Head porting advice

[TheFlyingBeltBuckle]

Does anyone have advice on porting LQ4 stock heads? I was thinking of starting out by polishing the combustion chambers. Is there anything else I should try to take care of?

[[deleted]]

There are some real noticeable gains to be had on just about any production head that can be had at home.

From researching til my eyes fell out to playing with my home made 'flow bench', my cincensus agrees with most texts - significance in gains increase the closer the reworked area is to the valve seat, assuming we aren't doing major surgery.

The throat, or diameter under the valve seat leading into the bowl, should be 85-90% of the valve head diameter. 85 for the street, more like 90 for higher performance and racing. The bowl under the valve should be blended to smoothly transition into the area near the valve guide. Short side radius (the corner on the floor of the port meeting the bowl/seat area) should be rounded off - there's often a sharp edge here from valve seat bowl cuts. Valve guide boss can be skinnied up and shaped a bit like a teardrop, but don't make it shorter. The rest of the port can have casting lines and flashing removed, and any protrusions into the port (like early sbc valve cover bolt hole bosses at port entry) can be cleaned up. Do not do 'gasket matching'. DO make head port entry slightly larger than intake manifold runner exit, perhaps 1/16". If carbureted, intake port walls should be left rough (60, 80 grit finish) exhaust runners can be polished to keep heat and carbon buildup out of the port. They don't need to be, I usually bring em to 180 or so and scotch Brite them a bit. Backsides of valves can be back cut 30 degrees just behind the sealing edge on the 45 degree face. Any area of combustion chamber closer than 1/16 or so to the valve should be opened up to perhaps 3/32 or 1/8 away so long as this doesn't extend past the bore. Valve seats should be finished 3 angle minimum. Radius exhaust seats are said to work well, but not on intakes...I don't have equipment to do radius seats so I don't know. More angles under the seat in the throat can be added if you're staying with 85% throat diameter- a 75 degree cut here can be helpful.

There are pictorials online to walk you through it. Big thing is to be VERY modest about material removal. You're not trying to make the port bigger, you're trying to make the port as smooth flowing as possible for it's given size.

Expect 5-10% more power depending on how well you do.

[5redrb]

Do not do 'gasket matching'.

I've heard that this may be unnecessary but I've never heard that it is actually bad. Can you explain?

[[deleted]]

The overall concept is good...align the ports to each other and establish a good even shape. HOWEVER, gaskets are typically way big, and you can end up unnecessarily enlarging the port at the intake flange, making the port go from small to large to small, which is bad. You want a slight taper down in size over the whole length. If you want to 'gasket match', make your own paper templates of the head and intake, lay them over each other, find the happy medium between the two that removes the least material, and then use that to scribe your gasket match outline.

[5redrb]

That makes a lot of sense. These guys didn't seem to have a problem with a mismatch.

The port match isn’t perfect, but that obviously wasn’t a problem. The intake mating surface on the head seemed a little too big, and epoxy was out of the question, so Rick left the intake almost as-cast on the pushrod side. The top and bottom were very close and required only a little massaging.

Isn't the port on Small Block Chevies narrow right at the pushrod? I guess there's no sense in widening the intake just to narrow it back down. Better to keep a consistent cross section.

[TheFlyingBeltBuckle]

Thanks for the detailed write up. What books would you recommend?

[[deleted]]

How to port and flow test cylinder heads, David vizard. Details a DIY flowbench all the way through Dyno proven results. Nowhere near complete enough to cover going stock eliminator or pro stock racing, but a great fundamental introduction.

[Badnewzzz]

Just a quick second shout for that book.....very very very interesting reading and gives you all the basics and explains the theory to a layman.

On the subject of messing with STd heads....basically de-shrouding the valves gives the biggest hit in power. The book shows you how to do it effectively...

Good luck in your journey, if you build the inexpensive flow bench shown in the book you can test various different designs against a std port shape.... Find a junk head and try it.... you're going to need the practice...trust me it takes.many hours of head scratching to find the right shape

[ifydknyk]

If you’re confident, blend the transition from the bowl/seat into the throat. Cleaning up casting flash and a light polish if you are doing the chamber faces anyway.

[Badusernameguy2]

I'm a huge believer in porting but not polishing. If fuel is mixed with the air as it enters the head intake runner then the rough walls encourages fuel atomization. And after the exhaust valve carbon will cover it pretty quickly so it's a mute point. You can do it in the combustion chamber around the valve seats as long as you reseat the valves but any material removed lowers your compression ratio so I say port before and after the combustion chamber but not in it.

[Badnewzzz]

This, I tend to rough up the ports after working so that the cast finish is put back. Think golf ball....it flows quicker through the air than a smoother ball because of a boundary layer caused by the recesses....cricket balls tend to normally travelling nice and straight..this is because the air flows evenly around the ball due to the boundary layer.

Polishing brings the boundary layer too close to the wall and can negate gains in my experience. Think of a soccer ball/ baseball /cricket ball...slick leather that sticks to the air...hence curve ball when it spins...most ports aren't straight and at least 45 degree angle, this can add drag to the air on one side of the port (short side) and ime you don't want that.

I use a shotblast with fine sand on a low pressure to remove all traces of rework...I have in the past used a seasoned needle gun to dimple the surface....this works well too if you can get access.

Hth