S54 question - high compression and low octane *advice/opinions needed*

[mainstreamer00]

Hi everyone,

I have a general question that I'm looking for advice on - any anecdotes or thoughts are appreciated.

The scenario is engine seal failure (cylinder 3 blown) on an engine running 12:3:1 compression on 91 octane.

From your experience, is the decision to run such high compression on such poor quality octane advisable, or asking for mechanical compression pre-detonation?

I can elaborate, but am running into an accountability question on a newly rebuilt engine experiencing such issues.
Hassan has also suggested the above to being the cause of the issue.

Further backup or thoughts would be much appreciated here.

[digger]

It makes zero sense to run that high compression on medium grade pump fuel. You’ll make more power with lower compression as you won’t have to limit the timing. Limiting the timing hurts more power than what you gain with the higher compression. People way overestimate the benefits of compression ratio

[mainstreamer00]

Agreed^

So in the case of pre-det causing potential damage to seals/ringland etc (won't know until tomorrow), would it be more likely that a) the car was experiencing detonation due to mechanical compression of the cylinder b) some issue with the initial tune's timing parameters (car was knock sensing as well)

Sorry if this is vague, just trying to understand the issues more clearly.

[digger]

You can tune a NA 14:1 on pump gas with very conservative timing, appropriate AFr, appropriate temp compensation it’s fairly silly approach but can be done as you can tune a turbo with richer mixture and much less advance which is kind of analogous in some ways

A number of non tune issues exist:
- fuel delivery issues global or individual cyl
- fuel consistency ( tune was different quality fuel )
- mechanical issues
- etc
- etc

[AdamM3]

I have had an engine that was 12.5 -1, it also had pretty large cams so the mechanical compression was probably helped a little by that. I ran it on 93 octane and it was definately knock limited and caused me to run way less then optimal timing. This engine had forged pistons and survived for a decent amount of time but i probably could have made a little more power with slightly less compression and more timing. It still made about 200whp on a 1.8L. It can be done but tuning needs to be on the money, this engine was very easy to get into detonation for sure. Then there is the issues of fuel quality, how good the knock sensing system is(it will be leaning on the knock sensing system at this compression level and octane level), etc.