I'm with quattrokid - surely if you've got custom exhaust pipework & manifold + a straight thru silencer, you've got about as much exhaust breathing as the engine can handle unless you go up to a larger bore system ? i.e. on the list of your engine's limitations, the exhaust system should now be last on the list, not first. Unless there's summat else bothering you about it, that I'm missing ? Is your custom exhaust system made in tubing smaller than the stock bore ? This is the only way that I can see your exhaust system might be 'strangling' the motor's breathing & power
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