In the last two installments, we made Project Time Attack Miata safe to drive, cheap to maintain, and lighter than Nicole Richie. We've had a blast driving it at the track. It gives great driver feedback and--even with its scant 100 wheel-hp stock drivetrain--has been able to keep up with much more powerful cars. A smart owner would enjoy the car in its present form, or maybe install some modest bolt-on upgrades to give it a little more grunt, without losing the crisp response and character that make it easy to drive.
No one ever accused us of being smart. This time, we're unleashing a whole new level of complexity by adding turbo power. And lots of it. But this doesn't come without some undesirable consequences: reliability and heat. Both require significant investment in monitoring equipment, heat exchangers, and other items--which add weight to a car we've worked so hard to make light.
The robustness of the 1.8-liter four-cylinder BP engine has proved itself repeatedly in past Miata project cars. Designed for use in boosted applications overseas, the turbocharged BP engine can provide 250 wheel-hp (with proper engine management) and still run many miles of track before giving out.
To make more reliable power, the stock bottom end has to be strengthened. Since any budget was cast aside long ago, we looked once again to the folks at Flyin' Miata to help us build the ultimate Miata engine. Our goals were not only to build an engine that would withstand high boost for long periods, but also spool a healthy sized turbo quickly, making gobs of torque across the rev range.
Flyin' Miata has been boosting Miatas for a long time, working out a formula for ultimate power that centers on a custom crank, cut from a single solid 350-pound billet of steel and designed to increase stroke by 4mm to 89mm. This is the only Miata crankshaft we know of that is not reground from the stock piece. While the process of welding on additional material and rebalancing has proven to be inexpensive and reliable, starting from a uniform piece of steel provides greater strength and longevity compared to something that's cut, welded and ground many times. A freshly cut crank is also easier to balance.
The added stroke, combined with an increased bore of 84.5mm to accommodate larger Wiseco pistons, increases the BP's displacement to 1995cc with a boost-friendly 9:1 compression ratio. This is not the largest bore size seen in a Miata, but we hesitate to go larger for two reasons. First, a larger bore decreases cylinder wall thickness, thereby cutting our margin of safety for a fairly expensive bottom end. More importantly, since the engine is now more undersquare, the bore/stroke ratio has decreased from 0.976 to 0.949, which significantly increases piston velocity, crank load and sideload. Coupled with the increased mass of larger pistons and boost, the new engine will generate significantly more load on the rings.
Usually, this is compensated by lowering the redline to preserve engine life. Flyin' Miata's design, however, also moves the wrist pin inside the lower oil ring, which keeps the rod ratio consistent, with the stock engine taking some piston/rod acceleration load off the crank pins. Combining this piston size and mass with a stock rod ratio (when linked to the crankshaft by Carrillo A-beam rods) has created an engine that is still happy to spin at 7500 rpm.
While building a bottom end that can take such abuse serves a purpose, we also wanted to maximize the amount of air coming from the turbo into the combustion chamber. One of the most significant incremental improvements Mazda made for the Miata engine came in 1999 when it reworked the top end. This redesigned head features intake ports raised from 39 degrees to 51 degrees, creating a straighter flow path. When matched with the revised intake manifold, flow efficiency is increased dramatically. Mazda also replaced the cam angle sensor with a magnetic sensor on the front of the intake cam sprocket and added a toothed wheel on the crankshaft for more accurate engine management. We simply inserted the old cam angle sensor (CAS) back into the new '99 head, which allows us to use adjustable cam gears on both camshafts. We also used the newer-designed solid lifter camshafts with longer duration and increased intake lift, which will also mitigate some of the high-rev valve float issues of the old hydraulic design.
Into this head, Flyin' Miata has stuffed Supertech 1mm oversized stainless steel intake and exhaust valves, springs and titanium retainers. With healthy polishing and port matching of the intake and exhaust manifolds, this will allow up to 15 percent more airflow at peak lift, according to Flyin' Miata.
Strangely, when we built this engine, Flyin' Miata was between turbo kits. The old design (used on our previous car) had been discontinued, with the new kit still to be completed. While it would have made sense to use a full `end-to-end' build from one shop, we called the fabrication specialist at Boostlogic in Austin, Texas. Boostlogic recently purchased a Miata and prototyped an equal-length runner tubular manifold from stainless steel. This design was matched with an external wastegate that dumps to atmosphere and a three-inch downpipe, which has the potential to produce more power at the limit than traditional log-style manifolds utilizing internally gated turbos.
The downpipe and exhaust are also made of polished stainless steel. While stainless steel is heavier than some options, it does not transfer heat very well. This means that the heat from the exhaust gases will stay in the exit plumbing and not transfer its energy into the engine bay as quickly as with other materials. Boostlogic also built an intercooler kit large enough to support 350 wheel-hp. The 6.5x3.5x1- inch bar and plate core barely squeezes between the frame rails in the mouth of the car. While thickness helps in dropping the intake charge temps, it also generates a larger pressure differential, which restricts the velocity of flow getting to the radiator. We'll see what happens when summer arrives.
Boostlogic's intercooler piping kit uses polished aluminum tubes with a HKS-style BOV flange welded on the hot side just before the intercooler. And now that the engine management uses speed density (MAP) instead of a flow meter, we chose HKS's Super Sequential Blow Off Valve. During manufacturing, something went out of alignment on the jig, which positioned the downpipe just slightly into the header when everything was tightened up. This minor interference was easily remedied by couple of well-placed whacks with the universal BFH (Big F*ing Hammer).
After talking with several different sources about turbos, we ended up selecting a Garrett GT2871R with a .68AR. The goal isn't ultimate power, but good throttle control and roughly 300 to 325 horses at the wheels. Of course, this is shooting from the hip, since our previous Project Miatas only reached 187 wheel-hp and the new target is just a stab in the dark as to what the `right' power level is to maximize lap times without breaking driveline parts.
No comments:
Post a Comment