I am planning on purchasing an HPD trail pipe and clutch kit for my Scrambler 400. Would there be any loss on the claimed improvement of 7 to 9 HP is I kept the stock silencer? If so, how much? I am trying to justify the additional $60-$120 dollars.

 

After some testing with diffrent pipes,other quads and this form, I'd say yes about the horse power but the loss would only be minimal.
You should still feel a power gain.
Hope this help's you out,Rictor

 

I'm currently running the HPD Pipe & Stock Silencer / Spark Arrestor trying to keep the exhaust a bit quieter. As for horsepower loss, I would guess than a loss of 1 to 2 Horsepower would be a reasonable estimate with the stock silencer compared to a " High Performance " silencer.