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Old 05-21-2015, 06:57 AM   #1
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Compression Too High? Possible Detonation

I'll begin by saying I couldn't find any related answers via a search because I think this issue hasn't been addressed directly or not in depth. I rebuilt my Viggen about 5,000 miles ago and it has always ran well. However, most noticeably at idle or when not under load, I can hear a soft, low-pitched "thud", which I assume could be detonation. In general, this has not affected the performance of the car. All I can tell is that it is completely random, and sounds like a rubber mallet hitting something soft, no rattles or loud bangs.

The motor has JE forged pistons, .5 bore over, has been rebuilt before I built it and across the board has compression at 225 psi +/- 5. I have heard of people running compression at or slightly above 200 but not 25 over. Could this possibly be compression that is too high causing detonation?

Like I said, the engine runs well. It has been tuned slightly to Stage 0/1, but the sound has been there since the first start up. Any opinions or comments are appreciated!
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Old 05-21-2015, 11:48 AM   #2
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Detonation and piston slap noises sound more like a "tick" or a "click" sound, like a marble rattling in a glass jar sort of, so it doesn't sound like that's it.

My built C900 has compression around there and it runs fine.

Is it just one random thud sound at a time?
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Old 05-21-2015, 01:49 PM   #3
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My wagon usually reads around 210psi
Even @ 225psi, your compression would be <9.5:1
At idle and cruising(almost no load) it would be nearly impossible to detonate based on that compression. (some hondas and LsX engines are 11:1+)
**edit** I was off- Ls6 is 10.5:1

You may need a bushing replaced. You may need a crank pulley replaced. you may need an engine mount replaced. I guess what I'm saying is that a thud is more likely movement than detonation.
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Old 05-21-2015, 02:42 PM   #4
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It's totally random, no pattern or relation to engine speed and it may happen under load but it is most noticeable when there aren't any surrounding noise, like when sitting in my driveway at idle. That's why I pointed to detonation but I have never heard a diagnosed detonating engine so I had to ask. If it is more of a movement, maybe its something related to the belt? If the car isn't moving I don't think there would be much noise coming out of the mounts/suspension (although it is way overdue for a refresh).
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