IIRC my quench height is .052"
I have over 100k miles on my stroker.
That's a pretty safe value. Is that with a zero-deck?
So, upon reading a Road & Track article, I came across a couple interesting tidbits about reducing knock. One thing they mentioned was the stroke to bore ratio having an effect on reducing knock. They said that an undersquare engine is less prone to knock due to less combustion chamber surface area, which reduces heat loss through the head as well as a faster combustion. But I would have thought that a larger combustion chamber surface area would conduct more heat from the combustion chamber (similar to how "cold" spark plugs work), thus reducing combustion chamber temperatures as well as knock. But the article is saying that a smaller combustion chamber's lesser surface area absorbs less heat from combustion in the first place (also due to a faster combustion) and results in an even cooler, more knock resistant combustion chamber. Makes sense.
Then I realized something about my XJ. My exhaust is extremely cool. To the point that you can easily grab the tailpipe without burning your hand. Meanwhile, you'll melt your soul if you even think about touching anywhere near the head. Which is why so many of us have heat soak issues along with cowl and/or hood vents. This leads me to believe that there is a LOT of heat being wasted through the combustion chamber. Yet the 4.0 is very undersquare. It's a curious thing. Ideally, 90+% of the gasoline we burn in the 4.0 will be converted to kinetic energy rather than heat, but I know that that's not going to happen without basically, a redesign of the 4.0. However, there's obviously room for improvement.
I wonder if one could use a higher compression ratio/more advanced timing in order to take advantage of the inherent reduction in knock by one of our strokers to come up with a more efficient engine. I guess my point is, those who are building very low compression strokers may be actually wasting potential efficiency. But then again,
any engine that's not operating on the verge of knocking is wasting efficiency. This also explains another reason for diesel engines having such a long stroke compared to their bore along with having extremely high compression ratios :idea: So apparently, simply the mere "stroking" of the 4.0 will automatically help reduce knock. Just another reason so build a stroker! :yelclap: party1: This of course is in theory. In reality there may be things about the 4.0 that don't let it respond that well to a change in stroke to bore ratio. I'm sure there are others who can speak from experience on this point, such as:
My 4.7L Jeep stroker is running 10.4:1 compression and due to getting the quench height adjusted correctly, I run the cheapest pump gas and get no ping. I also get 21 mpg on the freeway.
:thumbup: Anyone else have any input?
What other properties of a cylinder head will reduce knock? I would imagine that a good polish would help, along with smoothing out any rough edges, obviously. Short of sodium cooling, I can't really think of anything else :dunno:
On a side note, Mazda's "SkyActiv" technology is actually pretty cool. It keeps the intake valve open through 20-30% of the compression stroke before closing, in order to reduce power consumption of the compression stroke. Yes, this results in a lower compression ratio and a
less volumetrically efficient engine, but also increases its fuel efficiency. Less air = less fuel = smaller effective engine volume. The trick is - that in the day of variable valve timing, this Atkinson cycle can be "turned off" when more power is needed. The whole system is very similar in effect to a "multiple displacement system," except in my opinion, this is better executed.
