Lowering the 250's compression? - Ford Forums - Mustang Forum, Ford Trucks, Ford Focus and Ford Cars

mr_mcgeec · 01-10-2004, 11:48

I want to turbo my XF unleaded EFI and i only want to run 6-10 PSI, the compression ratio is currently 8.7:1. How much will i have to lower my compression? What's the best way of lowering it? and what's the best pistons to use?

Thanx,
Sean McGee

ebxr8240 · 01-10-2004, 17:30

8:7 is ok at 10 Lp so long as fuel etc and spark/ timing is set right.But some unshrouding of chamber around valves lowers compression and helps flow...

ACL have a race series rebuild kit for ohc motors I guess they have them for X flows..Fit arp bolts and resize/ ballance assembly.I guess you could machine say 1/2 mill from centre piston 12mm from outside edge of piston to lower comp even more without weakening it...

xf6250 · 01-10-2004, 23:17

U can do as ebxr8240 has explained, or u can go and buy a decompression plate and thats another way of bringing down the comp.

mr_mcgeec · 01-11-2004, 04:19

Thanx for the help, theres just a couple more things. Is it possible to use a 3.3 crank to lower the compression? and how can you make the 250 safe to rev harder than it does, and what would the redline be?

Thanx again,
Sean McGee

ebxr8240 · 01-11-2004, 04:38

By using 3.3 crank makes the motor that size. But a well built 3.3 with 4.1 head milled to gain correct compression. But thats going to cost even more...Pluss 3.3 or 4.1's don't have very good valve gear for extream revs..With the compression you have is o/k just make sure spark and fuel are up to it...

xf6250 · 01-11-2004, 22:53

Yeh, totally agree with ebxr8240, make sure spark and fuel is plentiful, as for redline u shouldnt have any problem revving it to 5000rpm.