I was told that cars that naturally aspired engines that are built usually have low compression pistons. He went on to say, supercharging an engine with low compression pistons could be bad. Apparently you need high compression pistons to handle the higher crank case pressure.

What my question is: can someone give me an overview on how this works? What is the Physical difference in the pistons? What about the piston rings? How can you tell the style of pistons apart? What about the connection rods and stuff?

Please enlighten me fellow mustangers.

Mike

 

he is telling you backward a S/C motor likes lower comp.while a N/A motor runs better with more comp.

 

Compression is simply a relation to the cubic area in a cylider when the piston is at its lowest point compared to how much space is left after the piston is at its highest point in the stroke. The most common way to change compression ratio is to use a different piston, one that will take up more room in the cylinder (like flat top pistons or domed) to increase compression, and the opposite (like dished or with valve reliefs) to decrease compression. Of course other things like rod length, etc can affect the compression ration, a piston change is the most common way.

The purpose of compression is to increase the cylinder pressure - since the more pressure = more power but too much pressure can be a bad thing and cause pre-ignition (like a deisel - which don't use spark plugs, they rely on 18+ to 1 compression to self ignite the mixure (often with help from glow plugs). So going to a lower compression piston will decrease cylinder pressure enough to use higher amounts of boost to obtain more power without pre-ignition.

Hope you enjoyed this essay.

 

I did, now I kinda understand why someone would want a higher compression
but as to lower ...?

 

Well to answer your question their is pretty much a "magic number" for cylinder pressure in relation to ignition timing to prevent pre-ignition. You wan't to get as much air into the cylinders as possible without going over, if you see what I'm getting at. You will get to a point where you can add no more boost, can't go any higher on the octane rating, and you can't pull any more timing without getting detonation. At that point if you want to increase boost and hp you will need lower compression pistons.

A cheesy generic example for a 5.0 engine:

This particular NA setup produces 300 hp with 93 octane fuel 9:1 compression and 15* initial timing
Change the compression ratio to 12.5:1 run 104 ocane race gas and get 375hp MAX
Drop the compression back to 9:1 add a supercharger at 6# drop the timing to 10* and you get 400hp MAX
Increase boost to 10#, add intercooler, drop timing to 8* and you get 500hp MAX
Increase boost to 16#, lower compression to 8.5:1 and you get 600 hp
Increase boost to 20#, lower compression to 8:1 and you get 700hp

Of course its a little more complicated than that, but you get my drift. Compression ratio is overrated by the way, it doesn't make a HUGE difference, but when you're maxed out with all other mods and going up a point adds 10 - 15 hp I suppose it's worth it.

 

Hrmm, kinda gotcha

 

it's about the "sqinch" compression also needs octang to be affective in complete combustion...

 

The old "roots" type blowers are the ones that you really have to worry about lowering your compression. Holley says that a roots blower running at 6# with an 8.1:1 compression ratio is like running a 12.1:1 compression on an N/A motor. With a turbo or a centrifical supercharger that would take about 15# of boost to do the same. That's because the roots blower compresses the air in the cylender and the others compress the iar inside themselves.

 

that statement is not true at all.

 

What's not true about it?