Porsche 924 Le Mans engine renovation - part 1
Many of you may already be familiar with our work and our current works. This blog post is about our Porsche 924 Le Mans project. The topic is none other than the engine of the car.
I would not get into its development and production history now, but the Porsche 924 is a model with a turbulent fate. The model was intended as a successor to the 914 as an entry-level vehicle, which uses a lot of Volkswagen and Audi specific parts.
The model became a success story for the factory, with 150,684 units manufactured. It pulled the company out of a serious financial crisis, however, its judgment was not too worthy of this vehicle. The public almost never accepted it as a real Porsche, and even mocked it as a "farmer Porsche".
Perhaps this has led to these vehicles moving from affordable to very cheap price category. This process peaked somewhere in the late '90s and early '00s. At this time in Hungary, the 924 reached those kind of owners it should have never done: the constant overdrive and maintenance-free operation caused the death of these cars. It was then that the model's life came to an equally unworthy rest. Many cars ended up at the end of the garden, under the walnut tree, with a statement "I'll do it someday".
After the economic crisis of 2008, when vintage cars rose in value, one of the driving models was the Porsche 911. This type went through such an increase in value, and is still going through, that it was necessary to look for an alternative for less wealthy people who still want to own a Porsche.
That was the moment when people started evaluating the 914, and after it had already become a desired car, the 924 followed. This change happened somewhere around the 2010s, and the price of these cars also started to climb nicely. On the other hand, economically, the restoration of these cars is not a profitable business, so it is worth better to buy the ones in a preserved condition.
But then why do we restore such a car? There are several versions of the 924, two of them are limited editions and their price is already quite high and still continues to rise. These are the 924 Martini Edition and the 924 Le Mans models.
In the introduction, we gave a small picture of how those who might not have known the fate of the type should know why our project subject is in such a bad condition.
Disassembling the engine was not brain surgery, as we brought it in a "basket" together with the car. Finding the parts was more of a headache. Apart from the so-called laced block, everything was stored in boxes, oil cans cut in half, and mason jars, of course not cleaned. Unfortunately, as a result of the quality of the previous disassembly and storage, additional errors appeared in addition to the initial problems.
During the disassembly and condition assessment, we encountered the following problems: the cams of the camshaft were worn and battered, along with the valve lifters, all the valve guides were worn, the skirts of two pistons already showed the first signs of seizing, the pin bushings were worn in all connecting rods, the crankshaft had worn bearings on resting pins, and the cylinders in the engine block shown initial damage caused by the coolant remained on top of the piston.
"Oh that's all?" we could say, "it's been worse!". So let's dissect the facts a bit.
The problems in the cylinder head itself are negligible, the worn valve guides are replaced with new ones, the cylinder head is ground, and the valves are milled and polished. The valve lifter capitals and the camshaft errors can actually be considered a type error in these old Porsche engines, since they have a valve clearance adjustment system that cannot be used to set an exact value and even easily stops during operation.
During the repair, we ground the camshaft cams back to match the original cam profile, luckily it wasn't so worn that we couldn't do this. The valve tappets cannot be repaired, they must be replaced in any way possible. However, we want a reliable car that performs without problems for many years for its future owner.
That's why we replace the factory valve lifters with a spacer plate system designed and manufactured by ourselves. With this change, we do not affect anything in the engine, except the durability, which of course we increase. Therefore, the renovation of the cylinder head did not cause any difficulties.
Let's look at the crankshaft. I mentioned the crankshaft earlier when listing the faults. Unfortunately, as we expected, it is about bearing worn. The difficulties began here. When measuring the crankshaft, we found that this engine had already been rebuilt, the idler pins were ground to the first oversize and the drive pins to the second oversize. What is the difficulty in this? In itself, it is not much until the studs can be ground to the last oversize, in this case the third. However, this would also require oversized bearings, which are no longer available for this engine. In other words, to be more precise, you can still buy a standard size or first oversized one for a driving pin, but there is absolutely nothing for a resting pin.
Of course, in this case we show off our knowledge, we found the first oversized one for hundreds of thousands of forints. No thanks! The next solution could be production, due to our subcontractor background we can also produce bearings, but this also has limitations. One is the cost, and the other is the support bearing, because they cannot produce flanged support bearings, and this engine needs exactly that. Then let's look at another type of solution. Convert the engine block to the bearings of a completely different car manufacturer's engine. Sounds crazy right? However, it is not impossible, of course it requires professional knowledge and experience. The first step is to select the right new bearing. It is important to choose something that has a supply, because if we don't buy something like that, we will be there for the next renovation, just like we are now.
We managed to find a Mercedes-Benz V6 engine family whose bearings would be suitable for renovation. The only obstacle is that the socket size of the Porsche and Mercedes bearings does not match, which means that you cannot install the Mercedes bearing one into the other. Unfortunately, in order to achieve the correct nest size, we would have to expand the block's resting bearing nests by line drilling. It may sound complicated, but it is actually considered a routine operation during motor rehabilitation. However, thanks to the aforementioned good contact system and our circle of subcontractors, we managed to cut short the entire crankshaft misery. Finally, one of our excellent partners indicated that he happened to still have the drive and rest bearings for this engine block in stock in all sizes. In this way, it is enough to just grind the crankshaft, and we can avoid extra work.
And then let's look at the other problematic cases: replacing the worn pin bushings in the connecting rods is not a big deal, we successfully got hold of the last stocks in the country, so we can no longer be surprised.
The bigger problem was the cylinders and pistons. The engine block must be drilled to an oversized size by all means, because the antifreeze eroded its surface. "Oh, it's not a big deal!" we could say again, this is true, but this would require oversized pistons, which of course is like Columbo's wife, everyone talks about her, but no one has ever seen her, because she doesn't exist. Of course, we have a solution for this as well, we will bush it back to its factory piston. This solution is also good for us because it allows the pistons to be turned a little smaller, in order to eliminate the sticking marks and create the perfect factory surface. Then, of course, we fit the new bushings to our reworked pistons for perfect operation.
But wait a minute, what is reverse bushing/sleeving? The cylinders in the engine blocks have different designs, there are some when the material of the block is machined and the piston is in it, and there are designs where a completely separate bushing is pressed or only pushed into the engine block by hand.
In our case, we are dealing with the design where the material of the engine block is machined. Thus, in this case, re-bushing means pressing in a new cylinder. Of course, for this, the engine block must first be drilled, but only to the extent that our new cylinder bushing can be pressed in with a solid fit.
This is how a pile of scrap metal is transformed into a Porsche engine. If you are wondering whether it will still be able to function after this, follow our fight.