How does quench area change affect same compressio

[Guest]

In an effort to possibly use my shorter than intended stroke crank, I'm trying to decide how significant an increase in quench height will be in the face of a resulting slight increase in compression.

I understand the dynamics, however this is a unique scenario. First this is a turbocharged engine which by nature will have turbulent inflow into the cylinder above what natural aspiration would produce.

Second, the quench height and stroke (3.31 to 3.48 ) would be increased and the piston volume decreased at the same time from 18 cc to 9 cc with a resultant slight increase in static compression of ~.1. The piston will go from .031 below the deck to .093 below the deck. The gasket thickness is ~.064 stock taking the OE .095 total quench to .157.

I can't find the info I need to make a decision. I'll have considerably increased quench which usually is a bad thing however, the uniqueness of fast burn heads, higher resulting compression despite the increased quench and the turbulent airflow due to turbocharging to counter the lost mixing effect normally caused by optimum quench.

I've read some say it doesn't matter as much on a turbocharged engine. My personal experience with contrary to popular belief theory is with putting iron heads over aluminum head pistons in a 3100 resulting in considerably lower compression turbocharged. Despite the suggested 7ish compression ratio, in my Fiero with the 4-speed manual turning about 2800 rpm on the interstate, it still pushed the car to just shy of 28 mpg with new tires and a tune up, about the same as the original 2.8L with 8.8:1 compression and no turbo. Doesn't make a lot of sense to me.

That's a 3100 block and pistons with the Fiero 2.8 intake and heads.

So, will the increased quench have a drastic effect eventhough the compression ratio increases at the same time?

[86FieroSEv6]

The effect will be more dramatic below 3k rpm. If you blend out the quench shelf near the valves it should reduce some of the turbulence to the point of acceptability. The sooner you can get into boost the better.

[Joseph Upson]

Reduction in turbulence is detrimental, reduced quench area provides its benefit by increasing turbulence which promotes mixing of the air fuel mix and then forces the mixture toward the center of the chamber for a better chance of an organized combustion process as no combustion takes place in the mix in the quench zone.

I believe it will be a less than positive effect as a result of what I've stated above, good compression but poor quench resulting in increased detonation risk due to a quench area too wide to prevent spontaneous detonation or ignition from a second source in the perimeter. Guess I'll have to buy another engine to get a crank.

[86FieroSEv6]

Then why are we smoothing out our piston domes and combustion chambers? Why are we changing valve angle and locations? While quench area plays a role in the mix, combustion chamber shape, valve position and through flow are more critical and I wouldn't focus on quench so much in a sub 500 horse engine. I've never seen a text book in the lane next to me.

[60dgrzbelow0]

Some parallel articles with on point information:

http://www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech102.html

http://www.car-forums.com/s9/t11032.html

http://racingarticles.com/article_racing-10.html

http://www.gofastnews.com/showthread.php/1404-squish-quench-in-HO-turbo-16V-4cyl-motors

[Joseph Upson]

Then why are we smoothing out our piston domes and combustion chambers? Why are we changing valve angle and locations? While quench area plays a role in the mix, combustion chamber shape, valve position and through flow are more critical and I wouldn't focus on quench so much in a sub 500 horse engine. I've never seen a text book in the lane next to me.

Are you asking a question, or making a statement? My situation is the result of a crank that was offset ground to the wrong stroke. I'm not smoothing anything, changing valve locations/angles directly, or modifying combustion chambers, I'm assessing the practicality of using what I already have for an intended build. The consistency in what I have read as well as what is supported by some of the threads linked to above is that turbulence is a good thing especially that associated with small quench areas. I'm just trying to assess my unique situation regarding whether I should use the 3.48 stroke crank with what I have and its acompanying increased quench area, or aim for the intended 3.56 stroke I sought to begin with to restore the stock quench and higher compression I also wanted.

[geoffinbc]

If you don't have tight quench then you don't have fast burn heads. They go hand in hand. The shape of the fast burn chamber is only created when the piston comes up to the cylinder head deck. This forces the gases near the wall of the cylinder into the center of the chamber. When the burn starts it has less distance to travel so the mix in the chamber burns fast. Hence the name. If you allow the gases to stay over by the walls then you just have an open chamber.

Fix your problem don't band aid it. Get a new stock crank. Did you tell the shop to machine it wrong or did they screw up? If it was their fault it sounds like they owe you a crank plus machining.

[Joseph Upson]

If you don't have tight quench then you don't have fast burn heads. They go hand in hand. The shape of the fast burn chamber is only created when the piston comes up to the cylinder head deck. This forces the gases near the wall of the cylinder into the center of the chamber. When the burn starts it has less distance to travel so the mix in the chamber burns fast. Hence the name. If you allow the gases to stay over by the walls then you just have an open chamber.

Fix your problem don't band aid it. Get a new stock crank. Did you tell the shop to machine it wrong or did they screw up? If it was their fault it sounds like they owe you a crank plus machining.

You are pretty much repeating what I've already said, my question is about what one can anticipate with my situation though. There's no band-aid consideration, just a concern about the practicality of using the current parts all of which are forged in the stated circumstance. By what you have said and popular rule of thumb the factory 3900 already has the equivalent of an open chamber, gasket thickness = .064, piston location at TDC -.031 for a quench height of .095 which is well beyond recommended yet with a 9.8:1 compression it still runs fine on low grade fuel. We don't know for sure if the quench standard changed with the new head design or not, given that fact and the additional knowledge that quench science likely originated and still focuses in discussion mainly on early V8 motors and is super imposed on modern day engines. Consider the argument that whenever an engine is boosted you have to lower compression. Like I said, I understand the theory, my question is its validity in my circumstance. When the cometic head gaskets were listed in WOT-Tech I recall seeing some pretty thick gasket options. Again, my situation involves a prallel relationship between quench and compression ratio coupled with forced induction. I'd like to use what I have if it is likely to work without a problem instead of butchering another motor for a crank.

The offset grind mistake is my fault, I broke my rules and trusted the machinist understood what I intended when I set the crank in front of him and showed him what to do instead of drawing it on paper so his mind couldn't edit and recreate my instructions. The crank looks great, it's just the wrong stroke.

[86FieroSEv6]

Well then, what exactly is the stroke change you have induced? Personally, I think that as long as you are in the ballpark for your desired compression ratio and piston:valve clearance is fine then go for it. Nattering over a small loss of quench is a wild goose chase.

[Joseph Upson]

Well then, what exactly is the stroke change you have induced? Personally, I think that as long as you are in the ballpark for your desired compression ratio and piston:valve clearance is fine then go for it. Nattering over a small loss of quench is a wild goose chase.

3.48 stroke, piston to valve clearance will not be a problem for sure thanks to the additional .054 sink in the hole, the replacement pistons are .010 taller recovering that much from the number posted earlier. I've heard of as much as .120 so maybe .144 with modern technology and relatively high compression will be okay. The oil squirter should compensate for the loss in cooling in the quench area of the piston due to the increased quench. I really want the high compression though so I think I'll go ahead and try again. The goal was 11.5:1 compression with boost for overall quick response and improved efficiency with the 6 speed tranny.

[Joseph Upson]

I've figured it out, instead of dumping more than ~$600 in another attempt with a third motor and regrind cost, I'll address the problem with the .064 multi layer metal head gasket which is a 3 piece part that has a center piece comprising about .050 of the gasket thickness, maybe I can have a machine shop plain it down to about .030 or better yet I can take the two thinner layers from my new gasket set and combine them with the ones from the old gaskets for what will probably amount to about .025 over four metal sheets that should seal well, heck if it is determined that I can get away with using the two sheets alone it will put me almost right at the stock quench height, as well as push my static compression up over 10:1. I can then use the funds to balance the assembly instead.

[95Grand-Am]

I don't know if it is possible but could you find a longer rod to move your pistons up a little?

[Joseph Upson]

I don't know if it is possible but could you find a longer rod to move your pistons up a little?

Are you kidding! I'm dealing with SBC H-beam rods that have already been narrowed for the crank. I did however explore that option and the next rod option up is 5.850" at over $400 for the set and therefore too long. Even with a thicker head gasket this approach would be as expensive as buying a broken, or cheap motor for the crank and having the crank ground again after you total up the rods, plus narrowing and a thicker gasket provided cometic has them available in time. The crank is a nice piece in its current state, it just needs to go back and have the radiused shoulders reduced a little so that the chevy 2.00" bearing doesn't get pinched as it is wider than the V6 bearing which is a good thing in terms of durability.

[86FieroSEv6]

I think you'll be alright. Just make sure the deck is absolutely flat. Head gaskets over .040" thickness demand this to prevent leaks.

[Joseph Upson]

I think you'll be alright. Just make sure the deck is absolutely flat. Head gaskets over .040" thickness demand this to prevent leaks.

The stock gasket is multilayered steel at .064, I'm looking to make it thinner. The engine hasn't reached 10k yet so there shouldn't be an issue with the deck being flat.