intake exhaust flow ratio's

[SW]

This seems like a good place to bring this subject up, and kind of falls in line with the mid lift thread.I know opinions on this subject vary and i hope i don't start a battle here, but i'm curious as to what everyones opinions are on intake/exhaust flow ratio's on naturally aspirated motors, and importantce on low/mid/high flow.I know that some people like to see numbers as high as 80% and others tell you that anything over 70% is overkill.I know on some of our best performing comp. eliminator motors, that the exhaust valves were very small and the ratio's were under 70%, but this could also be attributed to our combo.Just thought i would throw this out there and see what other peoples experiences are.
Shawn

[speedtalk]

The trend seems to be lower Exh %. Me, I take what I can get and slap a camshaft bandaid on it Darin Morgan has some interesting thoughts on this - he doesn't even worry about CFM on the exhaust.

[SW]

Does Darin go into that info on your interview with him?Something about cross sectional area?I'm going to have to order that one! I know that we had a 1.55 exhaust valve in the wedge heads and the port was very small.The low lift numbers were really poor,by normal standards.That probably had a lot to do with the 55* exhaust seats.With the smaller tube headers and a good merge collector you could hear the difference.The motors just accelerated much better.I also know that when you flow that same small cross-sec exhaust port that after about .400 lift it has that "shriek" for lack of better words, to it.I think the trend that we are seeing is smaller,higher velocity exhaust ports with less flow,less exhaust duration and overlap to reduce pumping losses and create a flatter torque curve. Any comments?
Shawn

[learner]

Here is my thoughts on this subject. In my experience i've noticed that enlarging the exhaust port will raise the torque curve in the rpm range. Though a cam change was never made, i'm fairly positive that a smaller exhaust lobe and/or different l/c would have countered that. I like a small or relatively small port that is very efficient. I know the cam specs have to reflect the port effeciencies along with all the other variables. I've known of people changing heads without any gains until the cam change was made. I don't know exactly where to draw the line as far as size/efficiency that are needed based on rpm and engine sizes but i like the ex. port to be as small as possible because the engine will accelerate better and is more responsive. In 2002, a friend of mine in Utah took me through Chapman's shop and i saw some of the stuff they were doing with the heads. The intake ports were also quite small. A lot smaller than you would think anyway. Nice shop by the way.

[maxracesoftware]

i had a customer who had a single-pattern roller cam profile
i kept increasing Exh-to-Int Flow Ratio in stages
everytime the car..went much faster down DragStrip
the last increase , i was just at 90.0 % percent ratio and couldn't get anymore ...then we tried same Intake lobe with 6 deg at .050 more exhaust duration...the result=> car slowed down a little .

E/I Ratio also depends and is affected by how much of a difference you have in duration spread between the Intake and Exhaust lobes..and also at what point you are opening the Exhaust valve

if you are opening the exhaust valve really soon , the engine won't like a high E/I Ratio

if you have a lot of spread between the Int and Exh lobes's duration wise..the engine won't like a high E/I Ratio

Its very possible to use a relatively high E/I Ratio sometimes on a too-large cross-sectional area head ...and cause that head to work better

Likewise, if head cross-sectional area is State-of-Art like ProStock and highly scienced out .....you can't use high E/I Ratio method, because you can't afford to waste flow out of exhaust....and you already have the correct velocity in the ports without having to rely on high E/I Ratio help

[speedtalk]

Hi Chris, Thanks for joining the board. I know who to call on for oiling system questions now.

[speedtalk]

My forte is valvetrain.

Great, even better.

[maxracesoftware]

Hi Chris,

Good example right now is the LS1 head. The exhaust port is excellent..some at GM was paying attention to the Honda when it came to %'s, the exhaust is around 80%
---------------------------------------------

yes it has great exh ports
but
i wish GM would also have paid attention to LS-1 and LS-6 's Intake short turn ... the "water" is too close on many castings
and also on outboard intake port wall

this head could still be cast better for more HP / Torque in those areas
LS-6's outter wall at bowl was a good move but still didn't fix the short turn problem.

Chris, with the 80.0 % E/I Ratio whats working for you with cam durations at .050 and centers ?

[learner]

Over lifting the exhaust...yes, the current trend. People don't seem to realize that may increase top end hp but it severly reduces torque.

[maxracesoftware]

Where are you located in LA Max?
---------------------------
Louisiana www.maxracesoftware.com

Meaux is pronounced like "Moe" Cajun French

the new AFR replacement for LS-6 heads , is it going to have a much longer installed heights ?

2.080 +?? intake valves +
interlocking seat inserts ??
spring pad diameter ??