Hi Guys
Don’t know if someone can give me a advise , just looked at a twin turbo which started ok but when you rev it there is a large flat spot followed by a knock noise. It revs up ok if you bring it up slowly with no noise
The car is what I want to buy but issue is holding me back
Cheers Mick
Sorry forgot to mention battery was flat and I was running it on a battery pack
Cheers
A flattish battery can give all sorts of issues and I don’t think a battery pack would be strong enough, but I’m not sure. Where on the engine does the knocking noise come from, without more of a description it could be a number of things, if its loud and low down could be crank shells and big end bearings, but like I said, more of a description please 
Terry 
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The battery was so dead when I took the pack off the car stopped dead , the knock comes from the top end and sounds like a pre ignition knock near the Y pipe
Definitely not bottom end knock .
Thanks for any advise
If it was me I’d ask the seller to put a new battery on it before making any decision… Even with a duff battery once it starts the alternator should produce enough power to keep it going. I wouldn’t be buying it if didn’t run without the aid of something else but that’s me… If it’s going cheap for a few hundred I may think again.
If you rev it hard your drawing more power than revving it gradually and the slow rev increase may be charging it enough to produce more sparks allowing you to get the revs up. Planting your foot may be drawing more power than the battery has.
Even if you take a good battery with you and stick it on would be better than nothing. If your not mechanically minded and it still knocks with a new battery I would walk away but try to take a mechanic friend with you.
Jus
Thanks Jus
I already own a GTO which I have fully rebuilt the engine box etc but its a non turbo auto model . I am mechanically ok but have never had a turbo or come across this issue . Thanks for advise and I will take a battery and fuel back with me tomorrow but if it is still the same I will walk away
Many thanks for the help
Does the knock get faster and louder with revs ? how long has it been standing ? could be something as simple as a collapsed or noisey lifter
Craig 
Guy said been stood a couple of years before he took it for mot, its not a knock as an engine mechanical knock . I would say its a pre ignition knock when you open throttle quick as I can build revs up as long as I do it slowly with no issues
Cheers
Thanks for advise everyone , I put the battery off my n/a and a gallon of high octane and as sweet as can be
Deposit paid and collect Monday
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