rod lenghts: explain this to me like i am a 2 year old.

[nicolas]

i got a bit cross yesterday as i got into a discussion about rod lenghts. so the guy tells me rod lenght has no effect what-so-ever on an engines caracteristics or performance. only more friction with longer strokes and if vw could have fitted rods off 10 inches they would, but they were restricted due to the tyres being in the way.

so i begged to differ and said that it did matter, i tried to explain the different angle made the piston move differently (faster and snappier), but he waved it away. so i don't mind that, but i have come a long way and have used parts off my brain to come and understand different lenghts DO make a difference in an otherwise exact same engine. but i couldn't explain it. 

so help me out please.

[John Rayburn]

Rod length will effect where your tune lies. In identical engines, a shorter rod version will have snappier bottom end and torque and tend to rpm less (great for a full weight street car) while longer rod version will have a lazier bottom end, will come on later, and rpm more.

[nicolas]

Rod length will effect where your tune lies. In identical engines, a shorter rod version will have snappier bottom end and torque and tend to rpm less (great for a full weight street car) while longer rod version will have a lazier bottom end, will come on later, and rpm more.

i said exactly that. he said 'no way!'

so i did get it right.


BTW where are the empi bumperbars? 

[Rasser]

A perfect example of this would be a type 1 1.6L engine, compared with a type 4 2.0L engine. The 2.0L type 4 engine have smaller rod length. I bet that back then Volkswagen knew what they were doing when they decided to put short rods in the engine that were going into a full weight camper or van.
Actually we should be talking rod ratio, and not rod length ?

I seem to remember that a rod ratio above 1.6 is "safe" - can anyone come with some guidelines regarding rodratio ?

Great thread Nicolas, Not many people think about rod ratio when building their engines!

[John Rayburn]

I run a 1.66 rod ratio , 84 with 5.5 rod and it is real snappy. I think Bill Schwimmer is at 1.63. Jeff Denham has told me he'd like to see if he could get away with a 1.59 ratio in one of his engines. The type 4 analogy is great, and I always felt the same thing, that they ran the short rod to get the heavy vehicle moving.

Rod length will effect where your tune lies. In identical engines, a shorter rod version will have snappier bottom end and torque and tend to rpm less (great for a full weight street car) while longer rod version will have a lazier bottom end, will come on later, and rpm more.

i said exactly that. he said 'no way!'

so i did get it right.
                                                         I took the bumper bars off many years ago when the reproductions showed up on every single car ,at every single event. When enough cars take them off, I'll put mine back on. Oh, and your friend is wrong, it's like arguing about air density, you can have a counter opinion, but air density has physical law to back it up.

BTW where are the empi bumperbars?