Guys, I've been suggested to port & polish my stock intake manifold (carb) on my 300 I-6 Bronco as opposed to putting in a performance intake manifold (since they're pretty hard to find in my neck of the woods...). Would this work?. BTW I would also port & polish the head and install headers... thanks

 

A port match and polish wouldnt hurt the intake, but I would try and get a better intake to do it to than the stocker first. Offenhauser makes a intake for the 6 that works rather well.

 

You can find more 300-6 info in the 300-6 engine forum.

I have seen porting do much more damage than good. If you don't watch your fit-up tolerances you can end up with a gasket sticking out on one side of the port or a port mismatch that produces an obstruction to airflow that is much worse than a standard cast port. Remember that gaskets vary from one manufacturer to another, lot to lot, and even individual gaskets in a lot. Make sure that any port resizing will result in a transition from a smaller opening into a larger one.

Are you wanting to convert from EFI to a carb?

 

Whatever you do, don't polish an intake. polishing is only for the exhaust side, or possibly the combustion chamber on heads. It will cause fuel to stick if you polish the intake.

 

I am researching the port and polish idea in the FE forum also and there is some good stuff being said there, too. It's not related to the 6 cyl, just port info in general.

 

Thanks guys, so then I could port but not polish the intake? Like I said, they don't sell them performance intakes here, at least not for the 300, and I would have to get it from someone in the States. Maybe if I increase the port diameter as deep as I can on the intake manifold while porting intake and exhaust ports on the head I might get good results? (more performance, something which the I-6 is not known for...)...

 

When you "polish" the intake, only do this with the rough sandpaper rolls, not the fine grit. Do not attempt to do any major increase to the size of the ports. Only attempt to match the gasket (insert the bolts through the holes so that the gasket is aligned exactly as it would be installed and then mark the intake with a scribe after using Dykem to better see your scribed mark). In addition to the gasket match, remove the casting flash and give a smooth transition to any sharp corners, this will decrease the turbulance in the manifold. If there is any significant difference in the sizes of the port from the top under the carb to the head mating surface, again, slowly transition the changes to smooth out the flow. These changes should improve the flow of your manifold by quite a bit.

 

Expanding more on the porting issue, over-porting will hurt the motor where you use it the most in a truck.... at lower RPM's. The main objective with "street" porting as I call it, is to smooth out the flow and allow it to pick up velocity, which will result in more air/fuel mixture being able to enter the cylinders. By opening up the port, you end up with less velocity at lower RPM's, which will have a loss of power at low RPM's Most all factory parts (heads and intakes) can benefit from some mild port work, but the amount that would help a 5-6000rpm street rod motor will actually be detrimental to a good stump puller in a truck. As for the polishing, a polish on the intake will also allow velocity to increase. At times, a good port match on the ports and gaskets and a good polish job will achieve all that is really needed to bring a motor awake. The debate between polish and satin finish is an interesting one, I myself go with the full polish, even tho most swear by the satin finish to increase turbulence in the air fuel mixture. Reason being an article I had from years ago by Jerry Branch of Branch Heads (the guy who helped design Harley's Evolution head) with the results of some tests from Kawasaki of all places. The basics of the article and test results were that (from using sensors the size of a human hair) satin finishing the port did nothing to enhance the swirl or turbulence or whatever people call it these days, of the air/fuel mix. The roughing up of the port walls didnt affect the flow thru the main part of the port, and actually allowed a place for the gas to puddle on the port walls. Another thought on the "turbulence increases airflow" theory.... how many carbs do you see that have a satin finish in the bores? JMO tho.

 

Ya know, I just dont know about the mirror polish vs a rough polish on the intake side. In the hundreds of heads that were done in the shop that I worked at, we did do a mirror polish on both the intake and exhaust side. After doing further reading in recent years though, I dont see that you will hurt anything by polishing th intake side with a coarser grit, if you get "puddling", that you try to attribute to a rougher finish, well I would have to say that you've got some serious issues with your fuel system there! My take is that the rougher finish will not hurt anything, and very well might help a little.

 

This is why Unless you know what your doing, head and intake work is very critical, without a fow bench!! either you know what your doing or not! Its that simple..For some of my Family that are into street machine's I have done heads for them with better results..I am in NO way A port and Polish man..I just knock off some imperfections out of the cast and will on Occation Polish the chamber's...

Of coarse port matching..The rest I leave up to the Big boys that have done 1000s of pair with a flow bench..and know what the hell there doing!!

I know a little about Porting..But just enough to get into trouble!!! I wont do the Heads on my Mystery motor.. I'll leave that to the Pro's...

Russ...

 

A flow bench should always be used on any major porting job, your right Russ. But as far as the rough vs mirror polish, you wont have any difference in CFM at all between these two finishes, so the flow bench wont help in this matter.

 

Oh I agree, if you dont have certain RMS finish on the intake ..its not going to atomize the gas proper...But the Exh..can be a mirror finish...

But like I had said the boys that really know what there doing have 1000s of hour's test diffrent RMSs on the heads with a flow bench...And All BS aside thats the bottom line....Flow bench's dont lie!!

 

umm, I have 1000's of hours on porting work, I did it for many years and there is no flow difference in the rough finish vs the mirror finish on the bench, we flow tested them both ways.

 

The one thing I want to emphasize is: DO NOT gasket match. You must have an allowance for part fit-up and gasket variations and slip or you will end up with a gasket protruding into a port or a port mismatch that will produce a protruding "shelf" that will disrupt air flow. The airflow must always be from a smaller port into a larger one. Remember there is slop in the fit of all the parts and there are also variations in gaskets. To be safe the intake should be at least 1/16" all around smaller than the head and gasket.

 

Good advice on the 1/16 Eric, when you do your port matching be sure and leave this allowance for size difference between the heads and intake or you will have a terrible turbulance that will cancel out all of the hard work put into the porting.