B18 rods vs. B20 rods size... interchangeable?

[ncctek]

are the rods in a b18b the same as a b20?
would the rods from a b18b work in a b20? and visa versa?

or would that change the stroke and all..

im looking at buying a set of forged rods, but not 100% sure if im going b20 bottom end, or b18 bottom end

or if im going vtec or not... but is it safe for me to buy the rods for lets say a b18 and end up using them for a b20?
or buy them for a b20 and use them for a b18?

thanks guys.

[HBoy]

The rods are the same for those two motors but rods have nothing to do with changing the stroke.

Yeah, rods locate the piston relative to the crank, and it's not something you'd want to change much without also changing the pistion to account for the difference.

As an aside, I was reading about an interesting trick - building a so-called 'long rod' engine involves using a longer rod, and having a piston in which the wrist pin is moved upward towards the crown to make up for the added rod length. The result is a reduction in angularity between the rod and crank and rod and piston, which helps to smooth out the engine, reduces reciprocating drag, and actually lets you apply more work per combustion event to the crank (more power for same fuel). Neat stuff.

With the discussion about long rod motors don't forget about how it changes dwell time. Depending on the cam you have that can make a huge difference in power output.

-Michael

Michael, is dwell time the degrees over which the piston is at TDC?

Yes. Don't have a lot of time to get into the details but there are a lot of variables here that work together. For example if you don't have a lot of dwell at the top then an increase in compression ratio makes a bigger difference than in an engine that has a lot of well.

-Michael

That's interesting. Is it because the increased dwell time means there's more of a chance for the increased compression to slow the rotation of the crank and so increased compression means increased losses just before combustion?

No, the dwell time at the top helps on lower compression engines because it gives more time to evacuate the exhaust before the piston begins to descend. Mechanically it doesn't make that much difference in rotational losses. The smaller the vertical movement of the piston, the smaller the rotational force required to move the crank. Engineers have worked most of this stuff out for you and the tiny differences from making these changes are difficult to find unless you end up building a poor combination. The most common problem I see is too aggressive of a cam on a stock compression ratio engine - fixing the compression is more effective than playing with rod length. With all the stuff working properly together we're talking about a difference in a few foot pounds of torque. Unless you're building an all-out race motor don't worry too much about it.

-Michael