rebuild help

woodman · 02-11-2003, 04:18 PM

Am I correct in assuming that if I do a stock rebuild ('75 1.8 Ljet) but bore the 93mm barrels to 96mm and use 96mm KB pistons, the CR would be affected? More specifically, does that slightly larger comustion area alter the CR thereby requiring mods to the deck height or head?

I plan on having the heads rebuilt and cc'd to have each chamber match and to be able to calculate the CR. Is it reasonable to shoot for 8:1 or so and run 91 octane pump gas? I'm trying to figure out all the calcs to make sure what I'm going to have done will work out.

Deck Height: Is this a function of calculating the CR, or is it set first and the combustion chamber modified to arrive at a predetermined CR?

By the way, my case is split and I'm going to have the machining done as soon as I can confirm the numbers.

Thanks guys!
Steve

tryan · 02-11-2003, 05:21 PM

cr is a ratio of the volume at the bottom and top of the stroke. after you cc the heads (and pistons if they are dome or concave), you set your deck height to get the desired ratio.

hardflex · 02-11-2003, 06:01 PM

you are correct in your assumption on higher compression from larger volume, however, that's minor compared to the change due to piston shape. 1.8's had a low cr, 7.3 to 1, and probably a dished piston. the 96's are probably flat topped and cr with minimum deck ht would be high 8's, WAG. so you set your deck ht wherever you want the cr to fall.

I like this little calculator for planning this stuff. http://www.johnmaherracing.co.uk/enginecalc.htm

you'll need to know volume of the 1.8 heads (i'm guessing about 58cc,double check me with a search) and the size of the dish, if any, in your pistons. add those together for the chamber cc's, to find your deck ht and compression ratio.

Zeke · 02-11-2003, 10:25 PM

I told my head builder what deck height I has planned and what CR I wanted. With the formula, we calcualted the camber size, 53cc in my case.

The deck height can be used to *adjust* the CR, but should fall into a range or .045 to .035 for performance reasons. With the design of the type IV piston and head, a large deck will dog the engine from what I've read over on the TypeIVum. The combustion wants to stay in the center of the cylinder, not out on the edge of the piston. A better flame spread is accomplished by a narrow deck (squish).