I suspect that my engine has a pre-ignition issue. Although my son Mike can hear such an event, he's 3 hours away. I know from previous experience that we can both stand next the same engine and I can't hear what he can hear.

Modern engines with engine management systems rely on anti-knock sensors to adjust the timing so that they can run as close to pre-ignition as possible without it actually happening. So what about those of us is older engines like the Type IV? Is there a knock sensor that can be attached while setting the timing so that we can accomplish the same thing?

The guys over at Megasquirt are trying to use knock detection to retard timing. I know MSII will log knocks but currently there is no retard. I've heard that it is very difficult with out engines because they are really noisy. As I understand it sensors are more or less tuned for each type of engine.

I used a stethiscope to try to hear knock but all I heard was a whole bunch of racket so., I finally gave up. I suspect the top of a cylinder under the tin might work but what a hassle.

Dave

This came up after I finished my rebuild. This is essentially a stock 2.0L engine, but with flat-top Euro pistons and the compression set at 8.1:1. After about 500 miles I was experiencing run-on after I shut it off.

My consulting mechanic said that it could be pre-ignition and that one of the ways to test for this was to run a couple of tanks of 89 octane through it. I did that and the problem seemed to go away, but the whole point of this engine was to have a little bit more power, but still run 87 octane fuel. The next step is to retard the timing a little bit - like to maybe 20 degrees BTDC, instead of 23 degrees - but of course to make sure that the timing is set in the right place one would need an anti-knock tester.

The other reason why this would be good is that one of the local PCA guys thought it was really strange that I would build this engine and set it with such low compression. When I told him that it was because of the goal of being able to run 87 octane fuel, he said there are lots of engines built with higher compressions that run regular fuel and that all you have to do is adjust the timing to make it work. Of course there is no chart to lookup to tell you "if you run this compressio ration, then use this timing".

You don't really NEED a knock sensor. As Dave says, these basically don't exist for air-cooled engines, which are indeed very noisy, esp. pushrod air-cooled engines.

Retarding the timing can make the knock go away, but it also increases running temps. You may simply have to run 89 or 91 octane all the time. Engines that run higher compression with 87 octane usually have better combustion chamber designs than the Type IV, and/or a better cam design. You can't compare a 911 or a 944 engine with a Type IV.

Run-on after shutoff can be timing, but it can also be mixture that's too lean, or an engine that's running way too hot. Do you have a CHT gauge?

No CHT gauge, but I've been planning to add one anyway since oil temps don't really tell you what is going on with your engine as well as head temps do.