Thursday, November 3, 2011

I Was Wrong

Not exactly ground shattering news, huh? As they say: we all make mistakes. I certainly have to plead guilty, ...but sometimes being wrong about something is a little easier to take than others.

On the several occasions I have been asked about doing further porting work on Harley's Screamin' Eagle CNC ported heads, my answer has been "not enough material left to gain much." That was based on the annoying (to me at least) habit of the factory making the "choke" (the necked down area just below the valve seat) too large. The best and brightest porting minds in the world will tell you that in most cases 89 to 90% (choke diameter to valve diameter) is as big as you should go for best flow. They will add that in few rare cases you might go as large as 91% but absolutely no more! Harley seems to be fond of percentages somewhere north of that limit. The Screamin' Eagle 110 heads are a good example with a choke percentage of 91.7%. Their CNC ported MVA (Maximum Valve Area) remain true to form with a 1.950" choke working out to 92% of the 2.120" intake valve.

Recently though, I had a long time customer ask me to see if there was anything to be gained on the CNC MVA heads from his brand new Screamin' Eagle 120R engine. A careful perusal of the S.E. catalog revealed that the next step up from the 120R heads (aka the CNC MVA heads) is their "Pro Hurricane" head sporting a 2.175" intake valve. Hmmm ...that same 1.950 choke with a 2.175 valve would give an percentage of 89.7%. At least that would give me something to work with. On the chance that the overall length of the Hurricane valve would be usable in the MVA head, I ordered a pair. When the valves arrived, I was happy to see that they would work out well.

After some careful measurements and calculations for valve to valve clearance at the cams TDC lifts, the next stop was the Kwikway 044 seat and guide machine. Machining my usual multi angle high performance seat to the correct depth for the larger valve took only a matter of minutes, and then on to the flow bench. I intentionally skipped the one other modification I had planned in order to see what the larger valve by itself would do. The results were encouraging: a 19 CFM increase at .400" lift! However, at .550 and .600" lift it showed a 5 CFM loss compared to the original valve. Not to worry, though, I was pretty confident that it was turbulence that caused the loss in flow, and I would address that with the other change I had planned.

And this time I was right! Coupled with my second modification to the heads the result was a nice increase in flow at all lift points except .100 & .200". Not too shabby for a glorified valve job.

.100"----before 74.8 -----after 71.3 (-3.5)

.200"----before 145.0----after 144.2 (-0.8)

.300"----before 201.7----after 210.8 (+9.1)

.400"----before 251.7----after 270.9 (+19.2)

.500"----before 292.0---after 299.7 (+7.7)

.600"----before 311.1----after 315.5 (+4.4)

.650"----before 314.8----after 323.0 (+8.2)

So what was that second mod, you ask? Sorry, but if I am going to stay in business, I can't give away all my speed secrets. Suffice to say the picture below is an "after" picture, and you'll be hard pressed to see anything except the factory CNC machined surface

Which leads to the next question: is there more to be gained with more work to the rest of the port? Guess I'll have to change my answer to "probably."

1 comment:

Knucklenutz said...

Awesome is such a terribly overworked word St. Lee, but it is what it is !