Long Rod v. Short Rod

Let the debate begin.

Interested in seeing what peeps gotta say.

 

our we talking in general with the same crank stroke?

 

Yes.

Building a Honda BVTEC/HVTEC/KVTEC with same stroke but you can play with the rod length.

 

for the mad scientist

I thought this was going to be another Bore vs stroke debate.

But same crank stroke and different rod lengths should be an interesting topic. . .

<- - - - Will be Reading patiently

 

If custom rods were used I'd go with a longer rod. Could be similar to a stock rod length weight. More dwell time at TDC. Take advantage of that with quench (higher compression too depending on the bore). Possibly move the powerband up if everything else is setup right since the engine won't want to choke as much. Decrease side loading for some improved frictional losses. Assuming the pistons are custom you can play around with the design to get some weight off. Thats my basic thoughts on it.

 

here's a discussion i had awhile back with CC
https://honda-tech.com/zerothread?id=1891097

on paper, a longer rod, netting a higher rod/stroke ratio, is supposed to allow you to rev the engine higher at a safe level. This is because less force is being applied as a result of piston thrust (side loading) on the cylinder walls. also, the rod bearings will be taking less of a beating due to "better" attack angles on the power stroke which is likely the biggest worry, in my opinion, when wanting to rev a motor - spinning rod bearings.

also, from another perspective with the longer rod, its going to dwell at TDC longer which means its going to sit at peak cylinder pressure longer than a short rod. I could see that as a good thing and a bad thing. The good - power loading. It hold peak cylinder pressure longer which could mean more power. The bad - If you have a hot motor, sitting at peak cylinder pressures is going to create more heat which induces hot spots which in turn increases the likelihood of pre-ignition.

and now the shorter rod. Pretty much the exact opposite of the longer rod. Shorter rod is reducing the rod/stroke ratio and on paper means the motor cannot safely be spun as high as the longer rod. Shorter rod on the same crank is going to net a lower static compression which means you are going to have to have a custom piston made with either a lower pin housing location, taller compression height, or a bigger dome to get the same compression as the longer rod...but then even then, you dont know if the air will flow as well being as they are two different piston designs.
The shorter rod is going to have a shorter amount of time at TDC which to me means you could possibly throw my ignition at a high compression motor on pump gas and get away with it since the dwell time is shorter and the piston speeds away from TDC are going to be greater than that of the longer rod.

my opinion, this theory between long rod vs short rod can be control through engine balancing, bearing clearances and oil control.

i could be completely wrong in everything i said but i guess that is what is so great about a discussion of opinions

 

for most of us, the combo that nets ~ 1.58:1 R/S ratio is the best compromise between longevity and max usable power output (when displacement is constant).

Longer rods are heavier, but weight is somewhat offset by reduced angle/load against cyl walls/bearings/journals/etc. longer rods mean longer piston dwell near tdc and slower breathing per rpm.

 

Whatever fits between the crank pin and the wrist pin.

 

What is the ideal rod-stroke ratio?

1.49 B20A
1.54 B18B
1.58 GSR
1.74 B16A
1.78 B16B
1.99 VW Air-Cooled Type I

 

there is really no ideal rod stroke ratio...it's all depending on what type of torture the motor will be subjected to whether it being road racing or street ripping...

just need to be realistic when building any combination they are two different worlds and both function differently during the combustion process everything depends on what an individual wants out of their set up...

everything will mostly come down to the proper clearances, oil, and maintenance as stated earlier...and lets never forget about a good tune

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">What is the ideal rod-stroke ratio?

1.49 B20A
1.54 B18B
1.58 GSR
1.74 B16A
1.78 B16B
1.99 VW Air-Cooled Type I
</TD></TR></TABLE>

ideal to who? and what really determines ideal?

 

No ideal??

How about 1.37 R/S is that worse than 1.74?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">No ideal??

How about 1.37 R/S is that worse than 1.74?</TD></TR></TABLE>

where is the evidence other than paper theorists that a long stroke and short rod is actually bad?

if its so bad, how did randy rev his h23vtec to 9k for over 2 years and never run into problems?

my answer: there is more to rod/stroke than just a long stroke/short rod is bad. You have to include all the clearances of a motor into longevity just like you do in any engine build.
My opinion is that if you spin a rod bearing on a low r/s motor, you cant immediately blame it on having low r/s.

 

With the quality of materials available today for bearings, rods, craks, pistons, piston rings, valve springs, and cylinders you are not limited to what books of the 60's, 70's and 80's recommend.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">What is the ideal rod-stroke ratio?

1.49 B20A
1.54 B18B
1.58 GSR
1.74 B16A
1.78 B16B
1.99 VW Air-Cooled Type I
</TD></TR></TABLE>

i was under the impression that a b20 had a 1.54 r/s ratio like the ls?

 

B20B (84x89) yes. B20A (81x95) no.

 

B20A has a 95mm iirc stroke.

 

Im a complete noob to this but I am more than willing to know.... can you use a B20A crank with some rods to make a stroker kit?

 

yeah ive pondered this subject many times in my head but logic and real world application seem to clash pretty hard. like stated there are some pretty r/s ratio motors humming along at more than 9k so its hard to say how much of an impact rod length plays. I remember reading about this article in hot rod magazine where they built a smallblock 400 with a 327 crank, since they were able to used a 6.25" rod vs 5.7" or 5.9" they noted better detonation tolerances and torque didnt suffer too bad just shifted the powerband a little father up. it was called the 350 chevy should have built. Like stated tho, with todays technology advances in the auto field, we can get away running less than "ideal" setups including r/s ratios

 

Yes but it takes some work. search for user 1700anddropping. He has been banned, but he discussed it on here with some other members that have also done it.

 

even though you can get away with more with todays material,i think the cylinder walls will wear more on a lower r/s ratio motor[most dont care including me ]
i would like to play with a b16 block,rods and ls crank with custom pistons. you can still have the 1.8,a better r/s ratio,but i may shift the powerband higher though. seems like a fun/cheap setup,besides custom pistons.

 

So far looks like people prefer short rod or don't have a preference.

I have come up with an ideal for Honda BVTEC in particular.

What am I famous for? Starts with a "C."

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Master of the Universe &raquo;</TD></TR><TR><TD CLASS="quote">So far looks like people prefer short rod or don't have a preference.

I have come up with an ideal for Honda BVTEC in particular.

What am I famous for? Starts with a "C."</TD></TR></TABLE>

excuse my dumbness,are we talking ideal as in longevity or performance. if performance i would say somewhere around 1.61[best of both worlds IMO]
longevity would be the higher the better,but know torque. crotchrockets have something like a 2.1 dont they?

 

We're talking you want to win a race every now and then.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by 4g hatch &raquo;</TD></TR><TR><TD CLASS="quote">

excuse my dumbness,are we talking ideal as in longevity or performance. if performance i would say somewhere around 1.61[best of both worlds IMO]
longevity would be the higher the better,but know torque. crotchrockets have something like a 2.1 dont they?</TD></TR></TABLE>

h22 ftw