[Brian Baker]

Originally Posted by gearbanger

Brian, why don't you just tell us what the answer needs to be.

Because I don't know the answer. I'm trying to open the discussion up on technical terms and so far I'm getting smart assed remarks. 1/2" broached the subject before going off a bit on a tangent, but nobody else has addressed my questions.

[DiamondJim]

45 degree seats will help d-ports with higher lift flow. HIS, Rhoads rocker arms?? Like Fasteddy said, cam it up! -Jim

[taff2]

The way I look at it is like this, if you have a head that maximum flow is at .550"lift ,and you have a cam that lifts the valve to .550",then you will only see the maximum flow point once. If you fit a cam with .650" lift to the same head,you will now see the maximum flow point twice-once as the lifter rises up the lobe and once as it goes back down the lobe. Whether you will see a power increase depends on if you can grind this cam to time these two points to occur when the piston is descending at it's highest speed or when the pulse from the header pipe reaches the valve/combustion chamber.

[gearbanger]

taff2, that is pretty much what I had posted earlier but apparently that we "don't know that the hell we are talking about".

I think maybe the correct answer is, There is no reason to ever put a cam in a d-port motor that has over .550 lift. Okay good deal, we've got that figured out now.

[Brian Baker]

No no no...you guys are getting your asses up on your back about this (well some of you). Go back and reread my questions. I fully understand the concept of having a little more lift than where the peak flow occurs. What I want to know is:

1.) How far past the lift where peak flow occurs do you go before diminishing returns are evident?
2.) Continuing from question 1, is there a formula to figure this out?
3.) How does velocity in the port correlate to flow levelling off (or dropping) at higher lifts past peak flow?

[fasteddy]

I agree with the concept of drawing breath twice per event-if the heads do not go turbulant and lose flow after the max lift number.

My iron stalled at .600--did not tank but leveled off. I put a .640 lift roller in 'er. Works well-- making 650hp.

[taff2]

Ah Brian,that's the art of the cam designer,beyond my knowledge I'm afraid.

[DiamondJim]

Originally Posted by Brian Baker

No no no...you guys are getting your asses up on your back about this (well some of you). Go back and reread my questions. I fully understand the concept of having a little more lift than where the peak flow occurs. What I want to know is:

1.) How far past the lift where peak flow occurs do you go before diminishing returns are evident?
2.) Continuing from question 1, is there a formula to figure this out?
3.) How does velocity in the port correlate to flow levelling off (or dropping) at higher lifts past peak flow?

Good questions. I would bet SS racers have more info, they have been getting all they can from d-ports for years. Maybe Lynn McCarty will chime in. -Jim

[gearbanger]

If it works like water or natural gas systems, then the velocity is proportional to the friction losses. So as the velocity gets so fast, the friction loss starts to eat up some of the increase in flow due to more valve lift. I havent seen any flow numbers reported on any head that went down as the lift went up so I cant really say that that condition really happens or not. I can see getting to a point where the lift is high enough to where the valve is not a restriction any more so you could take the valve out if you wanted and it would not increase the flow. It is hard for me to imagine a point where the flow would actually go down if you lifted the valve more.

[johnta1]

You are basically wanting to know what the optimum cam would be for this head.

You would need all the specs of the engine to see what it would need.
Bore, stroke, intake, crank, pistons, rings, flow rate of head, port length, plus quite a bit more.

Or do what the real guys do, lots of testing.

[Brian Baker]

Originally Posted by johnta1

You are basically wanting to know what the optimum cam would be for this head.

You would need all the specs of the engine to see what it would need.
Bore, stroke, intake, crank, pistons, rings, flow rate of head, port length, plus quite a bit more.

Or do what the real guys do, lots of testing.

No, I'm not wanting to know that. Listen, it doesn't matter what kind of port we are talking about being the principles that I am asking about can be applied to any port....we just happen to be talking about factory d-ports as a point of reference.

I'm in a nutshell asking how to determine optimal lift for a given port design based on test data (flow and velocity).

[johnta1]

Doesn't work that way.
Need more data.

You can use the ballpark figure of

minimum of lift: 24% times intake diameter
maximum of lift: 41% times intake diameter

Using 2.11" intake valve - .506" min lift; max .865" lift

This is for racing.

[Skip Fix]

Brian I want to think Jim McFarland in one of his articles had something about valve lift at max piston velocity to get the best cylinder filling.

One thing to remember also when looking at flow and velocity if at higher lift the keep flowing more but the velocity is getting too high it may not help. I have a friend with some ported High Ports that at the pushrod bulge when they are at .800 and flowing 380-390 the velocity is getting really high so could be an issue.

I think it is still like John said "more area under the curve" so higher lift if not detrimental to flow and a better stability lobe is what can be used. Even though D ports nose over at .500 or so unported I know the SS guys in the 80s early 90s used high lift rollers in them also to get that.

[tempest455]

N/SS guys are running over .800" with 980 heads.

[ponjohn]

Originally Posted by DiamondJim

Good questions. I would bet SS racers have more info, they have been getting all they can from d-ports for years. Maybe Lynn McCarty will chime in. -Jim

That is what a dwell cam is for.

I would argue that the time passing through max lift twice is probabaly not as effective as having max lift for a determined time frame.

[Half-Inch Stud]

Optimal Intake Event for N/A engine:

1. Hit your self with some Geometry knowledge, & Open Excel. Or open HO-Racing's PMD Engine Theory thesis.
2. Study the Plot of Piston Velocity vs bore position. (feet/Sec vs Degrees TDC).
3. Ponder on the Maximal Velocity being where the Intake Value Ought Be Wide-Open.
4. NOT SHOWN: Ponder the OverLap Event being a STRONGER VACUUM pull than the Peak Velocity Event.

5. Wrestle the #3 & #4 events above to ponder if Intake Closing is as critical as alleged
for just ANY Cam & Application.
================================================== ===============
The interested student (<-- I always loathed/despised such textbook words) should be able to figure on EC & IO, IC, & EO events(Lifter up@0.050") as Rules-of-thumb for the 4.21"stroke PMD. Then knock 15degrees off for the 3.75"stroke PMDs.
================================================== ===============

Fundametal problems:
Intake: Bunched air really unbunches quickly, thus IC event vs RPM is difficult to analyze/predict due to low-lift flow improvements.

Exhaust: The Hyper-strong Exhaust Vacuum pull during Overlap can instantly become "Pressure" due to offfending Exhaust pulse from another CYL. Thus, Exhaust System & Head-Flange Exhaust leaks can really lean-out a single CYL from the Bunch. Power-Imbalance "feel" & Plug Readings become relevant.