Thread derailment!!

In reference to Transmans post about the carb tuning and bsfc.
It’s not a good idea........ since its not directly indicative of whether or not the carb is rich or lean.

Example....... imagine the motor on the dyno, steady state run at 5000rpm, making 500hp, using exactly 250lbs/hr.
So, the bsfc is .500.
Let’s say there is a wideband reading in the pipes and it’s showing 12.5:1.

Now............we attach a big hyd pump to the front of the crankshaft.
One that uses up 100hp@5000rpm.
So, with the pump attached to the front of the motor(eating up 100hp), the dyno only registers 400hp on the load cell.

The motor is still actually making 500hp, using the same amount of fuel and air it did before, and is still the same a/f ratio....... but since the dyno only registers the power as 400hp(because there is a 100hp parasitic draw on the front of the motor)the bsfc is now .625.
But...... the motor is not running any richer.

Bsfc is an efficiency reading...... the result of measured power vs fuel consumed.
Nowhere is the ratio of air to fuel referenced to arrive at a bsfc number.

Sure.......”generally” high bsfc numbers track with rich a/f ratios, but they don’t always.

I have tested a number of motors through the years that really didn’t like lean mixtures.
You’d richen them up....... a/f ratio reading would confirm....... motor would make a big move in inscreased power....... bsfc would go down.

What high bsfc numbers are really telling you is....... you’re not making good power for the amount of fuel you’re using.

Most of the stuff I test doesn’t make the best power when they’re getting into the danger zone for being lean.
That being said, I usually tune for best power, while making sure there is some safety margin built in by erring to the richer side.
And then the bsfc numbers just end up where ever they are.
If the motor makes pretty good use of the fuel those numbers will be decent........ and if it uses a lot of fuel but doesn’t make great power...... the numbers won’t look that good.