shim under or above the barrel?

[Martin S.]

I want some of what you're on if you think short rods are the way to go in a bug engine, har har 

TYPE 4 engines. 
Not big strokes but short rods for torque to push heavy buses.

Now there's a real performance engine, sheesh, VW Vans??

From CIP1 website where you can't even buy rods as short as he's using in the 2276:

'A note about rod lengths. We offer 4 different rod lengths because rod ratios are very important. Most engine builders today agree that longer rods produce more power, and less ware on internal engine components then shorter rods. Top engine builders try to achieve a rod ratio of approx. 1.6-1.8 to 1. A 82mm stroker crank should have a rod that is 145mm (5.7inches) to achieve a 1.76 rod ratio.' http://www.cip1.ca/ProductDetails.asp?ProductCode=C13%2D8310%2DARP

[DaveN]

Why don't you build two engines.  One with the recommended 5.7" rod and one with the Porsche length rod. dyno them and show everyone your results?

[Martin S.]

That would be a good magazine article. Most people already have their mind made up by the time they start paying the engine shop for machining, etc. For myself, I went with what my engine builder recommended. 

[1946vw]

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn’t care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There’s no magic – a rod’s function is to connect the piston to the crankshaft. Period. David Reher

[Martin S.]

Wow I didn't see that coming! Totally agree about the short deck height. But I don't get your intake manifold and head port reason. I like it because it makes your engine efficient (burns everything and thus runs cool) and creates POWER without having to advance timing. The tighter the better. .035" seems reasonable for a bug motor.

[nicolas]

Ok, maybe a bit of a trivial question, but i am weighing the pros and cons on a solid 1mm shim under the barrel or a copper 1mm shim on top between barrel and head.
which is better, are there benefits doing one or the other?

Nicolas, one thing I learned about 20 yr ago was to machine the chamfer off the top of the cylinders, to increase seating area. No matter copper gasket or not.

i could do that, makes sense as far as seating surface goes. i'll dig into this deeper.

Why don't you build two engines.  One with the recommended 5.7" rod and one with the Porsche length rod. dyno them and show everyone your results?

yes, that is also an option. i am more then likely going to rebuild this thing within the next few years and if i can come up with a good set of 5.5's or 5.6's i might give it a ty. why not.
for now however i want to get this engine build and hopefully be able to taste at last PG tips turbo engines 

[Martin S.]

Why not? There are a few reasons for a street engine if you want to do a proper job with the cooling system. The cylinder tin had to be extended using another set of cylinder tin as donor metal and the shroud had to be modified, stretched and tweaked to fit on the cylinder tin and not have huge air leaks. The header needs to be cut and stretched out to fit the wider motor as well as the intake manifold. Plenty of extra work is involved! Here's a pic looking very frankensteinish...

[Bruce]

I can remember Mark Herbert was a fan of short rod motors.
The piston dwell time at TDC and BDC was less with a low rod ratio.
I think the big reason Mark liked them was that piston acceleration rates are higher with a short rod, so this will force a port to flow better than if you had a long rod.

For Nicolas' original question, go with the shim under the cylinder.  The shim on the top just creates twice as many surfaces that can have a sealing failure.

[neil68]

I would put copper on top then to keep as much of piston as possible in cylinder at BDC

For my 2332 cc with Mahle cylinders, I used copper head gaskets for this very reason.  When checking the piston skirts, more cylinder shims wouldn't have been the best idea, as the piston would have pulled out the cylinder further.

[Martin S.]

I can remember Mark Herbert was a fan of short rod motors.
The piston dwell time at TDC and BDC was less with a low rod ratio.
I think the big reason Mark liked them was that piston acceleration rates are higher with a short rod, so this will force a port to flow better than if you had a long rod.

Bruce, do you believe that?

[Taylor]

If I had to make a list of the ten most important specifications in a racing engine, connecting rod length would rank about fiftieth. Back in the days when Buddy Morrison and I built dozens of small-block Modified motors, we earnestly believed that an engine needed a 1.9:1 rod/stroke ratio. Today every Pro Stock team uses blocks with super-short deck heights, and we couldn’t care less about the rod ratio. A short deck height improves the alignment between the intake manifold runners and the cylinder head intake ports, and helps to stabilize the valvetrain. These are much more important considerations than the rod-to-stroke ratio. There’s no magic – a rod’s function is to connect the piston to the crankshaft. Period. David Reher

Wow I didn't see that coming! Totally agree about the short deck height. But I don't get your intake manifold and head port reason. I like it because it makes your engine efficient (burns everything and thus runs cool) and creates POWER without having to advance timing. The tighter the better. .035" seems reasonable for a bug motor.

Martin, you missed the most important part of Brian's quote.  The part that said it came from David Reher.  He's not talking about the space from piston top to cylinder top deck.  He's talking about NHRA pro stock V8 block deck.

[Martin S.]

Ok now that makes more sense to me. The priority for those engines is the intake. That's not the same as for a VW engine where the priorities could be increased cost or fitting the motor in the car. We're lucky that modifying the rod length, along with barrel length is fairly straightforward. You're simply missing out on power/rpm if you don't go for it. My engine builder had a one track mind and that was to build it the best with the parts I felt like paying for. He said I could easily open up / match the intakes, bolt on some dual carbs and go win some races with the engine.