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Location:
Lost in the K hole
Registered:
May 2002
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 Which is Better....
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Thu, 11 July 2002 12:51
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ok,
plugged a few numbers into dyno 2000 and this is what it spat out. the only difference between the two is the top graph is running a 75mm stroke crank, the bottom one an 85mm stroke crank.
its only a guide, but its gives an impression of the style of the engine output. question is, 1300kg car, w58 manual, 3.9:1 diff, which engine would drive better. subtle, but still, a desicion has to be made.
75mm stroke crank
85mm stroke crank
cheers
ed
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Location:
Brisbane / Gold Coast
Registered:
May 2002
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Re: Which is Better....
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Thu, 11 July 2002 23:52
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Unless you're planning on running more than 8,000rpm all the time, the long stroke crank looks better.
But there's not really enough information there - Is that just for a stroked crank and so more capacity engine, or, an engine with the same capacity and a smaller bore/bigger stroke?
If it's the stroked engine then it's no surprise that it'll make more power but if it's the same capacity then I'd be very dubious of that program - There's not a lot of difference (within limits) between a short stroke/big bore engine and a long stroke/small bore engine of the same capacity, all other things being equal.
In practice though, the short stroke/big bore engine will always make more power as it'll have less internal frictional losses, less inertia, and the valves will be shrouded less.
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Location:
Lost in the K hole
Registered:
May 2002
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Re: Which is Better....
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Fri, 12 July 2002 04:06
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bill,
to clarify, the bore is fixed at 84mm in both examples. im just toying with the idea of swapping in a shorter stroke crank to bring the revs up (and lose torque). so in the lower graph, capacity is 2.8L, in the top graph (shorter stroke) capcity is 2.5L.
im not sure how the rod length is going to cahnge yet. could be up to 5mm longer in the short stroke crank. i hear that having the longer rods will improve torque a little, thou i cant see how. and ideas? i mean, its still a 75mm stroke, but would just have a longer rod so the piton would reach the block deck.
cheers
ed
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I supported Toymods
Location:
Brisbane, QLD
Registered:
May 2002
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Re: Which is Better....
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Fri, 12 July 2002 04:17
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Hmmm.... I might be completely mistaken, but wouldn't a longer conrod increase the compression?
Cheers
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I supported Toymods
Location:
I renounced punctuation
Registered:
May 2002
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Re: Which is Better....
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Fri, 12 July 2002 04:37
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Am I reading the graphs correctly? Are you getting ~300 hp @ 7700 from the 2.5 l hybrid, vs ~280 hp from the 2.8l?
And is that 17 psi boost you are running there?  What happened to '5-6 psi just for a bit of extra oomph'? 
Looking at the graphs (and having experienced a 4M/M hybrid), I would prefer the bigger capacity (read long stroke) for the street. Also we only ever took our hybrid to 7000 (limited by cam and valve springs/bounce). To go over that you really need to sort out the top end.
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Location:
Lost in the K hole
Registered:
May 2002
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Re: Which is Better....
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Fri, 12 July 2002 04:42
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nishad
5mm longer conrod simply serves to compensate for the 5mm less stroke of the crank, brining the piston back up to deck height. in fact even if the tdc pisition of the piston in the bore is the same, thus giving the same combustion camber volume, the compression ratio will be less due to the decrease swept volume of the shorter stroke crank.
cheers
ed
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Location:
Lost in the K hole
Registered:
May 2002
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