do these things work? do they really give more hp?
and are they good for the engine?

Cheers

Yes - they boost the octane rating of the fuel.

If you have a car that adapts the ignition advance to the fuel then yes, you will get more power.

The coupe does not however do this, so unless your car is mapped for higher octane fuel then you wont get more power, you will however get more tolerance for detonation, which will protect the engine if you are pushing it.

really its a waste of money then isn`t it?

IMO - yes. Unless you are going to track day the car.

playing devil's advocate for a moment,(as I only use V-power or tesco 99) has anyone worked out if it's cheaper to run bog-standard fuel and dump in a load of (high-quality) octane booster???
This is essentially what shell etc do to produce their premium fuel afterall, as all petrol comes from the same distilleries, just in different trucks, with, sometimes, added extras......

i watched a programme once when they tested several so called octane boosters... guess what none of them worked, in fact a couple were less effective than to begin with not worth the money.........

its all down to mapping

If you run V-Power AND octane booster AND you have the car mapped to use the higher octane, it WILL produce more power

However, just throwing a bottle of octane booster in the tank will do two things

1) make the engine more resistant to detonation (so you may be able to run a bit more boost)
2) empty your wallet a bit

If you REALLY want to get the most from your fuel, switch to V-Power and then get Leighton to live map the car - you'll be staggered at the difference

I run it on track days, some aren't worth the sticker on teh bottle, some are quite good, google a test of them and you will see which are the decent brands

My car isn't mapped for it so I may be wasting my money but for track days I just consider it a bit of extra insurance.

well then ill have to get it mapped,
ive always used V-power.

Er with V power, it's also the cleaning additives in the petrol that make a difference... I get more mpg than using standard petrol, and also for the next few tanks afterwards even on standard fuel...