It's been over a year since I dropped my car off at Auto Talent to build the 4.7L low compression, supercharged S65 stroker motor. The custom pistons had been ordered in April 2010, and the plan was to wait for the pistons to arrive and then drop off the car as soon as they arrived.
RD Sport had built the original 4.6L high compression stroker motor. A custom ESS VT2 supercharger was then added as an afterthought, and we came up with the idea to hit 200 MPH in the Texas Mile. I knew it was a long shot already because I knew I would need about 725whp in order to hit 200. We sat with 615whp -- knowing that 200mph wasn't going to happen. But we tried anyways and hit 186.1mph. We learned whenever we turned up the boost past 6.25 PSI, two things happened: 1) the fuel system couldn't supply enough fuel, and 2) the high EGT's of the 4.6L supercharged motor would burn O2 sensors. The only solution was to redesign parts of the fuel system and build a low compression motor. So I decided to order the pistons in April 2010 and patiently waited for them to arrive.
Once the piston landed in customs, I drove my car down to Auto Talent. Once the motor was out, we inspected everything and began to notice some changes we wanted to make. (Pictures shown here[/url].) After pulling the motor, we received our first piece of bad news. This was going to be an omen of many delays that were to come in the future. Basically, Mahle made the wrong pistons. Mahle made 94mm pistons for high (12.0:1) compression, not the 10:1 low compression that we ordered. It was going to take Mahle another eight weeks to manufacture and ship the replacements. So, the waiting game started.
During the eight weeks, I didn't sit idle and uninvolved. I had a big decision to make: who was going to build the motor? I considered my top choices Auto Talent, RD Sport, or possibly my former racing engine collegues at Van Dyne Engineering in Huntington Beach, CA (formerly from Drake Engineering -- from the famed "Drake Offy" racing motor). Using Van Dyne had a certain appeal because I know their standards are very high and they do things quite a bit differently than most street engine builders. The downside was Van Dyne had never built a BMW S65 motor. I untimately worked out a deal where Van Dyne would build the motor to their standards, and Zolti from Auto Talent would be directly involved as a participant and consultant during the motor building process.
In hindsight as I would later discover, chosing Van Dyne might have been the smartest decision I made. Today, the bottom end has been completed: it is now built. But the road leading to this point was filled with delays, setbacks, and challenges. Over the course of the next few days, I will write one article focussing on each of the challenges and decisions we faced. As usual, I will provide plenty of pictures of the build and the painstaking perfectionist process that we have been going through. For the most part, I will share everything we discovered -- with one exception. Certain technical details of the measurements we made and the tolerances we measured and adjusted will remain confidential because some of these measurements and adjustments will make their way into future products.
The pistons eventually arrived in early July, 2010, and the motor had already been out of the car for a few months. By this time, I had already decided to use Van Dyne Engineering to build the bottom end, and I was still undecided about using Van Dyne for the top end as well. I drove down to meet Federico (RD Sport), Stewart (Van Dyne), and look at the new pistons. It was a great opportunity to see both sets of pistons side-by-side (high compression and low compression).
I snapped a bunch of pictures of both sets of pistons, and talked to both of them about my project. Below shows pictures of the pistons side-by-side. The high compression stroker motor pistons are those with the higher dome, while my low compression pistons are the ones with the dish. At some point in the discussion, Federico pulled me aside and told me Carrillo wanted to remake my connecting rods. It was my option, but it would mean another eight weeks of delays. (That story, and pictures, will follow in the next article.)
The entire set of piston pictures can be seen here. New Pistons
For the most part, the connecting rod bearings looked fine when we disassembled the motor. Other than one or two uneaven wear marks on one or two of the bearings, they all looked like they were in good shape. As I would find out a year later, the uneven bearing wear would be addressed when we assembled the bottom end. (That will be the subject of another update.) I thought everything was in decent shape, so I was taken a bit by surprise when Federico (RD Sport) pulled me aside to tell me that Carrillo wanted to redesign the connecting rods. The complete set of disassembly pictures and pictures of all bearings from multiple angles can be seen here. BMW M3 Motor Tear Down - Robert & Mary Ann Collins
The problem with the connecting rods
When Federico pulled me aside, he told me that he had sent one of my connecting rods back to Carrillo for analysis. After inspection, he said Carrillo hadn't designed the connecting rods for a boosted motor, and wouldn't warranty the connecting rods if I continued to use them. If was my choice, and I could continue to use my existing set of connecting rods and might not have any problems, or take Carrillo's advice and redesign them, spend another few thousand $$$, and wait another eight weeks. To me it was an easy decision: we must redesign the connecting rods to be safe.
Federico and Carrillo had seen something in the connecting rods bearings so small that I never saw it. On three or four of the connecting rods, precisely where the connecting rod cap bolts together, three or four of the bearings were worn to the copper. At such high power and high rotational velocity (high RPMs), the connecting rod cap was deforming, becoming oval shape, and pinching the connecting rod bearings. Pinching the bearings was causing them wear down to the copper in this location. When I inspected the bearings, I didn't even notice it because it's a very small area -- about 1mm width. The pictures below show the wear near the bearing caps.
The new connecting rods
Once the new connecting rods arrived, I was very eager to see what had changed. I opened the box, removed a connecting rod, then stared at it for a few minutes because I didn't see any difference. Only when I placed the rods side-by-side did I begin to see the difference. Precisely at the spot where the connecting rod cap bolts together, there is a little extra metal. That's about all I noticed. However at the 2011 SEMA show in Las Vegas, I happened to be talking to a guy in the Carrillo booth. I described my project to him and how Carrillo redesigned the connecting rod. Just by coincidence, I was talking to the guy who actually designed my connecting rods and he remembered the project very well. Not only did he add a little more metal at the junction, but he thickened the H-Beam a little, changed the H-Beam radius, and slightly modified by bore size for the connecting rod bearing.
All of these latter changes aren't really noticeable to the naked eye, but you can definitely see the extra metal in the new rods when you compare them to the older ones. A complete picture set of all of the new connecting rods can be seen here: New Connecting Rods - Robert & Mary Ann Collins
Calculating Rod and Piston Velocity
While I was waiting on the connecting rods to arrive, it seems that I had a little extra time on my hands. So I decided to see how the stock connecting rod compares to the RD Sport RS-46 connecting rod (they are not the same length). So I wrote a spreadsheet and compared rod length, piston velocity, piston acceleration, and rod angle. Since I had specs for the factory S65B40 (OEM Motor), S65B44 (GTS Motor), and RS-46, I decided to compare all three.
I apologize that I don't remember the units of measure when I did this (it was a long time ago). But from these graphs, it's pretty easy to see how each one compares against each other.
Impressive work Robert,how many miles did it take to cause the damage on the rods, perhaps i should check mine.
20000 miles on the stroker + supercharged motor. I lost track of how many of those were pure stroker, and how many were stroker +supercharger. If I'm not mistaken, I think it was about 11000 stroker and 9000 stroker+supercharger. Included in those miles were two track days, the Mojave Mile, and Texas Mile.
[/quote]one other thing, have you checked if the 4 wheel drive tranny from 335xi would fit on a M3, i am starting to get tired of wheel spinn ..[/quote]
I haven't checked, but I doubt the M3 chassis would accomodate the AWD setup very easily. I also think there would be mandatory ECU programming and vehicle coding as well.