Polishing intake runners

NoDOHC · 11-14-2010, 09:38 AM

What is inportant is that the intake runners do not change in cross-sectional area throughout the entire length of the runner. This way the air can maintain a constant velocity. The runner should be slightly larger than the intake port, so that the air is accelerated through the port, resulting in better fuel mix.

I polished my intake runners to 600 grit where dry and 160 grit where wet (after the injectors). If you polish the runner too much where they are wet, you will see a symptom called fuel pooling, which is the fuel forming rivulets on the polished surface. Fuel pooling will make the engine idle very poorly.

160 grit seems to have a small problem with fuel pooling, but nothing next to 600 grit.

My engine will occasionally misfire at low fuel and air flow rates (idle, lightly loaded cruising), but the misfiring is infrequent and barely noticable.

If I were you, I would probably not go past 80 grit past the injectors if you want a good idle.

If you follow the rules of intake porting, there are major gains to be had from it.

EDIT: Here is a thread where this was discussed.
http://www.rotarycarclub.com/rotary_...ead.php?t=8550

11-14-2010, 09:38 AM	#1
NoDOHC The quest for more torque Join Date: Sep 2008 Location: Sheboygan, Wisconsin Posts: 855 Rep Power: 18	What is inportant is that the intake runners do not change in cross-sectional area throughout the entire length of the runner. This way the air can maintain a constant velocity. The runner should be slightly larger than the intake port, so that the air is accelerated through the port, resulting in better fuel mix. I polished my intake runners to 600 grit where dry and 160 grit where wet (after the injectors). If you polish the runner too much where they are wet, you will see a symptom called fuel pooling, which is the fuel forming rivulets on the polished surface. Fuel pooling will make the engine idle very poorly. 160 grit seems to have a small problem with fuel pooling, but nothing next to 600 grit. My engine will occasionally misfire at low fuel and air flow rates (idle, lightly loaded cruising), but the misfiring is infrequent and barely noticable. If I were you, I would probably not go past 80 grit past the injectors if you want a good idle. If you follow the rules of intake porting, there are major gains to be had from it. EDIT: Here is a thread where this was discussed. http://www.rotarycarclub.com/rotary_...ead.php?t=8550 __________________ 1986 GXL ('87 4-port NA - Haltech E8, LS2 Coils. Defined Autoworks Headers, Dual 2.5" Exhaust (Dual Superflow, dBX mufflers) 1991 Coupe (KYB AGX Shocks, Eibach lowering springs, RB exhaust, Stock and Automatic) Last edited by NoDOHC; 11-14-2010 at 09:41 AM.