In previous installments we discussed our plan for the project and also our competitive performance analysis and goals. Now its time to jump into real race car building, and at the heart of any competitive race car is the engine. This is especially critical in low-power applications like STS2.
We dropped the motor off at Applied Racing Technology several weeks ago, and stopped in this week for a progress report and to snap some pics. ART is currently in the process of building five motors, mostly for road racing Spec Miatas, and we are in the middle of that queue. We had a good talk with ART co-founder Ed Gilfus about our motor and ART’s philosophy on building fast Miatas.
When ART builds a Miata motor, they focus on three main areas for performance: head flow, compression, and bottom-end sealing. Even within the factory service specs there are areas for improvement in each category. For example, ART has done a bunch of flow bench work to come up with a specific valve job that flows the best, yet is still within factory tolerance numbers. The combustion chambers are also matched in volume by adjusting the sinking of the valves. And then there are the straightforward things like shaving the head to the minimum service spec to optimize compression. In addition, attention is paid to parts that just plain wear out. Valves are checked for length, square, radius and seats. Valve springs are all checked for rate and square. Cams are checked for lobe height and obvious signs of wear, and lifters are assured of proper operation. Any of these that is worn excessively is replaced. For a top-shelf motor, many of these items are replaced routinely for optimal performance and longevity. While this is way into the decreasing returns curve of performance for the dollar, we have chosen to go there to establish an upper limit of performance for the motor within the rules.
After cleaning and examining the head, we were fortunate to see very little casting flash in the ports and nice transitions to the machined valve pocket areas. Some of the other motors ART is currently building are not as nice in this area. Our head’s gasket surface is also very flat. The bad news is that our head appears to have been serviced once before and not so carefully. The thickness is right at the service limit on one side of the head and about .003-.004 thicker on the other end. Our combustion chamber volumes match this trend with about a 1 cc spread from the thin side to the thick. Unfortunately, it is virtually impossible to fix this by shaving at an angle since there is no excess material on the one end. It would take an amazing machinist, and a lot of luck, to achieve this without going under the legal limit. We’ll just have to live with it.
One note on cleaning: To remain legal, it is critical not to use any sort of abrasive cleaning process in the port areas or combustion chambers. Anything which alters the surface texture of those areas can be construed as a modification and is protestable. ART uses a combination of hot tanking and a very mild bead blast using materials that are softer than the aluminum alloy used in the head. This is sufficient to remove the carbon buildup without altering the head itself.
Since the STS2 rules allow it and we are going top-shelf with our build-up, we’ve chosen to go ahead and bore the block the allowed .010 first factory sizing. While this is typically unneccesary (and not legal in Spec Miata), we want to get perfect bores for optimal bottom-end sealing. When boring a block, ART uses a custom-made torque plate to simulate the stresses of the head and make certain that the bores are still round when the engine is bolted together. Into that block we’ll be fitting Mazda factory overbore pistons that are parts-bin matched to within a gram of each other. Since ART builds so many motors, they order parts in large batches (see picture) and can select from within those batches to match up component weights. Nice.
We are fortunate in that our crankshaft is very straight. Ed says he sees a lot of Miata cranks that have excessive warpage and require reworking. Our rods are also perfect right from the factory, very closely matched in weight and length. Ever-the-perfectionist, though, Ed will be assembling the lightest rod to the heaviest piston to get all the assemblies perfectly balanced, yet still within the rules.
When it comes to clearances on the Miata, ART goes fairly tight. Since our autocross application does not see the heat expansion that endurance racing Spec Miatas do, we can improve our piston sealing without risking heat-related damage. Bearing clearances will also on the tight end of the range because of oiling concerns.
While we were there, we looked over ART’s injector cleaning and flow-matching setup. Basically, you send them a set of injectors, they clean and flow test them, and match them up in groups of four from their stock. Once again, another advantage of using a shop that does some volume in your application.
And one final surprise was sitting on the floor by our motor. A nice low-mileage factory-optional viscous limited slip and the corresponding stub axles! Woo-hoo! The Spec Miata folks all typically run the more effective Mazdaspeed clutch-style limited slip but STS2 cars are limited to factory-only viscous units. This piece was pulled from a Spec Miata built recently by ART and thrown on the junk pile. Their junk is our treasure!
Next we catalogue our suspension starting point based on two year’s worth of CSP development.
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