Recently have been doing a lot of reading and searching for info on CR.... but could not find what I was looking for....

My question is this, when increasing CR, is there a differnce in achieving a given target CR by use of HG only and acheiving the same CR with pistons only?

What I'm trying to ask is if there is a difference in how the engine makes its power (possibly in different RPM ranges?) or would it be the same? Or is it more of a case that yeah you have acheived your desired CR with lets say a HG change but it is not as reliable as what acheiving the desired CR with use of pistons?

Sorry if I'm not making sense, I'll try to elaborate further if needed.

Cheers!

 

Dropping in pistons won't change your cam timing, and you'll still have plenty of room to shave the head if you ever warp it. Other than that, given the same CR, they will not have differing affects on your torque or power curves.

 

Yeah that is what I was thinking..... so in theory.... it is a case of

"Compression is Compression!" no matter how you acheive it.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SPEEDCORE &raquo;</TD></TR><TR><TD CLASS="quote">"Compression is Compression!" no matter how you acheive it.</TD></TR></TABLE>I don't think so. If you raise your compression by changing pistons, your new pistons would have a slightly different shape on top. So your combustion chamber has a slightly different shape.

It's probaby not enough of a difference to measure, but theoretically it's different.

 

By milling the head or running a thinner HG, you reduce quench area.

 

Ok I understand that.... but interested if there would be a difference in power between acheving the CR between the two? And if there would be a difference in reliability between the two also..... ie. I for some reason think that getting the CR to a desired level with the use of pistons is moe reliable than the use of a HG.

For arguments sake and to sort of bring some more info to the discussion lets use the following as an example,

AUDM B16A2 which is 10.4:1
Target CR is 10.8
Either with use of HG only, or
Use of - PCT's only.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote &raquo;</TD></TR><TR><TD CLASS="quote">
Quench, or squish area is typically the flat area on the top of the piston that's almost level with the top of the block deck. It must have a corresponding flat area on the deck surface of the head to qualify as quench.
If you look at a combustion chamber, you will usually see these flat areas, and they will have the volume of the actual combustion chamber between them. When the piston is compressing the mixture, as the piston nears the head, the flat areas on the head and piston come together and force the mixture from those areas to "squish" into the chamber, where the spark plug and burning mixture reside, so you achieve a more complete burn.
The quench area also runs cooler than the rest of the chamber / piston. These lower temperatures are where the "quench" comes from.
When properly designed, the quench areas can have a tremendous effect on the quality of combustion, and allow higher compression ratios, and due to this they are considered "artificial octane" by scientific types.
Bottom line is "properly designed, quench is good".
.......................................T.O.O. .......................................
ps. As it is like (more octane), and promotes a better burn, why do the turbo people do away with it?? Go figure.

</TD></TR></TABLE>

 

hmmmmm interesting...... the article said that there is corresponding "flat" areas on the piston and the combustion chamber and this is the quelch area ... I always thought the quelch area was pretty much the combustion chamber as a whole...

I understand that if the whole of the combustion chamber IS the quelch area, how the quelch area is reduced with a thinner HG..... would this not be the case with a higher compression piston also? More material on the top of the piston to increase the CR but also reduce the quelch area?

Correct me if I am wrong but wouldn't the quench area be high temp ... as you are compressing air/fuel and compresing a gas = heat as a by product?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JimBlake &raquo;</TD></TR><TR><TD CLASS="quote">I don't think so. If you raise your compression by changing pistons, your new pistons would have a slightly different shape on top. So your combustion chamber has a slightly different shape.

It's probaby not enough of a difference to measure, but theoretically it's different.
</TD></TR></TABLE>

correct, gas flow inside the combchamber is altered. especially with HIG dome pistons...

 

quench is not the whole combustion chamber. Re-read the quote.

That is from Larry at Endyn, it is correct.