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Bob uk
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Posted on Friday, 19 September, 2014 - 08:12:   Edit PostDelete PostView Post/Check IP

In the early days of motor taxation in the UK the RAC formula was applied which used bore size only to guess the power. Ignoring the stroke.

So to keep road tax low the makers favored small holes and long strokes.

An Austin seven was rated at 7hp. Which is not correct because the seven is about 25 bhp, with luck. A 20hp RR is nearer 50bhp (guess)

Long stroke means that the piston travels further per revolution and therefore the piston speed is higher and the engine will idle slower. The longer throw of the crank adds weight and inertia plus the counterbalance is heavier thus a smooth idle is obtained without a heavy flywheel. Except that a heavy flywheel was added anyway to aid starting.

Because the stroke is long or longer, the gap at tdc which dictates the compression ratio can be deeper allowing room for the valves. In diesels because of 20:1 compression ratio tend to be of the longer stroke for this reason. The smaller bore doesn't have the room for large valves so breathing is restricted.

Longer stroke means the power stroke is acting on a longer lever which translates to more torque. Because the piston speed is higher, it's a double benefit which gives big six cylinder diesels mega torque.

But it limits revs not so much mechanical damage but the piston at high revs and thus piston speed is trying to travel faster than the ignition flame front. Plus add the extra weight require to bits to withstand the stresses and it gets into vicious circle. So there are limits.

Short stroke on the other hand gives more room for big valves and keeps piston speeds low. This allows high revs and lighter reciprocating masses which allow quick revving. But they lack low down torque.

Because the way the maths work revs x torque equals hp. The holy grail is to have lots of torque that doesn't drop when the revs are increased. Interestingly max torque occurs at the rev speed range where the engine is most efficient. Unfortunately the revs put the car too fast and air resistance comes out to play. 50 mph is on most cars very fuel efficient.

So race stuff revs high to get Bhp and tractors rev slow to get the torque.

Car makers tend to stay in the square ratio because the compromise of long short happens to work the best.

Ford 1300 crossflow engine had the same bore size as the 1600 crossflow. The 1300 could be made to rev much higher and produce more bhp than the 1600 but the 1600 had bottom end grunt which made it better putting the power down out of the corners. The 1000 cc version had a 1300 stroke and small bores which gave it good torque for a small town car which doesn't go on motorways. On motorways the car is under powered but nippy around town.

Other makers such as Fiat would use bigger bore and shorter stroke so that the engine revs a bit to give the owner a zesty engine.

Finally a weird engine from Honda which went well but a maintenance nightmare.
Instead of round pistons oval pistons that are 2:1 ratio in shape two pistons moulded together in an oval bore. The engine was a V4 or a V8 if you like. It revved high and had very big valves. How mads that.

Apostrophes are either possessive or in place of a missing letter. So how mad's that is the correct form.

(Message approved by david_gore)