5.965 Rod on 3.625 crank - thoughts

[getchasum]

Planning a stroker 4.1 build, I have the 3.625 crank and a line on a great deal on a set of H beam 5.965 Rods. Sure I'd prefer a 6.35 rod But.....

I was hoping some of the engine builders would chime in to give me their thoughts on the short rod with a stroked crank?

 

[Paul Lohr]

This is a pretty good read on the topic of rod to stroke ratios. Even if you don't understand all of it, I think you will get the main points by reading it over once.

Piston Motion: The Obvious and not-so-Obvious, by EPI, Inc.

Figure 9 is especially telling - this graph shows a piston's velocity, acceleration, and travel when it is connected to a rod and stroke that develops a 1.54:1 R/S ratio in comparison to a 2.56:1 R/S ratio. The difference is very little even though the R/S ratios are very much different.

If you use a 5.965" rod and a 3.625" stroke, your R/S ratio is 1.65. Say you go with the 6.350" rod and the same 3.625" stroke, your R/S ratio is 1.75. Remember the figure 9 graph - this was a comparison between R/S ratios on extreme ends of the spectrum. The differences in piston velocity, acceleration, and travel were small (of all the characteristics, 5% was the minimum and 10% was the most). The difference in maximum side loading is 2% according the Figure 10. Still not all that much. Think of how small the differences would be in piston velocity, acceleration, travel, and side loading if the figure 9 graph compared a 1.65 and 1.75 R/S ratio.

I am open to any corrections or additions.

 

[turbofabricator]

The general consensous amoung top engine builders it to NOT dwell on the piston dwell time at TDC. In other words, it really doesn;t make any measurable differnce. Put the longest rod you can in an engine, but don't design an engine around rod length. Look at IHRA Pro Stock engines. They have rod/stroke ratios in the 1.57:1 region and they still make HUGE power. There just isn't any room to install a longer rod. Simply put, even if you could run an engine on a computerized dyno, you are not going to read ANY difference in power output or even longevity between a 5.97" and a 6.35" rod. There are guys that'll argue the math, but not many can quantify with running engine numbers. I have spent hours and hours researching the issue, and usually the guys that claim it makes a difference have no real working numbers to show. Just a bunch of theories and math. A bumble bee can't fly on paper, either.
If you have a good deal on the 5.97" rods AND you can get a good piston with the right deck highth for reasonable money, run 'em. NOthing wrong with using a longer rod, though. "IF" it makes a little less power using a shorter rod (like MAYBE TWO HP), TURN THE BOOST UP!

 

[Nick Micale]

Ken said it all in his post, do not get involved the "math" of what is theoretically best, real world numbers are to small to measure where you are going with that build.

I recently built 4.1 3.625" stroke with the shorter rods, and do not know how you could tell any difference, maybe at 7-8000 RPM a couple HP, not that it went there :smile:

 

[drturbo69]

Building same 3.625 bms narrow 5.96 bms rods j&e 3.860 stage 1 block M&A HEADS.
DOC

 

[getchasum]

Thanks for all the reassurance Guys......Sometimes the files that one creates in their one mind is the hardest thing to over come.

Interesting read, Thanks for the link. :smile:

 

[tonysmach]

I am not an engine builder, but would the shorter rod take less machining for clearance in the block than the longer rod when building a stroker engine?

 

[turbofabricator]

No, rod length has no effect on stroker clearance. IT is the rod bolt that hits the block at the oil gallery. Most of the time trimming the block is all that's needed. Some blocks with more core snift require the use of a thin wall tube epoxied in pace to seal the gallery if you trim too much. The other issues are the threaded end of the rod bolt will hit the cam lobe. Though, with the current stroker kits, I haven;t had this problem.

 

[tonysmach]

Ken, what stroker kits do you use where there is minimum clearance issues?

 

[Paul Lohr]

Glad this topic came up - it forced me to dig into something that I have needed to look at. I would like to share a summary from an SAE paper that might shed light on why the rod/stroke ratio is not all that important (assuming the rest of the engine setup does not change).

Effect of the Ratio Between Connecting-rod Length and Crank Radius on Thermal Efficiency

They numerically increased the rod to stroke ratio in the test engine and realized decreased thermal efficiency. Putting the fuel in at a later time (using stratified charge or direct injection) and therefore being able to push the combustion off until later helps to reduce thermal efficiency losses. I think this is because the piston's average motion is higher - we are lessening the time where the heat of combustion is exposed to a slow moving piston. The dwell time of the piston at top dead center is an opportunity for heat energy to escape through the cylinder walls, combustion chamber, and piston.

 

[turbofabricator]

Ken, what stroker kits do you use where there is minimum clearance issues?

I've used both Full Throttle and DLS kits with good results.