Polish n Port

[Richf]

good book , i have a copy for £10 posted if you like

[Andy W]

seen a few cases now where a stock GTS head doesn't alow much more than 500 hp

so yesterday i attacked the new head for the Stagea with a die grinder .......

[skid]

^^^ is this going to be a new service your going to be offering Andy??

[Dan]

Originally Posted by Andy W

seen a few cases now where a stock GTS head doesn't alow much more than 500 hp

so yesterday i attacked the new head for the Stagea with a die grinder .......

And today?

[Andy W]

after a bit of tidying up, i decided i better spend some money on a polishing kit to do it properly

but at least the 'hump' is gone

[Checkpoint]

Originally Posted by Dan

Well the head will be being stripped down once I know where my finances are.

I will be doing it myself as pointed out I am a mechanic... But, Ive never ever looked at porting and polishing. Looks like I might have me a go at it though (Polishing that is)

Worth reading up on it tho.

Porting a forced induction engine is different from porting a naturally aspirated engine.

[Mort]

If you are serious and want a better result than "attacking your head with a die grinder"... have a word with CNCHeads. They've just developed a new gas flowed cylinder head for the Skyline and are working on a big valve version. I used this firm for a new head on my Escort Cosworth when I lived in the UK 18 months ago. Top notch work....

Hi to all by the way.... I'm considering getting a skyline so I've been lurking about here for a while. Good site

[GRowsell]

I used to take the heads off of my old Datsun's (looks for an old man smiley) in an attempt to Blue Print the engines after being told by the guru of Mountune Racing that it was always worth getting an engine to the state the designer intended before playing too much further (also read it in various tuning bibles over the years).

As AndyW says (and his pic blurrily shows), the casting process often leaves some huge ridges which disrupt the air-flow in a way the designers didn't want. Removal of these lumps is relatively easy, quick and free, especially if the head is off anyway.

As for polishing the internals... I have read and been told mixed opinions on this, but here are the two opposite versions:-

Slightly textured surfaces can actually help overall airflow due to the minute air pockets acting as a friction reducer for the overall mass of air to pass over... (fast boats have used textured paint to trap air bubbles to act as friction reducers... but then that's water/air versus water/GRP friction, not air/air friction versus air/aluminum friction... so forget that one )

or

A fully polished/flowed port gives no place for turbulence to occur therefore 100% of air is going in the same direction...

To me, inlet's seem as important as exhaust ports, because the fresh air needs to be available at a constant pressure when it arrives at the valve, so if the flow is restricted in one port (by casting lumps causing turbulence etc) or the tract lengths are different, then the compressed air 'bank' in the inlet manifold could end up 'pulsing' slightly and one cylinder gets slightly more or less air.

Is this why the same cylinders often fail on Skylines?

Assuming the designer got the flow characteristics right around the valve area, and that the distance of airflow to the valves is equal from the 'air bank', then all we can do is ensure the air gets an unrestricted path?

With exhausts, people seem very aware about getting the gases back out the exhaust tracts and into the main exhaust evenly (to stop pulsing) and spend lots of money on gently curved, equally length-ed masterpieces, but often don't worry about the inlet... I'm aware that there are big pressure and temperature differences, but every little helps?

Just my thoughts... awaits other opinions

[Andy W]

what do you mean blury, looks fine to me 'Hic'

the inlet side i left alone other than smoothing down the casting marks and some fairly horrible ridges below the valve seats

[Richf]

Good post Gavin as for textured surface think "golf ball"

FWIW on the bikes i used to dabble with, head work was very important on the normally aspirated stuff but the turbo charged bikes often had lots less done , slightly larger stainless vales were fitted and the heads lightly ported.

One of the guys i knew had a meltdown at one meet and was forced to use a bog stock head he had as a spare, it had the smaller stock valves and had received nothing other than a clean up with a die grinder to remove the casting marks

Result

Best ET he ever recorded , from then on he only ran unported heads albeit fitted with stainless replacement valves for safety

Conclusion

1) The Japanese really know how to design cylinder heads

2) For forced induction most people need no more head work than a basic clean up (I'm sure the Japanese would do this but its not cost effective)

3) Its easy to screw up a head if you dont know what you are doing and its not just down to flow figures from a flow bench