They say Rome wasn't built in a day. The same should be said for any well-built Porsche 911 motor. They both took some time to be designed. So, hot-rodding my own 911 engine proved to be a maddeningly slow process. I fought through the weeks of running around town chasing parts, and of meticulously cleaning components, by thinking of the long-term benefits. I longed for the day that the machine work would be completed and the motor could be reassembled, which brings us to this installment of the project.
During this hunting and gathering stage of my engine project, Steve Becker from B. Precise Machining more or less left me to my own devices. He wisely preferred to keep his hands clean from the 16 years of grime that had accumulated on this particular motor. I soldiered on with the grunt work, in anticipation of "The Day." A 911 engine cannot be assembled on a Saturday afternoon. Time and care must be taken to ensure all dimensions and tolerances are within specifications. Attention to detail pays huge dividends, both in long-term reliability as well as in attaining maximum performance. A comprehensive discussion of all the intricate nuances of building a 911 motor is beyond the scope of this article (it would literally consist of hundreds of pages) but what follows is a summary of "highlights" illustrative of the general assembly steps required for building a bulletproof long-block.
Bottom end assembly
Once the case halves, crankshaft and rods were picked up at Ollie's Porsche Machining, a final cleaning was in order to ensure that any stray metal particles created during the machining process did not find their way into any oil passages. The case-mating surfaces were also checked to ensure that there were no dings, nicks or scratches that might cause a potential oil leak. In the interim, Becker disassembled the oil pump and intermediate shaft assembly for a thorough cleaning as well as to check for signs of wear. After proclaiming that all was well, we were ready to roll!
First things first-the main bearings were installed into the case using a combination of motor oil and moly assembly lube, and we checked to ensure that the oil holes on the bearings lined up with the oil holes in the case. The same assembly concoction was used for the insertion of the intermediate shaft bearings. After inserting the oil pump o-rings in their respective locations, we proceeded to lay in the lubricated oil pump and intermediate shaft assembly, and rotated the shaft to make sure it turned freely. Once the assembly was in place, the new locking tabs for the oil pump were installed on the studs and bent up into place before the retaining nuts were torqued. The two new timing chains were then prepped and installed on their respective intermediate shaft sprockets.
Steve first trial-fitted and then lubricated the #8 nose bearing and installed it on the crankshaft while I lightly coated the mating surface on the case and flywheel seal itself with Curil-T sealant before installing the seal. Once the crankshaft was set into the case and the nose bearing correctly seated, we turned the crank slowly back and forth to check for any signs of interference. Finding none, we then mated the case halves using Three Bond 1104 (aka Yamabond) sealant. Almost every professional engine builder has an opinion on which case sealer works best and most defer to the factory recommendation of Loctite 574. However, Becker has successfully built RSR, 935, 956, 959, 962, GT2 EVO and GT1-98LM motors using his favorite case sealant, so who was I to argue for anything different for my street motor? The case through-bolts, with beveled washers and new sealing o-rings, were correctly oriented through the case and the capped case nuts torqued down using a cross-pattern technique designed to ensure uniform compression. I chose to add a little visual flair to my engine by having the case fastening hardware, mounting brackets, and linkage pieces gold cadmium plated. The results are stunning, especially the contrast between the gold case cap nuts and the aluminum engine case. Finally, the numerous perimeter case nuts were tightened to spec.
After the case sealant had cured overnight, we resumed the next evening with Becker fitting the new pilot bearing and resurfaced flywheel to the crankshaft and torquing the new flywheel bolts. At the other end of the motor, I installed the intermediate shaft cover and torqued the crank pulley to its final value. I then proceeded to install the four inboard chain ramps and made sure the lone brown ramp (the others are black) was properly located in the lower right position. Becker installed a new oil pressure switch, breather cover and oil thermostat. These are infamously known as the "Big 3" due to their close orientation and propensity for oil leaks. The "Big 3" are also virtually impossible to service once the motor is installed in the chassis. I installed the new oil pressure relief pistons and springs into the crankcase, noting the vertically mounted pressure relief valve uses a longer spring with guide sleeve than the horizontally mounted relief valve. The shorter spring used by the horizontally mounted relief piston does not require a guide.
Most engine builders wisely elect to install the connecting rods on the crankshaft before sealing the case. Porsche special tool P221, or a homemade equivalent, enables the #4, #5 and #6 connecting rods to be held in an upright position during the sometimes awkward case mating process. We elected to wait until after the case was sealed before installing the rods. Long before we began the assembly process and sealed the case, the rod and main bearing journals on the crankshaft were checked with a micrometer, and connecting rod dimensions were verified with a bore gauge. Ollie's impeccable machining and Becker's insistence that every measurement be double-checked gave us 100% assurance that all was in spec. After prepping the rods for assembly, and fitting the bearings to both halves, a small amount of motor oil and assembly lube was evenly spread on the inside surface of each bearing. The rod halves were then mated to the crank using ARP's heavy-duty 9mm rod bolts. The rod bolt threads were treated to ARP's special assembly lube and the bolt/nut combo torqued to their ARP specified value of 35 ft-lb. Inserting the oil drain plug into the case was the final touch.
Top End Assembly
With the short-block completed, we turned our attention to the top end of the motor. Having removed and discarded the old cylinder head studs, the new ARP head studs were threaded in the case before the big-bore 100mm Mahle pistons and cylinders were installed. If using new head studs, the threads in the case should always be tapped and chased to remove any leftover material to prevent galling of the new head stud threads. Armed with Loctite 262, I put a small dab on the threads of each stud and proceeded to thread the studs into the case by hand until they lightly bottomed out in the case. It is important that 911 head studs be equal in length, and this was measured and verified.
While I was tackling the head studs, Becker measured the combustion chamber volume of the #1 cylinder head so we could begin to determine the compression ratio of the motor. He carefully filled the now twin-plugged cylinder head with a mixture of WD-40 and motor oil (with both spark plugs installed) and measured 88.8 ml. The #1 and #4 pistons and cylinders were then assembled on the case to measure the deck height (the distance between the top of the piston and the top of the cylinder). The deck height should always be checked, especially in this instance where the case spigots were machined for a larger bore and the cylinder heads having been surfaced and milled in order to gain a few extra tenths of compression. Instead of using 0.25mm thick cylinder base gaskets that are included in most gasket sets, special 0.5mm base gaskets were employed to arrive at the deck height of between 0.75mm and 1.2mm, which is commonly recommended for performance street engines. With the thicker 0.5mm base gasket installed between the case and cylinder, Becker measured the final deck height at 0.75 mm. This measurement was on the tight end of the spectrum, but it provided enough clearance to avoid concerns about my expensive Mahle pistons slamming into the valvetrain when the motor is warm and the components are fully expanded.
Becker used the same WD-40 and motor oil mixture used to measure the head displacement to measure the cylinder volumes as well. The filled cylinder volumes measured 71 ml. Plugging the numbers into the formula (see the table) yielded a measured compression ratio of 9.5:1, the same ratio advertised by Porsche for a stock U.S.-spec 3.2L Carrera. Ideally, I was planning to run the motor in the 9.8:1 range with twin-ignition, though I rationalized that with no knock sensors to retard the ignition timing, the lower compression ratio would help alleviate the potential for detonation (pre-ignition) due to sweltering summer days, or a tank of bad fuel. The pistons and cylinders for #1 and #4 were then removed from the case to prepare for final assembly.Another careful step came next: installing the individual pistons onto the connecting rods. The Mahle pistons I selected are a high-domed type with cut-out reliefs for the valves. The larger intake valve reliefs found on high-domed pistons must face upward and the smaller exhaust valve reliefs downward. The stock CIS and Motronic pistons have an odd dome shape and should be installed with the dome facing up and to the left. Installing the pistons 180 degrees backwards could lead to devastating piston-to-valve interference if not caught during the cam timing process. Such an error would require another teardown and some expensive parts.
Before the pistons could be fitted into their respective cylinders, the piston rings needed to be properly aligned to minimize compression loss. Becker oriented the oil ring gap so that it faced the top of the piston and aligned the gaps 120 degrees apart in preparation for fitment into the cylinders. He then delicately installed the pistons into the cylinders using a ring compressor. (This is a "feel" process that an engine builder gains with experience. For the rest of us, it is common to break a ring.)
Before the cylinders were slid down the head studs and held in position with special Porsche P140 tools, Permatex High Tack Sealant was sprayed on all six copper cylinder base gaskets to help eliminate another potential hot spot for future oil leaks. It is important to note that the longer cooling fins on the cylinders are pointed downward. Failure to install the cylinders in their correct orientation will lead to overheating and subsequent engine damage. Finally, the freshly powdercoated cylinder deflector tins were fitted between the individual cylinders to ensure that the proper amount of air generated by the fan would sufficiently cool this large-displacement 3.5L motor.
While I was putting the finishing touches on the cylinder deflector install, Becker was mating the cylinder heads onto the cam towers, once again utilizing Three Bond 1104 liquid gasket instead of the factory recommended Loctite 574. The heads and cam towers were mounted as a unit (holding the oil return tubes in place) onto each bank of cylinders. After the head stud threads were treated with ARP's special assembly lube, I installed the washers and torqued the 12-point head nuts using a cross pattern technique. The new Web-Cam camshafts were installed into the cam towers and lubricated with Web-Cam's assembly lube. I mounted the now pressure-tested and ultrasonically cleaned oil cooler at the front of the engine next, and Becker installed the chain boxes and the cam end cover plates (with new o-ring and gasket from the gasket set), and tightened the three cam end plate retaining bolts on each bank. He and I then each took one side of the motor, fitting the thrust spacers, spacer shims, woodruff keys and cam sprocket flanges.
The cam sprockets were properly oriented before hanging the dual timing chains and sliding them into place. The cam sprockets are identical left to right, but the dished surface faces the pulley end on the left bank and the flywheel side on the right. It is also wise to make sure the camshaft sprockets run parallel with the intermediate shaft sprockets to minimize wear of the timing chain and related components. The last two black chain ramps were finally fitted along with the idler sprocket assemblies. A special tool is used to keep tension on the chains during this step on a 911 engine assembly, as the oil pressure-fed chain tensioners should not be installed until after the cam timing has been set. The large cam bolts should only be tightened by hand at this point, as they will be removed again later so the cams can be properly timed.
The rocker arms and shafts for cylinders #1 and #4 were ready to be installed in order to carefully check the piston-to-valve clearance. This step is overlooked by many but should be performed by all, especially if new pistons and/or camshafts are being installed. The intake and exhaust valve clearance is adjusted to 0.004-inch (or 0.102mm as measured the European way). Remember to check that the motor is at TDC for cylinder #1. Web-Cam recommended a minimum clearance of 0.050-inch on the intake side and 0.080-inch on the exhaust side for the 20/21 cam profile. In this assembly, the piston-to-valve clearance is a non-issue as the high-dome pistons used in this particular application are generally used in conjunction with much higher lift and longer duration camshafts. The Web-Cam 20/21 grind is one of the hottest profiles available for use with the standard CIS/Motronic piston dome configuration, where the clearance can be awfully tight, but with the larger valve relief of these RSR high-dome pistons, the clearance is easily over 0.080-inch. To check the clearance, simply remove the retaining nut on the valve adjustment screw and turn the screw until it touches the tip of the valve. The M8x1 screw will open the valve 0.040-inch or 1mm for every full 360-degree turn. Simply count the number of turns until the valve rests against the piston. The process was then repeated for cylinder #4. If the minimum clearance is not attained, you must stop immediately and address the problem before continuing with the rebuild.
Timing the camshafts is a very important process that must be done correctly in order to have a motor that makes the most power possible. The camshafts are literally the heart of the motor and determine the engine's personality. The complete timing procedure is much too in-depth to discuss in this article, but trust me when I tell you these cams were properly timed at 2.0mm overlap and well within Web-Cam's specified range of 1.9mm-2.2mm for the 20/21 profile. Once the settings were verified on both banks, the large cam bolts were torqued down to an incredible 90 ft-lb. The remaining rocker arms and shafts (with new RSR seals now riding in their grooves) were installed and the valve clearances adjusted.
After I removed Becker's special chain tensioning tool and installed the oil pressure-fed chain tensioners, the chain housing covers and valve covers were placed into position with new gaskets. The cam oil lines, oil temperature, and oil pressure senders were the last components to be fitted on the motor before the long block was ready to accept the intake, exhaust, and electrical components. It looks like a 911 motor again!
While Becker and I are nearing completion of the motor, Dwain Dement and the guys at Vision Motorsports are hard at work prepping the chassis with some final tweaks in anticipation of reinstallation. Next time we'll wrap up the assembly by installing the intake, exhaust and electrical components needed to finish the rebuild and fire up this beast. Vision lead tech Mike Olsen will guide us through the motor installation into the chassis and the proper break-in procedure after initial start-up. The engine mapping wizardry of Steve Wong at 911chips will optimize the fuel and timing curves to unleash the 3.5L motor's full potential. Finally, we'll come full circle and return to the Dynojet 248C to measure the improvements made over the original 3.2L output.
Robert Bentley Service Manual,
1984-1989 Porsche 911 Carrera
How to Rebuild & Modify Porsche 911 Engines
by Wayne Dempsey