Engine Design

Over the years I have read a book or two on engine design Mostly about car engines ,, Head work , Bore & stroke in this case ,, A square engine is where the Bore matches the Stroke , Over square is the Bore Larger than Stroke it will make HP at lower RPMS as where Under square engine Where the Bore is smaller than stroke makes HP at higher RPM ,, I beleive the Chevy 302 enigne is Square ,, That is why it runs so well and stays together well as an example ,, My question is ? The 810 over bore kit seems to be the Over Bore of Choice ,, Is there a mechanical reason ? Or is it as simple as it is the only one that is made for the 650/750 KZ engines ,, Any Thoughts ?? :whistle:

I believe the 650 "overbore" size is 720 and the 750 is the 810. The more popular an engine the more sizes you have. When i had an RD400 there were 4 sizes available from stock and most places had all of them.

I believe the same was true with the H-1, overbore was 0.20 0.40 0.60 0.80 .

It is much easier to just overbore an engine and put in larger diameter pistons than it is to make a longer stroke crank, then you would also need special pistons to get the pin height correct or maybe custom rods

the 302 chevy had a 4" bore like a 327-350 and the short stroke of a 283 thats why they wind so hard!:woohoo: ive got a 67 302 chevy block if somebody is looking for one.:whistle: 900-1000 kawasakis have the same stroke,but the counterweights on a 900 crank are lighter thats why they rev easier than 1000s,1000 topend on a 900 bottemend is a really healthy combination.

well add this to the mix... rod lenth, longer rods in an engine increase torque by allowing the piston to dwell at TDC longer therefore burning more A/F mix at higher compression. relieve stress on rotating assembly by decreasing rod angles therefore increasing revability. Foe example a typical small block chevy with stock rods(5.7in)will dwell at TDC for about 3 degrees, now add longer rods, say 6in, now the piston dwell will be around 12 degrees and the crank/rod angle will be reduced allowing the engine to work easier with more explosive power...but nobody makes parts like that for motorcycles

you start getting into differant rod ratios and things get technical fast!:blink:

Post edited by: wireman, at: 2006/02/16 01:32

Foe example a typical small block chevy with stock rods(5.7in)will dwell at TDC for about 3 degrees, now add longer rods, say 6in, now the piston dwell will be around 12 degrees

I was just going to let this one slide, but it just kept nagging me:laugh:

If a piston stays at TDC for 12 degrees of crankshaft rotation, then it also will spend 12 degrees at BDC, now assuming that this 12 degrees is actually 6 degrees BTDC and 6 degrees ATDC, and the same applies at BDC, then that means that the piston now has 12 degrees less crank rotation to get from TDC to BDC. See the problem with that line of thinking?
That also implies that if you lengthen the rod enough, the piston will spend 180 degrees at TDC and 180 degrees at BDC and would have 0 time to travel from one point to the other.

Now extending the rod does give the piston a greater straight line to transmit the force of the gas expansion, and less piston to cylinder wall friction, but it also increases the reciprocating weight.

You don't have to take my word for any of this, you can cut a circle (to represent the crank throw) and several straight pieces od different lengths (to represent the rod) out of cardboard, use a straight pin to stick them together and observe the results of different rod lengths yourself.

longer rods put less load on cylinder walls also,ill call my dad today hes the family rocket scientist when it comes to strokers and rod ratio kind of stuff,outa my league! :blink:

Its good to see all with their thinking caps on :huh:

By the way, this is probably what was referred to above:

Compared to a short-rod engine, the long-rod engine dwells longer at, and decelerates and accelerates slower from, the vicinity of TDC.

leaving out the word "vicinity" changes the statement

hotrod.com/techarticles/pit/index14.html

And more really good info at www.stahlheaders.com/Lit_Rod%20Length.htm

Shoe48 wrote:

, Over square is the Bore Larger than Stroke it will make HP at lower RPMS as where Under square engine Where the Bore is smaller than stroke makes HP at higher RPM ,,

I thought it was the other way around. My 650 is oversquare, revs 9000 rpm high to make power. My Harleys had long strokes, made all their power at low rpm...:whistle: