Ben Diss

iTrader: (1)

I've been reading a bit on thermochemistry and fuels. Wrapping my head around the combusion event has been a bit of challenge but I think I understand now how knock occurs and how different fuels behave in this regard. One thing that is talked about is how combustion chamber geometry affects engine knock. Some engines, due to their combustion chamber geometry (not necessarily compression), are more prone to knock than others. Some fuels, even though their anti-knock index may be the same, are more prone to knock in knock-sensitive engines than others. A fuel sensitivity index is given as FS = RON - MON where low numbers indicate a fuel that's not sensitive to engine geometry.

How does the LSx engine fall in this regard? Is it sensitive to knock or not? Is there a difference between 243 heads and L92 heads? How would using larger than stock valves effect an engine's sensitivity to knock? What about different spark plug designs?

I guess what I don't understand is what kind of combustion chamber geometry is more or less sensitive to knock?

Thanks.

gectek

iTrader: (2)

I thought you said you read up on this. Normally those effects are only for older head designs with a huge open chamber where alot of hot spots can occur and there are alot of sharp edges. Case in point the old open chamber large cubic inch pontiac heads like the 400-455. They had many many different head designs that had a large range of cc dimensions, from around the low 100's to the 120's or so. The larger chamber would produce more tendency towards knock when the cr reached a "breaking point" iirc it was about 9...that is one of the reasons the stock CR on those engines was around the mid to low 8.x range. If you have a good quench area, not alot of sharp edges or paths where the burn cannot reach efficiently and carbon desposits and hot spots do not build up, then you shouldnt have a problem. One way to stay away from knock occuring is to run lower coolant/oil temps and run a colder range plug. There are also several other ways

The new chamber designs of the LS heads are not prone to any type of knock tendencies that I have heard of. While GM may have taken the short road on some things, they did not mess up on the cyl head design. The chambers are constantly being redesigned. The chamber is very small now, almost reaching the diesel type chamber soon probably. Then the chamber will be only on the piston face.

If only they were splayed...

Ben Diss

iTrader: (1)

Thanks. That's good information. I've got two followup questions.

- whats quench area?

- whats a splayed piston?

Also, reading through this and thinking a bit, it seems like notching pistons for proper valve clearance would increase the knock sensitivity by creating texture for uneven airflow in the chamber and by reducing mass unevenly.

gectek

iTrader: (2)

splayed valve not piston
and no notching the piston, when done right, will not effect knock sensitivity that much...only when done improperly, will it effect anything. the reason people notch them is to save time and money on custom piston setups, or breaking down the engine and replacing the pistons. they just run a big cam. the cam will increase the overall flow characteristics of the entire chamber if designed properly, and you will not have a problem with the flycut pistons at all. it everything is poorly designed and poorly fabricated/machined, then yes you will have all sorts of problems.

for quench, check out this
http://kb-silvolite.com/article.php?action=read&A_id=35
or just search for quench or quench area to get a good explanation