Ive switched to 97 and then 98 vpower recently after getting some occaisional mild pinking at low revs /low gear under load (the car is remapped and supercharger pulley/free flow cats and exhaust modded).

Seems to be improvement all round.

I know there is meant to be a knock senser but not sure how it works. Does it always adjust in real time so in any one trip if knocking is experienced the timing is retarded but only for a while?

Tia

 

Knock sensors are simply piezoelectric elements that produce voltage when it senses knock. It will retard timing and torque when the ECM receives voltage from them. When the elements stop producing voltage, ECM returns to its normal advance curve.

 

David, I've never heard a ping. An interesting correct in thinking that a properly functioning sensor will mask any ping or pinks before a human could hear them? John

 

Mark:

If the higher octane fuel works for you, just use it. With the modifications you have made, the octane requirements of your car might very well need the higher octane. Even if the knock sensor works perfectly to reduce detonation, the car will not perform well as it intervenes to save the engine.


Some random observations:


I had a V6 Camry which called for high octane fuel, but the knock sensors, etc. allowed it to operate perfectly adequately on regular fuel. Only when demanding maximum power would the electronics intervene, so unless I was towing the boat on its trailer where a tank of premium did make a difference, the 20% cost saving of using regular worked well.


In contrast, in the late 70's, when premium leaded fuel became no longer
available in British Columbia except for 2 stations in Vancouver and one in Victoria, all of us callow young fighter pilots based up-island with our various sports cars were left in a spot. My 1971 240Z pinged horribly on regular, and retarding the ignition timing enough to stop this turned the car into an undriveable slug. One of my enterprising colleagues, who had previously successfully used water-alcohol injection on his turbocharged Datsun 510, using positive boost to pressurize the reservoir, experimented with doing the same on his 240Z using a windshield washer pump activated by a switch under the throttle pedal. The lag in the delivery of the fluid made this unacceptable. So the answer was that I got a couple of barrels of 100/130 avgas and a hand pump in my carport and everyone was happy again. One of the properties of the knock inhibition of octane is that it is not linear, more like exponential. For example, if you mix equal parts of 90 octane and 100 octane, you do not get 95, you get 97-98. So all it took for every fill of regular was 1 gallon of the 100/130 to make all our cars very happy. Well, except for one of my buddies 1964 XKE, which preferred 2 gallons mixed per fill. Those barrels lasted quite awhile between re-fills at the nearby private airport.


Two asides to this:


When buying avgas at the airport into the barrels tied on a boat trailer, they insisted they could only selll into aircraft. Taxes and such. So I lied through my teeth and claimed it was for a float plane in the harbour and used an invented registration. That made them happy in case of an audit.


We also had at that time a maritime patrol squadron on the base that flew the Argus with four Wright 3350 turbo-compound engines that used 115-145 avgas. Try as I might, but I was never able to convince the base supply officer to sell me any of that. (And I was not going to put myself in jeopardy by stealing it, although it could have been easily done through co-operative airmen). That would have worked exceptionally well but never got to try it.


Further to this, I later flew the Grumman Tracker. It, too, was designed to use 155/145 in its Wright 1820 engines, but by then that fuel spec was no longer being refined in Canada. The US Navy still had it in Bermuda and Norfolk, but at home we only had 100/130. So the engineers did their analysis and the only change was maximum takeoff power was reduced to 53" manifold pressure instead of 58", but METO, climb and cruise all remained the same as before. If that is not direct evidence that you only need the octane when you need it, I don't know how else to convince you.


From time to time, friends have related how they have "treated" their ordinary car to a tank of premium and claimed smoother running, more power and better mileage. I try to be polite enough to not contradict their misapprehension, but file it in the same category as those heart-felt anecdotes about the latest tennis racket, skis or winter tires.


But in your case, I think you are well served by pumping in the fuel your Jag needs,


Cheers


Andy

 

Long before.

 

Hot spot in cylinder may not help any knock sensor?