Now for the interesting part...

I took a gel casting of the inside of the intake runner:

(Port side to the left, valveseat side to the lower right)

Then cut it longitudinally every 0.5":


You old-timers who took college chemistry might remember this from old-fashioned chart-recorder chromatography...

I traced the cut bits onto silicone paper:


Using known areas of cut silicone paper as a standard weight, I weighed each cutout:


Applying a minor correction factor of 1% (since I could measure port and valveseat directly with calipers), I came up with this:

Location; CSA (cross sectional area, sqin):
0" (at the port); 1.73 sqin
0.5" in; 1.36 sqin
1.0" in; 1.32 sqin (point of tightest restriction)
1.5" in; 1.53 sqin
2.0" in; 1.63 sqin
2.5" in; 1.99 sqin
3.0" in; 1.89 sqin (this is where the "boss" for the valveguide starts)
3.5" in; 1.89 sqin
4.0" in; 1.93 sqin (past the vavlveguide boss and into the bowl)
4.5" (at the valveseat); 1.93 sqin

What I found shocking was the unexpectedly tight restriction in the region about 0.5" to 1.25" in from the port (interestingly, right where the injector would be if this was drilled for injectors like an L67 head). I'll try to explain what I think the implications of this restriction are in my next post.

First off, everything I've read suggests that the point of tightest restriction through the intake tract wants to be about 20-25% up the intake from the valveseat, in order to give a "velocity boost" to the air before entering the bowl:

Measured Intake tract distances:
Head: 4.5 in
LIM: 3.25 in
UIM 5.9375 in

Total: 13.6875 in

Distance from valve to restriction: 3.5 in
3.5 in / 13.6875 in * 100% = 25.6% (PERFECT)

Next, using this calculator to determine intake air velocity in Mach number:
http://www.wallaceracing.com/machcalc.php
Given an intake restriction CSA of 1.32 sqin:

rpm Mach
2400 0.28
2800 0.33
3200 0.38
3600 0.42
4000 0.47
4400 0.52
4800 0.56
5200 0.61
5600 0.66
6000 0.71
6400 0.75

Supposedly, a mach number of 0.5 is optimum for VE, 0.55 is a VE "chokepoint", and >0.6 gives turbulent flow (and a severe dropoff in VE).
Well, what do you know, given those parameters, the table above matches up VERY nicely with VE graphs on all of my mods so far:
(i.e., best region about 4000-4800rpm, big dropoff above 5200 rpm)


This strongly suggests that in order to be able to get anything out of this engine above 5200 rpm, the intake runner on the heads will need to be enlarged slightly at the restriction point. Likely, it will want to go from about 1.32 sqin to about 1.46 sqin:

rpm Mach
2400 0.26
2800 0.3
3200 0.34
3600 0.38
4000 0.43
4400 0.47
4800 0.51
5200 0.55
5600 0.6
6000 0.64
6400 0.68

This should move the optimum band for power to the right about 400 rpm (and perhaps up by who knows how much?). Add upgraded valvesprings, and this type of headwork should allow shift points to increase to around 6000 to 6250, and actually allow the engine to make some power at the higher rpm'*.

This porting would likely require removing some material on the floor (short side radius) of the intake, about in the circled area:



I'm looking at the exhaust runners next.

Wow. Very interesting approach. I have to read this later.

Extremely well done. I really like the research you've put into the L36 during the short time you've been in the club. It brings some new perspective to the "usual" modifications we generally suggest to L36 owners.

I have never read anything about the optimal Mach number for the air entering the intake, and your calculations intrigue me. Do you have any links or references for the relationship between air velocity and volumetric efficiency? I may have to do some Googling.

If your assumptions are accurate, this may well explain why my car feels sluggish in the upper RPM ranges. After retarding my cam timing in an effort to flow more air at higher RPM, I may be exceeding the optimal Mach number past 5000 RPM. It makes sense... we'll just have to see how the car reacts to some tuning and a throttle body. Perhaps a throttle body is unneeded at this juncture, considering the information at hand.

Very interesting stuff. Please keep it coming!

Very interesting. I have my eyes on this thread. Once the tranny is in, i am going to really drive into motor mods.

I love it!!!