Leelites1
Member posted 05-11-2002 09:48 PM
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hello...engine builders are finding that a piston that has a dish(Chevy Inlines have a dish) is better in buiding power....confusing huh?...well some still believe dome pistons are the ultimate-finding out that they are surprised too... new research has shown different results-yet you can find engine builders that knew this MANY years ago.....what you try to acheive is a queuch area(place where mixture of gasoline and air is swirled for optimum combustion)....the GM Vortech concept does this....even piston design in pistons now are dished....mmmmm so where does the compression come from?.... well smaller chambers in the head..as the piston moves to TDC, the fuel/air mixture is pushed to the dish where the optimum mixture occurs and most power is created.....the new Jim McFarland-designed Swirl/Quench hypereutitic piston -is designed to increase swirl and turbulence....(piston looks like a 250 and 292 piston dish design)....this info comes from the April 2002 issue of Chevy Performance,pages 82-85 for those needing to verify....this concpet has been around for years---even NASCAR incoporates this concept....so where does compression come from->>>>>SMALL CHAMBER or milling head....mmmm interesting huh..if you have a small chamber and a dish pstion ,swirl occurs and optmum combustion occurs....see how this works?
Lee
ps dont get upset at the messenger
[This message has been edited by Leelites1 (edited 05-11-2002).]
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Homey_D_Clown
Junior Member posted 05-12-2002 06:39 AM
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Here's a concept for you............
NOBODY CARES!!!!!!!!!!!!
[This message has been edited by Homey_D_Clown (edited 05-12-2002).]
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StrangeRanger
Member posted 05-12-2002 08:46 AM
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Ever looked at the inside of a FORD 1.6 litre Kent engine? I'll bet not. The head is perfectly flat, no chamber at all. the chamber is entirely contained within the piston dish. That concept's been around the Ford factory for about 40 years. There's even a high swirl Cosworth version that fits the OHC conversions. So who exactly discovered what?
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MustangSix
Member posted 05-12-2002 10:18 AM
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Using a dished piston allows more quench area, but you need a suitably small chamber to maintain CR.
Jaguar's Michael May used the concept to employ a 12:1 CR on the HE V12 engines (11.5:1 on us engines).
But for another extreme example, look at Chevy 348/409 engines - NO combustion chambers, like the Kent 1.6. Valves are flush with the head surface and the deck is angled to form the chamber.
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Jack Collins
'66 coupe
250 EFI Crossflow / T-5
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Jay
Junior Member posted 05-13-2002 06:01 PM
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I'll try and give my .02 on this subject, but I don't expect my post to stay active and around for long. If you're lucky enough to read this, you might want to copy it for your records.
Here is my opinion...and that is exactly what it is, just an opinion. I have only come to this decision because of past experience and I hope it helps out.
I prefer to use a piston top that will have a mirror image of the combustion chamber designed "into" the top. I dislike the complete dish (more like bowl) design, as this results in a very inefficient burn and creates an excessive amount of tumble in the chamber.
I always run the piston design described above in conjunction with a very small chamber. You will need to keep a few things in mind when setting up your combination.
Excessive cylinder pressure will encourage engine destroying detonation with NO piston immune to its effects. The goal of any performance engine builder should be to build their engine with as much detonation resistance as possible. An important first step is to set the assembled quench distance to .035".
The quench distance is the compressed thickness of the head gasket plus the deck height, (the distance your piston is down in the bore). If your piston height, (not dome height), is above the block deck, subtract the overage from the gasket thickness to get a true assembled quench distance. The quench area is the flat part of the piston that would contact a similar flat area on the cylinder head if you had .000" assembled quench height. In a running engine, the .035" quench decreases to a close collision between the piston and cylinder head. The shock wave from the close collision drives air at high velocity through the combustion chamber. This movement tends to cool hot spots, average the chamber temperature, reduce detonation and increase power. Take note, on the exhaust cycle, some cooling of the piston occurs due to the closeness to the water cooled head.
If you are building an engine with steel rods, tight bearings, tight pistons, modest RPM and automatic transmission, a .035" quench is the minimum practical to run without engine damage. The closer the piston comes to the cylinder head at operating speed, the more turbulence is generated. Turbulence is the main means of reducing detonation. Unfortunately, the operating quench height varies in an engine as RPM and temperature change. If aluminum rods, loose pistons, (they rock and hit the head), and over 6000 RPM operation is anticipated, a static clearance of .055" could be required. A running quench height in excess of .060" will forfeit the benefits of the quench head design and can cause severe detonation. The suggested .035" static quench height is recommended as a good usable dimension for stock rod engines up to 6500 RPM. Above 6500 RPM rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine.
If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a dish piston. Dish (reverse combustion chamber), pistons are designed for maximum quench, (sometimes called squish), area. Having part of the combustion chamber in the piston improves the shape of the chamber and flame travel. High performance motors will see some detonation, which leads to preignition. Detonation occurs at five to ten degrees after top-dead-center. Preignition occurs before top-dead-center. Detonation damages your engine with impact loads and excessive heat. The excessive heat part of detonation is what causes preignition. Overheated combustion chamber parts start acting as glow plugs. Preignition induces extremely rapid combustion and welding temperatures melt down is only seconds away!
For a successful performance engine, use a compression ratio and cam combination to keep your cylinder pressure in line with the fuel you are going to use. Drop compression for continuous load operation, such as motor homes and heavy trucks, to around 8.5:1. Run a cool engine with lots of radiator capacity. Consider propylene glycol coolant and low temperature thermostats. Reduce total ignition advance 2 to 4 degrees. A setting that gives a good HP reading on a 5 second Dyno run is usually too advanced for continuous load applications. Normally aspirated Drag Race engines have been built with high RPM spark retard. The retard is used to counter the effect of increased flame travel speed with increased engine heat. "Seat of the pants" spark adjustment at low RPM will almost always cause detonation in mid to high compression engines once they are rung out and start making serious horsepower. Set spark advance to make best quarter mile speed not best ET, usually 34 degrees total advanced timing.
"Compression Ratio" as a term sounds very descriptive. However, compression ratio by itself is like torque without RPM or tire diameter without a tread with. Compression ratio is only useful when other factors accompany it. Compression pressure is what the engine actually sees. High compression pressure increases the tendency toward detonation, while low compression pressure reduces performance and economy. Compression pressure varies in an engine every time the throttle is moved. Valve size, engine RPM, cylinder head, manifold and cam design, carburetor size, altitude, fuel, engine and air temperature and compression ratio all combine to determine compression pressure. Supercharging and turbo-charging can drastically alter compression pressures.
The goal of most performance engine designs is to utilize the highest possible compression pressure without causing detonation or a detonation related failure. A full understanding of the interrelationship between compression ratio, compression pressure, and detonation is essential if engine performance is to be optimized. Understanding compression pressure is especially important to the engine builder that builds to a rule book that specifies a fixed compression ratio. The rule book engine may be restricted to a 9:1 ratio but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 ratio but is usually not restricted to a specific compression pressure. Optimized air flow and cam timing can make a 9:1 engine act like a 10:1 engine. Restrictor plate or limited size carburetor engines can often run compression ratios impractical for unlimited engines. A 15:1 engine breathing through a restrictor plate may see less compression pressure than an 11:1 unrestricted engine. The restrictor plate reduces the air to the cylinder and limits the compression pressure and lowers the octane requirements of the engine significantly.
If you have a 10:1 engine with a proper .040" assembled quench and then add an extra .040" gasket to give 9.5:1 and .080" quench you will usually experience more ping at the new 9.5:1 ratio than you had at 10:1. Non quench engines are the exception to this rule.
Please feel free to use this information as you want, but contact me before publishing this in any catalog, spec sheet, or any other means of advertising.
Thanks again and I hope this helps,
Jay
[This message has been edited by Jay (edited 05-13-2002).]
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OHCFbird2
Member posted 05-13-2002 06:39 PM
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That was my whole point- to get the chambers small enough for a dished piston to work (LIKE MODERN ENGINES), it would require major surgery. .060" off of a 250 / 292 chevy head ain't gonna do it. Know how/why I know these things, Lee? I used to work in the cylinder head lab and dyno cells at several world-class builders. I have hundreds of hours on flow benches and on dynos, as well as having grown up in the industry.
Know what the biggest thing I ever learned was? That you never assume anything about a part or combo. Each engine is different and respond differently to mods.
If you'd ever like to have an adult conversation as to how swirl really works (it begins in the port, not on top of the piston), email me.
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JEFF
Pontiac OHC-6 Forum
