Originally Posted by J Wikoff
I'm feeling better mid-range power, the kind that rips the tires loose when you mash it and it downshifts to first.
Every LIM I've ported for myself or others has had the same effect. Including the two we just installed over the weekend. The L36 LIM took me just over two hours, but another hour of 'detail work' in the runners would have been nice. Other work and a tight mod schedule prevented it, but in the long run, would have made little difference:
You'll notice in that topic the VERY good alignment of the LIM to the head ports on install. If you don't have your motor opened up during porting, you'll need to rely on very good experience to keep the port spacing right (based on port-port centroids). This is critical to maintain equal port velocity across all 6.
If done correctly, a very noticable midrange pull will be felt. It may or may not raise the HP peak, but it gets into it quicker through the midrange, 'flattening' the HP curve a bit, and provides a much improved throttle snap (response) off the line. This is true for SC or NA LIM'*.
Remember, the WORST from the factory, and gains the most is the L36.
Next is the S1 L67 (and L27).
The best from the factory is the Series 2 L67. This gains the least matching to factory heads, but if you don't mind 30 minutes of cleanup, you will notice it.
There are great variances in match from LIM to LIM of the same type. As well as casting flaws that push one cylinder 'out' a bit compared to the rest on some LIM'*.
There is a little more spooky detail work you can do with the PROPER precautions before installation. AFTER insuring a good port match on the LIM, you'll be left with the ideal dimensions of:
LIM.......0.85x1.85" with corner radius matched to head ports (improved to ~1/8")
This STILL leaves the gasket cavity at 1.0" by 2.0" (roughly) causing a small mismatched cavity between where the AF mixture can swirl, eddy, and start to expand before necking back down into the head. This can be improved by LIGHTLY chamfering or rounding off the entrance of the cylinder head port. EXTREME caution should be used if this is done with the heads on the car. Stuff the ports, tape everything off, and have a good shopvac and magnet ready for each port. An exaggerated detail I worked up in CAD:
This is the original concept I worked up a couple years ago.
Refined to the dimensions I port to now (and stated in this topic) it would be this AFTER porting BEFORE chamfer:
AFTER porting and AFTER chamfer:
Think of it as not much more than smoothing off the sharp entrance to the cylinder head intake port to ease the transition and maintain port velocity. This has been done on several cars with great success, including the two shown in the link above from this weekend'* work at my house.
I'm not responsible for a anything you screw up on your car, and I'm warning you now that if you've never done this before, do it the first time with someone who HAS, and has also DEMONSTRATED success with it.