Bruce Smith's Tech Articles from the 356 Registry
Useful stuff for your early Porsche and old cars in general.
Since the invention of the carburetor years ago, there has been a need to clean the residues and reactants that accumulate inside and out. The crud that forms in carburetors and fuel pumps generally falls into two categories: organic and inorganic. Most of these contaminants can plug jets, nozzles, and pathways, preventing proper mechanical operation. This article described the source of contaminants and some simple methods for cleaning.
For all but the exceptionally blessed, owners of old Porsches have cars with some non-factory paintwork. Whether piecemeal, re-spray, or full down-to-the-metal restoration, it’s likely that some paint repair exists. The extent is often a mystery. Whether for purchase, sale, or curiosity, who doesn’t want a detailed map of their car’s finish? The downside is that you risk discovering things you hadn’t imagined. This article describes methods and tools available to measure the paint on your car.
The centrifugal spark-advance distributor has been used in the internal combustion engine for over 100 years. The familiar concept hasn’t changed much since the early days, using a timing mechanism based on pivoted arms cast outward during rotation. Pivots, weights, and contact surfaces are important maintenance elements to ensure proper performance but since the springs can be prone to distortion and failure, they need particular attention in a decades-old distributor. This article describes role of distributor springs, and how DIY modifications can effect distributor advance.
So you’ve decided to tear down your carburetors and rebuild them. Now the challenge is figuring out whether the jets in your carb are what you need, or even what they are marked to be. Incrementally raising and lowering main jets sizes while monitoring driving performance through the rev range can result in selections that improve performance and/or gas mileage. This could be a worthwhile effort, especially if your engine is no longer at factory specs. In this article, steps are provided to resize jets both up and down to meet the needs of your car and carburetors.
If you’re doing work on carburetors or fuel pumps, at some point you’ll find that a regulated low pressure source could be useful. Float level setting, flow rate testing, or leak checking are usually easier on a bench than in the car. A low voltage electric fuel pump could be a solution but the output is usually fixed or, if not, setting a particular pressure isn’t easy. In this article, some simple approaches are described that can deliver fluid at the low pressure levels needed when testing fuel parts.
There are a few decisions to make when restoring your car’s hardware. And there can be advantages to DIY plating –no lost parts, small batch runs, and possible cost savings. This article describes things to consider and what to expect when attempting your own coatings. A review of coating types is also presented, together with some de-mystifying of what the factory used when our old cars were new.
Why would anyone be interested in an article about fitting a sub-$50 non-original part into a vintage car that increases in value every 24 hours by nearly double that? Making an aftermarket VW distributor that will fit into a Porsche 1600 motor work nearly as well as the one that was designed for it becomes a worthy challenge. And since having a spare distributor for your old car is a good idea, the notion of doing so is more manageable when the cost is far below the $500 needed for a rebuilt Bosch original.
Mechanical fuel pumps in an old Porsche are non-positive type, delivering fuel to the carburetors only when needed but keeping the fuel line pressurized at all times. This article is the first of two parts describing 356 and 912 fuel pump types and the how-to of repairing and rebuilding, including common problems encountered with 50+ year old pumps.
If you’re taking apart an old fuel pump, you’ll probably encounter one of two possibilities. In the best of cases, you may find that it needs a bit of cleaning, maybe some flattening of mating surfaces, an inspection for worn pieces, and a basic re-assembly with a rebuild kit. This is the hope for cars that have been maintained regularly and driven often – sometimes not the case for our best loved old cars. The more likely scenario is the need of more serious repair. Since an old Porsche pump probably isn’t ready for a simple swap of the parts found in a rebuild kit, it’s worth addressing remedies for some of the issues encountered and described in the first part of this article.
When your ignition system is off, not much else about your motor is going to run very well. Spark distribution, timing, advance, and dwell are popular discussion topics where opinions sometimes vary. But most issues can be separated from speculation since there will only be a small range of optimum settings for any given engine. The goal is to achieve the ideal, which is sometimes difficult without a full diagnostic setup, including a dyno. But the DIY mechanic can get pretty close, especially with an old Porsche that isn’t highly modified beyond its original state.
Your car’s power and torque are fundamentally governed by its cylinder compression. Measuring the ability of an engine’s cylinders to develop and hold pressure is a fairly standard process, easily falling in the DIY category. This is done through a compression test, a leak-down test, or both. These tests differ with regards to the tools used, the testing methods employed, and what’s actually being measured. We review both here with some insight into the equipment used and the types of results to expect.
The breaker points are often considered the weak link of the system for several reasons, including wear that will impact dwell, bounce that can prevent coil saturation, worn cams that will degrade lobe accuracy, and arcing. In the 1970s, there were over thirty vendors with add-on electronic kits including familiar names like Motorola, Borg Warner, Per-Lux (now Pertronix), and Radio Shack. With today’s market limited mostly to pre- 1980 era cars, few suppliers remain but they are able to offer technology that has now had many years of field testing.
There are a few repair options out there which have been around for a very long time. For internal threads, helical screw thread inserts (STI) were first devised in the 1930s by Aircraft Screw Co., Heli-Coil, and others.These were developed to secure a hardened bolt or stud into a soft metal boss like aluminum or soft alloys. By lining a tapped hole with a coiled wire insert, the softer boss could withstand torque and repeated tightening without damage. External thread inserts such as Time-serts or Keen-serts are solid metal bushings that are threaded both inside and out. When installed with a special driver, they are screwed into a tapped hole and complete the thread cutting to secure them in place. Being solid, they are generally larger than helical inserts, which can limit their application in tight spaces.
If you’re thinking about a parts washer for your home garage, there are a few options to consider. Commercial shops usually use large drum or tank versions sold by companies dedicated to servicing them and handling the disposal and replacement of cleaning solvents. For a home garage, there are choices that might be better suited. Matching the washer and chemistry to your needs requires the comparison of the equipment available as well as the handling and safety of the chemicals used.
After fifty-plus years of use, the distributor in your old car has probably had some rebuilding done to it. Along with the periodic replacement of breaker points, it might have received new shims, insulators, friction plate, and maybe even new springs along the way. A fresh coat of paint to the outside may give you the warm feeling that it’s like new again and ready for another 50 years. But these ‘kit’ parts and paint don’t address what could most worn out about it: the main shaft in the distributor bore.
Dr. Bruce Smith is an engineering professor at the Rochester Institute of Technology and a Fellow of IEEE, the OSA, and SPIE.