Can we please put an end to the "long connecting rod" myth??

[hoffman900]

Dot Heton,

It's a great discussion. There is nothing magical about what the engineers and gang did with these motors - this is what engine builders and engineers do. Computers allow us to quickly calculate and plot out these things.

Because it might come up, the total area for Piston CFM demand for each is as follows:

136mm rod: 30813.1cfm @ 28"
145mm rod: 30812.8cfm @ 28"

This is a for a 75mm bore x 74mm stroke @ 7200rpm.

 

[Tomterrific]

Since the XS650 has a (approximate) 3" stroke, a rod almost half the length of the popular long rod (3") would have the piston at BDC at only the first 90°. The next 180° would dwell at BDC with the last 90° zooming up to TDC.

Ports too big? Cam too wild? Short rods rule!

Tom

Ps. This is a joke for those who get it.

 

[Brian G]

This is great. I'm glad it went from name calling to a thoughtfull process.

+1 on that! Still, the thread contains both passion and science - a good combination in motorsports! Thanks to all who contributed to a great read - I'm frequently amazed at the knowledge and experience which often remains unseen in these forums. Reminds me of the 'Triples Online' forum for BSA/Triumph triples, where several of the members either worked in the factories, raced or built race engines. They don't post a lot but when they do it's worth paying attention. They don't always get along either...

Regards,

 

[xjwmx]

What an interesting and revealing thread! The early defenders of the long rod could not say why it was the benefit they were claiming it was. Which means they could not have effectively used it anyway. Certainly couldn't have chosen a length optimal for their purposes.

But, it's not wise to fly in the face of conventional wisdom, which says here there is in fact some benefit somehow. Conventional wisdom can be spectacularly wrong, but only pretty rarely and pretty never about something that lends itself so well to analysis.

 

[Jack]

All this technical data makes for a good read but it's my believe coming from experience, that the three factors where most overlook when building a long rod motor is getting with a cam manufacture who'll design a cam around the engine combination,second most all overlook how the breathing characteristics change,no matter how small the degree changes,pumping loses will occur and it's this lose that most are feeling when the power delivery has been alter from the rod change. One way to combat this change is modify the intake ports to excel in to delivering a higher percentage of velocity rather than focusing on CFM gains cuz L/Rs don't require the same peak flow # as a short rod motor,it's basically follows the same ruling when installing a longer stroke crank .Anytime you change pistons speeds, whether by rods or cranks,don't over look the head,it must comply to the engine package.
Lastly long rod motors love "BIG CARBS" when that piston dwells at BTC , you need to take advantage of the incoming charge inertia by packing as much fuel and air as possible before the intake valve closes. One of the baddest long rod motors to ever hit the streets was the little monster chevy 302 with the cross ram or my all time favorite Mopar 340 behind a four speed packing 1000CFM thermo quad.

 

[hoffman900]

Jack,

This is a good discussion.

Piston velocity peaks .5 degrees later with a longer rod and intake draw is practically a wash 1 cfm @ 28" depression. Obviously, things are greater in a running motor, but for continuity sake (most heads are flowed at the same depression) we'll use it.

I would love to see people step outside the box in cam manufacturers. There are plenty of high end cam shops that aren't Web or Megacycle.

My impression is that pumping losses will be less - peak piston velocity is down with longer rods. Not a lot, but 20 FPM is something. A good exhaust will have scavenged most of the exhaust out of the cylinder before the exhaust stroke anyway.

Carbs are a function of port velocity.

For example, comparing the two in PipeMax (it shows other velocities as well, I just picked this one).

Average Intake CSA to reach 260FPS:
136mm: .888 in^2
145mm: .884 in^2

Average Exhaust CSA to reach 265 FPS:
136mm: .709 in^2
145mm: .706 in^2

This makes sense as a longer rod has a slower peak piston speed, which you mention in your post.

I would think increase charge inertia would mean going down in jet sizes as air speed through the carb venturi would be greater and create a larger depression. Has anyone had to decrease their jetting due to long rods?

I'm just not seeing how much of a difference in measurable flywheel power longer rods would make. The differences are minuscule.

I wish I had software like EngMod 4T to model and see graphical representations of pressure traces between the two. Anyone want to donate $400 to the cause?

Thoughts anyone?

Cheers,
Bob

 

[TwoManyXS1Bs]

Wow, I scratched a scab and got an arterial bleed! Sorry guys...

Hoffman: Thank you so much for the Ricardo friction report, and for your airflow analysis.

MrRiggs: Your sidewall thrust graph closely resembles mine, it's nice to know that there are other likeminded individuals out there.
Now, take the sidewall thrust values and multiply them by the torque arm (distance crank center to wristpin center) to get engine block anti-torque curves. These must match crank torque values, a good/easy way to confirm calculations. And like you said, the discussion was originally intended to address the mechanical advantage differences issue of long rod vs short rod.

I agree this effect is miniscule, not worth the effort for street bikes and gridlock blocked urban drivers, but is a fun study. I could've improved my et's if I would just drop my car keys at the launch line. Racers look for any miniscule advantage, a half car length at the end of the indy 500 is an extremely small fraction compared to the whole race, but it means 1st or 2nd.

I dragged-out my old engine simulator, but, of all things, I forgot how to do a screen 'snapshot'

 

[hoffman900]

I was curious so I did a quick search on the site.

As far as long rod impact on traction, I'm sure it has some, but what I could find shows flywheel weights were installed as well. Flywheel weights (or billet heavier flywheels) are sold specifically to preserve inertia and smooth out the powerband. I'm sure in conjunction with a heavier flywheel they have a net impact, but by themselves, I really don't know. Though, in racing, every little bit helps.

I've read the XR750s have the heaviest flywheels of any flat track bike and that's part of their success.

That being said, if I were building a race engine, I would not hesitate to put long rods in with pistons for the decrease load on the piston, rods, and crank by running a much smaller slipper style piston.

 

[goodgollyjosh]

I am a bit of a lurker on this forum and I am not here to argue or take sides, in fact, let me say that I have learned so much from you guys regarding the xs. It is also clear to me that there are some very bright minds on here regarding engines in general. We should consider ourselves fortunate, I doubt the Harley guys have such a technical group at their disposal. Now, regarding the rod controversy, I have the pleasure and sometimes frustrating task of working with automotive engineers everyday at one of the largest independent research facilities in the world and I have had the opportunity to pick the brains of former F1 engine designers. There are benefits to longer rods regardless of oppinions on dwell time at TDC. The benefits are clear from a friction and weight stand point as others have pointed out. When turning 18K rpm, a Ferrari F1 engine is battling 4 bar FMEP when a typical passenger car engine averages 2 bar friction at 6k rpm. Taking some of the side loading out of the equation and also taking some of the reciprocating weight and transfering that into rotating can be quite benificial. This particular engine had a stroke of 41.4mm and a rod length of 113.5mm, that is a rod to stroke ratio of 2.7, just food for thought. Thanks for a great forum guys!

 

[TwoManyXS1Bs]

Hey, goodgollygosh, welcome to the forum! I think I have an idea of where you work. My father worked there as a research physicist, and I used to fish in the test engine cooling pond (if it's still there). Back in the `70s, your group developed and patented an unusual central/shared chamber dual opposed piston dual crank engine, and I've always been curious what happened to that thing. Ever heard of it?

 

[goodgollyjosh]

Wow, it's a small world. I have heard rumors about that engine, I don't know what happened to it. I do remember the guys talking about Smokey testing his hot vapor cycle engine up here back in the day. I was fortunate enough to meet him at his last PRI show before he passed. Fishing in the engine cooling pond, thats hilarious. That pond is long gone. All of the water for the dynos and engines are self contained now in cooling towers and various heat exhangers... gotta love OSHA great to hear from a local!

 

[TwoManyXS1Bs]

Boy, I swear, the 'net' crashed 3 times while trying to post this.

OK, dragged-out and dusted-off an old engine simulator. This simulator is idealistic, in that it ignores friction losses, thermal losses, pumping losses, uses an idealistic fuel burn combustion formula, and is primarily concerned with vector forces, and only analyzes 1 cylinder. It's an enlightening and educational tool, as it allows you to dynamically change various parameters, so you can see the effect in real-time. Unfortunately, it doesn't do before/after overlaps like mrrigs chart. I understand Jack is building an 83mm long-rod, so these runs are named in his honor. Attached are (2) screenshots, one of 136mm rod, the other of 145mm rod, @ 8600rpm, all other parameters are the same. If you look closely at the sidewall thrust curves, you might see the 3% reduction in the long rod. The simulations also produce tabulated text reports, best viewed with Windiff. However, the reports are 107kb each, and the forum attachment limit is 20kb. Any ideas on how to post these?

 

[hoffman900]

Very cool!

I just upload my screenshots to a photobucket account. Take a screenshot, paste in MS Paint, crop, and save as a JPG.

Can you export your data as a .txt file which can be opened in Excel?

The data I used to create my graphs were from PipeMax that I did that to.

Let me do what I did again for 8600rpm as that should be about right for a no hold barred race motor at redline.

edit:
Well, I did the above, but the magnitude between the two didn't change any.

However, gains from less friction would change as it increases with the square of speed.

 

[hotdog]

This is great - the more info the better for my 144mm LR motor....

 

[TwoManyXS1Bs]

Yeah Hotdog, you get a freebee. The tabulated .TXT report files that were produced could be imported into excel, but it would be huge. As they are, Windiff will take both files and do a line-by-line comparison, which is real handy when grinding down thru 720 degrees of formatted data.

Now, did I goof-up again? Is Jack doing a 78mm bore and Hotdog is 83mm?

You know, my days are largely blissful senility interrupted by moments of terrifying clarity...

Hotdog, if you want them, I could e-mail you these 2 (107kb) files.

 

[hotdog]

My bore is 80mm (750cc) with the longer CR500 (144mm c-c) rods.

Thanks for the offer - I will PM you my address.

 

[TwoManyXS1Bs]

Ok, .txt files sent. I'll do another sim of 80mm bore with 144mm rods. What rpm you want?

 

[hotdog]

Ok, .txt files sent. I'll do another sim of 80mm bore with 144mm rods. What rpm you want?

8500rpm - thanks.

 

[TwoManyXS1Bs]

Hotdog, I should've asked this before...

I used .846 lbs (298g piston, 65g pin, 21g rings) from another post's listing of Ross 80mm pistons. Shall I retain that value for the top reciprocating mass, or would you like another?

I used 40° as the spark event, another post here reported using other values 36° - 32°, what spark timing you want?

What compression ratio you want?

Got you at 8500rpm, full throttle. Runs will be for 136mm and 144mm rods.

 

[hotdog]

Hotdog, I should've asked this before...

I used .846 lbs (298g piston, 65g pin, 21g rings) from another post's listing of Ross 80mm pistons. Shall I retain that value for the top reciprocating mass, or would you like another?

I used 40° as the spark event, another post here reported using other values 36° - 32°, what spark timing you want?

What compression ratio you want?

Got you at 8500rpm, full throttle. Runs will be for 136mm and 144mm rods.

Yes use those figures, they are for the custom Ross slugs I've had made. Thanks.