Yes mate,
Big differences in a great many cases.

It all depends how close what you told us you had, resembles reality.
If your spec is exactly what you thought it was, and you have it all setup just how I want it to be done, the cal will be as good as it can get safely via mail order.
We have operated this way for 20yrs with very very few issues.

You have to remember that an off the shelf calibration for a speed density ECU has a whole bunch of compromises built into it to take into account that the customer, generally, has no way to check AFR and probably won't even bother with a boost gauge and just turn it up until it feels fast. (Yes - I hear that all the time)

The problem is, so many YB's are NOT quite what the owner thinks they are... So as default, a mail order cal is a little richer than optimum, has a little less spark than optimum and has breakpoints that allow the customer to mess about with his boost a bit without blowing the thing to bits.

With regards wierd mods...
Exhaust and intercoolers arent making huge differences to the calibration, but inlets and throttle bodies certainly do, usually, the best gains are in the stat, run and driveability ranges as the plenums completely screw up the gas speed and the manifold depression. (Normally for the worse)

But its not just that, its spec changes you didn't know you had, low compression, light head porting, cams, cam timing, cam lobe wear, fuel pressure. Literally, everything has an effect on these old speed density ECU's as they are absolutely dumb.

They know only temperature and pressure - they have no concept of flow. They "think" a standard head, cams and intake with a T3 is exactly the same as a huge ported head, BD16s and a GTX35. Unless we tell them we need more fuel at 1bar boost (Like 400bhp worth), they will still supply the fuel for 260BHP.


Every little thing that affects airflow, affects the calibrations suitability and ultimately, the vehicles driveability and throttle response. So dialling it in live until we stop making tractive effort and our AFR is right where we would like it can bring big improvements to some calibrations that, on paper "should" have been fine mail order.

Also, the mail order calibrations are 13x16 fuel maps and 8x16 spark tables as per Weber standards. Our latest firmware we use for all our live maps are 16x16 fuel and 11x16 spark with improved idle control strategy in the firmware that is FAR better than the old one and "usually" eliminates cold idle hunting completely which is a big plus for most owners.

As with all things, a lot of this is just an evolution of a system. As time goes on things get refined and ultimately, passed down to the customer.

As you will see from this absolutely pornographic movie, the AFR on these old 1980s Weber systems can be every bit as perfect as a standalone unit if the time is taken to live map them properly. Got to love this MK1... and I am generally not an Old Schol fan, but this car is something else!