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Slightly Unstable
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Discussion Starter #1
i know people have cut down their cans to make them shorties before and im debating doing this to my akras. being that i havent done this and i dont want to **** them up does it have to be done on one end or the other? its been many years since ive repacked an exhaust; and even then it was an old crappy d&d, and i typically change bikes so frequently i never kept one long enough to need repacking. my thoughts are to drill out the rivets at the bottom end of the can and pull it apart cut off the inner core as needed (probably doesnt matter top or bottom) then cut the cf with a nice hacksaw blade to make a clean cut. repack the can while its apart and rivet it back together at the bottom end. my reasons for the bottom end is that the carbon fiber is slightly discolored at the bottom end (on the bottom of the can that you cant see by eye which i imagine is do to the heat of the rc's exhaust) so id rather remove that portion than the upper portion. and being that its a carbon fiber sleeve does it have (or need) additional reinforcing where the end caps rivet to the sleeve or can i just pre-drill nice little holes for the rivets and not worry?
 

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TOP GUN Instructor
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Hey bmfgsxr,

NOOOOOOOOOOOOOOOOOOooooo :( Please DON'T cut & shorten Your Akro's! :banghead

:twocents Just go snag yourself some shorties from Mark or Simon, again. :twocents

Did I say... NOOOOOOOOOOO Please don't cut those!

Shoot man,.... You could sell them and buy the GP canisters you like!
 

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Slightly Unstable
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496 Posts
Discussion Starter #3
i know, ive been kicking around the idea.. even had a couple posts back and forth with mark in the classifieds about this. i kinda dont want to be bothered with the hassle of selling shipping, etc... although i know it would be a bit of a sin to cut down akras...
 

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Are your cans just starting to look too long for your bike or did they suffer damage or cracking due to age?

for someone with the proper tools (band saw to make a clean accurate cut, drill bits to take the rivets out, rivets and rivet gun and repacking material) it's EASY!

But as mentions earlier, you are killing the resale on cans becoming harder and harder to find. Take an inch off and they will look the same, take 6 inches off and they will look phuct up to you and everyone else that looks at the bike.:twocents
 

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Akrapovic, as well as other quality manufacturers, spends a lot of time and money designing and testing their exhaust systems to insure best response/performance. Their engineers create a "mathematically correct" system based on the calculated volume and flow characteristics, then can easily run the system up on a dyno 100 plus time making minor modifications in tube length and volume to achieve the best response and performance under a variety of conditions. Hacking off a few inches might make it look better (individual taste) or sound better (louder) but could easily cost you 10-20% in performance.
 

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+1 for whitw wings and guru lou.
 

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No need to reinforce the ends, I shortened a CF can on my old R1 with no issues. I wouldn't predrill holes though... it's easier to not screw up if you insert the end caps, mark the holes, and drill with the cap installed. But I would sell the Akras before I cut them down.
 

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Slightly Unstable
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Discussion Starter #11
they are just looking too long for me, although now im looking at just selling them and picking up a set of the motogp style cans (if i get the $ i need for the cans).

gurulou, i realize that akra spends the $ and r&d to get the best compromise, of power, sound, overall performance and im guessing some sort of noise levels but i could be way off base.

so my question would be this, if these were cut by say 4-6" couldnt the bike just be re-tuned for optimal performance (a/f ratio)? and really, this would be just a bit less restriction and by richening up the mixture you could possibly flow a little more air through the motor and hence increasing peak power (while possible losing in the low/mid range). afterall, it is a slipon which are typically designed for street bikes and i would guess (just using my own logic) that most aftermarket exhaust manufacturers create their slipons for a good balance of power over the entire rev range. i also realize its an aftermarket exhaust and by simply adding one these bikes are typically non compliant with epa regulations but i would think that to some degree they make these cans to at least be under certain decibel levels and that must be one of their compromises when designing these systems.

at this point i dont see me actually cutting these down, but it is an interesting topic on performance gains/losses when doing something like this.
 

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You've brought up some good points and I have to say that I don't know and it would depend how much is cut off, and would be truly hit and miss from there.

Here are a couple of examples from experience:

1. Many years ago I attended an advanced dyno school at the DynoJet facility in Las Vegas. At the same time, some of their personnel were developing new maps for some of the Yoshimura exhaust systems that were coming out on the new model year bikes. Yosh personnel were also there with some "engineering test parts". (I'll add that I do not know if these engineers were from the manufacturing side of things or from one of the race teams.) They were testing exhaust headers of different diameters and mid pipes that differed in length by about 1/2" increments. There were noticeable differences in where the power peaked out and how much peak power they got out of the system.

2. Our shop gets a number of race bikes in from people that use the track at Loudon. A lot of racers use Hindle or M4 exhaust systems for various reasons, mostly because they see other, faster racers using them. I will give credit where credit is due and admit that the design of those systems yield good peak power, but they all have some severe "holes" in hp and torque in the mid-range. Their "seat-of-the-pants dyno" told them that the systems were great because they could feel the increased acceleration; what the dyno showed was that the power curve was in the middle of one of these "power holes" and the increased acceleration they were feeling was happening when the slope of the power curve increased to try to catch up to where it should have been in the first place.

No amount of tuning could get rid of those "holes". Part of the design of an exhaust system deals with extraction of gasses from the exhaust port. If you don't have the length of the pipe correct you'll get an increase in back-pressure and less power or, conversely, you can get too much extraction and the engine doesn't have sufficient design to draw in a full compliment of replacement air/fuel mixture or you lose some of the heat of residual gasses which helps the travel of the flame-front during the combustion process.

In fact, designing in increased back-pressure at a specific rpm is one of the design tricks that manufacturers use to meet EPA noise regulations. Those tests are done at a specific gear and a specific speed. Thus, the factory ECUs are programmed to get good gas mileage and reduce noise and emmissions at those specific test parameters (something like top gear and 40 mph), thus introducing a designed "hole" in their performance curves (usually a flat spot on a dyno curve). That's where the aftermarket companies like Yoshimura and Akrapovic attract a lot of customer base and DynoJet/Power Commander allow you to re-tune the ECU for better performance and protect the engine from running too lean at high power settings.

Sorry for the diatribe, but there's a lot going on. But the short answer to your question is maybe/maybe not.
 

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Slightly Unstable
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Discussion Starter #13
well this is a far more productive conversation that our last one for starters. ;)

im going to attach my dyno sheet from my bike with the pair system removed, flapper removed and soft rev mod installed, i also have the akra slipons and k&n filters. there is still a decent dip around 5k which i would attribute to the header design at this point for the purpose of epa requirements, etc... (just a guess).

now this brings me to my next question about your thoughts on the motogp style cans for the rc's. obviously they are going to be far less restrictive than the akras or any other full size slipon so i wonder if that is going to make the hole at around 5k worse and would the potential peak hp gains be worth it (this is all personal preference really, but im just putting my thoughts out loud). now, with this said how would i get rid of this dip i have at 5k? im guessing a proper header would do this?

 

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Let's put that last conversation behind us and just agree that we'll never come to an agreement about that....

That's a pretty bad hole and is an example of the kind I would see in a Hindle or M4 exhaust. As you can see, if you're a racer and you never let the engine get below 5500 rpm, it would never make a difference to you. It makes decent top end and is nice and linear above 6K rpm. However, I have never seen that type of curve on an Akro system.

When I make dyno runs on street bikes, I feed full throttle in at 2500 rpm and watch the results. Not having made this one, and not knowing anything about the operators at that site, it almost looks like they had the bike at 3k rpm, punched the throttle open and started the program recording at approx the same time. The sudden opening of the throttle bodies could have leaned out the mixture and had the engine bog down on you. You might also have a bit of a vacuum leak, which would have exacerbated the problem.

I know the weather is getting cold, but did you get a chance to ride the bike since you had the dyno run done and have the opportunity to just open the throttle from 2500 rpm and hold it wide open until it hit 7k? If so, did you feel that hole in the power delivery?

The last thing I noticed is that they recorded this in uncorrected hp. Not a big deal, but SAE is the standard since it corrects for temperature and baro pressure differences. In your case, the SAE reading would be about 122.3 hp.

Do you have a power commander on this bike. I'm assuming that you do since the fueling looks as good as it does.
 

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Slightly Unstable
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Discussion Starter #15
i did get to run it at one event prior to packing it up for the season, although i dont race anymore i only do trackdays (having a wife and 5 kids doesnt afford me the time i had when i was single). it felt like a considerable improvement from where it was prior to the tune and the on off throttle transition is much better/smoother than before. i have to dig up the corrected before and after overlay but that sounds about the right ballpark. i gained about 4rwhp and 2ftlbs overall from just a tune. bike has the pcIII usb with stock headers and akra slipons. you mentioned a vaccum leak, since i removed the pair system where may a leak be coming in from? btw, the stock curve was virtually identical iirc. bike has 14k for miles and fresh plugs prior to the tune.
 

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Slightly Unstable
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Discussion Starter #16


here is the corrected chart with before and after overlay.

what are your thoughts on the motogp style cans, and have you ever tuned an rc with a pair?
 

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the core size is the main factor in the power the can makes not the packing material


they don'd spend millions on R and D on cans. take a akra can from 2001 off a R1 and then take one for last year off a suzuki and they are identical for core size and length. the perf holes may be different but after that there is packing to insulate the sound and sleeve there is not mcuh else too them.

the shorter the can the louder but not the least HP

if you are concerned about torque loss make sure you have a DB killer and or smaller core. I offer smaller core mufflers custom in 5 different corte sizes if that is a worry too

there is a few dyno runs of various GP cans for our bikes and i have yet too see one lose HP. try a search on here i am sure you can pull some up

there are plenty of R1 's with full exhausts and GP cans and all of them made HP too
 

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The MotoGP style cans are going to be loud may or may not have a positive or negative effect on power. Since you're limiting yourself to track days you may want to find out if the tracks you go to have noise limitations on those days. If so, you may want to get some mufflers that have provisions to insert some king of bolt in baffle or restrictor so that you would remain in compliance.

As for that hole in your dyno curve - thanks for posting the original run. That hole could also be caused by the fuel mixture going very rich or something going on with the timing.

As for the vacuum leak, besides the PAIR system, check to see if one of the hoses pulled off for the flapper might be open or leaking, or if any other vaccum hoses got pulled loose from raising and lowering the fuel tank.
 

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Slightly Unstable
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Discussion Starter #20
im in good shape regarding restrictions at the tracks i go to so im not concerned about that.

ill have my bike completely naked this winter as usual so ill check all vacuum lines. thanks for the tips.
 
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