Hitchcocks Forum

If a standard iron barrell Indian 500 Bullet and a Bighead Bullet both have the same pipes, why does the Indian Bullet sound so much deeper and have more thump? Is it to do with the lift of the cams,the ignition timing, or the ports? anyone know?

Hi Chris, it is probably all down to whatever types the cams are and the compression ratio. An earlier exhaust valve opening timing will be noisier than a later one, as the gases will start coming out under higher pressure, giving a louder, sharper note. a later opening point will be quieter and softer. Any respective increase in compression will make the exhaust note sharper and louder, while a softer, more 'boomy' note will come from a softer tuned engine. Regards, Paul.

Thanks Paul, that explains that. I must admit I prefer the soft boomy sound.

I'll keep an eye on the bikes for sale section, then...



A.

Ones already gone Adrian

Something to think about - at say 1200rpm ('cos it's easy to divide) the motor fires 600 times per minute or 10 times per second, yet we hear a relatively slow, "pop, pop, pop". What we are hearing are not individual firings of the motor but compression waves through the exhaust pipe. I found that opening the baffles produces more of a softer, "phut" at idle, it reduces back pressure which may be good for more top-end but not good for mid-range. Closing the baffles then makes a harder, "pop", it increases back pressure and produces more mid-range.

Get what your saying jefrs, but it's amazing how different the sound is as both bikes have identical pipes and bellmouth monobloc carbs. I hadn't noticed it before.

It could also be different wall thickness of downpipe or can. I eventually got a good head to pipe seal on the C5 by using two copper gaskets p/n 582638 and importantly used the torque wrench as intended (I mention the p/n because they're not for the EFI and not listed unless you search on the p/n), and that shut the motor up and changed the exhaust bark a lot. Then I found, rather counter-intuitively, that opening the baffle fins made the retort quieter, the "phut", but lost mid-range grunt and did nothing much to high revs ('cos it wouldn't get there). Closing the baffle fins together brought back mid-range but lost top end. The the fins are now at approx 60° from the side of the baffle cylinder which seems to produce best all-round and a nice deep "phop" note in-between the two extremes. It's loud but it's not brutal, the neighbours like it for sounding proper.



Tuning an exhaust is a bit of a black art, it originated by listening to the note, hence "tuning". Now we can have lots of complicated maths and computer modelling at design but often it comes back to (you have to work with what you've got) if it sounds right, it probably is.

Hi I have a iron barrel and a new b5 both had exactly the same mega on them , iron sounded nice bit loud but not excessive b5 was as noisy as hell both had same amount of packing same brand and thickness, is this something to do with efi against carby. Any thoughts.

Definitely cam overlap I reckon. Big overlap sharper sound.