Signs of Pre-ignition and Detonation?

[dajohnson53]

In a recent discussion on octane, people talk about the danger of preignition and detonation if octane is too low.<br /><br />What exactly are the signs of pre-ignition and detonation in a large (V6) two stroke engine?<br /><br />I've been using premium (90) fuel because I was told I should do that. This is an early 80s OMC V6. Now I think I understand that not only is high octane a waste of money if not needed, but it might actually decrease performance. But the danger is preignition and detonation, which are very bad. Problem is I don't know what they sound or look like.<br /><br />Thanks.

 

[Scaaty]

Re: Signs of Pre-ignition and Detonation?

It mostly depends on your motor timing. Too advanced and you can hole a piston, and you won't get any warning. It is extermly difficult to diagnose or detect pre-ing/detonation. Best bet is to back off the timing and sacrifice some performence and try regular. Me, I would stick to what the motor calls for, and don't risk it. Pistons are cheap, putting them in cost a bunch.

 

[dajohnson53]

Re: Signs of Pre-ignition and Detonation?

Well that's always been my thought - buying premium does cost a few more bucks a year, but it's a minor cost considering the risk of pre-ign or detonation. Since I didn't know what to look or listen for, I have been just buying premium. I don't know how to do timing, so adjusting that is not an option. I have a feeling that messing with it via a pro. mechanic would more than counteract the cost savings or potential performance increase?

 

[Dhadley]

Re: Signs of Pre-ignition and Detonation?

With the additive package in the fuel today the hi test burns hotter. Heat promotes coking and therefore stuck rings. Among other things.<br /><br />We've always found it best to run the recommended fuel and plugs with the factory compression and timing. If you raise the compression, then thats a different story.

 

[JB]

Re: Signs of Pre-ignition and Detonation?

Yes. What DHadley said.<br /><br />Tuned to factory specs, there is no "danger" of preignition in 2 stroke engines.

 

[jim dozier]

Re: Signs of Pre-ignition and Detonation?

Its hard to hear detonation in an outboard. You know it as pinging under acceleration in your car if you use too low octane. If you ever see aluminum droplets on your sparkplug that may be a sign (but usually too late). First indication is often engine damage That said, it is usually excess carbon hotspots or bad (too much advance) ignition timing as the culprit, not low octane.