I know that everyone says it's a good idea to port match your gaskets to the heads and manifold but has anybody cleaned up the runners in their manifolds too or is the rough casting better left alone ?
So do you think it is best to leave the intake alone or is cleaning up the casting and any irregular shapes beneficial ? It was just something I have never done nor heard of but popped in my head as another possible performance boost.
I've read that leaving a step ( a little higher than the bottom of the header) on the bottom of the head exhaust port helps speed the flow.
I port match and blend back into the intake runner and intake port. A burr grinder leaves a rough enough finish, I don't spend time with sandpaper rolls.
Grinding a bit out of the bowl , just under the valve can help but better to leave that to the pro's. None of this port polishing is anything you can feel in the seat of your pants. shed some pounds...better heads...more displacement and tune , tune , tune till you get it right. And have fun !!
I had a thread here step by step as I ported my slant six head. I forget what I called it but it should be archived. Not too smooth. Like he said burr or grindstone finish is good but DO NOT POLISH. Why? because the air sticks to a polished surface just like a race boat sticks to too smooth water. A little bit rough prevents this. Most of the ponies hide in the valve bowl BTW. In most cases the port is not the restriction. Biggest culprit are the valve guide bosses. When porting an intake manifold the intake should always be slightly smaller then the head so any mismatch and there will always be some will not put the edge of the head out into the airstream. don
Before I did a bunch of work up inside the intake ports or plenum under the carb I would have a flow bench handy.
I have a set of 2.02 small chamber heads (1967 Chevy) that have been professionally ported. The intake runners are rough surfaces but the exhaust runners are very smooth. My Offy 3x2 intake manifold runners are similarly rough. The intake ports are matched to Felpro 1256 gaskets. Posted using the Full Custom H.A.M.B. App!
I had a flow bench here for many years. Built it myself but eventually you know what to do OR more importantly what not to do. Turbulance is the enemy. The straighter the area under the valve seat in the bowl is the better the flow will be. There are limits to what you can do but keeping that in mind sure helps. If you could imagine a shape like 1/2 an egg shell under the valve seat you would be able to produce a decent job but there are limitations and sometimes water nearby. When beginning to learn to port go gentle till you are comfy. If doing an all out I make sure I have spare cores and endeavour to do what I want fully prepared to sacrifice a casting however after many years of porting , probably 49 if memory serves me, I feel the surface often. If it is getting thinner it gets warm and I know to quit. This is a bit instinctive perhaps but it is so for me. As for flow benches they are very useful however even if the worlds worst porter flows his head on a bench it doesn't do anything more then a voltmeter does for battery. It is a measuring device nothing more. Best thing you can do is every time you are around a set of real good heads take a peak in the valve bowls if you can. If someone ruins a good aftermarket head ask for it and look at it. study it. Make port molds even if just with silicon. Or as I tell my friends who want to port "If you want to port you have to learn to think like an air." Some people just have a gift for it. Others should stick to cutting potatoes perhaps. Remember bigger is NOT better. The right shape is better. don
I have always been told not to polish the intake runners. Do you guys think it is worth it to polish the exhaust runners and combustion chambers or is it not worth the effort ?
Mirror polishing the combustion chamber and exhaust port will help until they get a coat of carbon on them. By reflecting heat back into the combustion chamber .
When Don says, not too smooth, I believe he is thinking about the kind of smoothness you would get with 240 sandpaper. Not a mirror polish but not covered in burrs like a raw casting.
Thanks guys ! And I looked over Don's thread, I can tell he is the real deal. I am building a 265 for my 55 Chevy, look at my build thread when you get a chance. I am trying to build the best street/strip combo I can utilizing the engine and heads I just bought using modern cam technology and some inspiration of tricks the Nostalgia Super Stock guys use. A low to mid 13 sec goal is well in reach.
The problem is boundary layer air and fuel suspension on the intake side. Any pipe with air running through it has a thin layer of air that builds up close to the wall and just sits there. A rough surface helps reduce this by causing minor turbulence which forces the stuck molecules to move so you get a thinner layer. The other problem with a smooth intake that has air and FUEL mixture running through it is a smooth wall causes some of the fuel to come out of suspension and stick to the walls from surface tension. These spots attract more spots until the puddle gets large enough and a now BIG glob of liquid fuel pops off the walls and now you have large drops of fuel in the intake instead of homogenized air and fuel mixture. It it even worse around corners with smooth surfaces just like the roadrunner vs the coyote cartoon. Just like in the cartoon the lighter roadrunner and air make it around the curve but the heaver coyote and fuel can't make the curve as easily and smashes into the port walls and the crash makes for more liquid pools of fuel flying around the intake system. With rougher walls the blobs of now liquid fuel get smashed back into smaller blobs from the turbulence and the mixture stays more homogenized. The exhaust carries just air and unburnt particles of spent combustion by-products so the above problems don't apply so smooth is better in the exhaust side.
correct . and sometimes the polished surface will prevent the carbon from adhearing too but try to leave the quench area a little on the rough side to help promote turbulance , on a intake surface we often just leave it 300 grit , and only remove any bad casting flash in the ports
I generally leave the chamber except of sharp spots. I believe it helps carbon stick. I also glass bead piston crowns for the same reason. I have come to believe we are too quick to make stuff pretty ie polished and shiny. I took a step back from that several years ago and have never been tempted to go back to the old ways. Perhaps not all will feel that way but I have been able to produce some pretty competitive engines for very little $$$$$$ and that is what makes me tick. don
On the Intake you want it to be a little rough so that it causes the fuel to atomize better because the air is more turbulent. But the Exhaust can be smoothed out as much as your Headers will allow for flow.
Saw a demonstration years ago I never forgot. Local hot rodder and porting maestro poured a few drops of gas on a sheet of glass and blew on it, the gas drops rolled across the glass. Then he did the same with a sheet of emery paper and the gas vaporized at once.
That sounds very interesting and I guess proves the point in front of your eyes. Take a look the pics of my new "white" 265 in my 265 engine build thread and look at the pistons. The dome and top of the pistons are very rough. I have never seen that before. Was that done to them on purpose to help vaporize the fuel/air mixture ?
The Jalopy Journal
