Spark Retard with a Turbo

[kevinthenerd]

I sat down and figured out some calculations to see why people lower their compression ratio in turbo cars. Intercooling is a beautiful thing. I got to thinking... in a street engine (which is closer to 10:1 often), can you get all of the benefits of decreased compression ratio with spark retard? I guess my question is... Would knock ever occur in the complete absence of spark (like with preignition from carbon hotspots), or do always need some kind of spark to screw things up? My thought is... with no combustion whatsoever at TDC, you can use only part of the down stroke and basically have an anti-Atkinson cycle where you could take advantage of a long intake stroke with a short expansion stroke. The only major problem I could see is that the combustion will have to take place while the piston is moving quickly, which is very bad for efficiency. Perfect efficiency would only occur with a constant-volume burn.

[UncleBob]

there are many many different flavors of detonation. Its not a one-size-fits-all deal. Too much compression with too much boost with too little octane will cause detonation no matter what you do to the ignition timing. For that matter, too much boost with too little octane rating will cause detonation no matter what you do

The exact parameters you can get away with are way too complex to answer generically. As a general rule, the higher the octane, the lower the CR, the lower the timing, the more boost you can run safely....within reason. What combination you find of those variables, and how you figure out whether you've gone too far in one of them, or a combination of them.....well, that is the magic of tuning engines.

[KiwiBacon]

Bob was talking about the different flavours of knock. I like to think of two distinct ones, the preignition (ignition before the spark) and the detonation (spontaneous ignition started by the spark).

One problem with spark retard is the still burning fuel which is dumped out the exhaust ports. In the extreme case burning up valves, turbos and other exhaust items.
It's possible that many people who think a lean condition melted their engine, could have had the knock sensor retard their timing to the point where normal combustion was the problem and the fuel mix may have been fine.

Also interesting is the possibility of harnessing detonation to gain efficiency. There's an interesting thread on Engtips right now regarding Smokey Yunick and his high mileage experiments with it.

[534BC]

One benefit or reason for more power from low compression that is rarely mentioned is that if pressures (or detonation threshold) remains constant , the low compression engine is physically a bigger engine.

[kevinthenerd]

Originally Posted by 534BC

One benefit or reason for more power from low compression that is rarely mentioned is that if pressures (or detonation threshold) remains constant , the low compression engine is physically a bigger engine.

That shows up in the math as a natural log term, but yeah.

[KiwiBacon]

Originally Posted by 534BC

One benefit or reason for more power from low compression that is rarely mentioned is that if pressures (or detonation threshold) remains constant , the low compression engine is physically a bigger engine.

I don't get what you mean by "more power from low compression"?
Are you referring to forced induction with the "physically bigger engine"?

[beef_bourito]

he's saying that with two engines, the same size, same pressure at TDC, one with lower compression, the lower compression ratio engine is almost like a larger engine because of the increased boost, and therefore increased air/fuel in the cylender.

[534BC]

Yes, that's it. Lower the compression and increase boost.