Mini A-Series Engine data for builds

Engine block - 1275

Planning a race engine build typically often begins with finding a suitable A-Series short block, using the short-block from SPI seems to be the best choice. Said to be the strongest blocks. A used short-block or complete engine should be inspected to ensure that the bore size is suitable for a build (ideally std bore) and standard height to allow all aspects of the build to be optimized towards the project goal.

If the donor is used and has undergone previous machining, beware of:

• bores - if they are oversized, check that they are properly off-set when they are > .060, blocks can be sleeved but there's a point where sleeves can not be fitted properly when the bores are too big and not properly offset

• height - if the engine has been "optimized" for performance the block height could have been adjusted by shaving the block. Blocks that are no longer std height need to be properly measured and matched with the desired crank-stroke, rod legth and piston pin height

• height - some folks skimmed the gearbox face of the block. That sucks as the fit for the drop gears has to be looked at closely. Straight cut drop-gears allow for blocks being installed without gasket between short-block and gearbox. But if the surface has been skimmed, using gaskets and proper setup (even though there is no real "data" to set up the drop gear train) is important

Block height info below has been collected from verbal info-sharing, so take with a pinch of salt. I found that most "virgin" SPI blocks I ended up with had a height of 8.881"

Static compression ratio calculations - 1275

Not specific for Mini A-Series engines, but the spreadsheet has mini specific data pre-populated. Important data to gather is the head chamber volume, piston to deck height, bore diameter, piston diameter.

Use the spreadsheet to enter the "desired data", or the data you get from the machine shop, into the metric fields. The spreadsheet then provides the static compression ratio for the engine configuration. It's generally debated if it's needed to add the "ring land volume" or if that can be ignored. Play with the numbers and see how it affects the compression ratio.

Tools for the build

Transmission gearing calculator

There is a wide range of gearsets available for the Mini, std ratio, GT, close ratio gearsets. Combine that with drop gear ratios which allow to drive the input gear either faster or slower, as well as with different final drives, and the RPM to speed ratio for each gear and top speed can be adjusted finely. The final drive ratio and drop gear ratio influences the top speed, so depending on the race track find a configuration that avoids running out of RPM at the end of the long straights (too early).

It's generally said that the input gear ration can be adjusted "quickly" by swapping the primary gear (24 or 23 teeth for straight-cut drop gears), but I have not yet actually adjusted it at the track. Maybe one day when we will compete for a national championship I might...

The calculator has a bunch of gearsets' data added, the data is from the gear set supplier's website. Ah, and, no. There is no manual for this calculator, happy exploring!