Topic: Changing Compression ratio Please comment

[Graywolf]

As many of you know I blew a piston. So now I am rebuilding the entire engine. I plan to install Lsj  ecotec diamond racing pistons.  This will raise my compression to 9.5 to 1  Stock is 9.2 to 1 for the Lnf turbo What do you think???   Gray Wolf

[Uranium-238]

All I know is normally people lower their compression ratios if they're running FI, and raise them for NA. I do know that increasing boost effectively increases the compression ratio.

Check this calculator out.

http://www.rbracing-rsr.com/compression.htm

So, at a stock 16 PSI, 9.2/1 equates to 19.21/1 at sea level. That's pretty intense. A 23 PSI tune yields 23.59/1, a 27 psi tune 26.1/1 and 30 psi a whopping 27.98/1.

Of course, this doesn't take airflow into account. I'd say your choice would have to be based off of what kind of turbo you want to run.

[shabby]

I plan to install Lsj ecotec diamond racing pistons.

You're going to use non direct injected pistons?
Going from 9.2:1 to 9.5:1 doesn't seem like that much of a big deal, i think the bmw 335i runs around 10:1 compression if not higher.

All I know is normally people lower their compression ratios if they're running FI, and raise them for NA. I do know that increasing boost effectively increases the compression ratio.

Never heard of anyone lowering their compression ratio on an already forcefed engine, they always bump it up for more power just like the n/a guys. Unless of course you go from n/a to forcefed then you lower it.

[Uranium-238]

But depending on his turbo plans that may not be the best idea. I'm by no means an expert, but I do know that everything needs to work in concert. Bumping up the compression might be brilliant with turbo a, but pointless with turbo b.

[DirtyMike]

arnt our piston's top designed a bit differnt because of the DI? why not get the wiseco lnf pistons and keep the same compression?

[Treeman]

You know, 238, I was wondering recently about our effective compression ratios, which are well into diesel territory.  I know we have wonderful things like ECMs and DIs which control detonation but, thinking about it, it is absolutely amazing the boost some folks are getting from these engines.  What keeps them from turning into diesels?  Or is the sparkplug superfluous at these pressures?

[Uranium-238]

93 octane and tons of KR, I guess.

[Critterman]

I think you will find diesel compression ratios way above what we are seeing.

[Critterman]

Here you go from Google is your friend:

Filed In:Auto Repair
 
Car Repair Help
 The Diesel Engine
   In 1892 Rudolf Diesel developed and obtained the German patent for the diesel engine. His goal was to create an engine that was highly efficient. Much more efficient than the gasoline engine that was invented in 1876 and was not very efficient at all. Especially at that point in time.

   There are two main differences between a diesel engine and a gasoline engine.

A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously.
A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.
   Gasoline engines use either a carburetor or a fuel injection system to deliver the fuel to the cylinder. With a carburetor the fuel is mixed as it enters the intake manifold, long before it gets to the cylinders. In a fuel injection system the fuel is injected just before the intake stroke at the intake valve

   Diesel engines use direct fuel injection (DI), that is to say the diesel fuel is injected directly into the cylinder. The diesel engine has no spark plugs. The air it takes in is compressed and the fuel is injected directly into the cylinder where the heat caused by the air compression ignites the fuel. In the old days this meant that it exploded and expanded very quickly, making a noisy engine. This is why most diesel cars were IDI (indirect injection); the rough behavior was fixed by injecting the fuel into a small pre-combustion chamber that is connected to the cylinder by a narrow passage.

   This slows down the explosion, as the gasses have to escape from through the narrow passage into the cylinder. This gives a softer bang and a smoother engine, but the gasses have to work harder, which lowers the efficiency a little. However the newer breed of DI engines use other techniques to tame the behavior of the engine, such as two stage injection, electronic control, and acoustic shrouds and shock absorbing engine mounts to mask the rattle.

   The injector on a diesel engine is its most complex component and has been the subject of a great deal of development and innovation. On any specific engine it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process.

   One major difference between a gas engine and a diesel engine is in the injection process. Most car engines use port injection or a carburetor rather than direct injection. In a car engine all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine. If it compresses the air too much, the fuel/air mixture spontaneously ignites and causes knocking. A diesel compresses only air, so the compression ratio can be much higher. The higher the compression ratio, the more power is generated.

   In the United States and Canada, diesel engines are most commonly found in trucks and buses. In Europe, where fuel prices fluctuate around $3.00 to $4.00 per gallon, the fuel efficiency of the diesel has made it a popular choice for cars, and most European-market cars are available with a diesel or a turbo-diesel engine.

   Compared to gasoline-powered vehicles, diesels are more fuel efficient, and they can travel significantly farther on a tank of fuel than their gasoline counterparts. Diesel engines produce more torque, and they tend to be more durable. They don't need an electric ignition system, which reduces their complexity. However, they also create more noise, they can be difficult to start in extremely cold weather and they sometimes require more frequent routine maintenance than gasoline engines.

   Most passenger car diesel engines have a glow plug of some type. When a diesel engine is cold, the air compression may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electric heater that glows red-hot and helps to ignite the fuel when the engine is cold so that the engine can start

   Copyright © 2000 - 2003 Vincent T. Ciulla All Rights Reserved

[Treeman]

So their description of a diesel engine sound remarkably like ours including the compression ratios and direct injection.  What is the timing of our direct injection - is it during the intake stroke?  Still makes me wonder if a spark plug is optional at even stock boosts (comparing 238's calculations with Critter's article).

[Ben L]

Not sure our DI's are "compressing only air" right before the intake stroke, but what do I know?

Fascinating stuff at any rate. . . .