let's talk compression : UgH!

[terryly]

Found this on Cal-look lounge interesting may help
http://cal-look.no/lounge/index.php?act ... 9710;image
http://cal-look.no/lounge/index.php/topic,26676.0.html
Terry

[Buggsy]

I've probably read every post in the Forced Induction since 2001. It is very rare to hear someone say they picked the wrong cam in their new turbo engine. Why? Because the gains you get when you go turbo are so dramatic, it just amplifies what you were running before. If you're a high RPM guy and like big cams, and you go turbo, the gains will be huge horsepower at high RPM. If you like smaller cams and the driveability that goes along with that, the gains will be huge low and mid range torque. Almost everyone that goes turbo has had a built engine or two before, and has an idea what they want from their car. When you go turbo, you get twice as much of that. You also gain on the other end of the RPM scale too, so don't worry about the cam to much. If all else fails, you really can't go wrong with something around 280 to 285 advertised duration in a street car.

[Chip Birks]

I've probably read every post in the Forced Induction since 2001. It is very rare to hear someone say they picked the wrong cam in their new turbo engine. Why? Because the gains you get when you go turbo are so dramatic, it just amplifies what you were running before. If you're a high RPM guy and like big cams, and you go turbo, the gains will be huge horsepower at high RPM. If you like smaller cams and the driveability that goes along with that, the gains will be huge low and mid range torque. Almost everyone that goes turbo has had a built engine or two before, and has an idea what they want from their car. When you go turbo, you get twice as much of that. You also gain on the other end of the RPM scale too, so don't worry about the cam to much. If all else fails, you really can't go wrong with something around 280 to 285 advertised duration in a street car.

Can this get stickied? Just this post

[petew]

Same motor, same capacity (1600), same turbo, same heads, same cam (TCS-10)...
I first had dished top pistons with 7.5:1 comp ratio.
I then swapped over to stock Mahle flat tops (for other reasons) so the comp ratio went up to 8:1.
What I noticed straight away was how much snappier the motor was off boost. Additionally, it got into boost earlier thanks to slightly hotter exhaust temps. For a street motor I wouldn't go below 8:1 unless it's a huge stroker.

[buguy]

I'm surprised you could feel the difference with just that little bit of bump.

[theKbStockpiler]

It looks like a Turbo specific cam is not nessesary. Here's Crower's webpage with a few cams that have a LSA of 110.
https://www.crower.com/camshafts.html?cat=127

[petew]

I'm surprised you could feel the difference with just that little bit of bump.

Surprising but yes. It was pretty laggy prior to that. Course I've since improved the tune and it's better still. But think about it, every extra bit of compression you add is a little more efficiency and punch. ACVWs are not know for efficiency to begin with. A little more makes a big difference, hence the joy of throwing a turbo on.

[madmike]

I've been saying this for awhile

[buguy]

I guess it just finding that balance that works best for each person.

[Buggsy]

Realize that Petew gained his squish area back when he got rid of the dished piston. That may have been more of a gain than the compression increase. Some guys don't seem to know that using a dished piston is no better than running a big deck. If you don't have two fairly large flat surfaces coming within .040 to .045 of each other, you don't reap the benefits of a close deck. I am making the assumption that the entire top surface of the piston was dished and not just a mirror image of the combustion chamber. You can get this squish area with domed pistons (squishy pistons) also. Stating the obvious to most of us, but I have seen indications that some were not aware.

[petew]

^Wow. I didn't know that. Makes sense. I think you're comments on the shape of the piston are spot on too. Can you explain why the "squish area" makes more difference?

[theKbStockpiler]

without the squish area the air/fuel mixture does not mix at well unless at high RPMs. It acts like a crude pump to force it out to the inside of the cylinder.

[buguy]

So instead of squishy pistons can I just add material to my combustion chamber that mirrors the shape of my piston? FYI I've never even seen a VW piston!

[Chip Birks]

Vw pistons are pretty boring. Just round with a flat top.

[terryly]

not yours squishy guy