|
I have read with some amusement some of the posts about rebuilding Porsche 911-cylinder heads. The loyalty to certain shops is apparent and commendable.
I am the owner of Performance Developments and we have rebuilt many of these heads for many well-known shops and customers for over 20 years. Most of our work would be called 1/3 party work for independent service and repair shops. Lately we have gotten back into the 911 air-cooled work with Jeff’s 911 project “saving the un savable”.
I thought I could give my opinion on what I feel a good head repair is and what you should expect to have done.
Firstly, if you are the builder of the engine, you must set an overall budget and an acceptable level of workmanship. No point installing Timex parts in a Rolex repair. This should then determine the costs and level of repair each part of the engine will require. An engine is the sum of its parts. An engine should be looked at as a collective mass of individual parts. Each part must do its worth of work to collectively produce whatever result you require. Cylinder heads are the most important part of an engine when it comes to the transfer of air in and out of the cylinders.
The performance all comes from the heads.
Nice clean shiny heads do not mean that the repair is good. But, if time was put into the appearance, it generally tells a story of the repair shops dedication to quality. You cannot inspect a head for damage and cracks if its covered in dirt and grime. Heads should be crack checked (zyglow) before any repair work is started along with retainers mag'ed or zylow checked. There should be an order of sequence when repairing these heads. General Inspection upon delivery, some scope of the work required, a quick test of the sealing capabilities of the valves could also be done now.
Disassembly tells a lot. You can immediately tell if there is a guide issue. You can see any defects in the seats, and the valve faces. Valve margin widths can be seen where the valve has ended its life and any repair to the valve can no longer be done. You can also get some idea of the spring forces as the number of shims under the spring base can tell a lot.
Cleaning takes the time, removing all the carbon and dirt takes time. Any nondestructive crack checking should be done now. If the guides need to be replaced, often a second crack check inspection should be done post guide installation as old cast guide bosses can crack. Removing the old guides should be done with care. Some hammer them out when others do it in a more careful manner. This is where your choice of shop and the cost of the repair starts to show. Hammering out the guides and hammering them back in is quick and time is money. The guides should be sized to obtain the clearance to the Valve stem. After the valves are cleaned and polished, the stems should be measured for size and the valve head checked for concentricity to the stem.
Guides very rarely go back in concentric to the existing valve seats, so any seat work typically requires the seats angles to be re cut deeper into the chamber. The top angle that transitions onto the chamber roof often must be changed to suit. This is when the chamber gets cut and from here on out things start to go south on the performance. There is no other way but to replace the seat, so again, the cost of the repair dictates the work done. I have seen seats sunk so low that the air flow was severely compromised. For engines that make little HP, a loss of 5-10 horse power is huge.
Most of the time street heads will use a 45° contact face to the valve. These can be stretched wider for longevity and typically the Intake will be 1.00mm – 1.30mm and the exhaust at 1.50mm – 1.70mm. Seat width has a lot to do with the installed seat pressures as well. The wider the seat the less seat pressure is requires as the transfer of heat from the valve happens quicker. Conversely, narrower seats require less closing force so they don’t wear out quicker.
Once the seats are all cut and the heights are checked, valves should be installed and the stem heights checked to ensure the correct rocker geometry is held. Any adjustments need to be done now. Then the heads are probably faced for height and sealing surfaces to the Cylinders. Final cleaning which may include ultrasonic cleaning will happen now.
The retainer heights are measured, springs checked for equal or almost equal rates and the installed heights calculated to obtain the same installed pressures. Over the nose pressures and a coil bind checks are done at the same time. Then the heads are assembled with new seals, bagged and shipped to the customer. We log all measurements and work done on every set of heads so that in the event we have to replace a head or something similar we have a blue print of what is required.
This takes time. I know that a clean shiny head gets people all excited. I have read where they want to make a display out of them. Whoa, it doesn’t take much to make them all excited! Like it has been said, the devil is in the details.
Machine shops are like watch repair shops. You own a Timex, you would never take it to a Rolex repair shop. The opposite applies as well.
|