exhaust size effect on boost pressure

[Holden]

I was getting 13psi with 4-2-1 headers, stock cat and 2.25" exhaust. Changing the cat to a 2.25" straight pipe lowered boost to 12psi. Changing to a Kami 4-1 2.5" collector header lowered boost to 11.5psi.

I've dragged with open headers (my straight pipe has a cutout plate), but wasn't looking at the boost gauge. I'm sure it lowered boost some.

BTW: Go back and replace everywhere I said "boost" with "manifold pressure". My "boost" is about 10psi and that doesn't change.

[JDR]

I would like to just uncork it at the head just to see what it would do.

[JDR]

I am sure we have all talked about this one before but has anyone mesured what the results are at 12psi or more by uncorking the exhaust on boost pressure and or horse power. And how was it done just removing exhaust or upgraded to 3inch.

[Holden]

<BLOCKQUOTE><font size=2 face="Verdana, Helvetica, sans-serif">quote:</font><table border="0" width="90%" bgcolor="#333333" cellspacing="1" cellpadding="0"><tr><td width="100%"><table border="0" width="100%" cellspacing="0" cellpadding="2" bgcolor="#FF9900"><tr><td width="100%" bgcolor="#DDDDDD"><font size=2 face="Verdana, Helvetica, sans-serif">Originally posted by JDR:
I would like to just uncork it at the head just to see what it would do.</font></td></tr></table></td></tr></table></BLOCKQUOTE>

The ECU will instantly throw a code and go into limp mode. Then your radiator fan will melt!

(I shut mine down before melting anything, but the ECU freaks without an O2 sensor.)