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If I have preignition, can I hear it?
(I know what it sounded like in cars 40-50 years ago).
If I don’t hear anything, is any preignition going on too little to hurt a car?
I do not want to open and endless peripheral discussion, but I am interested in answers to these questions.

Car model, year, mileage…

I do not want to open an endless peripheral discussion

Golly, did you ever come to the wrong place!

If you don’t hear any pinging from the engine, there’s no preignition occurring.


Ha ha ha. Once you ask a question, you have no control over the responses so just sit back and read and enjoy. If you don’t want “endless peripheral discussion”, just use the telephone. After all, it’s not like you get a bill in the mail. Sheesh, kids.

thanks Tester

There are basically two types of preignition. Perhaps “unwanted ignition” would be a more accurate term. Yes, you can hear both of them from the driver’s seat.

“Pinging” is a form of mild preignition caused by a secondary wavefront caused by something other than the sparkplug crashing into the sparkplug-initiated wavefront. Pinging sounds like marbles rattling around in a tin can. It generally only occurs when accelerating.

“knocking” is a more severe form, where the cylinder ignites before the piston reaches the point in its cycle where it’s in a position to properly convert the explosion into rotating energy. The piston is still coming up and the explosion tries to push it back down. It sounds like someone banging on the engine with a hammer.

“Pinging” will damage an engine if allowed to continue.
“knocking” will destroy an engine if allowed to continue.

On distributor equipped engines pinging is usually caused by too much advance in the timing or an EGR system fault.

On non-distributor equipped engines pinging is most often due to an EGR system fault.

Preignition sounds like a diesel engine sounds. Modern engines have a knock sensor just for this purpose… Upon detection it immediately retards ignition timing.

You wont hear preignition much these days…its been fairly well tamed. If you do…you gots problems.


It probably depends on the car, but I’ve always been able to hear it on my cars. Usually I first notice it only on rapid accelerations or going uphill. I can’t speak to whether a little can damage the engine as I find it annoying and I always have been able to fix it when it happens. Usually just worn spark plugs, the gap has opened beyond specs.

No engine wants ANY sort of detonation…it is counter intuitive to its design …engines are only able to tolerate it at all because they are over engineered typically…otherwise catastrophic engine failure would result.


Back before they became so sophisticated pre-ignition could be disastrous in race car engines because the driver wouldn’t hear it over all the other loud noises.

Everything about racecars (other than in endurance races like LeMans) was and still is on the verge of catastrophic failure. The only way to win is to do a tightrope walk on the limits of every part.

Perhaps the most dangerous vehicles ever were the early slingshot dragsters. The drivers sat immediately behind a bomb, and right on top of differentials always on the verge of exploding under them. Scatter shields helped, but too many winning dragster pilots died from pushing the edge. Don Garlit’s safety innovations, especially designing the cars with the driver in front of the engine rather than behind, saved a whole lot of lives.

I recall seeing the race that finally drove Garlitz to move the driver in front of the engine. The other driver in his pairing had an engine explosion so violent that it blew the driver’s cage completely off the car in a huge fireball. Big Daddy got out of his car at the end of the track and sat on the side with his head in his hands. The camera stayed with him for a long time, and he did not move.

Another thing: Warren “Professor” Johnson initiated so many safety changes in the top fuel class that he earned the nickname Professor.

OK, now the flood begins.
I have a 1999 Lexus LS 400, which calls for premium. (It says 91 octane is required, but 87 can be used in a pinch). Anyway, I put a tank of 87 octane in it. I can hear no pinging, even on hill climbing or strong acceleration. I guess this means it’s OK. So, my second question, which probably only one person will actually address, is this: running on regular, will the gas mileage be worse, possibly worse enough to cancel out the savings on gas?

It is really quite simple. Check your miles per gallon with two tanks of 87 and then two tanks of 91 and you will have your actual answer for your vehicle.

This car is not driven much. I am saving it for my son.

There should be very little difference in fuel economy. I think if you very carefully check your mileage over several tanks with the 2 different grades there will be a slight difference depending on driving style. Those with a more spirited driving style will notice a drop in both fuel economy and engine performance. Those with a more conservative approach will notice very little difference.

Just because you don’t hear pinging or knocking doesn’t mean it’s not there, the knock sensors on your engine are sensitive enough to detect and adjust for it far before your ears hear it.

This question reminds me of the discussion here earlier about what happens when you engage the parking brake while driving. It’s interesting from a mechanical standpoint, but why would you purposely engage the parking brake while driving?

Why would you not just use the premium gas your car requires?

+1 to ase’s post.
I was unable to find out for sure, but I believe the '99 LS400 engine was a 10.5 compression ratio with a turbocharger boost. That would mean it’s prone to unwanted ignition under load when using 87 octane. But that’s also a very well sound insulated (and sound isolated) car, and you can’t rely on your ears as a trustworthy reference. The knock sensor system will compensate to a point, but why take the risk? Prolonged pinging can burn a hole in a piston.

In summary, the slight added cost of premium is very cheap insurance against very expensive engine damage.

And to think years ago the recommended tune up procedures from the car manufacturers said to:

  1. stand on the running boards while the car was driven under a load, (hood open)
  2. advance the distributor until you could hear engine knocking
  3. leave it there as that was the desired timing.