[talk_wrench]

Have just bought some RHS heads for my small block ford. They appear to be the beans. Really big, open, free-breathing ports. Brand new valves to match. (Before you ask they are 2.08 and 1.6 inches respectively). I'v also got a twin-carb tunnel ram to go on top.

Upon receiving both the tunnel ram and the heads, the finish provided is basically very rough on both, and has lots of casting marks everywhere. It is very clear that you are supposed to port and polish the heads to the desired finish.

I intend on port matching the heads to the manifold to get optimum gas flow.

I have removed all the casting marks, sanded off all the rough finish with sandpaper, starting on rough grade then moving down to a finer grade.

The job (in my opinion) looks pretty good. There are no irregularities from port to port, I have been very careful to keep all the sanding matching on each port, but the question now, is, how far do I go with this.

Two of my rodding friends say to leave a slightly textured finish is best, to provide turbulence to mix the fuel. They say a mirror finish is bad because fuel condenses on the walls of the head and destroys fuel mixture.

What is best? Mirror finish to achieve best gas flow, or textured finish to achieve best fuel mix?

Any advice gratefully received... Gwil.

 

[Torment]

textured.....definatly

 

[Shedster]

Yeh, don't go mirrored, that's old technology these days.

 

[talk_wrench]

Yeh, don't go mirrored, that's old technology these days.

What is it with mirrored thats such a no-no?

I'v heard that fuel condenses on the walls with a mirror finish, thus disrupting your mixture, but how can it condense on the walls when your head is around 90 degrees centigrade?

 

[Roadrunna]

Moved to the Tech area :tup:

 

[heavychevy]

Go textured on the inlets but I believe you can mirror the chambers and exhaust ports. I'm only repeating received wisdom from those who have done the testing. Mirroring the inlet only increases fuel consumption.

 

[100ev8]

rough walls give better air/fuel mixture . no good havin lots of fuel and air if they dont mix together

 

[scooder]

What is it with mirrored thats such a no-no?

I'v heard that fuel condenses on the walls with a mirror finish, thus disrupting your mixture, but how can it condense on the walls when your head is around 90 degrees centigrade?

Even though the heads will be hotter than 90 deg and the under bonnet temp is high you can get frost on the inlet and carb. I had it on a Consul engine cus it had an export inlet manifold (no hot spot) and on my old T with a Flathead cus it had no exhaust crossover (Navarro racing inlet).
Matt finish is the way to go especialy with a tunnel ram, I know of a test engine put together with clear panels let into the side of the tunnel ram to watch with it running, and it was reported that it looked like a rain storm with all the condensed fuel running down the inside!
Martin.

 

[talk_wrench]

I see.... mirror on the exhausts and chambers it is then.

 

[PeteF]

Satin smooth, no sharp corners into the flow.
Match the ports with smooth radius.

 

[kapri]

Even though the heads will be hotter than 90 deg and the under bonnet temp is high you can get frost on the inlet and carb. I had it on a Consul engine cus it had an export inlet manifold (no hot spot) and on my old T with a Flathead cus it had no exhaust crossover (Navarro racing inlet).
Matt finish is the way to go especialy with a tunnel ram, I know of a test engine put together with clear panels let into the side of the tunnel ram to watch with it running, and it was reported that it looked like a rain storm with all the condensed fuel running down the inside!
Martin.

Bare metal engine, no water to take the heat away and the carb would still freeze on top ( Beetle :tup: )

 

[fozwanger]

As said before, textured on the way in, smooth on the way out. You require a turbulant flow on the way in to keep the mixture "mixed" and to help stop the icing up.

The icing is caused by a process called Charles' law. Basically, a rapid expansion of gas saps the energy from it's surroundings as it requires energy to carry out the expansion. It's the water in the air (either in solution or droplet form in the air) and depending on your fuel, there is often a reasonable amount of water in the fuel too. (Hydrocarbon's have an affinity for water).
Smooth on the exhaust, there is no reason to have a turbulant flow here, you just want rid.
As said, make the path as direct and unrestrictive (no corners and nice radii) but keep the gases choppy.
However, if you expand a fuel rapidly (liquid) this gets very hot, purely for the opposite reasons for a gas (hence the reasons your injectors can get v hot)

Both of these phenomena are viscosity and moisture dependent.

P.S. Apologies for being an anorak,

 

[talk_wrench]

P.S. Apologies for being an anorak,

You're not an anorak. My best friend is doing a masters in chemistry so I hear (and now understand) this stuff all the time.

I'll bear this all in mind. Rough in, smooth out...

 

[fozwanger]

You're not an anorak. My best friend is doing a masters in chemistry so I hear (and now understand) this stuff all the time.

I'll bear this all in mind. Rough in, smooth out...

Whatever you do, don't use that on the missus!!!!!! :lol:

 

[talk_wrench]

Whatever you do, don't use that on the missus!!!!!! :lol:

Too late...

On a more serious note, mirror finish the exhaust I understand, yes. But the chambers? wouldn't sanding off the textured finish of the chambers decrease compression (albeit very, very slightly).

Also, what is the exact finish which is best for the intake, I mean, if you finish with a rough grade, you disrupt air flow, and with a fine grade, you risk your fuel condensing on the walls or not mixing properly....

Maybe I'm being really anal about this, but I built this engine last year taking the advice of others, where their attitude was "get it finished". I dont want that, now. I want to really take my time on this, and get everything done exactly how I want, which is as close to perfect as possible.

 

[scooder]

Polishing would increase the volume of the chambers but only a tiny amount so dont worry. The finish you need in the inlet ports is satin, not rough, not shiney but satin.
Its not getting anal its atention to detail.
If your witeling ally make sure to load the sand paper rolls,grinder,cutter with lard or tallow even used soap to help stop the abrasive loading up with ally.
Good luck and happy porting.
Martin.

 

[talk_wrench]

Polishing would increase the volume of the chambers but only a tiny amount so dont worry. The finish you need in the inlet ports is satin, not rough, not shiney but satin.
Its not getting anal its atention to detail.
If your witeling ally make sure to load the sand paper rolls,grinder,cutter with lard or tallow even used soap to help stop the abrasive loading up with ally.
Good luck and happy porting.
Martin.

Holy cow this polishing is taking foreverrrrrr...

Done 1 chamber and 1 intake in 3 hours...:sniff: I guess Ill be at this for weeks.....

 

[fozwanger]

I'm just about to start Porting my DOVE's, I know where you're coming from.
Water is a funny thing, it does everything that other fluids do not. When the water condenses on the sides the droplet "spreads out" making it flat and thin. This then reduces the shear angle between the surfaces and means that the greater contact area provides greater cohesive forces per net unit volume.
This then requires a greater force to dislodge the droplet for too reasons, the reduced shear angle and the attraction between the two surfaces.
Imagine if the droplet stayed more spherical, the angle would be steeper (droplet would stick out more) and a greater surface would be in the way of the gases moving past (a bit like a sail) and the lower contact with lower attraction would allow the droplet to be pulled off more readily.
If you can break the surface tension of the water then the droplets won't spread out (fairy liquid and oil),

 

[talk_wrench]

I'm just about to start Porting my DOVE's, I know where you're coming from.
Water is a funny thing, it does everything that other fluids do not. When the water condenses on the sides the droplet "spreads out" making it flat and thin. This then reduces the shear angle between the surfaces and means that the greater contact area provides greater cohesive forces per net unit volume.
This then requires a greater force to dislodge the droplet for too reasons, the reduced shear angle and the attraction between the two surfaces.
Imagine if the droplet stayed more spherical, the angle would be steeper (droplet would stick out more) and a greater surface would be in the way of the gases moving past (a bit like a sail) and the lower contact with lower attraction would allow the droplet to be pulled off more readily.
If you can break the surface tension of the water then the droplets won't spread out (fairy liquid and oil),

Right, um, even I'm stuck with this one... explain?

 

[fozwanger]

YOu need the droplet to stay round as when it condeses it spreads out(drop a drip of water on your kritchen worktop, it doesn't stay in a ball).

The water sticks to the surface, the flatter the drop becomes, the more of the drop is in contact with the surface, so the more it sticks. The edge of the drop is thin so in a manifold would be hardly touched by the passing gases (almost aerodynamic). If it was still a ball, the passing gases would knock it off (it would be obstructive and be caught by the passing gases)
Is that a bit clearer. SOmetimes I ramble on a bit and get a bit deep, I do it for a living (how sad am I?) :-(