mid-lift flow

bsfc9 · Post by **bsfc9** » Sat Jan 24, 2004 9:56 pm

reguarding the discussion about defining where the low/mid/high lift stations are, could one use piston position as a reference, as peak lift occurs between 100 and 120 degrees atdc this would be a high lift area reguardless of cam configuration.
I just finished listening to the CD with harold bettis which i found interesting and he kept referring to the point that the valve sees the mid lift twice and it seemed to me that he placed similar importance of this area as some "gurus". Also it seems to me that peak piston velocity would be occuring in the midlifts which would induce the greatest pressure differential across the valve which would increase flow. would the greater flow caused by a greater differential make up for the fact that the valve is moving very quickly through this region(if i dont have time to empty a bucket one cup at a time i'll dump the whole bucket). does any of this make sense--i hope so, if not forgive me. this is a great site don and i would not have thought my original post would have gererated so much interesting discussion. ok one final question in your "740hp" book you mentioned that some restrictor plate engines like reversion, whats that about? thanks

speedtalk · Post by **speedtalk** » Sun Jan 25, 2004 9:15 am

bsfc9 wrote:this is a great site don and i would not have thought my original post would have gererated so much interesting discussion

Good job on getting this thread going.

bsfc9 wrote:it seems to me that peak piston velocity would be occuring in the midlifts which would induce the greatest pressure differential across the valve which would increase flow.

I'm not saying mid lifts aren't important, I just got sick of people saying they're important because that's where the valve spends most of its time. I think your point is right, but keep in mind the inertia of the air column - it takes time to get it moving.

bsfc9 wrote:you mentioned that some restrictor plate engines like reversion, whats that about? thanks

Actually, I think they all like it. You want to kill a restrictor plate engine? Do everything you can to stop reversion. The typical wisdom on this type of engine is less overlap and less exhaust duration. The dyno tells another story.

bsfc9 · Post by **bsfc9** » Sun Jan 25, 2004 1:42 pm

don, i agree with you alot of people become fixated on midlift flow( and also on a high peak number). i think harold bettis hit the nail on the head saying the entire curve needs to be considered not just one part.

rmbuilder · Post by **rmbuilder** » Tue Jan 27, 2004 11:54 am

Great thread
I agree with assessing the entire curve, but I question ability of the unit of measurement (static lift/steady state flow) used to address the variables. Theoretically a port flowing less CFM @ a higher velocity has the potential toward (depending on variables) more efficient cylinder filling (provided you have sufficient depression) than a “lazy” port capable of flowing more CFM @ but operating at less velocity. What do you think about incorporating pitot/manometer (velocity/pressure) data with CFM to obtain one value representing potential volume transferred per unit of time/lift value from open to peak to close? It would seem this value could yield a more useful unit of measurement in determining the efficiency of an intake/combustion/exhaust tract. This value could then be evaluated relative to piston speed, displacement, crank angle, valve timing, and pressure differentials in attempt to optimize the potential of the system as a whole. In short, measuring airflow as a derivative (combination of values) rather than a singular function, similar in theory to horsepower being a function of work and time. Perhaps a dynamic measurement for
.001”>.400”<.001” flow
.001”>.500”<.001” flow
ETC
Bob

speedtalk · Post by **speedtalk** » Tue Jan 27, 2004 3:12 pm

rmbuilder wrote:efficiency of an intake/combustion/exhaust tract.

Right on Bob! Efficiency is the word. I look at CFM/Port Area, especially when I think I'm out to lunch on a head.

bsfc9 · Post by **bsfc9** » Tue Jan 27, 2004 7:16 pm

yea, port efficiency, thank you, i,ve tried to preach this to some but it sometimes falls on deaf ears. thank you

Guest · Post by **Guest** » Sat Feb 14, 2004 4:44 pm

Low-Lift Flow example=>

i've had heads have same exact flow at .200 to .400" lift on flow bench

but make 100+ HP differences

Low-Lift Flow is not where "all" the real HP is at
even though engine "sees" Low-Lift flow twice.

Post by **maxracesoftware** » Sat Feb 14, 2004 8:03 pm

i've had some NHRA Super Stock heads in my Shop ported by another head porter .

He illegally acid-ported the combustion chambers to unshroud around valves to pickup low and mid-lift flow ....these heads responded to those mods because at higher valve lift , this head was going into Sonic Choke conditions , so it liked the unshrouding low to mid lift flow .
Low to mid lift was Critical on those heads

on another set of heads that had ports not into Sonic Choke at high valve lifts ...low to mid lift flow was not as important or critical as the 1st set

the last head , ran faster down the DragStrip and made more average HP on my dyno

speedtalk · Post by **speedtalk** » Sun Feb 15, 2004 12:10 am

maxracesoftware, thanks for joining the board.

rmbuilder · Post by **rmbuilder** » Sun Feb 15, 2004 8:21 pm

Larry,
It's good to see you posting here!
Bob

Post by **maxracesoftware** » Tue Feb 17, 2004 1:45 pm

Thanks, Don and Bob

you have good Topics on your Forum