We have not seen any gains from polishing the combustion chambers, but it did help to reduce detonation.

Tony

 

Thanks NINE, I was reading a article earlier were a guy was claiming to have seen a 6% HP increase. The only way I could see him getting any gains was if he had a detanation problem that polishing would have corrected. My suspicion has been confirmed. <img src="gr_images/icons/wink.gif" border="0"> <img src="graemlins/fluffy.gif" border="0" alt="[Fluffy]" />

 

I think it would lower compression would it not?

 

Has anyone checked the dyno numbers on just a polishing of the cumbustion chamber, valves and piston top. If so what were the results? <img src="gr_stretch.gif" border="0">

 

NO compression is lost.

 

Tums,

As porous as the LS1 aluminum combustion chamber casting is, I find it very hard to believe you wouldn't loose compression ratio. In fact, I would be inclined to believe you could loose up to a half point. My LS1 heads where extremely porous in the chamber and I considered having this done, saw some knock retard that I just couldn't eliminate, and my conversations with 2 of the sponsors of this board told me the same thing.

Todd

 

No one here was talking about compression ratio, but about compression pressure.
"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, intake size, altitude, fuel, engine and air temperature and compression ratio all combine to determine compression pressure.

Understanding COMPRESSION PRESSURE

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 (usually not restricted to a specific compression pressure) 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. At one time COMPRESSION PRESSURE above a true 8:1 was considered impractical. The heat of compression, plus residual cylinder head and piston heat, initiated detonation when 8:1 was exceeded. Some of the 60's 11:1 factory compression ratio engines were 11:1 in ratio but only 8:1 in compression pressure. The pressure was reduced by closing the intake valve late. The late closing, long duration intake caused the engine to back pump the air/fuel mix into the intake manifold at speeds below 4500 RPM. The long intake duration prevented excess compression up to 4500 RPM and improved high RPM operation. Above 4500 RPM detonation was not a serious problem because the air/fuel mix entering the cylinder was in a high state of activity and the high RPM limited cylinder pressure due to the short time available for cylinder filling. Enough I think you should understand by now that pressure and ratio are 2 different things.

My theory why NO COMPRESSION PRESSURE is lost by POLISHING
The ideal for polishing is to keep the combustion energy from being absorbed by the piston / chamber. The reflectivity of polishing makes the chamber more efficent for the cumbustion process. The incoming air is not heated by the piston and the piston reflects the heat of combustion, you start to approach ideal conditions. This makes up for the loss that most people think would occur from the polishing creating a larger chamber. Since the process of cumbustion is more efficent the gains and loses balance out to equal very close to the same compression pressure at combustion. The extra benefit is the elimination of detonation. If you will look at Tony's reply it says they experinced no gains other than the reduction of detonation. They did not experience a loss which is what would have occured if what you are saying would have happened to the compression pressure. <img src="gr_images/icons/wink.gif" border="0">

 

Tums
Good explanation!