To further as to why someone would do this: in some SCCA classes ,here in the states, the internals are free so this would be a worthwhile mod. Bob can correct me here but in Solo II competion I believe Prepared class allows free internals but if you use a differant block then you end up in Modified. I've been contemplating it myself but will likely stick to the 1400 motor.

Tom

Interesting discussion. Piston speed is the main limiting factor in how high you can rev. F1 engines are turning close to 20,000 RPM , which causes some to say, "HOW"? They are very oversquare, so piston speed is low, despite the rotational speeds. The undersquare 6s in my Triumphs (and virtually every old Brit engine) run frighteningly high piston speeds, it's hard to believe that Kas Kastner turned his to 8600 RPM (and they didn't always last to the end of the race either). I got 2 spare A14s with my coupe, I'll be following this thread.

Scott

so the F1 engines really have a piston speed as though they ran less than 10,000 rpm, but the multiplication in the crank and rod makes it look like the piston is moving faster.

stupid question. do something like this make a .7 liter A series motor that revs really high --- in theory.

I'm sure having 12 cylinders helps make this work too.

Don't know the piston but Honda has made some wicked high reving motors, can't remember which 1 2 5 (RC145 maybe) turned 22,000 RPM's in the 60's. Of course the also had the NR500 oval piston 8 valves per cylinder GP bike that ran up to 25,000 in the 70's. Grnated the bikes had something like 44mm bore and 41MM stroke so what's the piston speed with 41mm stroke at 22,000 RPM.
Hey Scotdat a friend vintage races a GT6 and guess who helps him out.........none other than Kas Kastner.
As for the topic what is the max RPM of a A12 with A14 crank before it goes kablamo.

Tom

Quote:British cars are [or were] taxed on what they called 'RAC Horsepower rating' [Rediculous Auto Club] This number had absolutely nothing to do with the engines ability to perform work [power] but was in fact a mathematical formula that used bore size as part of the equasion. The bigger the bore, the higher the RAC rating & the higher the tax, which was paid each year as part of the registration fee.
In order to sell cars, British makers needed to keep the taxable power[?] of their engines as low as possible, so small bores & long strokes were the order of the day.
Talk about officialdom finding a way to stifle automotive development, but sadly, this seems to have been a British trait.

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Heard that yonks ago, I'd forgotten that.

What's the best solution??

-Put the A12 crankshaft on A15 block and machine the block to adjust the combustions chamber, will gain high revs and square engine but loose cubes;

- or put the A15 crankshaft and the A15 pistons on the A12 block, gain cubes but loose high revs

What do you think??

Who says you lose high revs? A15 short block will rev to 8500 RPM no problem. If you do all the modification to make an A12 run to 10,000 RPM, you can also do the same modifications to the A15.

The saying is: "There is no substitute for cubic inches" or "There is no substitute for cubic centimeters". The main reason to consider rod/stroke combination is when your racing class forbids larger displacement (or penalizes larger displacement). But that does not change the rule: the more CCs, the faster it is all else being equal. For the same engine weight, same carb/cam/head/exhaust the larger CC will make your car faster. The main thing that a smaller stroke will do is last longer because piston speeds are lower.

I would like to elaborate on a statement made previously. An oversquare engine does not necessarily lower piston speeds. A shorter stroke does! Also the rod/stroke ratio has a smaller effect on piston speeds. But if you simply make it oversquare by increasing the bore yet keep the same stroke, it won't affect piston speed at all.

You could also do this conversion if you wanted to keep the original engine number for a car. A friend of my Aunt has a 1969 Datsun 1000 sedan that he has owned since late 1970 (second owner) that had an A12 fitted in the mid 70's, because it didn't have enough grunt for the Auto. I'd love to buy this off him & fit an A14 crank into the A12 engine, because it's still slow off the mark & the extra torque wouldn't go astray. At the sametime, I'd still want to maintain the history/provenence of the car. Not too many 1000 auto sedans out there, I'm sure. Another car I'd do this conversion would be on a mates 120Y coupe which was a demonstrator car at Capitol Motors & had most options fitted. When he restores it, we'll do a 1390 cc A12, to keep the matching engine number. Pity the A14 he got with the 1000 ute was disassembled & the crank left out in the weather. Totally unusable. Still, he got an A15 & a 5 speed with it.

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This explains my 1964 Triumph Vitesse engine: 1600cc 6 cylinder (sleeved down 2000cc engine).