Building a peppy, torquey street motor

[Dave Erickson]

A few things are not mentioned that are part of building a good engine:

1. A good valve job.

2. Lightly flycut the heads to provide a flat surface for the cylinder tops to seal against (or seat them
by hand using valve-grinding paste).

3. Obtain even combustion chamber volume, with desired compression ratio and .025" squish for all 4 cylinders.



No. 3 is often overlooked, or at least the correct steps are not taken in the correct order, since it is a trial and error process. I would suggest this order:

A) measure the squish using .005" cylinder base shims. Correct so all 4 combustion chambers are equal. Correction is achieved by a) thicker shims, b) re-cutting the 30 deg squish area in the head, or c) flycutting the head. A head which is unusable can often be corrected by welding.

B) cc each combustion chamber. Pick the largest volume as the target and unshroud the valves on the other three until all 4 volumes are the same (don't remove metal from the squish areas).

[Maurice Cruz]

Many thanks Dave - this stuff is gold!

[Al Zim]

I hope you have not started your rebuild. Experience has taught me that building competition engines is more difficult than street engines because if you are not ultra precise you will not make horsepower nor will the engine last at high RPM's. With that in mind, you need to remember that Porsche was a design company and they had numerous engineers and draftsmen working on design of their engines! Perhaps this scan be duplicated on a computer but I doubt the computer has the experience the "factory" has/had. The CRITICAL MEASUREMENTS ARE the bore diameter on the big end of the rods, the bore diameter of the main bearings in a properly torqued case, the main and rod bearings journal size and the bore and size of the pistons and cylinders. Except for the piston and cylinders, each of these measurements are critical to how long the engine will live and how much horsepower the engine will make. The secret ingredient comes later! You have a Super engine configuration which I think is the best of all worlds for any 356. If the crank can be polished do that and then check the clearance form the rods with bearings (torqued) to the crank. A good craftsman may be able to adjust the rods slightly to improve (reduce) the clearance. If you do not have the c/912 rods you may want to purchase a set, the big end holds up better. BE SURE THE MARKINGS ARE ALL THE SAME ON THE RODS YOU PURCHASE. There was no standard from Porsche on rods all they asked from the manufacturer is they all be in the same weight group. You will not gain any substantial horsepower from a rise in compression ratio. At 8.5 to 1 you can run on regular gas. A plus in using the Mahle or Klobenschmidt is that they fit the cylinders better than the big bores. Check the clearance it it iw within specifications have the cylinders lightly honed and used them. The long skirt keeps the piston from rocking and the oil ring below the piston pin is a big PLUS. Install new rings and set the end play properly. These pistons will not work on a counter balanced crank and at 4500 RPM you do not need one. Continued

[Maurice Cruz]

Hi Al, thanks for weighing in and going through the effort of typing up a detailed post. I'm a long time customer of yours by the way

First up, this isn't a race engine, just a sweet street motor. And here in Aus I have access to 95 or 98 fuel in 99% of service stations. I aim to start the rebuild early 2017, so I'm just in prep phase now. I never take it past 5000 and most of the time putter between 2500 and 4500.

Second, the motor isn't a pure T6 1600 Super anymore. When the previous owner had the engine rebuilt in the 90s, he kept the stock crank and rods, but switched the cam and heads to SC. From what I've read on this forum so far the SC bits are ok for street.

If I'm understanding your view correctly, you are saying that I need to get all the bottom end clearances spot on, and also to stick with stock 1600S CR of 8.5 and stock pistons. Are you implying that a 1720 at 9.0 to 9.5 CR will over stress the stock Super bottom end?

[Martin Benade]

It is most people's opinion that a bit higher compression is worthwhile, and whatever pistons you use, make sure they match the angle of the later heads. The B pistons will not match up properly.

[Al Zim]

Two countries separated by a common language. Mr. Murray's comments regarding how much easier it is to build a race engine are totally incorrect. The more horsepower you make the more care you need in assembly. A) the super and the 912 cams are the same. B) the German fit of the piston to cylinder is much more precise than with the big bore, check your wear limit and measure the new new big bore possibly a surprise. They may be close to the wear limit when they are new. The 82.5 MM pistons have a larger surface sealing the head to the cylinder. C) If the depth in the head of the step is over 9.5 MM the heads will need to be fly cut. If you don't cut the heads they will leak compression since they will seal on the fin of the cylinder not the top. If you are using cast iron cylinders, you can use any push rod EXCEPT the ones that are all aluminum and the ones that have the wooden stick in them to control oil flow. These are just a few things that are critical to having a correctly built engine. Sooner or later your short cuts will make the engine explode. al zim

[David Jones]

Maurice, in reference to your quote,
"First up, this isn't a race engine, just a sweet street motor. And here in Aus I have access to 95 or 98 fuel in 99% of service stations. I aim to start the rebuild early 2017, so I'm just in prep phase now. I never take it past 5000 and most of the time putter between 2500 and 4500."
Australia still uses the same octane ratings as were used in Germany and referenced in the Porsche manuals so you can go by the recommendations in the various manuals. They do not however use the term RON but instead reference ROZ which is the same. With reference to fuels used in the USA whenever they are mentioned remember we go by the US standard which looks like a lower octane but is actually an average of two numbers. RON/MON which is an anti knock index octane number not the number you see on your pumps. I only make this point so you do not get confused when you read posts about folks in the USA using what appears to be a lower octane than you do on the same spec engine. We have 98 or 99 octane it is just that we call it 93.

[C J Murray]

Two countries separated by a common language. Mr. Murray's comments regarding how much easier it is to build a race engine are totally incorrect. The more horsepower you make the more care you need in assembly.

Hi Al, They all need careful assembly but a street engine needs to work over a 6000rpm range regardless of rpm, load, or throttle position. A race engine is used at wide open throttle in the last 2000rpm before redline. Any moron(even me) can tune an engine to do that but getting a street engine to behave under all conditions is difficult once you vary from the known combination that the factory provided. Isn't that why guys like you always say "factory is best"? Isn't that because you can't do modifications and then get it to run correctly like some can? I agree, if you don't know how to tune you are better off staying stock just like if you don't know what combination of modifications to apply you should stay with stock parts.