compression increase ?

got my Falcon 6 Perf. handbook & started reading. man, if you don't have this book, order it! it can help clear a lot of questions up. anyway, i'm thinking of swapping pistons to the 255 V8 pistons (in my 250) & was curious what type of compression increase i'd get. the motor has 38K on it and runs fine, so i don't plan on any milling or decking. are these dished or flattopped pistons? i plan on going with the Isky 264 cam-will the adjustable rockers really help? i'm trying to get some HP w/o alot of machine work. 99 HP stock is just too little.thanx for the input. 8) ------ps.-i live at sea level, so i don't wanna overdo it on comp.! and where the heck are the offy tri power conv. sold??? can't find it at Cliffys site!!

Heh 46stude, These pistons are flattop with 4 valve reliefs in each, the valve reliefs are about 1.5 cc deep total. When you install the pistons, you will raise the piston up .085" at TDC higher than your original. I am not sure if your engine has been worked on before, but you will need to measure deck clearance after you install the pistons and use the formula for figuring compression ratio from Dennis or Daves website. Don't forget to change the stroke to 3.91 for the 250 and add any boring or CC changes in the head if it has been milled.

Howdy 46 Stude:

I ran your numbers through the calculator and got a CR of 9:1. That's assuming a .030" overbore, 62 cc chambers, a composite head gasket of .055", and a new deck height of .075" That's based on a stock deck height of .150" and no drop down in pin height on the new pistons.

If you're going this far with a rebuild, you really should consider milling the head .010" and decking the block .010" just to ensure a true mating surface. The new pistons will likely not be a true 1.585" pin height. Most replacement/rebuild pistons are slightly less than listed, on the likelyhood of machining during the rebuild. This still leaves way too much space for a good, tight quench area.

These numbers are only based on the big generality. Measure all of your own for exact specific.

Click on our website below my signature and then click on the "Compression Calculator" on the left side of the page, then play, by inserting your own numbers.

Adios, David