Slash cut exhausts... can someone clear this up?

[Jayy]

I know people on YouTube talk a load of shit in the comments but I repeatedly see comments on slash cut exhaust videos saying something like, "There's no back pressure and that's bad for the engine" or "It will burn the valves out quicker".

How true is any of that?

I've had a similar very small tip on it for months on end, didn't notice any performance problems.

[alextg]

As far as I know only 2 strokes required a bit of back pressure to keep them running optimally. I don't think it wil have any effect on a 4t engine, the exhaust is simply designed to direct noxious fumes away from the rider isnt it?

[Bezzer]

Slash cut really just means the style of the exhaust end ie. cut at an angle not straight, it's what's inside that governs what the exhaust is doing.
A straight through exhaust is a straight through exhaust with next to no back pressure slash cut, megaphone or whatever other style end.
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I used to be indecisive but now I'm not quite so sure.

[Jayy]

[Shaft]

[alextg]

I'm not sure entirely mate, it would be best to ask a mechanic or someone with more experience, surely if you've been running it for a while some damage would be evident by now if it was bad?

[Shaft]

[sickpup]

[Bezzer]

[Shaft]

[sickpup]

[Shaft]

[Babba]

I used to hang around a with a young lad called Paul Slater who used to make exhaust systems in his dads shed.

In the days when your bike HAD to be the fastest we would try ANYTHING to wring just 1 more horsepower out of 'em.

Paul discovered a book written by an MZ engineer called Walter Kaaden, (actually, it might have been my copy lent to him) describing in some detail how the exhaust design helped MZ to win sooo many races. Something that had been mostly forgotten by the whole motorcycle industry.

Paul would knock up a bean can style expansion chamber exhaust for anyone with a few quid & enough time to hang around the shed while he trial fitted it.

I can assure you that some of the early one's really were made out of bean cans !

Paul & his brother Mick went on to found a company called Codnor Light Fabrications who produced exhausts under the brand name of Micron, & almost overnight revolutionised the design of performance motorcycle exhausts.

Exhaust design isn't my thing, but a lot of exhaust manufacturers were born out of Micron, most notably Scorpion & Art & I have known, ridden with & chatted to most of the major players.

I can whole heartedly assure you that exhaust design isn't simple, there is a lot going on in there & it's much more than just a tube that vents the gasses away. I can also assure you that unless the exhaust is specifically designed & mated to your engine & it's state of tune, then any gain in power is purely an accidental coincidence.

P.S. If anyone wants to slag off Micron 'cos they might have had a particularly bad experience, feel free. My mate Paul & his brother Mick took the money & left Micron when they were still very much the best in the business.

[Cheeseybeaner]

[UnspeedySam]

I just found out that the spanny I got for my RXS is an early Micron. Cheers!

It's a 'Condor Power Exhaust'
____________________
Riding: Does an Audi estate count?
Fixing: Honda VFR750 RC36 '94 and MZ ETZ 251 '90
Gone: ZZR600 '00, TRX850 '97, RXS100 '93, JS125-6B '07, BMW R1100RS '93, Kawasaki ZX-6R-J2 '01,Honda Bros NT400 NC25 '88

[Babba]

[Jayy]

So if I buy a slash cut and stick it on, best thing to do is get it on a dyno and set the power commander up accordingly?

[Cheeseybeaner]

[Cheeseybeaner]

[lozzypop1]

[temeluchus]

4 stroke exhaust design is all about carefully adjusting the acoustics to improve cylinder scavenging. The can is the minor part of the system, provided it isnt unduly restrictive, as lozz says its the plumbing before the can that matters.
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Some shite cruiser. Now with guns and FREEDOM!

[Kickstart]

Hi

It isn't the pressure that is important, it is the pressure waves.

Take a very simple single cylinder 4 stroke with a single exhaust. When the exhaust valve opens the exhaust gases rush down the exhaust. When they reach the end of the exhaust a pressure wave bounces back to the cylinder head. At the right revs this arrives back at the exhaust port just in time to bounce back again sucking the next lot of exhaust gas from the engine. Unfortunately the pressure waves move at a pretty much constant speed irrespective of revs, so at other revs the pressure wave arrives just in time to ram the next lot of exhaust gas back into the cylinder head.

Gets a lot more complicated than that with multi cylinder engines where the exhaust headers are joined together to in different orders and at different lengths.