Sixgun

What happens when you install a square port intake on oval port heads? Does it affect the performance much?

andy60

You will not see much of a difference on idle or low RPM. When you put your foot into it or at high RPM, is when you will have issues.

DZAUTO

This one has been beat to death over on the Chevelle site (TEAM CHEVELLE). At one time, all the "experts" talked about the resultant problems that will occur from what is called reversion, which is an airflow interruption that occurs where the manifold runners and head ports meet, because of the sudden change in port sizes.
Well, several of the big block Chevelle guys have rectangle port (or as you call it, square port) intakes on oval port heads. All of those guys have reported that they have NOT noticed ANY problems with a rect port intake on oval heads. Thus, I have totally changed my view on this configuration (contrary to what all the experts have stated in the past). My personal Chevelle is not yet a BB car. It has a healthy SB400 in it at this time and will remain that way until the perfect deal comes along on a BB-------------------with ALL the accessory brackets and hardware (including the fan shroud) for ps, alt and air cond.

AZDoug

Reversion is a fantasy of people that do not understand fluid dynamics.

Educated as a Chemical Engineer many years ago, gives me the background to comment on this.

What happens is that the air in the manifold actually sets up it own flow form in the pocketed areas where the transition takes place, making the flowing air think that a smooth transition exists between the larger runner and the smaller head. The pulses do not bounce around, or bounce back. I have not run a Reynolds number analysis on what is going on inside an intake runner, but I am sure it is turbulent flow and not laminar flow, with laminar flow existing in the transition pockets.

Note what will adversely affect flow would be an attempt to radius the head ports to match the runner ports.

When going from larger to smaller, you want a sharp of an edge as possible, as even a small 1/8" radius on the head will create drag and unwanted turbulence into the port.

Another contrary to conventional wisdom tid bit: Leaving the tailgate of your pickup down during driving actually increases air resistance on the truck, as there is no longer an "air pillow" generated in the back of the truck by the tailgate that allows the air flowing over the cab to smoothly pass over the bed area, instead, you get a bunch of drag forming eddys behind the cab with no tailgate, or those vented strap gates

Doug

Donny Brass

I saw that one on Mythbusters.........

rgs

I would completely agree with your assessment if the fluid flow was continuous, but what happens with the situation of continually interrupted flow as with valves opening and closing? Each time flow is re-established, the resistance of the transition between oval and rectangular has to be overcome. Once it is and if flow remains constant or near, fairly smooth flow continues. But that is not what we have with an engine. At 6000 rpm the intake valve is opening and closing 50 times per second. Each time the flow is interrupted and restarted. Each time the flow has to overcome the resistance of the mismatch. Another characteristic found is that when the valve snaps shut, the flow does not immediately stop, but due to it's velocity, continues into the chamber behind the valve. This creates a higher pressure pocket that then reverses flow and rebounds. Many design factors affect the amount of rebound, size of runner, length of runner are the two major ones. During this rebound, flow passes reverse direction through the intake runner and the mismatch transition, creating even more turbulence. (Some engines have active intakes to alter this characteristic. The Taurus SHO has an active intake which uses long small runners for low end and larger short runners for top end.)

Many years ago, I tried this on a 1977 Ford pickup. It had a 351M engine (oval port) and I tried a 351 Cleveland 4 barrel manifold (square) on it. It definitely had issues. As soon as I put the proper intake on, using all of the other same components, distributor, carb, etc, it ran great.

We go to great lengths to port match intakes to heads on our race engines. Current technology is to extrude hone the head and intake together to obtain the desired flow characteristics, eliminating all of the opportunities to create turbulence. We also use radius pocket cutters instead of stones for the valve seats. Historical valve seat grinding used 3 angles, then later 5 to try and smooth out the flow transition past the valve. These are just examples of how critical to optimum performance a smooth flow pattern is. Any rough transition will cause turbulence and reduce flow. We would never consider such a mis-matched combination on a race engine. Will it run? Yes. Will it run smoothly? Maybe. It definitely will be giving up power capacity.

Mr D.

Depends on what your definition of performance is. If you are doing ¼ mile runs and looking to bring to bring home the trophy, then don’t do it.

If you are looking to save some money on heads for your street machine, than go for it. I installed a 66 427 / 425hp setup on oval heads and had no problems what so ever. Did the same with a 69 427 / 435hp Tri-Power setup on a set of oval port edelbrock performer heads.

Hope this helps.

AZDoug

The flow does not actually reverse (usually; in some instances, at low enough RPMs, you can have flow reversion, usually due to the piston pushing the air back out the intake vale on the compression stroke before the valve can close), what happens is you get a shock wave traveling back up the runner during valve closing. The difference is subtle, but it is a difference. The shock wave of course, can help or hurt other intake runners depending on manifold design, cam, etc, which we have all seen with peculiar peaks in HP and torque

I am not disagreeing that port matching is best, but the theory that the air intake charge bounces off the flat surfaces ce of the head next to the smaller port, and back up the runner is a visualization that people come up with that has no bearing in reality, since the fluid is compressable, and it is moving at tremendous speeds. Will you notice mismatched heads on teh street? No, especially if you are running mufflers. On a clock that measures to .01 seconds thru the quarter, you will notice a mismatch, assuming everything else is right

I deal with high speed pulse flow everyday, I make silencers for firearms. The difference is, I try to impeed the flow as much as possible, and I have often used the reverse of what works best in an auto intake or exhaust as a starting point in flow impediment through a straight through pipe. And one thing I can tell you: a simple flat plate with a hole in it (the equivalent to a Rect intake and oval head), it a real poor design for impeding flow.

www.srtarms.com

Doug

00fxd

I put it down to being like a bumble bee and flying, it's not supposed to work. Obviously port matching is best. If you were to run run it on a dyno I'm sure that you would see a power loss, but I have done it many times resulting in very good running performance engines and would not hesitate to do it again. Use the rect port gaskets.

BADBIRDCAGE

Ran it myself years ago on a 396. Street driving only. Can't say a bad thing about it. Never pulled above 5200 RPM with the motor (except for the time the mechanical linkage jammed wide open and it seemed like about a half hour to reach the ignition key to stop the windup).

Great looking setup for tri-power on a 66 Impala SS.

I say go for it. And don't forget to use the rectangular port intake set.

Rich

AZDoug

My first car. '66 SS Impala 427, TH400, mist blue metallic with white top factory optional bumper guards.

When I left MT to go to CA in 1980, I could barely get $500 for it in almost cherry condition, as it was either the Corvette or the Impala got to come with me. Got to LA a few weeks later and saw the SS would have gotten me $1600, at least.

Wish I still had it. I waxed it so may times I can still remember every curve and line on the body. Can't say the same about the ex-wife.

Doug