine...you put a long system on it to help it where it needs the most help...in the lower rpm ranges. The longer system hurts high rpm power less than it helps low end power....for a net gain of area under the hp/rpm curve. Which should always be your goal...not the hp peak, but the area under the curve...the total area under the curve represents your usuable power during a race.
Strangely enough in the old days it was popular to run straight pipes about 8" long ( look at some old photos of bonniville machines). Why? because very short primary pipes don't help make power...but...they also do nothing to hurt power...so at the least the tuners knew where they stood as far as the exhaust system went, and they could concentrate on other aspects of engine design.
quote: ....for a net gain of area under the hp/rpm curve. Which should always be your goal...not the hp peak, but the area under the curve...the total area under the curve represents your usuable power during a race.
You said a mouthful there. To me area under the curve is the simplest, most important, least understood concept in engine building. Most of the time when I talk in terms of area under the curve I get that deer in the headlights look.
I can't contribute as much to this as JohnnyB but have used both end caps- on diffrent bikes. Both my Hondas use the closed end caps. Running a more open pipe made them lean out hugely at WOT and if you jetted up to try and match it the plugs would get soaked everywhere else. And they were ear piercing loud, not the good MotoGP loud.
My BMW sidecar is using a 2-1,Branson meg, and the open end cap with about 6 disks. The dimensions for the complete system were given to me by the guy who built the Butler and Smith R90S superbikes. Due to whatever combustion chamber shape BMW uses running them pretty much open seems to make good power and isn't as loud as my CB350 was.
Zacks dads BMW solo bike was alot louder than my solo bike was but maybe it had to do with his reworked heads vs. my stock heads. And it seemed his throttle could open further than mine
Thanks guys, thats a lot to digest over coffee. I asked because I ordered a reverse cone end cap, didn't mean to, may have been a 2am internet purchase? Anyway it came in the mail and I realized this may not be a good idea.
Now I need to order a closed end cap.
What sort of effect would a closed reverse cone have? I'm welding stainless day in and day out, popping a cap over the open cone would be easier than ordering a new one!
Rosko....good question...I did that once but I welded in a little plate on the cone...before it's a cone...if you know what I mean. On the inside not the outside. Where the cone starts on the inside.
Same reason as you...just wanted a solid end cap and the cone was all I had lying around. Welding up the cone at the outside end might do some funny things.
The symptoms you describe are very typical of megaphone-itis. The reverse cone probably shifted the hp up to a very narrow range, high up in the rpm. Probably was making good power up there...but every where else it was creating all kinds of reversion pressure pulses that try to shove exhaust back inside the cylinder, which creates a very rich condition...not to mention destroying low end power.
Mary's little Honda once had a case so bad that it would run like a bat out of hell between about 10,000 and 11,000...but would literally barely run anywhere else.
At one point I had a copy of a paper that was written by a group of engineers from the Honda factory back in the 60s. Titled something like "Effect of megaphone exhausts on small displacement four stroke engines". Went into a lot of explanations on what did and didn't work well for their bikes. If anyone knows anybody that's a member of SAE they should be able to order a reprint of it. Otherwise I'll keep trying to find my copy.
It is also interesting that the 500 single guys sometimes put that little inversion cone at the ENTRANCE to the exhaust pipe coming right out of the head. I am going to go and do a dyno pull to see what it does . . . it supposedly keeps the high rpm power in and helps out the low-end torque. Probably a compromise.
Really? I'd think any restriction around there would have seriously detrimental effects on the engine's ability to evacuate heat.
I once stuffed a pair of unwrapped 4" baffles into the tops of my headers, the theory being, fuckit, let's see how that affects the sound and power out of this thing. I made it about a half mile down the road before I noticed the headers were GLOWING. It really must've been an amazing sight for the cars around me. They'd also expanded a good half centimeter. Keep in mind, this is with baffles that have a 1" passage clear through the middle - relatively little restriction. It's truly amazing my head made it through that bit of genius.
Evil...that's called an "anti-reversion cone" and was all the rage in automotive headers for a while. Supposedly keeps the reverse pressure wave from entering the exhaust port. In theory a good idea, and if I remember right there was dyno evidence to prove it's worth.
Craig... I have that paper somewhere. Very complex piece of research.
it exactly is called an anti reversionary cone,the idea like branson says is to keep gasses going back down the cylinder in the milli seconds its opened and to keep the gasses from going the path of least resistence back toward the head in stead of toward the exhaust out the back .Keeps the engine from having to push that gas down the tube the next exhaust cycle.It has provin on the dyno many times to be worth some power across the band width.
And the reason I think you see it on big singles and on automotive engines is that their rpm range is low enough that it's a benefit. On higher revving engines I think they have enough pulses in the pipe at the same time that's it's not necessary or not as benefitial...although it does seem I read that they are sometimes used in formula 1 systems.