Sprague,
Efficiency, velocity profile, port speed and cross section area are all equally critical in either a forced induction or NA application. The dated notion that you can crutch a bad (inefficient) head with a blower, a cam, or a bigger bad head, will not yield a positive result. In lieu of a data dump the quote below sums it up;

“To date, I have yet to see a turbo charged, nitrous injected or even an supercharged alcohol Pro Mod engine run better, make more power or run faster on the race track with a larger port than would be necessary to produce max power in a normally aspirated engine.”

Darin Morgan
-Induction Research and Development
-EFI Calibration and Tuning
Reher Morrison Racing Engines

What I've seemed to find, is that while at times the head port size, the heads flow numbers, may not be textbook, and would lead you to believe the engine would be a turd, may not be the case with the right supercharger. You take a screw blown setup, that might have a bit larger than ideal intake port volume, would maybe have you thinking "this thing is gonna lose low wnd power ". But then when you mash the throttle and the blower makes instantaneous boost off idle cramming air into the cylinder , all of a sudden the theory goes out the window. As mike tkach recently built a mild 509, with merc iron heads that sport a port volume of 320ish cc , with poor cross section, a horrible exhaust port, and a whipple. What he found was an engine making in excess of 830ft lbs at 3000rpms, while ultimately it made 760ish hp. While sometimes the head itself may not change the outcome, the blower design, cam design, etc ultimately can. And to much greater effects than simply quibbling over 10-15 cc worth of port volume. What you do not want is to run out of head and cause boost to back up the intake, and hurt top end power.

As far as N/A vs forced induction, Yes the right head on both works. port velocity is maintained while the density or weight per cu ft changes . Apparently the port size , cylinder size , and piston speed, are governing the speed, while the supercharger is still packing more air in the cylinder. From what I read.

Another interesting quote from Darrin morgan.


""The chamber design and camshaft exhaust duration are two huge factors for both supercharged and Nitrous engines. They are basically the exact opposite of what you would do for an NA engine. In an NA engine the quench/squish area is overwhelmingly important. In a supercharged or nitrous engine its important to get rid of all the squish quench area. The pressure rise and flame travel is so fast they a high squish chamber will end gas detonate even though the squish quench is trying to cool the mixture"

while i understand what mr morgan is saying i am somewhat confused.how does a supercharged engine make more power than it,s n/a twin.i will give an example.a 9.1 cr 454 with a afr 305 head,same cam,same everything.why does the supercharged engine produce more hp&torque.i always thought it was because the supercharged engine filled the cylinder more efficiently than the n/a twin.if mr morgan,s statement is correct what is the purpous of supercharging?

what they have gone to in these applications is a hemi type chamber (chamber the same size of the bore) boosted applications. Or piston .200 in the hole with heavy (5 kits) nitrous. Not totally relevant for what we do unless you are looking for a serious boat motor :-)

You are flowing compressed air Instead of Atmospheric. The more oxygen/ gas you can get into the cylinder the more power it will make.

after rereading it several times i understand,re is only talking about the head.

I believe the mercury 1075 and 1350 have a quench of .120 ?

An interesting post on another forum by Curtis boggs from rcd



by*cboggs*»

I build a fair amount of forced induction heads, ..*
blown alky roots blown, turbo, procharger, etc.*
The old line that "it doesn't matter" is total BS, .. it matters as much*
if not more.*

Anytime you increase efficiency in any engine power goes up, ..*

As for the differences in the heads, .. the first thing on my list*
is dealing with the additional heat, copper/ally seats, Inconel*
exhaust valves, etc. I see too many people putting out of the box*
cnc heads on blown applications and having trouble, .. you HAVE to*
deal with the heat! I use solid or HIP processed castings for the same reason.*

In a blown application I think you can still make an intake port too big, ..*
while the blower will mask some of the issues of oversized heads*
there still can be too much cross section.*
The cross sectional areas and the valve/throat percentages can change*
and what you can get away with depends on whether it's a turbo or*
roots type blower. Yes you can go larger then a typical N/A head and*
find power but again going too large will depend on the application.*

Port design I think is the bigger issue, .. I'm more concerned about turbulence*
with the larger volumes we are moving, . . so i work on much "smoother"*
area and shape transitions.*

Exhaust ports are an area of concern, I'll typically use a little larger exhaust*
valve to intake valve ratio depending on the application, and will use*
a little more cross section and MUCH wider seat for a roots type blower.*
For turbo applications it varies but I want to keep velocity up but still use wide*
seats to move the heat from the valve.*

Cheers,*

Curtis

Race Flow Development*
Simultaneous 5-axis CNC Porting*
http://www.raceflowdevelopment.com

You know I was thinking about your previous post and about using the larger than optimal intake port and how it you said it still makes good power with the right blower what Darin said about the Pro-mod engines. You know top fuel is the limit. There is nothing more powerful. They run a 525ci motor. About 2 years ago I was in a friend shop and he said let me show you something (it was a top fuel head) I checked that thing out from top to bottom. I couldn't believe how small the ports were I mean with that kind of power and boost I would think you would be able to drive a tractor trailor through them. Surely not. I've worked on Nitrous Pro-Mod stuff been elbow deep in the motors I know what kind of port it takes to support 865ci and 9000rpm (tractor fits). But the top fuel stuff is small. I guess the theory of sizing the port to the cubic inch and rpm applies no matter what's on top of it or what kind of fuel its burning.

bb,i think top fuel is 500 inch limit unless they changed it.the nitro engine gets by with the smaller port because the nitro methane has it,s own oxygen,not relyant on oxygen from the air.i believe they only run the nitro engines to around 8000 rpm.