wat is the point to have longer/shorter connecting rods?

[MitchS046]

I've started to purchase parts for an 1915 street engine, but came across connecting rods with different sizes. some people advised me to go with stock length (5,4'') but no one could tell me why tho.....
is it so you can have more stroke with a sertain crank?
sizes I saw range from 5,5'' to 5,7''. in my eyes it isn''t worth starting playing with those sizes because of clearance issues.
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[modok]

Beyond 82 stroke, the pistons are pulled SO far into the engine that they may run into something. Good reason to use a longer rod, I'd say.

In the very OLD days, porsche(135.5- 136mm) and small journal chevy (5.7) rods were often used IN vw engines, so, those lengths are due to tradition.

5.5 is a handy size also. As you will find out IF you do not have the correct amount of material machined off the case for your cylinders.

[Wreck]

There is a thing called rod crank ratio . standard type 1 1600's have a relatively high ratio compared to some engines . divide the rod length by the crank stroke . different ratio's can cause different engine characteristics but not so much to make it obvious to the average driver . lower ratios can cause more cylinder wear .

When you change to a stroker crank people use longer rods to maintain a higher ratio for wear and also to give clearance for the piston and the crank when the crank is at bottom dead centre .

In your engine the stock length rods are the ones to use . Unless you are trying to build a narrower engine for some special reason .

[mark tucker]

these days its' eaze of assembly...the old dayz it was what the wercker said. some people dont mind a engine thats not happy the way it's built by a slap togeather the eazyest way. some do.

[nsracing]

She says the longer rod is more fun. The shorter rod is prone to 'pre-ignition.'

The above may not apply to engines.

[modok]

[jeffrey8164]

[BFB]

have yall really noticed much difference in performance with different rod ratios? i know the theory but asking about true seat of the pants feel
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[vwracerdave]

For a real street driver rod ratio is grossly over rated. Use the rod length that makes it easiest to build. On a full race engine that turns 8000 RPM then rod ratio does make a difference.
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[FreeBug]

Ease of assembly, indeed is the number one prevalent reason.

that said, long rods will reduce your max piston acceleration/deceleration, and in principle, longer rods let you stretch more rpm out of a given head.

I've always wondered in terms of "lugging" an engine, and how some engines are more prone than others to buck at low rpm...ho much of that is rod length, I'm thinking shorter rod=less lugging?

It's interesting to point out that the stock ratio on the type 1 is particularly high, at 2, and the type 4 went pretty much the same way. And yet, wear at the top of the cylinder is pretty bad with the vw, it seems to me. And that is with very long, well-fitted skirts.

What is the rod length on the 2.0 type 4?

But who cares, none of us are calculating peak piston speed or Gs anyways. Go with what fits best.

[Dauz]

Piston Slap