Yamaha FJ1200 potential silencer modification

[eliminator]

The plate that is riveted onto the end of the silencer was rattling, the rivets had worked loose.

I borrowed a rivet gun, and got some rivets. I drilled out the old ones (obviously losing the old rivet inside the silencer )

This revealed 3 off Ø5 that were for the rivets, but also 3 off Ø6.3. I couldn't resist starting the bike to hear if there was any difference to the standard silencer. Hmm, sounded far better.

My question is, if I were to remove both plates and run the bike like this, am I likely to cause engine damage by weakening the mixture?

The standard silencer outlet is Ø26 which is 530 square mm (if my dubious maths is correct!)

The additional holes equate to 3off @ Ø6.3 = 93square mm & 3off @ Ø5 = 60square mm. added together = 153

So the silencer opening would be increased by about 29%.

I have a Yamaha XJ900S Diversion, about 3 years ago, I fitted a Delkevic system to the bike. I fretted about removing the baffles and damaging the engine. Baffles were removed, bike sounded far fruitier, no problems were ever encountered.


Enlighten me with your thoughts?

[Sean]

OK this is my one and only tuppence worth as an old school back street biker.

The silencer is just that.....a SILENCER.

....it's sole purpose is to quieten the sound of an explosion of spent fumes coming out from the engine out ports. I believe the silencer has NO purpose or has any mechanical influence of stopping the exhaust fumes coming out too quickly ie by back pressure being applied to the spent fumes rushing out the down pipe into the breakout box to the silencer!

If the silencers are not some kind of pressure valves but simply silencers as I think ,it should make no difference other than in volume on the performance of your bike. What the bore size on the way out is irrelivant but should be equal on both silencers just to stop one being louder than the other.

It could be psychosomatic, if you think it sounds meatier than you might ride it harder (heard that before in my youth I think...)

Some body will come along and shoot me down in flames who knows what they are talking about for sure soon enough... ;D ;D

should have said realy - I haven't got a clue... ;D

[CD]

The exhaust back pressure is important as the speed of sound shock waves from valve opening can really affect the engine tuning. The header length to the pipe junctions being chosen to suit peak power revs and exhaust gas conditions. Changing the gas pressure and/or temperature will affect that tuning. So drop the back pressure and you might make the engine less powerful and/or uses more fuel.

Saying that, my 900 with larger bore stainless headers felt identical. The smaller bore (double wall) standard pipes "should" have boosted mid range but I could feel nothing different and fuel consumption was unchanged. When I fitted free flowing silencer (loud) cans the power peak definitely went up. I now have to watch the speedo or find the bike easily going to well over 90. With the old cans, it would always settle at about 85 - unless I bullied it along.

I've since put in some DB killers and the effect has slightly blunted that extra top end but its now not rattling the neighbours windows in the morning.

All on private roads of course.

[Pool Boy]

The perceived wisdom seems to be that a freer flowing exhaust can cause an engine to run lean and damage (amongst other things) the valves.
I have no idea how this happens but the explanation above seems plausable.
Some makers of aftermarket exhaust systems state that they can be fitted without re-jetting (or altering fuel injection) whilst others make no such claim and even suggest you check the fuelling.

So the answer, I suspect is that it MAY not do any harm but then again it might.

Try asking the same question here : www.fjfjrbiker.co.uk or check through the exisitng posts to see what other FJ owners have done.

[eliminator]

I've asked the question on the FJ forum as well. To date the most relevant suggestion is to run it, then do a plug colour check. Personally knowing how removing the baffle caused no engine damage to the 900 Divvy, I doubt if it will cause any problems with my FJ.

I'll be curious to see what difference (if any) it makes to the temperature of the silencers. The Divvy's can be touched after a run, if I did that with the FJ I'd burn my fingers.