DIY porting/polishing

[Alf]

What areas of the intake and exhaust ports benefit most from porting and polishing on a RB25DET head?.

I've done plenty of Chev na heads before but i've been told that turbo heads need work done in different areas.

The head will need to flow over 600hp so even a minor port will make a difference. :spcartman

[ace_nz]

you wont make huge gains from porting on modern cylinder heads, but tidying up the short side radius and blending the seat to port transitions will help low to mid lift flow, on the exhaust the flow likes to go high so around the valve guides and the long side needs attention, also a nice radius on the short side is always good without opening the port cross sectional area to much, velocity is extremely important. Back cutting valves helps as well.

[ace_nz]

this is an rb20 i did ages ago

[Cubes]

I had my RB25 head ported polished. Most of the work was done on the exhaust side however the usual was done on the inlet with the addition of deshrouding the inlet valves to help that forced air enter just that little quicker during low lift.

The exhaust was polished up to chrome and the inlet was roughened up to aid in low-mid rpm fuel atomisation.

[sweetr33]

im currently in the build of my Rb25/26 and we ported my head at home. Mainly concerntrating on the factory casting dags on the inside of both intake and exhaust ports. Also on exhaust side of head there is a lump in exhaust port, i was told to grind that smooth also and thats what i did, i have a feeling that it was there to speed up flow of exhaust gas into turbocharger to help initial spooling but this fela builds some tough imports like a 523hp rb25 r33 and more so i figured what he told me was alrite to do.

[Cubes]

A lot of people do remove this lump in the exhaust BUT it does serve a purpose for a street engine.

For all out race I was told to remove it but for street it does help the exhaust gas velocity. It should still be smoothed out and made in to a smooth lump instead of the factory hard edge.

Mine was left in place but reduced quite a bit.

Its really the same as 'Should I polish the inlet or roughen it up'.

[sweetr33]

very true mate, ben told to clean off dags and leave alittle rough on inlet.

[dnb]

great thread here!i am about to port my head so this is great.

the exhaust side seems to me to need the most attention as it is very small.

has anyone ground the area between the 2 ports to make a sharp edge rather than the rounded edge it has?

i was also going to make my exhaust ports finish in a round rather than oval because im running a tubular manifold

[sweetr33]

well if ur running a tubular manifold then u are best matching the ports to the manifold. So yes if ur manifold runners are round then match head port to suit./

[Mesmer]

If anyone wants a head to practice on i have a rb25 head that has no valve gear and has gone alittle soft.

Offers?

Oh and i got my head done a while ago. a small port/polish, valve seats, seals etc its just sitting around in plastic wrap waiting for the bottom end. I cant wait to see how it goes.

[Alf]

Thanks guys, some really helpful replies there.

What is best to use to do the porting with? and what is best to polish with?

I'm not quite sure with an alloy head as it's softer than cast iron heads.

[ace_nz]

great thread here!i am about to port my head so this is great.

the exhaust side seems to me to need the most attention as it is very small.

has anyone ground the area between the 2 ports to make a sharp edge rather than the rounded edge it has?

i was also going to make my exhaust ports finish in a round rather than oval because im running a tubular manifold

knife edging the divider can actually increase turbulance, look at the edge of aircraft wings they are not sharpe, port matching is not nessasary as long as the port is smaller than the exhaust primary, infact the lip left acts as a dam to reversion gases.

[GTR-Ben]

Hey Ace where do you work mate? I think I know already

[ace_nz]

yeah you know, got your beast fired up yet?

[GTR-Ben]

Very close man, dropping it in this weekend hopefully. Got the engine 99% done, just have to makes some little sleeves for the fuel rail, it had some big bosch injectors in there and the stock ones are about 4mm smaller where it fits the rail.

[proengines]

Alf,

I normally use carbide burs for shaping and stock removal, you're probably safer using a double cut bur as theyre not as agressive. Next I use commercially available porting rolls to remove the die grinder bur marks. Once you get close to the shape you need I'd use 60 grit emery on a stick in the die grinder, turn the speed down a fair bit and it will leave a beautiful finish. You can also use flapper wheels in the bigger parts of the port. I normally remove the guides before I start to give access to the entire port then fit new guides once the porting is done and its time to cut the seats. Dont just cut the guides off flush with the port, theyre pretty short to start with and you need them to keep the valve positioned properly on the seat.

Ive found that a 4 angle inlet seat and a slight 30deg back cut on the inlet valve along with a radiused exhaust seat (it does have a 45deg part for the valve to seal on) and no back cut on the exhaust valve work really well and with a basic port job will pick up around 20CFM flow@28" of water.

2 things though, first, be very careful around the short turn radius, you need it to remain a radius and blend smoothly from the valve seat into the port. Second, don't make the port too big by hogging it out, theres no need and you will lose air velocity.

Thanks guys, some really helpful replies there.

What is best to use to do the porting with? and what is best to polish with?

I'm not quite sure with an alloy head as it's softer than cast iron heads.

[dnb]

can you please explain this in a bit more detail proengines? cheers

"Ive found that a 4 angle inlet seat "

[sweetr33]

proengines dude what do u say about grinding away the lump in the exhaust port???

[proengines]

dnb, I can't work out how to upload photos or i could show you the profiles here, if you want, have a look at:

http://www.proengines.com.au/images/radius.gif

and

http://www.proengines.com.au/images/4angle.gif

The profile of the cutter is what is cut on the valve seat. These aren't the exact profiles I use but are pretty close. The valve seats on the 45deg section.

Sweetr33, I can tell you in a few weeks, I haven't been game to take the entire lump out on a customers head as I'd hate to have to call them and tell them I had hit the water jacket porting their head or lost flow and needed to weld it back in there.

I'm starting on a head for my own engine, it'll be a RB30/RB26 head and I have the head here ready to start when I get a chance. I'll take the hump out a bit at a time and see what it does on the flowbench. I think it will work better as the exhaust port only flows around 73-75% of the inlet with the hump there and it would be nice for power to get it up around the 78-82% mark. I'll also be trying some bigger valves so I'll let you know.

[Cubes]

Good stuff Proengines.

I noticed via your website you deshroud the inlet/exhaust valves.

Interesting. I was told by a few places not to worry about it.

I had my inlet valves deshrouded, exhausts were not.

You do some nice head work.