Some of you reading this might be thinking "Duuh, no shit, tell us something even more obvious" but for years, over a decade in fact, especially in the Japanese car tuning scene in the UK (but it seems to spread elsewhere too; I've heard it a fair bit even with Renault 5 GT Turbo owners lately), I hear people saying this like it's a fact, despite it much more likely to do exactly the opposite! This does seem to be a UK-only thing; overly wild cams on turbo engines aren't common elsewhere in the world that I've noticed. Whether this is better knowledge elsewhere, or less unscrupulous tuners selling cams by telling bare faced lies to convince people it's needed to improve spool, you decide...
Don't believe me? Well, there's enough dyno sheets out there to compare which prove it, but the production car world is a big hint that these magic 'anti-lag' cams are utter bollocks, as all production turbo engines are fitted with much milder cams than the production non-turbo version of that engine. From Fords to Porsches and everything in between, that's a fact. Do you really think they did it to make the cars laggier and have less low down grunt? No.
The fact is, cams are very give and take, and increasing cam duration (and overlap) reduces low down power, not increases it, and instead improves performance higher in the rev range instead.
A well-specced mildly uprated cam may well not noticeably slow spool over a factory turbo camshaft, and may indeed give a much stronger midrange when it's matched well to the turbocharger(s) fitted, but will it actually improve spoolup rpm? No.
In fact, if you over-do the camshaft you can totally RUIN a turbo engines performance, both spool and top end power, and this is something I've experienced a number of times (not from my own choice, I must add!).
I've seen (but wasn't consulted on, as if so I would've told them it was going to be a disaster) a magazine test of some "Fast Road" cams on a VAG 1.8T engine running the stock KKK K03 turbocharger. I knew even these fairly mild cams were far too big for the athmatic little K03, and lo and behold, the car lost a huge amount of performance from low rpm to high, compared to the standard cams.
Another good example was when a frien bought a Renault 5 GT Turbo modified by the previous owner. The engine was a good spec and fitted with a Piper 285 cam, which I know from previous experience is a fantastic cam for the little C1J lump as long as you've got a decent size turbo fitted to it, but this car had a standard Garrett T2 running 15psi boost. While the turbo didn't really spool much slower than standard, the longer duration still reduced lower rpm power compared to the standard cam, and as revs increased the restrictive little turbine of the T2 combined with the increased overlap of the cam (high pre-turbine backpressure and big cams REALLY don't mix!) meant the engine was completely dead on it's ass by 5000rpm- Lower rpm with the standard cam, making for a slower car everywhere in the rpm range.
When this exact same engine was fitted with a turbo of a more suitable size for the cam (A Garrett GT2560R running 23psi), the engine was transformed, and now makes well over twice the power, and revs hard all the way to 7500rpm, while only running 8psi more boost. Didn't spool faster though ;)
"Turbo Cams" are one hell of a big subject, and one I'll be gradually covering on this website in various forms no doubt, but this 'lag thing is the biggest issue...