Piston Design
Piston Design
(OP)
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?
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?





RE: Piston Design
RE: Piston Design
You also need to consider the taget rpm range, bore diameter, and piton weight. I have seen many references that a bore of approximately 4" is a good compromise.
RE: Piston Design
In addition squish to bore ratio is lowered with the squish band of the piston. This could have been avoided with a different piston design or with a larger combustion chamber. Instead the designers decided on the central bowl, my desire is to understand these reasons.
RE: Piston Design
I’m digesting your response. A unique combustion space is created when a dished piston is at TDC, the squish area becomes a small band and the chamber extends under some of the normal squish area of the cylinder head. With this mismatch the flame appears to have a greater distance to travel, why would this be desirable?
RE: Piston Design
What changes from application to application is requirements for power density vs fuel cost.
Sporty applications will favour power density and commercial applications will favour low fuel cost and long engine life. This means lower compression than sports models.
They choose to vary pistons rather than heads, I presume for manufacturing cost and inventory control reasons.
A reason for the shape of the dish might be to eliminate the need for left and right pistons or left side vs right side inlet valve pistons.
Regards
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RE: Piston Design
On a couple of other forums the general consensus was that you want a quech of about 0.04". I'm not sure what the whole rational is - but the indications are that a space this tight along with modern combustion chambers will result in an efficient burn and help prevent detonation (and combustion along the edges until the piston moves down some?)
RE: Piston Design
Are you looking at two engines in particular? The LS family has at least 4 different bore sizes and 3 different strokes.
RE: Piston Design
“ sporty applications will favour power density and commercial applications will favour low fuel cost and long engine life. This means lower compression than sports models”
I understand the idea of reduced compression relating to extended engine life. The part that puzzles me is low compression in relation to low fuel cost. I would think that increased compression would lead to higher engine efficiency and reduced fuel costs.
RE: Piston Design
“Are you looking at two engines in particular? The LS family has at least 4 different bore sizes and 3 different strokes”
jbthiel, I am comparing the 5.7L LS1 engine that was installed in the Camaro at the end of the production run to the 6.0L engines that were installed in Trucks during the same time frame. I’m trying to understand the rational used when determining the engine design differences.
I understand the differences in the application in regards to the vehicles but not how the requirements of these applications relate to the engine design differences.
RE: Piston Design
Hi octane fuels are more expensive generally by a greater margin that the increased thermal efficiency of the higher compression ratio they will support.
If this were not true, all manufacturers would increase compression and the lower octane fuel that was not cheaper would disappear from the market.
jbthiel
How so. All serious engine builders I know, cut the deck of the block to give what they believe is the optimum clearance. It varies with engine size, cooling, rpm and materials of construction, but so close they just touch at maximum rpm with hot pistons and a cold block, plus 0.002 or 0.003" is what most aim for.
This is to expel as much charge from this area as possible without anything actually hitting in operation. This charge being expelled then tumbles across the combustion chamber, thus increasing the speed of flame spread, thereby allowing for later ignition but still reaching maximum cylinder pressure at the ideal time.
Regards
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RE: Piston Design
Over the years there has been some pressure to reduce the octane levels of petrol. Australian car manufacturers succesfully demonstrated that this hurt fuel economy, consequently our octane levels for 'regular' fuel are increasing. Also, some modern engines are calibrated such that the extra efficiency from high octane fuels more than offsets the increase in price, eg WRX on premium gas. Of course another way of saying that is that their efficiency falls off with decreasing octane, which is not quite so good PR wise.
Cheers
Greg Locock
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RE: Piston Design
1. I prequalified that statement with "Assuming near equal compression ratio and combustion chamber shape." My point was along the production commonality line - if the heads and CR are the same the flat top will need to stay farther down the hole. I never said it was ideal. And usually if you change piston design you will be changing CR.
2. I agree with you on an all out performance engine. Off the top of my head I was thinking about 0.010", but when I searched a couple of different forums to confirm this the 0.04" value was being tossed around alot. That seemed too big to prevent ignition.
What do you think the minimum design clearance is for a production engine? I wouldn't think that they could hold less than 0.040" with all the tolerance stack-up. Maybe thats were the bigger numbers are coming from?
RE: Piston Design
0.040" static is not 0.040" running. Blocks expand, pistons rods and cranks expand. Rods stretch, piston skirts flex and pistons rock.
The optimum clearance depends on the size of the engine, the construction style and layout, materials used, accuracy of plant, and the engines cooling system.
A SBC with steel rods generally likes about 0.036" but an air cooled VW with forged pistons and 93 mm bore likes about 0.032"
Regards
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RE: Piston Design
The performance application is more focused on higher RPM power with an aerodynamic body style and gearing to match; this ultimately reduces engine load.
In contrast the Truck application demands an engine that will perform with heavy loads at a lower RPM. Body styles are less aerodynamic and consideration must be taken for additional payload and towing capacity.
Surly these distinct differences weigh in the decision making regarding piston selection and final compression ratio.
RE: Piston Design
The truck application will need to do higher mileage at higher load, so it will need lower rpm and generally heavier components or more durable material, but this does not effect piston dome shape other than its effect on compression. This might be reflected in block casting details and materials, crank material, 4 bolt mains, piston alloy and skirt design, valve train components etc
Regards
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Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Piston Design
Your two engines of interest use the same stroke and only differ in bore size by 2.5mm (~0.1"). One reasone may be that the LS1 is a gen3 engine where the 6.0L is a gen4.
RE: Piston Design
6.0L/364 CI Truck engine (Gen. III)
71.0 cc combustion chamber
9:1 compression ratio dished piston (estimate CR, I have not found actual)
300 HP
360 FP Torque
4 speed automatic
Consumer Guide Observed MPG 13
Fuel 87 regular
5.7/350 CI LS1 Camaro engine (Gen. III)
67 cc combustion chamber
10:1 compression ratio flat top piston
310 HP
340 FP Torque
4 speed automatic
Consumer Guide Observed MPG 17.4
Fuel 91 premium
RE: Piston Design
"The two primary drivers of mass production piston design these days is cost and emissions, with less priority given to octane tolerance, fuel efficiency, air flow, etc."
I agree with this totally, this is one of the reasons I'm interested in learning more. Trucks are widely used in my area and appear to be lagging in fuel efficiency.
RE: Piston Design
As I am reading your recent comments you are trying to associate the higher fuel consumption of a truck vs a sports car to piston design.
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 will say it again. The dish is to reduce compression so that cheaper fuels can be used, thereby giving lower running costs. All other factors being equal, lower compression also gives lower power output but longer engine life or equal engine life under a heavier duty cycle.
To quote an old joke. "Mummy mummy, why am I going round in circles? Shut up kid or I will nail your other foot to the floor!".
I am starting to feel like some one here has one foot nailed to the floor.
Regards
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
RE: Piston Design
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 :P 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.
RE: 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!
RE: Piston Design
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.
RE: Piston Design
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?