Raising compression

vw_bug_owner · Post by **vw_bug_owner** » Fri Mar 29, 2002 10:49 am

Is it possible to raise the compression by using less cc heads? Do they make less cc heads?

I think mine are 50cc... do they have 45cc? How does this work?

I have 041 heads with big valves. I am running 92's with 7.5:1 compression.

Lees73Super · Post by **Lees73Super** » Fri Mar 29, 2002 12:18 pm

No, you need to take your heads in to be flycut. If you have them flycut so that your deckheight is 0 then you can get copper spacers and set your compression much easier.

Lee

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One of these days ill get this stuff right!

73 Super, 2110, 120 cam, 2x 44 IDFs Hyfire CDI and some more fun stuff.

vw_bug_owner · Post by **vw_bug_owner** » Fri Mar 29, 2002 12:28 pm

The reason for my deck height being what it is.. is that the cylinders are long. So having the cylinders cut would be the issue right? Not the heads?

Hot Wheels · Post by **Hot Wheels** » Fri Mar 29, 2002 1:06 pm

Unless i really missed somthing here, I think i see a little confusion going on.

Deck Height is the distance from the top of the piston to the top of the cyl wall at maximum stroke. 0 deck height would put the piston flush with the top of the cyl, not good, there has to be a gap between the two. Everything ive read and done requires deck height to be .030 to .060 range, I like .040 myself. If you have a stock compression motor with a stock deck height in this range, you need to deck the heads to reduce the cc's in the chamber and raise the compression. There are some neat charts that show the effect of decking the heads on compression ratios.

It is concievable i suppose that an odd rod and piston combo would make a motor where you had to much deck height and needed to shorten the cyls. I have always had to shim if adjustment was required to gwt more deck height and then deck the heads to the desired ratio.

Sean

vw_bug_owner · Post by **vw_bug_owner** » Fri Mar 29, 2002 1:22 pm

I have a brazillian case, and jugs off my previous motor (which had a shim, but I don't have any on this motor.) I may be mistaken, but i think the deck height was like .150 or .105 or soemthing to that effect. When I put it together (4/5 months ago) I figured the compression and it was stock. I would like to raise it, but am new to the cutting of the jugs, or the heads. Which is a more practical solution. What is the price difference?

James2 · Post by **James2** » Fri Mar 29, 2002 2:20 pm

having the heads flycut is the easier solution. Having the cylinders cut and running .040 deck produces better results. You may have to have both done to reach your desired CR with min deck.

Searoy · Post by **Searoy** » Fri Mar 29, 2002 2:35 pm

In your case flycutting the heads is one option you could take to increase compression. It moves the seating surface of the cylinders further into the head. This effectively make the chambers smaller, since you are cutting away the "top edge" making the chamber shallower. It removes material from the head (a bad thing).

The other option is to cut the top of the cylinders, an equally effective option, should cost about the same but will leave more material in the head (a good thing).

I have heard (and use) deck hieght to refer to both the distance from the top of the piston to the top of the cylinder, and the distance from the top of the piston to the bottom edge of the chamber.

If you have .040" from the top of the piston to the top of the cylinder, and then add a .060 copper gasket you've increased the distance to the chamber's edge to .100.

If the piston sticks up out of the top of the chamber by .020 you've got .020 negative deck. I this case you can add a .060 copper gasket to regain a distance of .040 from piston to chamber's edge (nice).

You could also have .000 distance from piston top to cylidner top, but step cut the chamber so that there is .040 extra room for the piston to clear the chamber.

The bet I've heard is setting the piston/cylinder/rod combo up for a -.040 deck (piston above cylidner top) but step cutting .075" for a final deck of .035, and the combustion starts inside the head, not the cylinder. I'd like to try this one out myself, maybe with some super squish pistons in a single port head at 12:1 compression.

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*** Teach a Man to Fish ***
Searoy

"I tend to lean toward a tighter gap and a
looser skirt....a little slap never hurt." -- Joe of the West

Ryan67 · Post by **Ryan67** » Fri Mar 29, 2002 11:34 pm

I believe to raise the compression ratio you can have your heads fly cut. This is where they remove material from the combustion chamber to make it fit farther down on the cylinders in turn making the combustion chamber smaller. If you have some cylinder shims between the cylinders and the case you can also raise or lower the cr by adding or removing them.

vw_bug_owner · Post by **vw_bug_owner** » Fri Mar 29, 2002 11:58 pm

I understand that. I have no shims, and have stock compression. So, cutting the cylinders is the only option?

Hot Wheels · Post by **Hot Wheels** » Sat Mar 30, 2002 6:18 am

What bore, stroke and pistons are you using? What motor did the pistons come out of? Assuming type one motor, there are two main A&B pistons with different wrist pin heights as well for standard and stroked applications.

If you like the parts combo you have, I would have the cyl barrels turned down .100 to .110 so your deck height was to spec, then you could run stock compression or make the call to deck the heads and raise the compression. The big benifit to doing the setup with the deck height is the heads are not cut to some crazy cc that wont work on your next motor without major shims.

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Sean
Work hard, Play harder.