I don't know if anyone saw the new D-sport mag--but they did a comparison on a short vs. long rod motor. they didn't change any of the variables--they even kept the same program when switching motors. They were both Dart Blocks with 84mm bores--equaling 2.1 liters. One of them had a 1.62 r/s and the other one had a 1.47 r/s--Much to everyones disbelief the short rod motor--CONSISTENTLY made from 8-12 hp more everywhere in the rpm band. They theorized it was because the since the piston speed was greater the air was rushing in to the cylinder at a greater rate of speed. Pretty interesting stuff.

 

what about torque?

 

By just changing the r/s ratio and not optimizing the cam for each engine you may not see an advantage of one over the other.The piston is in a different part of the stroke relative to the cam in the two different combinations.What a dyno test won't show is the reduced stress advantage of the longer r/s ratio.I am by no means a high ratio fanatic.I have built many competitive low ratio engines.
Glenn

 

What was said above is 100% perecent correct. A low R/S engine will also like larger ports on the intake side due to the increased velocity of the first 90 degrees of crankshaft rotation on the intake stroke. Way too many variables to conclude that one is just 'best'.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by d16dcoe45 &raquo;</TD></TR><TR><TD CLASS="quote"> They theorized it was because the since the piston speed was greater the air was rushing in to the cylinder at a greater rate of speed.</TD></TR></TABLE>

just to clarify for those who are new to r/s ratios... the average piston speed is the same regardless of the r/s ratio... but the peak piston speeds are different, at a point in the stroke where some theorize that it'll have an effect on cylinder filling... same with the torque curve, the total area under the curve does not change.

i have heard of dyno tests on aircooled vw motors that showed no change in output for changes in r/s ratios... the advantage to testing with a vw motor is that the deck height is infinitely adjustable, so it stays the same to match the rod length... i'd be curious to know how the dsport test handled the c.r. changes that come with a honda block that has a fixed deck height and variable rod lengths.

 

Well--they used Dart blocks--perhaps one of them was the higher deck block; for the long rod, and the other Dart block was the standard short deck--to be used with the short rod. According to the article--pretty much everything was kept constant--valve size, cam, valve spring, compression, piston style, displacement--in fact they used the same tune for both engines--if each setup was optimized I don't know what the outcome would be--it was still pretty neat though. I am sure its true that the longer rod motor IS more durable because of less cylinder side loading--but how MUCH less durable is the short rod motor going to be? Would it really matter on a hot street motor that might only go 10,000-15,000 miles between rebuilds?

 

And actually the total area under the curve DID change for both torque and horepower--the increases they discovered were not just peak--they were everywhere from 4800 to 8600!! 4800 is where they started measuring--so in all situations in this particular test--the short rod motor would be FASTER than the long rod motor. 1.47:1 to 1.62:1

 

It is far more complicated than that.

Side loading isn't the main durability issue with low R/S motors, its piston accelerations. There are several good threads on here, if you look around.

 

I understand why a short rod motor is going to be less durable--side loading IS a big part of it--although with Hondas seem to be able to get away with it because the bores are so small, therefore the rod isn't it at a huge angle---but about the acceleration/deceleration issue--what is that mainly going to affect in a negative manner--the piston pin, the bearings, the rod--or all equally?

 

Piston accelerations are a MUCH bigger part of it. Side loading affects longevity far more that overall reliability. Its much less of a factor on a fully built motor with forged pistons, since they need to be reringed more often anyways.

These things affect all of those parts, but it usually manifests itself in the rod bearings before anything else.

 

On top of that short rod engines tend to be a lot more nitrous friendly.

 

If D-Sport built 2 Identical 2L Engines

Would the horsepower be within 4 - 5whp of each other?

Other than that it was a very good article.

 

yeah they are too many factors, it is nearly impossible to get an accurate test of this. The tune likely could have gave an advantage to only one motor. But there is another thing about a optimal R/S ratio, you can safely push the motor a little harder without increased stress. Also typically short stroke/high R/S ratio engines can produce torque at higher rpms therefore allowing more overall power. For example most b-series can make equal or more horspower than h22's because they have an advantage of producing higher specific output with rpms vs. the H22. But of course the h22 will still produce much more torque.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by d16dcoe45 &raquo;</TD></TR><TR><TD CLASS="quote">because the bores are so small, therefore the rod isn't it at a huge angle</TD></TR></TABLE>

The angle of the rod has nothing to do with the bore size, so long as the centerline of the piston and the crank are in the same plane, ie. no offset crank.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by danimal &raquo;</TD></TR><TR><TD CLASS="quote">
same with the torque curve, the total area under the curve does not change.
</TD></TR></TABLE>

what i meant there was the total area under the curve as a function of the rod angle, not the actual engine torque.

thanks for the heads-up on the article, i just got it today... michael did make all of the same disclaimers in it that others have posted here, but i suspect that this article will be mis-quoted for years to come

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by drdisco69 &raquo;</TD></TR><TR><TD CLASS="quote">
The angle of the rod has nothing to do with the bore size, so long as the centerline of the piston and the crank are in the same plane, ie. no offset crank.</TD></TR></TABLE>

i think that he might have been referring to the skirt of the piston hitting the rod(???), i have seen that on some non-honda motors, the inside of the skirt had to be clearanced.