Well I know that I am about to get shot here however, what could be worse that being told to "shut up already" openly on a forum...?
Look, one thing is for certain - bigger is not ALWAYS necessarily better. It is a well-documented fact that the ports on a 3.0 24 valve for example are TOO large for a stock 3.0 sub and that verifiable gains have been attained by switching to smaller 24 valve heads from the 2.5 litre 156. (Even with a 3.2 GTA sub and MILDLY ported small 2.5 heads - power-gains were realized!!!)
Jim is right - you don't want to touch the bottom end of the 12 valve Alfa V6 intake port. What I have seen is that your larger porting at the mouth of the induction port should taper rapidly towards about the valve guide-point and stop. As far as building up inside the port - seems excessive, but for now we only have theories and possibly bench-flow tests as a reference-point until Jim or somebody else releases practical findings of this mod! How much could be gained for the dollars any way, versus simply increasing manifold pressure for example...?
While I have not written a book on the subject (NO REFERENCE TO JIM) and while I don't proclaim myself "Guru" (.......) I HAVE spent a GREAT deal of time listening to a man that has done this on these (and other) engines longer than most folks here and I myself HAVE spent a great deal of time staring at a variety of 12 valve, 24 valve and even 4-banger heads - pre-and-post-opp! We are talking about applications that have been raced and reworked and raced and reworked and dyno-tested and flow-benched and again raced and dyno-tuned and tested and then some!
At the end of the day it is the results from a real-world application that holds water and not a theoretical computer simulation. Calculations will save you time and money and help you plan down the right path before you destroy many heads and engines through trial and error... In the end though, no matter how you got there - it's the "school-money" that buys the Whiskey!
I am pretty confident with what I experienced on MANY motorcycle engine modifications and with what I have learned in SA these past couple of years... Interesting conversations with head-specialists like Steve Hanoford helps too!
As far as Jim's quote above:
"....The problem with the 3liter are the intake runners, limiting flow to ~85cfm with the std valves. If you enlarge these runners to an ID of~39mm (from std 34mm) you will gain ~8cfm per cylinder or ~20hp total!! On top of this, if you reshape the intake valves giving them a flatter back, you will gain another 4cfm/cyl or ~9hp more! A total gain of ~29hp is no laughing matter. If you go through the expense of special profile large valves+larger valve seats+huge aftermarket runners+good head porting, you will spend a LOT of $$ for ~120cfm, worthy of a good race 3liter. Such a motor will go to more than 250hp with streetable cams. All in all, larger valves are a waste of money in this V6, if all you want is a 20-30hp increase. A good porting job, reprofiled valves, enlarged std runners, higher CR, Motronic (S)cams and a set of CSC headers will get you much more...."
versus:
"...Zamani, them pair of pants come only in two sizes right now, too small and huge. Ideally you'd want the ~50mm on top, ending at ~38mm at the bottom. You should see what serious engine simulation programs have to say about it! I'm sorry I was too late with this.
Jim K...."
Greek-man speak with forked tongue!
![Very Happy :D](./images/smilies/icon_biggrin.gif)
I guess that what Jim was trying to say is that TOO big is also not good AND the percentage taper-down and angle is more important than the runner's top-opening size!!!
Jose - you have that one runner there from Dawie - what is the ID at the bottom versus the top 50mm - I think that it is more like 40-42mm - maybe even less...? While mathematically it may seem strange, these work. They are proven so! The bigger issue here is the sub and the cams. Simply strapping these to a stock 2.5 sub is pointless - you WILL lose power down low and you MAY gain something up top - 4K rpm and above, so pointless on a street 12 valve any way.
Where they start to work well is on a big-bore 3.45 litres and up...
One my stock 3.0 12 valve with only headers, GoTech and port-matching on the bottom of the intake runner only (no additional porting) I saw documented gains on the dyno - well here comes the "dyno-variances police" again - I am sure! Bottom line - more bore, more stroke, more duration, more lift, more compression, more fuel and more revs and you WILL capitalize on these larger runner and plenum-sets on this particular engine...
...to Alfa, or not to Alfa? That is the question...