Port & Polish
 

Hi,

Just visited the Head shop that is redoing some work on my 78 3.0 heads. It is a long story invoving a well know shop (Sydney) messing up the valve guide install.

Anyway I said that since they have them can they do a port and polish. After a few week I was in the area about 5 hours away from home. So I went in to see them. They had a number of 2.7 and 2.0 heads having work done so I did not worry to start with. When I asked to see a job they had done he shows me this head that is ported and is quite rough on the exhaust port. There explanation is that the new theory is to leave the exhaust and head rough and the carbon that enevitably sticks to it will act as an insulator and keep the heads cool?

Any thought on that?

The next explanating as to why it was taking so long ( 4 weeks so far) is that they have to write the program for the porting? This really has me worried since porting seams to be a black art. If they are writing new programs then the job may be far from perfect.

Cheers

Mark.....

Do they have a flow bench?

Did not ask that question! Even if they do I am starting to feel that I am not too keen on them practicing on my my Dollar!

Mark.....

Mark, a rough finish is fine. Some builders prefer it. My preference is to have the ports rough where fuel is in suspension and polished everywhere else. The finish is kind of irrelevant on the exhaust side since carbon will build up anyway. A polished finish is only relevent in racing applications where the motor is torn down frequently or where carbon does not deposit.

As far as programming, it seems that they are indeed proofing on your heads. Sounds like the fist time for that port size. If they have been successful in the past then there is not really much to worry about. There is not a great deal of difference between them all.

Thanks for the reasurance. I will chase them up as to how they are determining the program,ie flow bench, and make sure I am covered off wit the risk.

Cheers

Mark....

for insulation????? Run Forest Run

Yea that's a little too much theory and a weird justification for the finish.

Mark:

FWIW,....We recently built a 3.0 race motor based on a set of STOCK early, large-port SC heads.

It had:

Stock valves
Stock ports (totally untouched)
Different valve springs & retainers
Webcam 171/149 cams
46mm PMO's on tall manifolds
JE pistons at 11:1 CR
Good 1 5/8" headers
Our own race muffler system
RSR-spec, twin-plug distributor
Careful assembly with some internal improvements

This thing made over 325 HP on our Superflow engine dyno.

The stock '78 SC heads were not a liability in this example,....:)

Caveat Emptor on people who claim to "improve" Porsche heads. Up to a point, certain ones work pretty well.

Regardless of whether this is acceptable or not, it is false logic. Insulation would keep the inside of the head hotter since that is where the heat is generated (inside the cylinder ad head). Insultation would prevent heat from transferrign through the head body to the cooling fins.

Mark; What is the build spec and intended use of the motor? It's redundant for me to agree with Henry, but in my analysis of 70+ different 911 engine configurations, it's become pretty obvious that the stock 911 port configurations don't leave a lot of easy gains. For street, DE or 90% of racing use, it's not clear to me that spending a lot of money on porting will net you much of a gain (assuming that you haven't increased the engine capacity by much, or are using T heads). If you're building a pro-race motor, or top level National Race motor, sure there's enough "black art" improvements left that will make a difference. I'm pretty confident that Henry and some of the other well known engine builders know what those black-art secrets are. But for it to be meaningful you'll also need your chassis, transmission and driver all to be optomised.

But if you're out just having fun -- why bother??? It's most likely better to spend the money on the track.

That's just my $0.02.

bbh03, Porsche implemented ceramic insulators in the exhaust ports of the 3.6 engines helping to control cylinder head temps. The theory is to stop exhaust heat from entering the head thru the port walls. Just like you state... sinc ethe wall does not absorb the heat the head operates at a lower temp. The carbon theory I am not sure about...

Good call from Steve(as always). Didn't even occur to me.

That's an interesting point Aaron

Porsche did however give up on the idea of insulating the combustion chamber. Apparently that caused uncontrollable combustian chamber temps. Don't ask where I got that from cuz I don't remeber, as usual...:)

please tell me this wasn't Bob Whyms or PR Technology?

To rely on a supposed layer of carbon build up for some sort of fire wall.......would be very inconsistent and very hard to assimilate what is happening there except for hope...if that type if engineering was applied in more places we would have more disasters..If you needed that protection you would want ceramic for a consistent game.

I tried that several times on Turbo motors combustion chambers using ceramic-metallic coatings with less-than-satisfactory results. :)

The guys at PR don't flow heads - they send them out to a place they won't divulge who, from what I've seen do excellent work. And it's hard to imagine Bob getting involved in shonky arrangements. But it would be very interesting to find out who it is :confused:

Hi all, Been away from the desk for a few day.......I'll try and cover off some of the points.

Steve- if the stock heads are good as is I will put a stop to the port & polish! Thanks for your example. What is the web mac used close to with reagrds to the GE grinds.

Jluetjen- The motor is a street car, used as a daily driver. However it will probably be sold in a short while. It is a bit of a bastard car being a stolen revovered slant nose targa. I am building a motor that will be fun an make it more of a marketable item. If it is not sold it will still be fun to drive. ( not much traffic in Canberra )

I did question the theory of the carbon, maybe on the exhaust port where the idea is to move the gasses out as quickly as possible with as little heat transfer as possible. But as it has been said before..Porsche did a pretty good job on the stock engines.

JohnJL - Bob Whyms did the initial very bad job on the guides - to their credit they are paying to have them redone. I now have to check to see if I really did need a complete set of lalves! He actually asked me if it was a race motor since the valves are worn in one area of the stem only? I have to question this sinece the stroke of the valve does not change? Ram heads are doing the other work.

Thanks

Mark......

Hope all goes well, Mark.

After this thread and Steve's comments I will stop the port & polish. They should be able to do a proper job on the guides and hopefully a good job on the welding of the chamfer......long story involving a 3.2 conversion and then back to 3.0.

Cheers

Mark.....