High RPM exhaust whistle

[chrisryder]

Helloo!

I have a LCB manifold and 1.5 inch twin box exhaust system on my minor. i've noticed that at high rpm (i dont know how high, not got a rev counter), just as/after i lift off (gearchanges) the exhaust emmits a high pitched whistle! this doesn't happen every gearchange, just when i change up later than i aught to.

any ideas what it is? is the air going supersonic? could it have something to do with the fact i have an 'over-run' valve in the throttle butterly on my HS4 carb?

like this one:[frame][/frame]

also, what's the point of this valve? it looks like it offers a massive air restriction. what is the advantage of it?

[bmcecosse]

It does indeed offer a huge air restriction...... it is there to allow air in to the engine when you close the throttle at high revs - for 'emissions' reasons Remove it - and simply solder over the hole - or fit a new throttle plate with NO protrusions. You can also see in that pic the restriction caused by the throttle spindle and how Vizard dramatically increases the air flow by thinning it all down, and cutting off the little screws.

[chrisryder]

is that a good mod to thin the spindle? or is that considered a 'race only' thing?

any idea about the whistle?

[bmcecosse]

Within reason - yes thin the spindle - but don't do the 'cut-in-half' mod -I think that goes too far. The little valve may well be the whistle source - so remove it and see what happens !

[chrisryder]

may just do that. not too keen on a blob of solder flying into my engine, i'll try and make a good job of it!

i quite like the whistle... everyone will think i've got a turbo