Piston Design

[DevinN]

Hi All,
Current GM V8 engines are available with different piston tops, performance applications like the LSx engines are fitted with flat piston crowns. In contrast many 6.0L Truck engines come with a dished piston crown.

What determines the piston design that is best for each application?

 

[jbthiel]

DevinN - According to GM the LS1 and LQ4/LQ9 are NOT the same engine family. The LS1 first appeared in 1997 and is considered a genIII. The LQ4 appeared in 1999 and the LQ9 in 2006 - both are genIV. Yes, all of the aftermarket and magazines say they are all LS1/GenIII's - but be assured that GM made changes. And seeing as the LS1 was the first engine in the Gen III/IV family, its hardware would have been set in stone much farther ahead of the release date than the LQ4, so in actuallity they are probably 3-4 years apart in design time. They also have significanly different intake manifolds (and probably exhaust manifolds). I'll bet the torque curves differ significantly.

LQ4 - 9.4:1 LQ9 - 10:1

And I most assuradly agree with Patprimmer - Trying to compare MPG numbers is useless. Even if both engines were in identicle vehicles you couln't nail the MPG differences to engine efficiently, there are way too many variables. If you want to compare the efficiency of the engines you need BSFC numbers.

And to again back up Patprimmer - trucks are made to tow things, not that many people do that anymore Truck engines need to be able to use cheap fuel and not detonate. Yes you can just back off on the timing, but then you loose a lot of power.

 

[DevinN]

"In my opinion, the difference in fuel economy will be almost entirely due to the weight and aero differences of the vehicle, but with slight differences due to compression ratio and barely measurable differences due to other influences of piston design"

"I most assuredly agree with Patprimmer - Trying to compare MPG numbers is useless. Even if both engines were in identical vehicles you couldn't nail the MPG differences to engine efficiently, there are way too many variables. If you want to compare the efficiency of the engines you need BSFC numbers"

Thanks guys,thanks for the help!

 

[SJWalker77]

It's my understanding that the "quench pad" has a certain limit of effectiveness. It's goal obviously is to increase turbulence for complete combustion, preventing pooling or left-over fuel in the ring lands.

It's also supposed to increased octane tolerance... though I have my doubts about the effectiveness at that since quench doesn't really occur until you get under 0.065" or so and iginition has already been triggered.

From what I've read about big block piston design it is much better to have your quench-area be a solid ring along the outside of the piston than to have a solid pad on one side of the chamber. Why? A few reasons I'd guess, but the big one seems to be the effectiveness a quench "jet" has in mixing the combustion chamber when it's injected into the middle of the chamber (i.e. over a dished piston), than accross the bottom of the chamber (i.e. over the surface of a flat-top piston).

Mind you, this is just conjecture from some hot rodders and engine builders I'v talked to, but some of them are very knowledgable and one came pretty close to winning the Engine Master's Challege a few years back.


Main reason the small blocks use flat-tops (like the LT1's -5cc) is to get the compression ratio they want. Using a dished piston (i.e. JE -31cc uses in S/C engines) would require MUCH smaller combustion chambers in the heads (i.e. say 20cc instead of 58cc) and this leads to 2 issues:
1) valve angle probably has to be reduced if you wish to keep the valve sizes large (therefore heads and intake/exhaust manifolds must be redesigned), and
2) increased likelyhood of an interferance engine design. Not a major issue, but defintely not a step in the right direction in regards to reliability.

 

[DevinN]

From what I've read about big block piston design it is much better to have your quench-area be a solid ring along the outside of the piston than to have a solid pad on one side of the chamber. Why? A few reasons I'd guess, but the big one seems to be the effectiveness a quench "jet" has in mixing the combustion chamber when it's injected into the middle of the chamber (i.e. over a dished piston), than across the bottom of the chamber (i.e. over the surface of a flat-top piston).

This is exactly the type of information I am hoping for. I saw pictures of what you describe from the EMC.

Some very high level racing engines designed for power and economy are now fitted with something called a concave dish piston. The cylinder heads are designed with as much squish area a possible with the valves determining the size and shape. The combination of the large squish area of the head and a circular piston dish creates exactly what you describe, a nearly circular squish ring around most of the chamber.

In contrast production vehicles have the piston dish and ring but the cylinder head design does not provide enough squish area to develop the "squish ring". I'm not in a position to make conclusions but it appears the auto manufactures could learn something here. I certainly want to.

Thanks, can you direct me to pictures of the said engine?