rod-to-stroke ratio defined

rod ratio is calculated by taking the connecting rod length and dividing it by the crankshaft's stroke.

A good rod ratio is considered to be 1.65-1.77. Some people say the ideal rod ratio is 1.75:1 or 1.77:1. Guess what? the Civic b16a 's rod ratio is 1.75:1 and the CTR b16b is 1.86:1. Co-incidence? I don't think so. Honda has lots of race engineering experience with high revs in it's Superbike and Formula 1 N/A 18,000+ rpm. They trickle this knowledge and tech down to us common folk in a street huh?

Unfortunately, the integras got a low or short rod ratio: b18b 1.54:1 and B18C 1.59:1...not ideal.

So what is the importance of rod ratio anyway?

rod ratio describes piston geometry (3 things):

1. piston speed away from TDC and BDC
2. piston dwell time at TDC
3. the amount piston sideloading against the cylinder wall as the piston travels up and down the swept volume.

So rod ratio can affect how high you can rev, since a low rod ratio places more side loads on the wall.

The rod ratio in a naturally aspirated engine affects how well the cylinder is filled and dictates cylinder head port size. The faster the piston pulls away from TDC on the intake stroke means you can get more suck to fill the cylinder. How fast the piston transitions or flip-flops from squeezing the exhaust gas out at TDC for the exhaust stroke to dropping down and begin filling on the intake stroke (i.e. TDC dwell time) affects your cam overlap and cylinder filling.

low rod ratio engines have short piston dwell times at TDC and fast piston speeds away from TDC (or faster piston speed dropping down on the intake stroke compared to a long rod ratio). So a low rod ratio motor generates high flow velocities for filling through the intake port at low-mid rpm. These engines like bigger cylinder head intake port sizes compared to a long rod ratio motor like the Civic Si's b16a.

Low or short rod ratio ALL MOTOR engines like a cam with more lobe separation angle, more duration, and more cam overlap, since it has short piston dwell time at TDC and needs help scavenging in fresh air/fuel.

This all has to do with revving ability, proper intake port sizing, and cylinder filling IN AN ALL MOTOR SETUP which depends on passive filling using lower pressure in the cylinder (called pressure gradient) compared to the atmospheric pressure (14.7 psi). In turbos and SC's, you push in the air to fill the cylinder and so rod ratio plays a very MINOR role in cylinder filling.

The importance of rod ratio in a boosted engine relates to how efficiently the inert burnt exhaust gases are removed from the cylinder after combustion. The piston speed away from BDC to push the exhaust gas out is important. Remember exhaust gases aren't burnt twice and cannot make power and so if they aren't removed, they just take up space in the cylinder ... preventing room for fresh air/fuel from coming in to do their job. Burnt exhaust gases are like unwanted tenants not paying rent: you want to evict them from the cylinder.

Short piston dwell time at TDC is less important in a boosted engine because you don't want big cam overlap. More cam overlap here would cause some of the boost to shoot over into the turbo manifold instead of going into the combustion chamber (assuming the boost pressure is higher than the exhaust manifold pressure). You don't need big overlap cams to help filling. In fact, boosted engines prefer big lift but short duration and overlap.

So a low rod ratio actually helps a boosted engine, since the piston speed away from BDC is higher than in a long rod ratio engine (to help evacuate the extra burnt exhaust gases) and the importance of dwell time on filling and cam selection is less important for filling.

JUST REMEMBER: don't rev the sh*t out of a low rod ratio engine (all motor or boosted)...the shorter rod ratio still causes the piston to sideload the cylinder wall harder causing more risk of a piston going through a wall, the higher the rpms go (keep it under 8100 rpm).

If you want to read more on rod ratio, check this site out:

<3 tuan & temp

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