DIY Port work rb25det head.....

[meet07]

Since my motor is being rebuilt for the second time I decided to do a little work to the head myself...you know remove the hump on the exhaust ports and smooth everything out in there. I also decided that I would like to port match the exhaust manifold as well. Im using a stec manifold and a cometic triple ply reusable exhaust manifold gasket. Placed the gasket on the head, used my fine tiped permanent marker and outlined what need to be removed. All the ports looked like they need just minimal material removed except for cylinder 4. With the gasket on that port just looks funky! Doesnt look right at all.... You could see the sut on the gasket where it didnt line up....Im wondering should I go ahead and removed the the material from the head so that it matches with the gasket? I know I need to do the manifold as well so im not worried about that but has anyone else seen this before??

I can take pics tomorrow if it helps....

[danielmcn]

I wouldnt port match unless there is a large difference in size, like the manifold is smaller and blocks off flow. Think about it like this. Air first has to go through the valve and then out the port. As the air moves up the port it will slow down as the port gets larger and larger. If you make the mouth larger it will cause the air to slow more, this will translate into slower spool and less power. If I were you, I would only do the manifold if its smaller then the heads. I would remove the humps and smoothe the ports out, but dont take too much from the humps cause you could hit the water jakets......


Good luck..

[robbie2883]

I wouldnt port match unless there is a large difference in size, like the manifold is smaller and blocks off flow. Think about it like this. Air first has to go through the valve and then out the port. As the air moves up the port it will slow down as the port gets larger and larger. If you make the mouth larger it will cause the air to slow more, this will translate into slower spool and less power. If I were you, I would only do the manifold if its smaller then the heads. I would remove the humps and smoothe the ports out, but dont take too much from the humps cause you could hit the water jakets......


Good luck..

ok...hold up...so by your reasoning back pressure is good for a turbo?

NO!!! if the manifold ports are smaller than the head ports (on the exhaust side) there will be a lip that will cause turbulence. this disrupts airflow and actually slows spool time down. not to mention lowers the ability to reach peak boost as easily. even if ports already match, just smoothing out the material can gain HP and spool just by smoothing out the airflow, which creates more volume, which uses more gas, which makes more HP!

[meet07]

Yea I know the phrase "the bigger the better" doesn't always mean its the best but for turbo it helps usually. when I talked to the guy at the machine shop about opening up the part around the valves he said that it would lower compression. I told him was turbo and he was like "oh". I want as much air going into the turbo as possible :-)

[danielmcn]

I wouldnt port match unless there is a large difference in size, like the manifold is smaller and blocks off flow. Think about it like this. Air first has to go through the valve and then out the port. As the air moves up the port it will slow down as the port gets larger and larger. If you make the mouth larger it will cause the air to slow more, this will translate into slower spool and less power. If I were you, I would only do the manifold if its smaller then the heads. I would remove the humps and smoothe the ports out, but dont take too much from the humps cause you could hit the water jakets......


Good luck..

ok...hold up...so by your reasoning back pressure is good for a turbo?

NO!!! if the manifold ports are smaller than the head ports (on the exhaust side) there will be a lip that will cause turbulence. this disrupts airflow and actually slows spool time down. not to mention lowers the ability to reach peak boost as easily. even if ports already match, just smoothing out the material can gain HP and spool just by smoothing out the airflow, which creates more volume, which uses more gas, which makes more HP!

You must not have read my post at all.... I said unless the manifold ports are smaller then the heads exhaust ports then dont mess with the size. Meet was talking about porting the head to match the gasket and then matching the manifold to that, this is not a good idea. Unless the manifold is smaller then the head ports, then just do the manifold. Fact is if you make the mouth bigger then you will lose spool time, its just a matter of how much your willing to lose.

[danielmcn]

Yea I know the phrase "the bigger the better" doesn't always mean its the best but for turbo it helps usually. when I talked to the guy at the machine shop about opening up the part around the valves he said that it would lower compression. I told him was turbo and he was like "oh". I want as much air going into the turbo as possible :-)

Your talking about unshrouding the valve? This will lower the comp, but unless you remove a lot it wont make much of a difference. Just make sure you place the new head gasket on the head lined up like it will be when its bolted down, and mark around the gasket. You do this to make sure you dont over do it and have a part of the gasket sticking nto the compustion chamber. That would not be good at all.

Also if you port the throught make sure you port the whole port alittle. Like if the throught flows 180cc and the neck flows 200cc and the mouth flows 240cc. Lets say you make the throught (closeest part to the valves) flow 195cc, but dont do anything to the rest you wont see much if any gain as the neck still cant flow more then 200cc and the mouth will still only flow 240cc. You have to port evenly, what you do to one area should be done to the rest. Just becarefull not to mess up the Short side turn, you can lose a lot of power messing this up. If you have or know of a flowbench that will be your best tool in porting. Put it on before then every so often to see what is changing and if your helping it or not.


One more thing, if your porting then do a valve job too. At least a 3 angle. This will pull it all together.

You got my number still?

[meet07]

I dont wanna remove to much material b/c im sure there is some professional way of seeing if all the ports in each chamber is measured the same......all I want to do is even everything out...thats all. but this gasket,manifold and port for cylinder 4 is all off like weird, freaky off....

Imma take some photos when I get home tonight to show you guys what im talking about. I mean you would think that the gasket is whats off but with the gasket on both the head and manifold its the same......??

Its alien weird I tell ya! Alien weird!!

I got to thinking that maybe this exhaust manifold gasket is just junk as far as design? I mean the gasket looks the same on both manifold and head so its gotta be the gasket right.......?

[julio]

I was under the impression that you wanted your exhaust ports on the head a tad smaller than the manifold to help prevent reversion.