Back in July, the DRtuned Racing Saab 9-5 racecar experienced a cracked piston failure, that has proven to be a common issue on the T7 engines. You can read up about the start of our journey here. Anyone familiar with the Saab powerplants will recognize the designations of Trionic 5, 7, and 8. The T5 engines (B204, B234) are known for their indestructibility, ease of tuning, and overall simplicity in design. The T7 successors (B205, B235) were seen as a refinement of the T5 design, with lightweight internals and block and better flowing cylinder head design. T8 engines (B207) are best known for throwing out the entire book of Saab engine design and simply slapping in something made by GM, and that’s about all I have to say about them (they’re great engines, and I think GM did a fine job with them, but they’re nothing special in terms of engines).

The (Somewhat) Common Saab T7 Piston Ringland Failure

The T5 engines, are also known for their capability to withstand upwards of 600whp on stock engine internals. Everything about their piston, block, and rod design was about strength and durability. The overall architecture was then applied to the T7 generation, but with more refinement and increased efficiency. Included in these refinements was a lighter piston design, and a lighter engine block, and the result is that we now have a slightly weaker generation of motors. If the stars misalign enough, otherwise good running engines have seen failures.

So what happens to cause the Saab T7 Pistons to fail? Well several things take place with the T7 engines that are a cause to the failure, but we’ll remain focused on the most devastating of these issues. First of all the lightweight Saab T7 pistons simply can’t handle the same level of heat and heat cycling. Anyone who has upgraded brake sizes understands that mass = heat dissipation. A small piece of metal heats up much faster than a big piece of metal, and it also cools down much faster than a big piece of metal. So now we have pistons getting hotter faster, and cooling down faster, and therefore we’ve created a lot of heat cycling stress in the metal (the outside will cool-down faster than the inside, and will expand/contract at different rates). Heat stress can fatigue weak points in the piston design, and any external stressors like high boost, significantly higher temperatures, or abnormal piston ring interactions will crack the fatigued zones in an instant.

Secondly, the piston ring gap is significantly tighter on the Saab T7 engine design. This is the distance between the ends of the piston rings when fully seated around the piston and inside the cylinder. This may seem like a good thing, in that less cylinder combustion pressure can bypass the piston and pressurize the crankcase, also known as blow-by, however it also means the tolerances for heat expansion are much less forgiving. Combine the rapid heat expansion and shrinkage of the piston being transferred directly to the piston ring, we’ve now created an opportunity for the ring to expand so far that the gap disappears and the ring ends make contact. Now we have a fully sealed cylinder, with combustion pressure and further heat expansion that has to go somewhere. If those forces can’t press the piston ring end radially (combustion) or tangentially (ring ends closer together from expansion), it will press them upwards or downwards. This vertical force is now directly on the piston ringlands that are already fatigued, and causes the entire piston to simply crack.

This sounds relatively gentle, and our experiences show the best case scenario, there is a high chance that the piston will in fact shatter or shed large chunks of metal into the cylinder. This will damage the valves, cylinder head, turbo, etc to a point that they are no longer repairable or reusable. We got very lucky in our case, as the cylinder wall is barely even marked up. A simple cylinder hone would be all it requires if we ever decide to use this block in the future.

While the circumstances for this to occur are quite specific, with the updated engine design these circumstances seem to come by naturally and quite easily. If you are tuning your engine above a typical stage 3 (~300-310 crank hp), it is necessary to build your engine to prevent this from occurring. If you drive your T7 car hard on even a stage 2 or 3 you will want to consider building your motor sooner than later. And finally if you’re like us and are crazy enough to race a newer generation Saab, you will definitely want to build your motor regardless of your power levels to guarantee reliability and prevent any catastrophic failures mid-race.

Forged Saab Pistons for Trionic 7 and Trionic 5 cars

eEuroparts carries a remedy if you are looking for this type of power, a set of forged Wiseco Pistons that come with rings.  The part number is KE184M90 and these are available as a complete set ready to go onto the ends of your existing connecting rods, provided they are in good shape.  When installing new pistons, there is a specific method to this which is gone over in this blog post, the most specific part of which involves making sure the piston rings are correctly sized for your particular block before just jamming everything in.  Its important to note that while stock pistons are meant to be a very consistent size at different temperatures (read: Hypereutectic piston), a forged piston will expand and contract with heat.  Because of this, you must be careful with a cold engine equipped with forged pistons because the tolerances will be looser at startup.  Oil consumption during this time will also increase while the pistons and rings warm up and expand to fill the space inside the cylinder and make a perfect seal.

To diagnose a cracked piston: Any sudden changes in power combined with loud noises (bangs, grinding, crunching, knocking, bad cringy sounds) etc etc, are never a good thing. In our case, there was no noise, simply a visceral loss in power with burning oil smoke coming out of our hood vents and tailpipe. Inspection in the engine bay revealed a lot of oil residue around the oil filler cap, the PCV pipe on the valve cover and intake pipe, and the steady puffing of exhaust gasses out of the filler cap when opened. These are symptoms of significant blow-by and burning oil in the cylinders. This should be immediately followed up with a cylinder compression test to confirm if it is indeed a cylinder issue and which cylinder is the culprit.

Further notes and side discussion: the idea has been tossed around that if you were to tear down a B205 or B235 engine, and re-gap the stock piston rings to the same specs that are used in the T5 engines, it would be possible to achieve higher horsepower figures on the ‘stock’ pistons. If anyone has tried this, or has further information to add to this discussion feel free to comment below or contact us directly via our website drtuned.com or our Facebook page.