Baffled using 99 octane fuel
 

Hi hope this is right place for this post. Drive a peugeot 406 estate with 2.0 petrol 16v and auto box. Gearing is on a par with a diesel. 60mph = 2200rpm. Auto box is ecu controlled. Why is it when i use 99 octane fuel my engine runs smoother and auto box is fantastic compared to using 95 octane fuel. Mods done are front air dam; radiator grill blocked so all air has to pass through radiator; lengthen air intake by 6"; fitted an exhaust that is 1/4" bigger in diameter and all tyres 4psi above recommended. Cannot think of a logical reason to why it is smoother and mpg is up 5+%. Tail pipe had a lot of carbon on it so i think its running rich. Mpg is up from 35-37mpg to 40mpg. Coasting distance is up as well as power for climbing hills box changes up sooner on 99 octane. Engine temp is bang on thermostat opening temperture. Can anyone shed any light on why this is as engine is designed for 95 octane.

Sounds like there might be some pre-detonation or detonation and the engine is pulling timing? But if there's soot on the tail-pipe, I'd check the plugs for fouling.

Any idea what the compression ratio is on the engine? Niky is probably right about knocking and the timing retarding.

Especially considering you've modified airflow inside the engine bay, it may be running hot enough to cause pinging, but not hot enough to cause any changes in the engine temps.

Check your air intake temperature.

During a mild frost period I ran with just a lower grill block and the air intake snorkel detached, so it would generally draw air from the radiator rather than the upper grille.

The intake air was cold and stable for the first 10 kilometers, then the thermostat would open flow to the radiator and it would very gradually climb to ambient, then at 30 kilometers it suddenly went through the roof, raising one degree every 4 seconds!
It ran to well over 50 Celsius and I would have stopped to reattach the snorkel if not for hitting the off ramp right then,which cooled it back a bit.

Also I noticed a drop in economy going above 35 Celsius. My car has EGR; I bet it needs cool air to function properly.

I used a cheap digital in/out temperature sensor with the out feeler in the intake nozzle at the time. Nowadays I read intake temp on my UltraGauge.

Hi stubby79 think it's 9.5 to 1. Air intake is insulated and covered with silver tape. I am drawing air from bottom of engine bay. With do a touch test on air box to see if that is warm after a 20 mile drive today. If it is warm then I will have to think about insulating airbox or changing air feed to cold air. Thanks for advice

Keep an eye on the general under hood temperature, especially around the 12V battery.
It will suffer if exposed to temperatures over 40 Celsius for a prolonged time.

If you are to route fresh air for the intake you might incorporate the 12V battery too.
Then if it overcharges and starts bubbling HHO al that goodness gets sucked into the intake and give you better economy than a unicorn.

Most cars nowadays use sealed batteries, which wouldn't allow its vapors to be released. But anyway, even though some folks might feel inclined to try this with a non-sealed battery, I wouldn't recommend it due to the acidity of the battery water.

Well not taken it out yet. But engine in tray was loose so removed to see if it makes a difference. Need new mountings fitted.

shouldn't allow its vapors to be released