Another insurance question

[flatheadfan]

There are probably some insurance grues out there who can answer this question. During a discussion last night a question came up regarding coverage. If you insure a generally stock car and then undergo a significant horsepower increase (via engine change or accessories) and didn't tell the insurance company about the change are you still covered in case of an accident or can the insurance company refuse to pay? Assuming you are at fault.

Tom