Hello,

Anyone experienced preignition at an idle.

On this one I've bumped the compression, intake & head ported, 500lift cam, dual springs, larger valves, webber carb, & header, I think that's about it.

I've tried richening the idle circuit, raising the octane, the timing has been set everywhere from +4 to + 24 with not much improvement. Can too much advance in an cam gear cause this?

This is the first one I've built with quiet exhaust, maybe they have all been doing it??

Any ideas would be greatly appreciated, I really don't want to knock the rod bearings out of it.

Thanks,

McQ

Set your timing back to stock for now and give this a read:

http://www.4x4wire.com/toyota/tech/cam_degreeing/

Hopefully Ted (engnbldr) will chime in.

Who did you get the cam from? Never heard of it.

Thanks for the input !

I've gotten most all of my stuff from Ted & Tod. The cam I was just using general numbers. It's a DOA 490 that Ted recommended. Also the block was decked. I've read that article about 5 times and my comprehension must be a little short for such proceedures. To be honest I haven't tried but I figured you have to throughly understand something to do it, and I clearly do not. If I saw it done once, I know I'd get it.

I was told by a fast toyota guy at a race one time that you need to advance pretty close to 3 degrees for every .010 removed from the block or head. He said when he degrees he's always right in that formula (after he has used a wheel).

The first advance degreeing device I used for cam timing was one that Ted was able to get for me that simply went on the crank and you lined it up to the "a" if you wanted to advance your timing. Thats it. No other options. So I thought it was probbly important but not life or death.

This block has .015 taken off and .010 off of the head so I set the advance to 7.

I realize I should be degreeing these. If you are pretty confident in a degreeing problem it's no big deal to just adjust it back down a couple.

Thanks for answering !!!

McQ

>>>*Preignition at idle usually means preignition at speed also. In full throttle situations that can mean the next step which is detonation, that is often found by hearing the loud noise and big cloud of smoke.

One point is not correct, the valve timing does not change 3? for every .010" machined off the head/block, it is closer to just 1?.

I would suggest moving the cam timing to a straight up timing setting as very early event valve motion can lead to detonation. Besides, early event affects lower RPM, and later moves the power peak up the RPM scale. Logically your combo is intended to run at upper RPM's.

Still, preignition can be cured by retarding the ignition timing, and I would say always unless the cam was radically out of time.....Kind of a curious situation, can you describe the problem or perhaps tell me how the diagnosis was reached?..The reason I ask is I would question that if everything else is close to normal.....*EB

Ted,

Thanks for the reply. WOW one drgree per ten... looks like it's fourth and long, a par six, and I'm going to try and make a lay-up if I don't get a strike ! I'll correct the issue this morning and put it straight up like you suggest. With my old formula, no wonder the race truck runs great between 6000 and 8000 rpms !!! I'll back it down too !

I came to this conclusion when returning home after a test run and adjusting the idle and rechecking the timing. About every three or four seconds you can hear a very sharp clear metalic "ping" not loud, not deep, and nothing like valve train noise. It appeared to be coming from the top end of the engine. It idles so rough it's kind of hard to tell but it almost appears to slightly affect the idle when it does it.(pings) When it's cold it doesn't seem to do it nearly as much which would also lead me to believe it's preignition.

Thanks so much for taking the time to address this issue. And thanks for all the previous guidence and for the top notch parts !!!

Sincerley,

Jeff MCQ