Exhaust querys

[Mikekitts81]

So I bought a second hand full akprovic system for my wr450f on arrival it wasn't in the best conditions like the ebay pictures showed it to be... The description did say a few scratches and dings but nothing serious.

Anyway on arrival there is a slight ding and knock on the exhaust that shows a dent in the header pipe internally that restricts flow .

So iam wandering if there is a way that one can reshape the header so it pops back out so I have a clear flow of air again or if it's just gonna be a dremmel and weld job to fix the problem. ?????

[Richard Simpson Mark II]

Probably depends on the shape of the dent.
You could try sealing the pipe at one end, filling with water at the other, sealing that end, and sticking in the freezer. Seen this done with mild steel expansion pipes on two-strokes.

[Mike Wright]

[daveuprite]

You can also get these blowing out kits, but I don't know if they're any good:

https://www.ebay.co.uk/itm/Tag-Z-Exhaus ... SwqjhcNOhM

[Mikekitts81]

Cheers guys will try both see which works... Don't suppose you can help my other issue full of them today went in garage checking bike over to a big hit on bash ate last time out and it's destroyed the bolts and plate... Anyway my question is I Got a second hand exhaust today looks like the packing hasnt been changed since it was new that's all that was left in it. anyway My exhaust header on the bike is 50mm straight through to silcener. But My new full system which is a aprovic system as a smaller head for the first quarter of the header and than opens up to 50mm for the 2/3s left that then goes to the silcener. So do we have any exhaust gurus in here Cause wouldn't it be better leaving the 50mm striaght through header on rather than The narrow to same diameter header or is this part of the experience havi g a different power curve like in the 2 strokes .???????

[catcitrus]

Cheers guys will try both see which works... Don't suppose you can help my other issue full of them today went in garage checking bike over to a big hit on bash ate last time out and it's destroyed the bolts and plate... Anyway my question is I Got a second hand exhaust today looks like the packing hasnt been changed since it was new that's all that was left in it. anyway My exhaust header on the bike is 50mm straight through to silcener. But My new full system which is a aprovic system as a smaller head for the first quarter of the header and than opens up to 50mm for the 2/3s left that then goes to the silcener. So do we have any exhaust gurus in here Cause wouldn't it be better leaving the 50mm striaght through header on rather than The narrow to same diameter header or is this part of the experience havi g a different power curve like in the 2 strokes .???????

That does seem a little odd--I can understand a separate Helmholz chamber to resonate and send a filling, positive wave back at lowish rpm to push some fresh charge back in (the canister on the side)--overcomes large valve overlap designed for power at high revs where you would otherwise lose fresh charge down the header before the exhaust valve closes (hence the advantage of VVT). The change in section will produce some form of reflection--particularly as actual gas speed reduces significantly down a parallel header as the gas cools (thats why on proper silencers the outlet can be so small as the gas has cooled to the point that you can reduce the csa by at least 1/2 and maintain gas velocity). IMHO Akras are vastly overpriced as they are simply straight through with a perf tube and absorption material that is simply only good for high frequency sound attenuation(think of the stuffing as as a series of interlinked very small expansion chambers)--not effective on the big pulses from a 4 stroke--for that you need tuned chambers and some reverse flow--like OE!

[Richard Simpson Mark II]

Excellent post, Catcitrus.

My old DR350S had a nearly silent exhaust because it was tunes so the reverse flow sound wave matched troughs with peaks on the primary flow.

If you disrupted the exhaust flow with your hand, it got much louder.

I once hd an interesting conversation with a busy-body about 'noisy motorcycles'...I pointed out that we were having a conversation and could hear each other perfectly well.

"But your engine's not running,"

I point to rev counter..."Yes it is"

[Richard Simpson Mark II]

Double post

[Toe]

I'd personally be inclined to go for the full Akrapovic knowing actual R&D goes into their systems, unlike many many other aftermarket manufacturers that have a silencer and make headers to fit them.
OE are built to pass noise tests and help with emissions, which they are optimised for. Aftermarket offers lighter materials, usually cooler temperature silencers, and will be more optimised for non-standard engines (mapped/jetting, etc).

Saying that I'm by no means an expert!

[Hugh]

Greetings,

Exhaust tuning is a bit of a" black art" requiring very sophisticated and expensive testing facilities. Changes to the shape and dimensions are a compromise depending upon the operation of the engine, in other words there is an optimum operating range and at either side of that the exhaust compromises torque and power output.

I have known exhausts get dented at a trials meeting and the bike's performance behaviour changed, some times for good and sometimes for worse

Back in the good old days of open megaphone exhausts you could watch racers come out of a corner and stick their boot over the exhaust outlet because of what was termed 'megaphonisis', in other words low down power and torque had been compromised so as to give performance at the top end. A twisty circuit causes manufacturers all sorts of problems for engine tuning especially if they do not have access to all the modern day 'electrickery'.

Back in the 60's Phil Irving wrote a book called "Tuning for Speed" published by Temple Press, another good book was "Two-Stroke motor cycles and how to get the best from them" published by Iliffe& Sons Ltd. There are other books of course but these two are perhaps the easiest to read. Sorry don't now where you might buy a copy other than trying the internet of perhaps Abe Books.

TTFN

Hugh.