randy331 wrote:groberts101 wrote:I
Just be prepared for a custom cam(and somewhat confused cam grinder) though, as shelf grinds don't take into consideration that some heads port designs can easily exceed 90% I/E flow ratios.
Do really believe a flow bench ratio of 90% E/I = a different cam than 85%?
The flow bench tells us EX duration requirements?
What would you change on the cam for a 5% E/I ratio change?
How does what the intake does to port flow on the bench affect duration?
How about what the actual headers to be used do to flow vs a short test pipe on the EX ?
How close an ex port flowz to the intakes on a flow bench at 28" determines a "good" ex port?
FlowZ rule?
Randy
Assuming you're not speaking rhetorically here?.. then hell yes, I most certainly do believe that a cams timing and lift curve can be further optimized around a more efficient exhaust port design. Is it worth it to the average power monger?.. probably not, but many chase much sillier things to gain 5 horsepower. You don't appear to be the average power monger though, so I thought my time may be slightly appreciated, or at least scrutinized for any potential gains, and decided to share my thoughts on the matter.
With these particualr heads, maybe it would be more realistic to ask that question as.. "Do really believe a flow bench ratio of 90% E/I = a different cam than 75%?"????? Changing the number spread to more realistically model what you're working with here should warrant no further question there.
"The flow bench tells us EX duration requirements?".. it most certainly does to some extent but it also points towards area under the flow curve which can be bought and payed for with more aggressive lobe designs and/or rocker ratio as well.
"What would you change on the cam for a 5% E/I ratio change?".. well, probably not much since I don't have big enough paychecks to play or race at that high a level. Based on my rewording of percentages given in the question above?.. I would likely shorten the exhaust side seat timing by a couple degrees at the very least. Or.. if the operational range was high enough?.. I might just leave it alone and add a couple more degrees to the intake side of things in hopes that any extra early tug @IVO may be beneficial to an already overly large intake port to help fatten the power band while hopefully helping it carry a bit further past peak.
"How does what the intake does to port flow on the bench affect duration?".. Assuming you're still speaking to the exhaust side of things here?.. it affects the total amount of crank degrees needed to effectively jump-start the intake tract early on and scavenge the entire chamber. But me thinks you already knew this.
"How about what the actual headers to be used do to flow vs a short test pipe on the EX ?" This is a tricky one for me as I'm constantly learning new things as I go. I guess it goes without saying that it depends on the header design, chassis restrictions, and whether or not they are just another set of "one size fits all cookie cutter's". Personally speaking, I try to match the port exit size as close as possible and keep velocity up while working to avoid major cfm losses in the process. But that's much easier said than done and I'm in the exact same boat here as everyone else is, and the worse the chassis restrictions and further off the mark the header design is?.. the more I'm forced to trade velocity for cfm to keep the power climbing and the power bands width will always suffer for it.
"How close an ex port flowz to the intakes on a flow bench at 28" determines a "good" ex port?" Heavily loaded question. I look at an exhaust port as a completely separate entity of the intake port and base its efficiency on the valve size, port area, and departure angle. If a stock style Vortec exhaust port with 1.54" valve and near stock sized outlet can flow anywhere near a bigger ported, higher angled, 1.60" valved exhaust port?.. I would consider it as being more efficient as the DC will be higher. The bigger port and valve setup would then need to be further optimized to match and finally surpass the DC of the Vortec configuration.
"FlowZ rule?" Depends on the engine and power requirements. In an air starved configuration?.. 99 times out of 100 it probably does. For the engines that most of us can afford to build?.. nope!.. not even close. Air-speed without premature choke for the power requirement = improved inertial tuning, higher BMT, and wider overall power bands. Along with less cam timing band-aids to get it all done.
In a nutshell.. a very efficient exhaust port design can reduce the amount of degree robbing necessary to maintain peak power potential while still affording more reasonable power band width. Many people just get that old saying "can't get it in if you can't get it out" confused with meaning they need a bigger valve/port/header/exhaust. When it comes to the exhaust side of things, smaller and faster is better if choke is still being avoided. Then we add more heat and things can improve even more. Such as slightly smaller SS piping with coatings.
