No it pulls hard from off idle to 6K where power falls off. The cam has 5* advance and I am running a MN6. Mid-range power in higher gears (cruising on the highway in 6th for example) are also much improved. In lower gears, the RPM jumps, but I think that is only the tires breaking loose

 

How does one figure this out without these simulations? For instance, I am trying to make all my power under 6K RPM, so should I basically go with the most flow and smallest port I can? Would something like port volume devided by flow provide a good indicator for head selection?

 

If you could somehow figure out average port cross section this would probably be your best indicator without a flow bench and measuring velocities at different points which none of our sponsors do for their regular heads (too time consuming i'd imagine). The problem with getting yourself in the head volume trap is that volume is three dimensional and there are many different ways about getting similar volumes with drastically different results. Your best bet would probably be to look at what other similar setups there are to yours then choose accordingly. If one is trying to maintain power under 6k rpm with a stock shortblock then the AFR 205 heads come to mind. Good luck!

 

Unfortunately, usually you can find the port volume and the flow data, but an area number that would be indicative of flow velocity is usually not found.

I agree on the AFR's, they are very high on my list

 

Awesome, I have been looking for this discussion on another board than my own I am working on a set of gen 3 3.4 V6 Gm heads right now and have been trying to find the best EI ratio, and why. I have read many different magazine articles online as well as other sources and have come up with nothing conclusive. Some say 70% EI ratio, others 75%, then 80%, then 85%, and 70-80% as a grey area, etc. No where do I see anything on powerband or cam specs other than some interesting information on theoldone.com on cam specs.

The stock ratio for low to mid lift on these heads are well over 80%, as close as 99% for .150 life. 94% for .200 and .250 lift. With the intake side ported/blended in areas for the highest velocity per cfm increase, I have got the ratio down to 95% at the highest and still in the upper 80s low 90s one exhaust. Im afraid to even touch the exhaust ports based on what I have read.

This is 1 quote from a magazine source.

"Single or Dual-Pattern?


A good rule of thumb is that normally aspirated engines whose heads have exhaust-to-intake flow-ratios under 70 percent (i.e., the exhaust flow is 70 percent that of the intake) like a dual-pattern cam to crutch the weak exhaust port. Over 80 percent, you're in single-pattern territory. Blower, turbo, and nitrous motors generally prefer dual-pattern grinds because of the larger-than-normal exhaust volume they generate.

Then there's that 70-80-percent "gray" area in between. The only way to find out is to try both styles of cams, different ratio rockers, or (if the cam is mechanical) lash-loops"

Here is a link to the rest of what I have found interesting so far on my site.

http://www.60degreev6.com/index.php?...ewtopic&t=7807

Now, i have found a way to lower the .050 and .100" lift values some on the exhaust about 5%, with the .150 and beyond averaging around 10% increase in flow (yes, I would have 102% EI ratio with this at 1 lift point). Point well taken on the exhaust doing too well of a job and having the intake charge flow out the exhaust, which is why I am thinking that a worse low lift value of exhaust flow could be beneficial.

I disagree on looking for heads with the best flow at the highest lift. This is one of the least important aspecs of the heads as they are only at that lift 1 time, vs the mid lift flow numbers since the valve is there longer.

Any speculation or theory or even documented power vs ratio changes would be excellent.

Side note if someone can, what % margin is considered flow matched? Search engines have failed me in my attempt to see what companies consider flow matched.

 

How does one figure this out without these simulations? For instance, I am trying to make all my power under 6K RPM, so should I basically go with the most flow and smallest port I can? Would something like port volume divided by flow provide a good indicator for head selection?
There's a lot of discussion possible on this, but per remarks below your post, you often can't get all the data you need without buying the heads and measuring for yourself. In a nutshell though, yes: Especially for your application, you can't go wrong with a high flowing, small port head! The problem comes when trying to decide between say a 300 CFM 210 cc runner piece and a 330 CFM 245 cc one. At this point you need either a ton of experience or a good simulation program and enough experience to interpret what it tells you, to determine which will work best for your application. The large port should certainly work well on a stout 400 +c.i engine, but how about a 327? Well, perhaps, if you're looking to twist it to 9,000 RPM, otherwise the smaller more efficient one will likely be better.
Once you are looking at a specific design, e.g. standard height/port spacing SBC heads, the volume is a pretty good indicator of minimum cross section, which all my experience and research says rivals flow numbers for importance. (Most long time racers can relate at least one case where an engine with a huge, high flow intake port fell spectacularly flat due to miserable gas velocity)

Moving on to E/I ratio, one of the problems is that a different ratio is 'ideal' depending on the application. It's no stretch to see that a supercharger or a heavy shot of nitrous is going to cause a spectacular increase in exhaust gas volume, which can best be dealt with via a big valve, high flow exhaust port, even if it means going to a smaller inlet valve to make room. The alternative of opening the valve early to avoid massive pumping losses on the exhaust stroke doesn't appeal, due to having all that 'lost' energy booming out the header.
Alternatively, as the compression ratio increases, more and more of the gas force is transmitted to the piston early in the expansion/power stroke, so the penalty of biasing valve size and port flow to the intake and crutching things with an early EVO is minimal. Consider a typical 'umpteen to one' CR Pro Stock engine. I have no idea (but perhaps someone will chime in...?) what the E/I flow ratio might be, but with say a 2.45" inlet valve and maybe a 1.6" exhaust, it would probably look spectacularly 'bad' to us peasants.
Last point: I may get some argument here, but in the search for E/I balance (if still needed after heeding above) I can't imagine it ever being a good idea to deliberately make the exhaust worse, unless the changes are directly required to improve the intake. Similarly, given the preceding exhaust flow discussion, it seems to me low lift exhaust flow is far more important than high lift, to get a fast blowdown while allowing a later EVO to capture all available cylinder pressure. As a matter of fact, Dynomation usually shows that using exhaust lift equal or greater than intake actually costs HP. [Allan Lockheed, creator of The Engine Expert simulation program, told me he once advised a Dodge-powered Modified owner that his EVL, (same in this case as IVL) was too high. The guy went from 1.7 to a 1.5 exhaust rocker on the spot and gained so much power he had to re-gear to stay off the limiter. The dyno later revealed a 40 HP gain.]
As far as exhaust flow being 'too good', and letting the intake charge flow through and out, this suggestion would be a case of 'shooting the messenger'. In the overlap period, the port merely communicates the pressure signal from the exhaust tract. If fresh mixture is being over-scavenged, the problem is a cam/header mismatch, not a 'too good' port.
Well, over the years I've found that I rarely learn anything while I'm talking, so enough of my blather and back to listening/reading.....

 

In my case, its only a couple CFM at .050 and .100 lost in exchange for 10s of CFM past that. Im just afraid of the 100% + exhaust ratio because of what I have read as "ideal" even though I have never read any particular reasons for such ideals. This particular head will be nitrous fed occasionally and NA the rest of the time, using 10:1 compression. Thanks for the clarification on the over scavenging during overlap.

 

Well, I can't add much to this discussion technically, but the input thus far has been great

 

Maybe just clean up your stock heads if you want to maintain or improve low end.I've built a 96 impalla SS with LT4 ported heads/hot cam and even though the car was faster it didnt feel as quick cause I lost low end.Then I built a 00 camaro ss with ported heads/and the similar spec cam as yours.again lost some street fun factor even though the car was fast.Now I got a magcharged z06 with stock heads/forged pistons Torque is good.If you clean up a set of stock heads and improve your intake manifold,I wonder if the fast manifold improves low end?port match,etc.I am willing to bet that you would like a set of stock LS6 heads,mildly ported,with nearly no change in intake but a little more on the exhaust side for daily driving.also,they breathe at high lift and the light valvetrain stuff will give you more revs wich will complement the cam you have.You get low end as well as more power up top simply due to a broader rpm range.If you gotta have ported or aftermarket heads:I was looking at some of the ported LS heads and it looks like the intake port volume is up to around 243cc on average where the AFR's flow nice and keep a 205cc intake port so figure the AFR more likely better bet for low end.GMHTP did a dyno of these heads on a camaro a few mos back so look for the article. Kickin up compression is also going to help a bit all around.I hate losing any low end torque--If you find a head that can do it all I'd be interested.