Rod length

[David Murany]

Ok so we have another development with the steel rod in the AA engine. Just recieved a HR rod and it is too short at 36MM. So it looks like Andys idea of a liner shim with the VAC rod may be the best way to go. Although the case might have to be port matched due to the liner sitting higher. Will post any additional info that I find.

Dave

Even when considering the extra work of cutting case ports to match the sleeve ports, the longer rod will be the more practical thing to do. The short rod could work, but the shoulder on the sleeve would need to be cut an extreme amount to correct the timing.

100% agree Andy. Trying to make the short rod work would be a real PIA!

 

[Brad Christy]

Andy, others,

Without getting into case volumes and such, isn't a longer rod better for performance, anyway? At least in theory? I know there are about a bazillion other factors that work into this equation, but this has always been my general understanding.

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

Model Machine And Precision LLC

 

[Andy Brown]

Andy, others,

Without getting into case volumes and such, isn't a longer rod better for performance, anyway? At least in theory? I know there are about a bazillion other factors that work into this equation, but this has always been my general understanding.

Thanks. Brad.

Titan Racing Components

BlackJack Hydros

Model Machine And Precision LLC

I think the longer rod is better in general. But it does depend on what one wants to accomplish. Long rod to stroke ratios make better top end power at the cost of low end torque and acceleration.

Short rod to stroke ratios provide killer torque and acceleration, but run out of revs quickly. This holds true for both two-stroke and four-stroke engines.

This considers all other parameters equal.

Shorter rod ratios tend to make better race engines for most drivers.

In terms of our two-stroke engines I consider ratios under 1.75/1 to be short and over 1.9/1 to be long. I designed all MAC engines between 1.8/1 and 1.85/1. I have run as short as 1.55/1 and as long as 2.14/1.

Here is a short list of engines and rod ratios that I know off the top of my head.

Old Picco 67 - 1.85/1

Picco 67 EXR - 1.96/1

MAC 67 - 1.8/1

Old CMB 67 Evo 1.87/1

CMB CAM 67 1.92/1

CMB 67 Greenie 1.8/1

Picco 80 - 1.65/1

MAC 84 - 1.8/1

Old Picco 45 -1.85/1

New Picco 45 - 1.75/1

Old CMB 45 - 1.78/1

New CMB 45 - about 1.83/1 ?

MAC 45 - 1.85/1

Nova 46- 1.68/1

K-90 - 1.73/1

Old CMB 90 -1.9/1

New CMB 91 about 1.8/1 ?