In the days when "our" cars were made, you could make from 5 - 20bhp just by "blueprinting" your engine, which is to say you were just eliminating manufacturing tolerances. The reason for the variation being that some engines were built naturally better than others.
We've all had cars that went better than the norm for no apparent reason, this is it! (I've also seen more than a few that were naturally awful, thirsty underperformers!)
Nowadays manufacturing tolerances are much tighter, gasflow is better understood and modern cars with ECU controlled Ei and EFi are much more tightly controlled for maximum efficiency in both power and economy. So they are relatively MUCH more difficult to extract those few extra horses from, especially for very little outlay beyond a bit of hard graft. But modern engines are SO much better than the stuff we mess with, that they are artificially detuned by the manufacturers map, for example, you can get a PD engined Golf diesel which produces anything from 99ps, all the way to 170ps and the only difference is the map! So a remap on a modern is the best way to go, if fact it's the ONLY way to go without spending huge amounts of dosh to end up with a massively powerful but temperamental and unreliable beast.
The aftermarket tuning industry grew massively from it's beginnings in the early 60s and is still huge today, you'd think THAT many people COULDN'T be wrong! But it's a sad fact that many hundreds of times more pounds have been put in than horsepower have been pulled out! The usual result of 60s and 70s "tuning" was only more noise, less mpg and an empty wallet!
This is the "Weber effect" People have been plonking Weber carbs on cars since I was a kid and mistaking a nice induction roar for horsepower! Why not, when the adverts told you it was so?
But an engine is an homogenised, symbiotic whole, more than the sum of it's parts, if you mess with one part, you MUST mess with all of it. To make the most of those twin Webers we all love, you need a different cam, exhaust, distributor, bigger valves, head work, port matching, etc. Then to prevent the motor self destructing under the strain of all the extra horses, you need to stiffen up the bottom end of the engine, better bearings, finer balance, improved oil circulation, cooling and so on. It's never ending really and VERY expensive!
THIS is why I decided, long ago, that tuning was a mugs game and the cheapest way to get more horsepower, was to import a larger and more powerful motor from a different car! It's the American method, "there ain't no substitute for cubes" (meaning cubic inches, the US capacity measure) It's worked for them for a lot of years and it works for me! Nowadays it's enough to fit a modern EFi motor of a similar capacity to get a large gain in RWHP aaaaaaand better fuel economy! "Greener" too with lower emissions and using recycled parts!
So IMO, yes, unless its been done properly (and very expensively) you ARE right to be a bit sceptical about tuning!
2 door '73 Toledo with Vauxhall Carlton 2.0 8v engine OWF 797M (The Carledo)
'78 Sprint Auto with Vauxhall Omega 2.2 16v engine EGP 247T (The Dolomega)
'91 Cavalier 2ltr 8v auto
'95 Cavalier 2ltr 16v auto
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